Yanzhuang Meteorite: Mineralogy and Shock Metamorphism 9811507341, 9789811507342

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Xiande Xie Ming Chen

Yanzhuang Meteorite: Mineralogy and Shock Metamorphism

Yanzhuang Meteorite: Mineralogy and Shock Metamorphism

Xiande Xie Ming Chen •

Yanzhuang Meteorite: Mineralogy and Shock Metamorphism

123

Xiande Xie Guangzhou Institute of Geochemistry Chinese Academy of Sciences Guangzhou, Guangdong, China

Ming Chen Guangzhou Institute of Geochemistry Chinese Academy of Sciences Guangzhou, Guangdong, China

ISBN 978-981-15-0734-2 ISBN 978-981-15-0735-9 https://doi.org/10.1007/978-981-15-0735-9

(eBook)

Jointly published with Guangdong Science and Technology Press, Guangdong, China, 2018 The print edition is not for sale in China Mainland. Customers from China Mainland please order the print book from: Guangdong Science and Technology Press. ISBN of the Co-Publisher’s edition: 978-7-5359-6835-7 © Guangdong Science & Technology Press Co., Ltd and Springer Nature Singapore Pte Ltd. 2020 This work is subject to copyright. All rights are reserved by the Publishers, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publishers, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publishers nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publishers remain neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Preface

Hypervelocity collisions between asteroids, presumably chondritic parent bodies, can cause the shock metamorphism of rocks and minerals in sphere of action of shock wave. The studies on the shock effects of rocks and minerals in structures, chemical composition, solidification and crystallization characteristics have great significance in the investigation of evolution of celestial bodies, geology of high pressures and high temperatures, as well as in material sciences. It is well known that the L group chondrites are more heavily shocked than H and LL group chondrites, and only detailed systematic petrographic and mineralogical studies on shock effects in L group chondrites, especially the L6 chondrites, were curried out during the last tens of years, but studies on shock effects in H group chondrites are rare for the rarity of strongly shocked H group meteorites. The Yanzhuang meteorite is a very strongly shocked stony meteorite, which fell on October 31, 1990 in Yanzhuang, Wengyuan County, Guangdong Province, China. Right after the fall of this meteorite, a group of scientists from the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (CAS) conducted field survey and collection of Yanzhuang meteorite samples. Five pieces in total weight of 3.5 kg of this meteorite was collected. The largest piece is of 823 g in weight. This meteorite is composed of light-colored severely deformed chondritic mass and dark-colored thick melt veins (up to 1.5 cm in width) and large melt pockets (up to 3  4  2 cm in volume). This implies that Yanzhuang meteorite had been subjected to violent impact event in outer space. A group of scientists headed by Prof. Xiande Xie in the Guangzhou Institute of Geochemistry, CAS, then conducted a series of study on collected samples and revealed that the Yanzhuang meteorite is a unique chondrite with most abundant shock-induced melt (more than 30% in volume) among all known shock-meltbearing chondritic meteorites. The modern micro-mineralogical experimental techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron probe microanalysis (EPMA), Raman microprobe spectroscopy (RMS), instrumental neutron activation analysis (INAA), X-ray micro-diffraction analysis (XRMD), micro-proton-induced X-ray emission (PIXE) analysis and laser ablation inductively v

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coupled plasma mass spectroscopy (LA-ICP-MS), have been used to investigate the mineralogy and the shock effects in the Yanzhuang chondrite. The micro-structural and micro-morphological characteristics as well as chemical composition of minerals (phases) were studied in detail. This meteorite was classified as an H6 chondrite on the basis of chemical composition and petrologic features, and evaluated it as a very strongly (S6) shock-metamorphosed meteorite. The mineral composition of this meteorite consists of olivine (40%), low-Ca pyroxene (30%), clinopyroxene (4%), plagioclase (4%), kamacite (10%), taenite (4%), troilite (6%), and small amount of merrillite and chromite. The results show that the shock-induced temperature and pressure distribution in the Yanzhuang chondrite was very non-uniform, hence, various transformations both in physics and chemistry, may have been favored in various places depending on the details of the propagation of the highly complex pulse of temperature and pressure. In the weaker action areas of shock wave (25–35 GPa): olivine and pyroxene display 4–5 sets of planar fractures, high screw dislocation density (from 103 to 107 mm−2), small size of mosaic blocks (from 20 to 2 lm) and greater difference in crystallographic orientation between neighboring mosaic blocks (from 1° to 10°); plagioclase has partly been transformed into maskelynite, several sets of Neuman lamellae have been observed in kamacite and taenite; mosaicism in troilite is widely spread. In the stronger action areas of shock wave (45–60 GPa): the brecciated and disorder structures in olivine and pyroxene grains were produced in the prevailing areas of stretching stress; the solid recrystallization and dislocation climbing of olivine and pyroxene were induced in the areas of higher temperature; olivine and pyroxene were transformed into diaplectic glass, melt glass and high-pressure phases in the areas of higher pressures, higher temperatures and fast cooling; kamacite and taenite were partly quenched into martensite, or partly melted and recrystallized; the extensive melt and recrystallization of troilite were produced. In the strongest action areas of shock wave (>60 GPa): the chondritic mass was melted in-situ and in whole rock, forming dark thick melt veins and large melt pockets which were composed of microlites of recrystallized olivine and pyroxene, FeNi + FeS eutectic blobs in different forms, silicate melt glass and some remained mineral detritus of chondrite; dissociation and vaporization of many kind of minerals including silicates, FeNi metal and sulfides took place to a certain extent; the solidified and recrystallized FeNi metal were mostly quenched into martensite or austenite. Besides, the achievements in micro-mineralogical investigations of the Yanzhuang meteorite also include the following new findings: 1. Natural diaplectic olivine glass and diaplectic pyroxene glass were discovered for the first time in this shocked Yanzhuang meteorite. The diaplectic olivine glass is identified by the stronger Raman frequencies at 1108 cm−1 and 1180 cm−1, and the diaplectic pyroxene glass by the weak Raman frequencies at 816–840 cm−1.

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2. As early as in 1992, high-pressure phase minerals including ringwoodite and majorite were found for the first time in H group chondrites. The ringwoodite in Yanzhuang occurs in two different forms: the first is allotriomorphic granular ringwoodite which nucleated and crystallized from diaplectic olivine glass, and the second is euhedral ringwoodite which crystallized from shock-induced silicate melt. It was found that back transformation from spinel (c-phase) to modified spinel (b-phase), and to olivine took place for more than 90% ringwoodite after the unloading of shock wave. Majorite nucleated and crystallized from diaplectic pyroxene glass. It was also damaged or transformed back to pyroxene after the unloading of shock wave. 3. It was found that the shock wave can cause the structural deformation and structure reorganization of some pyroxene and its high-pressure phase in Yanzhuang. In addition to itself two-bridging Si–O tetrahedral chain structure, some complicated combination of Si–O tetrahedral groups, including the Si–O tetrahedral with one or four non-bridging oxygen as well as fully polymerized Si–O tetrahedral, were partially produced in some “pyroxene”. 4. Condensation of shock-induced crystals of olivine, pyroxene, and troilite, as well as whiskers and needles of FeNi metal were found in holes or fractures in the Yanzhuang chondrite. A growth model for FeNi metal whiskers and needles was proposed on the basis of their micro-morphological features. 5. Extremely large eutectic FeNi + FeS blobs up to 1.1 cm in length were observed in the shock-produced melt of Yanzhuang. Three micro-structural and compositional zones and the new “step-type” distribution pattern of Ni content in FeNi metal dendrites in eutectic blobs were discovered, namely, the zone A of FeNi metal is composed of crystallites and non-crystalline phase firstly solidified in the episode of greater cooling speed, and subsequently, the zone B of FeNi metal is composed of coarser crystals crystallized due to the dropping cooling speed; and the zone C of FeNi metal is condensed at last from the remained liquid phase of metal. In contrast to the three-zone micro-structure of FeNi metal in melt veins, the FeNi metal dendrites in melt pockets, having greater volume of molten materials, only show two micro-structural zones, zone B and zone C. They occur in the symmetrical form of crust and nucleus. This implies that the FeNi metal solidified at lower cooling speed and in symmetrical heat radiation field. 6. Specific tetra-concentric-ring growth structure was firstly observed on the head of FeNi metal dendrites of various shapes in Yanzhuang. The development and growth of such growth structure are identical in three dimensional directions. This growth structure is characterized as being multilayered. The formation, propagation and interaction of tetra-concentric-ring growth structures in the same layer are responsible for the growth of dendrite tip and stem, while dendrite sidebranches are grown up at the junction of interaction of coupled tetra-concentric-ring growth steps between the adjacent two layers. Once independent sidebranches are formed, their stems, tips and sidebranches will be developed in the same mechanism of the growth as the tetra-concentric-ring

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structures. The repetition of above-described process will result in the formation of an array of dendrite. An assemblage with FeNi metal, troilite, Fe–Mn–Na phosphates and Al-free chromite was identified in a large FeNi + FeS eutectic nodule in the Yanzhuang shock-produced chondritic melt. A few Fe–Mn–Na phosphate globules have composition of Na-bearing graftonite (Fe,Mn,Na)3(PO4)2, and the majority of them corresponds to two phosphate minerals: Mn-bearing galileiite Na(Fe, Mn)4(PO4)3 and a possible new phosphate mineral of Na2(Fe,Mn)17(PO4)12 composition. The elements of P, Na and Mn in these minerals came from the dissociation of previous minerals, such as merrillite, plagioclase and chromite in Yanzhuang chondrite. Chromite in this assemblage is Al-free which is quite different with that of chormite (Al2O3 = 7.98wt%) in chondritic mass. The contents of minor components in this recrystallized chromite, such as MgO, CaO, MnO, SiO2, and TiO2, are also markedly decreased than the chromite in chondrite. The results of several instrumental neutron activation analyses show a similarity in bulk composition between shock-induced melt and unmelted chondritic rock of Yanzhuang, which suggests in-situ melting and fast cooling of the materials in both melt veins and melt pockets. While the major element concentrations of olivine, pyroxene, FeNi metal and troilite remain unchanged, some trace elements were redistributed between these phases. Ga is enriched in the metal; Co, Cr and Zn are enriched in the sulfide; Cr is enriched in olivine and pyroxene, and Ti is enriched in the plagioclase glass. Two special thermo-luminescence (TL) phenomena were found in the Yanzhuang meteorite through the determination of the natural TL, annealed TL and induced TL by b-radiation: (1) when the Yanzhuang sample was annealed at temperature up to 500 °C, the TL peak induced by b-rays shifted to lower temperature with the increasing of irradiation dose; (2) when the annealed temperature is greater than 600 °C, the TL peak temperature of the annealed sample was higher than that of the unannealed sample. This implies that the shock effect could change the TL characteristics of a chondrite. The measured equivalent b-dose for the melted and unmelted parts are 9238 Gy and 25753 Gy, respectively, showing that the thermal event ages for the two phases in the Yanzhuang meteorite are not equal, and their thermal histories are not the same, either. Four shock phases in the Yanzhuang meteorite were divided on the basis of shock effects of its rocks and minerals, namely, shock melt and recrystallized phase (M), very strongly shocked phase (S6), strongly shocked phase (S5), and moderately shocked phase (S4). It was found that the shock effects of different phases can be compared with those of the Jilin (H5) chondrite experimentally shock-loaded from 12 to 133 GPa. Hence, a new approach of using the typical effects observed in experimentally shock-loaded samples to evaluate the P-T history of naturally shocked meteorites was proposed.

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Yanzhuang chondrite was spalled off its parent body in a collision event between two asteroids at 2.6 Ma ago. The estimated speed of the impact was about 7–8 km/s. The shock peak pressure acted on Yanzhuang was estimated as >60 GPa and the shock peak temperature 2000 °C. Based on the studies in the shock effects of rocks and minerals, it was assumed that the Yanzuang chondrite is really the most heavily shocked ordinary H group chondrite ever found and a unique meteorite that contains most abundant shock induced melt among all known shock-melt-bearing chondritic meteorites. We are indebted to the National Natural Science Foundation of China for supporting the study of Yanzhuang meteorite under grands 41172046, 40772030, and 40272028. We thank the Guangdong Provincial Science Foundation for financial support under grand 91478. We are grateful to the Guangzhou Institute of Geochemistry, Chinse Academy of Sciences, for its profound concern of studying the Yanzhuang meteorite. We would like to thank Profs. Zhaohui Li, Jingfa Liu, and Ruiying Hu for their help in field survey and related laboratory study of this meteorite. Many thanks to Prof. Xiangping Gu of Central South University for his help in X-ray microdiffraction analysis. Guangzhou, China July 2019

Xiande Xie Ming Chen

Contents

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Samples and Experimental Methods . . . . . . . . . . . . . . . . . . . . 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Meteorite Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Experimental Techniques . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.1 Petrographic and Phase-Contrast Microscopies . . . 1.3.2 Scanning Electron Microscopy–energy-Dispersive X-ray Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.3 Electron Probe Microanalysis . . . . . . . . . . . . . . . 1.3.4 Transmission Electron Microscopy . . . . . . . . . . . 1.3.5 Raman Microprobe Analysis . . . . . . . . . . . . . . . . 1.3.6 X-ray Micro-Diffraction In-Situ Analysis . . . . . . . 1.3.7 Instrumental Neutron Activation Analysis . . . . . . 1.3.8 Proton-Induced X-ray Emission Analysis . . . . . . . 1.3.9 Laser Ablation ICP-MS . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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General Introduction of the Yanzhuang Meteorite . . . 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Falling Phenomenon of the Yanzhuang Meteorite . 2.3 General Characteristics of the Meteorite . . . . . . . . 2.3.1 Light-Colored Chondritic Rock . . . . . . . . 2.3.2 Black Melt Pockets and Veins . . . . . . . . 2.3.3 Metal–Sulfide Veinlets . . . . . . . . . . . . . . 2.4 Structures and Textures . . . . . . . . . . . . . . . . . . . . 2.4.1 Textural Types of Chondrule . . . . . . . . . 2.4.2 Fragmentary Structures . . . . . . . . . . . . . . 2.4.3 Fracture Structures . . . . . . . . . . . . . . . . . 2.5 Chemical Composition . . . . . . . . . . . . . . . . . . . . 2.6 Mineralogical Composition . . . . . . . . . . . . . . . . . 2.7 Classification of the Meteorite . . . . . . . . . . . . . . .

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2.8 Noble Gases and the Cosmic-Ray Exposure Age . . . . . . . . . . . 2.9 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

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Microstructures and Shock-Metamorphic Features of Minerals in Unmelted Chondritic Rock . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Cracks and Fractures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Mosaic Textures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Dislocations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1 Olivine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.2 Pyroxene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Brecciation and Structure Disorder . . . . . . . . . . . . . . . . . . . 3.6 Solid-State Phase Transition . . . . . . . . . . . . . . . . . . . . . . . 3.6.1 Diaplectic Glass . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.2 High-Pressure Phase Transition . . . . . . . . . . . . . . . 3.6.3 Structural Variation in Pyroxene . . . . . . . . . . . . . . 3.7 Solid-State Recrystallization and Intergranular Melting Recrystallization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8 Microstructures in Kamacite and Taenite . . . . . . . . . . . . . . 3.9 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Morphology and Petrography of Shock-Produced Melt Veins and Melt Pockets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Morphological Features of Melt Veins and Melt Pockets . . 4.2.1 Chondritic Melt Veins and Melt Pockets . . . . . . . 4.2.2 Fine Veins of Metal–sulfide . . . . . . . . . . . . . . . . 4.3 Mineral Constituents in the Melt Veins and Melt Pockets . 4.4 Petrochemistry of Shock-Produced Melt Materials . . . . . . 4.5 Pressure and Temperature Conditions for the Formation of Melt Veins and Melt Pockets . . . . . . . . . . . . . . . . . . . 4.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Transparent Minerals and Silicate Glass in Shock-Produced Melts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Occurrence of Silicates with Two Different Geneses in the Shock-Produced Melts . . . . . . . . . . . . . . . . . . . . . . 5.3 Occurrence of Silicate Glass in the Shock-Produced Melts 5.4 Chemical Compositions of Silicate Minerals and Glass in the Shock-Produced Melts . . . . . . . . . . . . . . . . . . . . . .

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Tiny Spherules of Metal Distributed in the Interstices of Recrystalline Silicate Minerals . . . . . . . . . . . . . . . . . . . . . . . 99 5.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

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Vapor-Grown Crystals in the Yanzhuang Meteorite . . . . 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Vapor-Grown Crystals in the Pores and Cracks of Meteorite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.1 Troilite . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.2 FeNi Metal . . . . . . . . . . . . . . . . . . . . . . . . 6.2.3 Low-Ca Pyroxene . . . . . . . . . . . . . . . . . . . . 6.3 Discussion on the Genesis of Vapor-Grown Crystals . 6.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Morphology and Cooling Rates of FeNi Metal–Sulfide Eutectic Blobs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Morphological Features of FeNi Metal–Sulfide Eutectic Blobs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Determination of Dendrite Arm Spacing or Cell Widths and Calculation of Cooling Rates . . . . . . . . . . . . . . . . . . 7.3.1 Determination of Dendrite Arm Spacing or Cell Widths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.2 Calculation of Cooling Rates . . . . . . . . . . . . . . 7.4 Cooling Conditions and Solidification History of Shock-Produced Melt . . . . . . . . . . . . . . . . . . . . . . . . 7.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Intra-microstructures of FeNi Metal in Eutectic Blobs . . . . . 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 General Features of FeNi Metal in Melt Veins and Melt Pockets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3 Intra-microstructures of FeNi Metal in Melt Veins . . . . . 8.4 Intra-microstructures of FeNi Metal in Melt Pockets . . . . 8.5 Formation Mechanism of Intra-microstructures of FeNi Metal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Spatiotemporal Pattern of FeNi Metal Melt Solidification and Crystallization Mechanism in Space . . . . . . . . . . . . . . . . 9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 Sample and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3 Spatiotemporal Pattern of FeNi Metal Melt Solidification 9.4 Growth Mechanism of FeNi Metal Dendrites . . . . . . . . . 9.5 Driving Forces for Formation of Tetra-Concentric-Ring Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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10 Fe–Mn–Na Phosphates and Al-Free Chromite in the Metal-Troilite Eutectic Nodule . . . . . . . . . . . . . . . . . . . . . 10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2 Fe–Mn–Na Phosphates in Chondritic Meteorites . . . . . . . . . . 10.3 Metal-Troilite Eutectic Nodule No. 1 in Yanzhuang Shock Melt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3.1 Mineral Composition of Shock Melt Containing Nodule No. 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3.2 Occurrence of Fe–Mn–Na Phosphate Spherules . . . . 10.3.3 Composition of Fe–Mn–Na Phosphate Spherules . . . 10.3.4 Raman Spectra of Fe–Mn–Na Phosphates . . . . . . . . 10.3.5 Synthesis of Fe–Mn–Na Phosphates . . . . . . . . . . . . 10.4 Al-Free Chromite in the Nodule No. 1 . . . . . . . . . . . . . . . . . 10.4.1 Occurrence of Al-Free Chromite . . . . . . . . . . . . . . . 10.4.2 Composition of Al-Free Chromite . . . . . . . . . . . . . . 10.4.3 Raman Spectra of Al-Free Chromite . . . . . . . . . . . . 10.5 Discussion on the Formation Mechanism of Metal + Troilite + Fe–Mn–Na Phosphates + Al-Free Chromite . . . . . . . . . . . . . 10.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Neutron Activation Analysis of Trace Elements in Yanzhuang . 11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 First Instrumental Neutron Activation Analysis . . . . . . . . . . 11.3 Second Instrumental Neutron Activation Analysis . . . . . . . . 11.4 Third Instrumental Neutron Activation Analysis . . . . . . . . . 11.5 Forth Instrumental Neutron Activation Analysis . . . . . . . . . 11.6 Fifth Instrumental Neutron Activation Analysis . . . . . . . . . 11.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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12 LA-ICP-MS Analysis of Trace Elements in Yanzhuang . . . 12.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2 Sample and Method . . . . . . . . . . . . . . . . . . . . . . . . . . 12.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.3.1 Trace Element Concentrations of Whole-Rock Melt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.3.2 Trace Element Concentrations of Silicate Melt . 12.3.3 Trace Element Concentrations of FeNi Metal . . 12.4 Redistribution of Elements upon Shock Melting . . . . . . 12.4.1 Siderophile Elements . . . . . . . . . . . . . . . . . . . 12.4.2 Chalcophile Elements . . . . . . . . . . . . . . . . . . . 12.4.3 Lithophile Elements . . . . . . . . . . . . . . . . . . . . 12.4.4 Rare Earth Elements . . . . . . . . . . . . . . . . . . . . 12.4.5 Platinum Group Elements . . . . . . . . . . . . . . . . 12.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 PIXE Analysis of Trace Elements in Eutectic 13.1 Introduction . . . . . . . . . . . . . . . . . . . . . 13.2 Sample and Method . . . . . . . . . . . . . . . 13.3 Elemental Mapping . . . . . . . . . . . . . . . . 13.4 Line Scanning . . . . . . . . . . . . . . . . . . . 13.5 Summary . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . .

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14 Thermoluminescence Characteristics of Yanzhuang Meteorite . 14.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.2 Shock Characteristics of Thermoluminescence . . . . . . . . . . 14.2.1 Peak Temperature and Sensitivity of TL of Chondrites . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.2.2 TL of Naturally Shocked Chondrites and Shock Pressure . . . . . . . . . . . . . . . . . . . . . . . 14.3 Shift of Thermoluminescence of Naturally Shocked Meteorite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.3.1 b-Radiation-Induced Shift of Thermoluminescence . 14.3.2 Annealing-Caused Shift of Thermoluminescence . . 14.3.3 Thermal Event Age . . . . . . . . . . . . . . . . . . . . . . . 14.4 Shock Effects and Thermoluminescence . . . . . . . . . . . . . . . 14.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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15 Comparison of Shock Features in Yanzhuang with Those of Experimentally Shocked Jilin Meteorite . . . . . . . . . . . . . . . . . 15.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.2 Experimental Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.3 Shock Features in Shock-Loaded Jilin Meteorite Samples . . . 15.4 Comparison of the Naturally Shocked Yanzhuang Chondrite and Experimentally Shocked Jilin Chondrite . . . . . . . . . . . . . 15.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Shock History of Yanzhuang Meteorite . . . . . . . . . . . . . . . . . 16.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.2 Four Different Shock Phases in Yanzhuang . . . . . . . . . . . 16.2.1 Shock-Melt and Recrystallized Phase (M) . . . . . . 16.2.2 Very Strong Shocked Phase (S6) . . . . . . . . . . . . . 16.2.3 Strong Shocked Phase (S5) . . . . . . . . . . . . . . . . . 16.2.4 Moderately Shocked Phase (S4) . . . . . . . . . . . . . 16.3 Deformation and Transformation Evolution of Yanzhuang Minerals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.3.1 Silicate Minerals . . . . . . . . . . . . . . . . . . . . . . . . 16.3.2 Metallic Minerals . . . . . . . . . . . . . . . . . . . . . . . . 16.3.3 Phosphate Minerals . . . . . . . . . . . . . . . . . . . . . . 16.3.4 Chondrite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.4 Variation of Composition in Yanzhuang Under Shock . . . 16.4.1 Major Elements . . . . . . . . . . . . . . . . . . . . . . . . . 16.4.2 Volatile Elements . . . . . . . . . . . . . . . . . . . . . . . . 16.4.3 Noble Gases 4He and 40Ar . . . . . . . . . . . . . . . . . 16.5 Formation Process and Main Shock Event of Yanzhuang . 16.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Chapter 1

Samples and Experimental Methods

Abstract This chapter describes samples and analytical techniques used for studying the mineralogy and the shock-induced effects in the Yanzhuang meteorite. These techniques are optical microscopy (OM), scanning electron microscopy (SEM) with energy-dispersive X-ray analysis (EDXA), electron probe microanalysis (EPMA), Raman microprobe analysis (RMA), X-ray micro-diffraction analysis (XRMD), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), instrumental neutron activation analysis (INAA), protoninduced X-ray emission analysis (PIXE), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Keywords Yanzhuang chondrite · Sample preparing · Analytical techniques

1.1 Introduction The Yanzhuang meteorite experienced heavily impact in space, its chondritic material was partly molten and vaporized, and minerals in its unmelted portion were heavily deformed (Xie et al. 1991, 1994). Hence, we selected Yanzhuang No. 3 specimen which contains abundant melt veins and melt pockets, and techniques suitable for the investigation of shock-induced fine structures within minerals, as well as the techniques for analysis of microelements to explore their redistribution during melting and condensation. These mineralogical techniques have been summarized by Edward. C. T. Chao, Xiande Xie, and Ming Chen in their publications (Chao and Xie 1989, 1990; Chen 1992; Xie and Chen 2016, 2018).

1.2 Meteorite Samples Samples used for this study mainly obtained from Yanzhuang No. 3 specimen of 508 g in weight (Fig. 1.1) and partly from other specimens. This No. 3 specimen was cut into small pieces for making thin sections as it is shown in Fig. 1.2. The prepared samples include: © Guangdong Science & Technology Press Co., Ltd and Springer Nature Singapore Pte Ltd. 2020 X. Xie and M. Chen, Yanzhuang Meteorite: Mineralogy and Shock Metamorphism, https://doi.org/10.1007/978-981-15-0735-9_1

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(1) Large polished section: A large slice of 38 cm3 in area and 6 mm in thickness was cut off from No. 3 specimen. This slice then was polished to prepare a section for study of the rock facies and fabric and for metallographic study of opaque minerals in Yanzhuang meteorite. (2) Small polished sections: A series of small polished sections of 1.2 × 1.2 × 0.5 cm3 were prepared for metallographic microstructural and chemical compositional study of opaque minerals. (3) Polished thin sections: Twenty double-polished thin sections of 30 µm in thickness and without cover glasses were prepared. Part of them was glued on the glass slide with epoxy resin for the study of transparent minerals using optical and phase-contrast microscopy, as well as for SEM, EPMA, and RMA, and the another part with Canadian gum for TEM study after ion thinning. (4) Several small pieces of 1 × 1 × 1 cm3 in volume with naturally broken surface were used for the study of surface microstructures of some selected minerals. (5) Several monomineral grains were selected for micromorphological, X-ray diffraction, and other analyses. (6) Small samples were taken from chondritic rock and different parts of shockinduced melt for instrumental neutron activation analysis of microelements. Besides above-mentioned samples, 200 g of Yanzhuang No. 3 specimen were preserved as duplicate sample, and the rest portion of this specimen was used for other analysis, such as thermal luminescence, noble gases, PIXE, and LA-ICP-MS.

Fig. 1.1 Photography of a cut surface of Yanzhuang fragment No. 3: 1-light-colored unmelted chondritic region; 2-brecciated chondritic region and partially melted, crystallized, and brecciated region; 3-shock-melt veins, and 4-large melt pocket

1.3 Experimental Techniques

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Fig. 1.2 Slice samples cut from Yanzhuang fragment No. 3. The English letters are numbers of slices

1.3 Experimental Techniques 1.3.1 Petrographic and Phase-Contrast Microscopies The petrographic microscope is a basic tool to be used to identify mineral assemblages, optical properties, structural and textural relationships, some internal fine structures, and deformation features in Yanzhuang minerals. Microscopic study is also an important prerequisite for utilization of many other experimental methods. In this study, the phase-contrast microscope is specially used to observe and analyze the shock-induced change of microstructures in minerals. It is an effective tool for observation of refractive index change of transparent minerals.

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1.3.2 Scanning Electron Microscopy–energy-Dispersive X-ray Analysis A JSM-35C scanning electron microscope equipped with an EDAX-9100 X-ray energy-dispersive spectrometer has been used for SEM–EDX studies of Yanzhuang meteorite. In analyses, the working accelerating voltage is 20 kV. We utilized SEM– EDX to study: (1) surface microstructure of minerals. (2) Metallographic microstructures of FeNi metal after etching. (3) Chemical composition of tiny minerals (phases) and linear/planar distribution of certain elements.

1.3.3 Electron Probe Microanalysis An EPM-810Q electron microprobe equipped with an EDAX-9100 X-ray energydispersive spectrometer were used for analyzing the Yanzhuang minerals (phases). In wavelength-dispersive analysis, the working accelerating voltage is 20 kV, the beam current is 20 nA, and the data were corrected using a ZAF program. Natural and synthetic phases of well-known similar compositions were used as standards. In non-standard EDX analysis, the counting time is 80 s. In this study, we use EPMA technique to analyze the chemical compositions of different mineral phases in Yanzhuang, as well as the linear or planar distribution of certain elements.

1.3.4 Transmission Electron Microscopy The H-700, JEM-200CX, and JEM-200EX transmission electron microscopes were successively used for study the mineralogy and fine structures of minerals in the Yanzhuang meteorite. They all were equipped with the EDAX-9100 X-ray energydispersive spectrometer. The working accelerating voltage is 200 kV. Samples for TEM analysis were prepared mainly by selected area ion thinning of thin sections. Only a few samples were prepared by powder dispersion of single minerals. In our study, TEM techniques were used for observation of microstructures of different minerals (phases) in the meteorite. At the same time, the selected area electron diffraction patterns were obtained to explore some crystal structure information. High-resolution TEM was also used for some samples to search residual high-pressure phases.

1.3.5 Raman Microprobe Analysis A Yvon U-1000 laser Raman microprobe (Ar+ laser, laser power of 400 mW and 488 nm green line) has been used for recording Raman spectra of different minerals

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and polymorphous phases of the Yanzhuang meteorite. A microscope was used to focus the excitation laser beam to 1–2 µm spot. The running speed is 2 cm−1 /10 s. The range of frequency scanning is 200–1300 cm−1 . Each Raman spectrum was obtained through 5–8 time of scanning. In this study, laser Raman microprobe technique was used to identify mineral species and observe the change of microstructures of different shock phases in the Yanzhuang meteorite.

1.3.6 X-ray Micro-Diffraction In-Situ Analysis X-ray micro-diffractometer is an effective tool for obtaining X-ray diffraction data of tiny size of minerals and materials, usually less than 1 mm, directly on thin sections or plane blocks containing the mineral or material of interest. A Rigaku D/Max Rapid IIR X-ray micro-diffractometer has been used in in-situ study of some Yanzhuang minerals. It is mainly composed of the following parts. The diffraction effect of the sample is recorded on 2D image plate of the area 470 × 256 mm2 arranged in 4700 × 2560 pixels. The pixel coordinates of a diffraction dot on the image are related to the incident angle (θ ) of X-ray and the dipping angle (β) of the normal line of the sample plane. Numerous diffraction dots with the same θ constitute a Debye ring. Intensity integration along the Debye rings yields one-dimensional 2θ -I data similar to the pattern of powder diffraction.

1.3.7 Instrumental Neutron Activation Analysis The instrumental neutron activation analysis of Yanzhuang samples was conducted for several times. As early as in 1991, the authors of this book had conducted neutron activation analysis of Yanzhuang whole rock with the help of Professor Zhou Rongsheng of the Chengdu Geological College (Chen 1992). Begemman et al. (1992) conducted instrumental neutron activation analysis of trace elements and determination of noble gas contents for both light-colored unmelted chondritic rock and black-colored shock melt of the Yanzhuang meteorite. Zhong et al. (1995) analyzed major elements, rare earth elements, and some other trace elements in light-colored phase, black melt phase, and metal particles in Yanzhuang meteorite using neutron activation technique. Chen et al. (1994) also used the instrumental neutron activation method to determine the trace element concentrations in Yanzhuang shock melt, unmelted chondritic rock, and magnetic metal phase. They also discussed the shock effects and thermal history of the Yanzhuang meteorite on the bases of its noble gas contents and shock features in the Yanzhuang parent body. Kong and Xie (2003) not only conducted the instrumental neutron analysis for the Yanzhuang unmelted and melted phases, but also determined trace element concentrations for FeNi metal, sulfide, and silicate phases. The detailed results of above-related analyses will be given in Chap.10 of this book.

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1.3.8 Proton-Induced X-ray Emission Analysis Proton-induced X-ray emission is a unique technique developed in the late 1960s for performing non-destructive analysis, which is based on the measurements of characteristic X-rays induced by energetic focused proton beam directed onto the surface of a specimen held under vacuum. A high-energetic Si(Li) semiconductor detector is used to measure the energy and intensity of induced X-ray to achieve X-ray spectroscopic analysis. The space resolving power can reach a few micrometers. This technique has been successfully used for analysis of various types of samples to estimate the trace multielement concentration in several types of systems. Wu Xiankang of the Shanghai Institute of Nuclear Research, Chinese Academy of Sciences in cooperation with Li Zhaohui of the Institute of Geochemistry, CAS, performed trace element analysis of Yanzhuang FeNi metal using the SINR nuclear microprobe facility with a 3 MeV proton beam (Wu et al. 1995). The sample is of 50–100 pm and the scanning area of proton beam is 180 × 150 µm2 and the induced X-ray is received by Si(Li) detector, and the elemental maps are given by a computer.

1.3.9 Laser Ablation ICP-MS In the study of our Yanzhuang minerals, an Agilent 7500a ICP-MS coupled with a Resonetics RESOlution M-50 laser ablation system in the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, was used to analyze the trace elements. RESOlution M-50 laser ablation system consists of an excimer (193 nm) laser, a twovolume laser ablation cell, a Squid smoothing device, and a computer-controlled high-precision X-Y stage. The two-volume laser ablation cell is designed to avoid cross-contamination and reduce background flushing time. The Squid smoothing device can reduce statistic error induced by laser ablation pulses. The accuracy of the X-Y stage is better than 0.1 µm. The laser beam spot diameters are 30 and 60 µm, respectively. Our results indicate that the relative standard deviations are mostly less than 5%, and relative deviations of obtained average concentrations from reference values are mostly less than 10%, while the external standards MPI-Ding glass is used (Tu et al. 2011).

References Begemann F, Palme H, Spettel B, Weber HW (1992) On the thermal history of heavily shocked Yanzhuang H-chondrite. Meteoritics 27:174–178 Chao ECT, Xie XD (1989) Micro-mineralogical techniques in geological investigations. Science Press, Beijing, pp 215 (in Chinese) Chao ECT, Xie XD (1990) Mineralogical approaches to geological investigations. Science Press, Beijing, p 388

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Chen M (1992) Micro-mineralogy and shock metamorphism of Yanzhuang meteorite. Ph. D Thesis, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, pp 95 (in Chinese with English abstract) Chen YH, Dai CD, Wang DD, Xie XD, Li ZH, Fang H, Cai ZF (1994) Chemical composition and shock effects of naturally shocked meteorites. Geochemica 23:25–32 (in Chinese with English abstract) Kong P, Xie XD (2003) Redistribution of elements in the heavily shocked Yanzhuang chondrite. Meteorit Planet Sci 38:739–746 Tu XL, Zhang H, Deng WF, Ling MX, Liang HY, Liu Y, Sun WD (2011) Application of RESOlution in-situ laser ablation ICP-MS in trace element analyses. Geochemica 40(1):83–98 (in Chinese with English abstract) Wu JK, Zhu JQ, Li ZH (1995) Quantitative micro-PIXE analysis of Yanzhuang meteorite. Phys Res B 104:445–449 Xie XD, Chen M (2016) Suizhou meteorite: mineralogy and shock metamorphism. Springer and Guangdong Science & Technology Press, Berlin Heidelberg, Guangzhou, p 258 Xie XD, Chen M (2018) Yanzhuang meteorite: mineralogy and shock metamorphism. Guangdong Science & Technology Press, Guangzhou, pp 202 (in Chinese with English abstract) Xie XD, Li ZH, Wang DD, Liu JF, Hu RY, Chen M (1991) The new meteorite fall of Yanzhuang, a severely shocked H6 chondrite with black molten materials. Meteoritics 26:411 Xie XD, Li ZH, Wang DD, Liu JF, Hu RY, Chen M (1994) The new meteorite fall of Yanzhuang, a severely shocked H6 chondrite with black molten materials. Chin J Geochem 12:39–46 Zhong HH, Huang JQ, Ling YY, Jiang LJ, Hu GH, Li ZH, Yi WX, Wang DD (1995) A preliminary study on the cosmochemical characteristics of elements in the Yanzhuang meteorite by INAA. J Instrum Anal 14:7–11

Chapter 2

General Introduction of the Yanzhuang Meteorite

Abstract On October 31, 1990, at 21:45 Beijing time, the Yanzhuang meteorite fell in the field of the Yanzhuang village, Wengyuang County, Guangdong Province. Ten fragments, totaling 3.5 kg, were recovered during the field survey. This meteorite is assigned to an H6 (S6) chondrite. It is composed of light-colored unmelted chondritic rock and black-colored molten mass. Constituent minerals in the Yanzhuang unmelted chondritic rock comprise olivine, orthopyroxene, clinopyroxene, plagioclase, maskelynite, kamacite, taenite, troilite, and small amount of merrillite, chromite, and ilmenite. The shock-induced melt is composed of microcrystalline olivine, pyroxene, plagioclase, FeNi and FeS nodules, and glassy materials. Keywords On-spot survey · Yanzhuang chondrite · Unmelted chondritic rock · Shock-produced melt

2.1 Introduction The Yanzhuang meteorite is a fall and was classified as an H6 chondrite. Right after the fall of this meteorite, the first author of this book organized a group of scientists including Zhaohui Li, Jingfa Liu, and Ruiying Hu from the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, to conduct field survey and collect the meteorite samples. This group then conducted systematic studies on collected samples. Especially, Ming Chen completed his Ph.D. thesis on the study of micromineralogy and shock effects in this Yanzhuang chondrite (Chen 1992). During the recent years, members of this group continued studying of this meteorite in laboratories using advanced techniques and obtained a series of interesting and valuable new results (Xie and Chen 2018). In this chapter, we describe the results of on-spot investigations together with a brief introduction of this Yanzhuang meteorite.

© Guangdong Science & Technology Press Co., Ltd and Springer Nature Singapore Pte Ltd. 2020 X. Xie and M. Chen, Yanzhuang Meteorite: Mineralogy and Shock Metamorphism, https://doi.org/10.1007/978-981-15-0735-9_2

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2.2 Falling Phenomenon of the Yanzhuang Meteorite On October 31, 1990, at 21:45 Beijing time, an incandescent fireball of pink and light yellow color was rapidly sweeping through the air over several counties in the central and northern parts of Guangdong Province. The fireball flight was observed by thousands of local residents at Guangzhou, Qingyuan, Shaoguan, Heyuan, Lianping, Wengyuan, and Yingde, while shock wave sounds were also heard. The flight direction of this fireball runs from southeast to northwest, that is, from Heyuan through Lianping to Wengyuan. This fireball violently exploded in the sky over the Lianping County. Residents in these areas saw this elliptical fire body turned into a string of bead-like bodies swept the sky and heard heavy but oppressive roar right upon the explosion. The fragments then fell in the field of the Yanzhuang village, Wengyuang County. Its geographical coordinates are N24° 34 -E114° 10 (Fig. 2.1). Right after the fall of this meteorite on October 31, 1990, the first author of this book organized a group of scientists including Zhaohui Li, Jingfa Liu, and Ruiying

Fig. 2.1 Sketch map showing the flying path and the location of the Yanzhuang meteorite

2.2 Falling Phenomenon of the Yanzhuang Meteorite

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Hu from the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, to conduct field survey and collect the meteorite samples. During our on-spot survey, the farmers in the village told us that the Yanzhuang meteorite was broken into several pieces, and one piece of them hit and penetrated the eaves of the Guo family’s house, and fell on the cement floor forming a hole of 30 cm in diameter. Another piece hit the eaves of neighbor family’s house and fell on the wooden floor of its balcony (Fig. 2.2). There was also a fragment hit the granite outcrop aside a village road forming a small and shallow pit of 2 cm deep. Three larger pieces (referred to as Yanzhuang meteorites No. 1, No. 2, and No. 3) and several smaller fragments of the meteorite were recovered after its falling. Though the total volume of this meteorite was unknown, meteorite samples recovered by our group weighted 3.5 kg in total with the largest piece measuring 823 g in weight (Xie et al. 1991) (Fig. 2.3). The most marked feature of this meteorite is that it contains a big amount of black melt veins and melt pockets. Obviously, this meteorite had subjected heavy collision by another asteroid in space. Though thousands of meteorites of various types have been recovered up to now on our Earth, the great scale of shockinduced melting and specific-metamorphic features of the Yanzhuang meteorite are very unique and rare in meteorite family (Xie et al. 1991, 1994; Chen 1992; Chen et al. 1995, 1994; Chen and Xie 1997; Xie and Chen 2016, 2018).

Fig. 2.2 Photograph showing the farmers of the Yanzhuang village show the damaged wooden balcony to Prof. Zhaohui Li (sixth on right) and Prof. Ruiying Hu (fourth on right)

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Fig. 2.3 Photograph showing the Yanzhuang meteorite samples recovered by the survey group from the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences

Our group then conducted systematic studies on collected samples. Especially, Ming Chen completed his Ph.D. thesis on the study of micromineralogy and shock effects in this Yanzhuang chondrite (Chen 1992). During the recent years, members of this group continued studying of this meteorite in laboratories using advanced techniques and obtained a series of interesting and valuable new results (Xie and Chen 2018). It is clear that comprehensive study of the Yanzhuang meteorite would provide important formation for understanding of the physic-chemical characteristics of asteroid surface and evolution of celestial bodies, and provide direct evidence for study of high-pressure and high-temperature geology and mineralogy of the Earth’s mantle, as well as for the study of the structure of cosmic masses. Especially, the study of heavily shocked and partly molten Yanzhuang meteorite is of great theoretical significance and application value for understanding the shock-induced vaporization, melting, decomposition, and recrystallization of minerals, and differentiation and redistribution of elements of cosmic masses, and condensation and solidification of molten phases.

2.3 General Characteristics of the Meteorite The recovered stones are roughly lenticular and ovoid in shape, with a maximum length of about 10 cm. The meteorite was covered with black fusion crust measuring in ~1 mm in thickness. The No. 3 stone is a representative among all collected

2.3 General Characteristics of the Meteorite

13

Fig. 2.4 Cut surface of the Yanzhuang No. 3 meteorite showing the black melt pocket (right dark region) and the black melt veins penetrating the light-colored chondritic rock of the meteorite

samples. It is of 10 × 5 × 4.5 cm2 in volume, and about one-forth of its surface was covered by black molten crust of 1–2 mm thick. The other three-forth are fresh broken surface exposed after the explosion of this meteorite. The exposed interior of the meteorite is composed of light-colored and strongly deformed chondritic rock, and black-colored completely molten phase in the form of veins and pockets (or blocks). These unique features are clearly seen on the cut surface of the Yanzhuang No. 3 meteorite (Fig. 2.4). The melt veins (0.1–15 mm in width) and the melt pockets (up to 2 × 3 × 4 cm3 in volume) connected and interweaved with each other and occur in the light-colored chondritic host rock. The tensile cracks in this meteorite sample are very well developed.

2.3.1 Light-Colored Chondritic Rock The light-colored unmelted chondritic rock is the main part of the meteorite, with coarse-grained minerals in it. Due to heavy impact, the meteorite has been severely fractured and become crumbly. The chondritic rock adjacent to black melt pockets or veins usually has darker color, and many fine metal and sulfide veinlets of 0.01– 0.5 mm in width filled the cracks and fractures occurred inside silicate minerals, thus forming so-called blackened areas (Fig. 2.5). The region right adjacent to the blackened area is the brecciated area, in which minerals were heavily smashed. Beyond the brecciated area is the area of unmelted but weakly deformed chondritic host rock, or simply call it weakly deformed area (Fig. 2.6). It should be pointed out that some local strongly deformed parts can also be observed in this weakly deformed area.

14

2 General Introduction of the Yanzhuang Meteorite

Fig. 2.5 Photomicrograph of the blackened area adjacent to a melt vein showing the smashed fractures of minerals and lack of chondrules (crossed Nicols)

Fig. 2.6 Photomicrograph of the chondritic mass showing the blackened, brecciated, and weakly deformed areas, and the very poorly defined chondrules in them (crossed Nicols)

2.3 General Characteristics of the Meteorite

15

Constituent minerals in the Yanzhuang unmelted chondritic mass comprise olivine (40%), orthopyroxene (30%), clinopyroxene (4%), plagioclase and maskelynite (4%), kamacite (10%), taenite (4%), troilite (6%), and small amount of merrillite, chromite, and ilmenite.

2.3.2 Black Melt Pockets and Veins The molten materials in the Yanzhuang meteorite occur in the form of black melt pockets and melt veins. They are very compact and hard and composed of recrystallized mineral crystallites, rounded or elliptic FeNi + FeS eutectic nodules/blobs, silicate melt glass, and small amount of mineral fragments. In the Yanzhuang No. 3 meteorite, the melt pockets are up to 2 × 3 × 4 cm in size, and the melt veins are of 0.1–15 mm in width. The veins connect with each other, penetrating the whole meteorite body. The detailed features of these molten materials will be described in Chap. 4.

2.3.3 Metal–Sulfide Veinlets In the regions directly adjacent to black melt veins or melt pockets, there are some fine veinlets of FeNi-FeS composition distributed in fractures in silicate minerals. They are 1–2 mm in width. Because of the development of such fine veinlets, these regions are blackened (Fig. 2.5).

2.4 Structures and Textures 2.4.1 Textural Types of Chondrule In thin sections, the Yanzhuang chondritic rock exhibits equilibrated ordinary chondritic texture. Both poorly defined chondrules and their broken fragments are present in the recrystallized meteorite groundmass. Chondrules are about 0.02–2 mm in size. In most cases, the boundaries between chondrules and the groundmass are indistinct. Although the meteorite was heavily deformed and fragmented by shock waves, and the chondrules experienced marked destruction, some chondrules can still be recognized in local regions of the meteorite. On the basis of texture characteristics observed under microscope, the Yanzhuang chondrules can be classified into the following textural types:

16

2 General Introduction of the Yanzhuang Meteorite

Fig. 2.7 Microphotograph showing a radiation chondrule and the fragmented area in the vicinity of a melt vein (crossed Nicols)

Monomineralic chondrules (olivine, pyroxene, and kamacite); Grated chondrules (olivine grate + recrystallized plagioclase matrix); Olivine aggregate chondrules (Microcrystalline olivine aggregate); Porphyritic chondrules (euhedral or subhedral crystals of olivine or pyroxene); Parallel fibrous chondrules (olivine or pyroxene, or both); Radiating chondrules (fibrous pyroxene aggregate) (Fig. 2.7). Besides, the cryptocrystalline chondrules, overlapping chondrules, wrapped chondrules, deformed chondrules, and fragments of chondrules are also present in the Yanzhuang meteorite. Among the above-mentioned types, the porphyritic type is predominant, followed by the grated type and the radiation type. The others are rarely observed in the Yanzhuang meteorite. In the blackened areas of this meteorite, the chondrule structure can hardly be recognized because marked recrystallization of chondritic materials took place in these areas.

2.4.2 Fragmentary Structures The fragmentary structures are widely developed in the Yanzhuang unmelted chondritic rock. Such structures can be divided into two types: The first type occurs in

2.4 Structures and Textures

17

Fig. 2.8 Microphotograph showing the fragmented region in weakly deformed area (parallel light)

the area closed to the black melt veins and melt pockets, where the meteorite experienced heavy fragmentation and displacement forming the graveling area in which the chondrule textures were completely destroyed (Fig. 2.7). The second type is the fragmented belts of 0.1–1 mm in width which cut the weak deformed areas of the Yanzhuang meteorite (Fig. 2.8).

2.4.3 Fracture Structures Fractures are widely developed in the Yanzhuang meteorite. They can easily be seen by naked eye or under microscope. These structures can basically be divided into three types: the first type is of irregular tensile fractures. They rather occur in large scale and often cut through the black veins and light-colored chondritic rock. The second type is the regular or irregular planar fractures located inside or in the interstices of silicate minerals. The third type is the fractures filled with metal and sulfide located inside or in the interstices of silicate minerals. This type of fractures only occurs in the areas close to melt veins and melt pockets.

18

2 General Introduction of the Yanzhuang Meteorite

2.5 Chemical Composition The Yanzhuang meteorite had experienced heavy impact before it entered the Earth’s atmosphere that caused the shock-induced melting and formation of melt veins and melt pockets in meteorite. Hence, this meteorite is composed of light-colored chondritic part and black-colored melt regions. Table 2.1 gives the results of chemical analyses for both the chondritic mass and the black melt of the Yanzhuang meteorite. From this table, it can be seen that: (1) The composition of major elements in the Yanzhuang meteorite is fitting well with the average H-group chondrite; (2) The melted and unmelted samples of Yanzhuang are quite similar in composition. (3) Small variations in FeO and FeS are revealed. Table 2.1 Chemical composition of the Yanzhuang meteoritea (wt%) Component

Chondritic rock

Black melt

Mixed sample

Average

SiO2

36.92

36.50

36.85

36.76

MgO

23.22

24.07

22.45

22.25

FeO

8.31

11.77

10.66

10.25

Al2 O3

2.36

2.61

2.36

2.44

CaO

1.57

1.73

1.81

1.70

Na2 O

0.92

0.94

0.94

0.93

K2 O

0.13

0. 13

0.13

0.13

Cr2 O3

0.45

0.45

0.44

0.45

MnO

0.41

0.45

0.30

0.39

TiO2

0.10

0.10

0.10

0.10

P2 O5

0.21

0.09

0.23

0.18

O−

0.07

0.13

0.05

0.08

H2 O+

0.08

0.13

0.07

0.09

Cu

0.067

0.090

0.010

0.029

Ni

1.82

1.81

1.81

1.81

Co

0.078

0.089

0.090

0.086

FeO

17.76

16.57

17.15

17.16

FeS

5.97

3.10

5.11

4.73

Total

100.38

100.68

100.56

100.55

TFe

28.0

27.67

28.67

28.11

TFe/SiO2

0.758

0.778

0.758

0.765

FeO/TFe

0.634

0.598

0.600

0.611

SiO2 /MgO

1.590

1.641

1.516

1.582

Ni/SiO2

0.049

0.049

0.049

0.049

Co/SiO2

0.002

0.002

0.002

0.002

H2

a Analyzed

by Peng Jinlian and Chen Wenhua

2.5 Chemical Composition

19

The FeO-content of light-colored masses (8.31 wt%) is a lit bit lower than the black masses (11.77 wt%), while the FeS-content of black masses (5.97 wt%) is higher than the light-colored ones (3.10 wt%); (4) According to the chemical composition and the content of total iron (TFe), the Yanzhuang meteorite should belong to the H-group chondrites. Based on the above-listed features, we argue that there exist close relations in material sources between the light-colored chondritic rock and the black molten materials. That is, the molten materials are of autochthonous origin. The melt veins and melt pockets are the products of in situ fast melting of the Yanzhuang meteorite.

2.6 Mineralogical Composition We analyzed the composition of Yanzhuang unmelted chondritic rock using the Rigaku D/Max Rapid IIR X-ray micro-diffractometer. The recorded diffraction rings are shown in Fig. 2.9, and the measured diffraction lines are shown in Fig. 2.10. From these figures, we can see that the unmelted part is composed of olivine (forsterite), orthopyroxene (enstatite), FeNi metal (kamacite and taenite), and troilite. Besides these four main minerals, our detailed mineralogical study also found small amount of clinopyroxene, plagioclase, maskelynite, chromite, ilmenite, and merrillite. The characteristics of mineral occurrences in unmelted chondritic rock are shown in Figs. 2.11 and 2.12 and described as follows: Olivine: Olivine is the most abundant mineral, usually occurring as irregular elongated crystals but occasionally as euhedral and subhedral crystals forming olivine

Fig. 2.9 X-ray diffraction patterns of Yanzhuang chondrite taken by micro-diffractometer, showing the recorded diffraction rings

20

2 General Introduction of the Yanzhuang Meteorite

Fig. 2.10 Diffraction lines measured from the X-ray diffraction pattern shown in Fig. 2.9

Fig. 2.11 BSE image of Yanzhuang unmelted chondritic rock showing its mineral composition. Olv = olivine, msk = maskelynite, Mer = merrillite, Tae = taenite, and Tr = troilite

2.6 Mineralogical Composition

21

Fig. 2.12 BSE image of Yanzhuang unmelted chondritic rock showing its mineral composition. Pyx = orthopyroxene, Msk = maskelynite, FeNi = FeNi metal, Tr = troilite, Ch = chromite, Ilm = ilmenite

porphyritic or grated olivine chondrules. The grain size is in the range of MgSiO3 . This order shows relative fugacity of these gaseous components produced from a shocked chondrite. The order may explain the occurrence of vapor-grown crystals in the Yanzhuang meteorite: (i)

At given temperature, the abundance of pertinent vapor components for the formation of low-Ca pyroxene might be the lowest among all produced components. Vapor-grown low-Ca pyroxene could occur only in the pores within melted pockets in which the peak shock temperature and the fugacity of each gaseous component vaporized from pyroxene were the highest. (ii) In the partially melted facies with shock temperatures about 850–1300 °C, gaseous phases Fe and Ni might be vaporized in situ or partially came from shock-produced melt. Fe–Ni needle-whiskers deposited in cracks as temperature decreased. (iii) Shock-induced volatile phases FeS and S2 could be the most abundant among the vaporized gaseous components. These gaseous phases might be transported a long distance from hotter areas to cooler areas, or from high-pressure areas to low-pressure areas. Vapor-grown troilite finally deposited in the cracks in the slightly deformed chondritic facies and the brecciated facies. Compared Fig. 6.9 Sketch chart of vapor components escaped from heated chondrites (after Gooding et al. 1977)

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6 Vapor-Grown Crystals in the Yanzhuang Meteorite

to other vapor-grown crystals, the troilite is one of the most abundant crystals deposited due to the lower thermal decomposition temperature of FeS. As the Yanzhuang meteorite had been heavily impacted, the severely shockmetamorphosed lithologies including the melt facies and the partially melt facies make up about 40% volume of stone (Xie et al. 1991,1994; Begemann et al. 1992). These lithologies experienced shock temperatures from >1500 to >850 °C. The shock temperatures were high enough to induce thermal decomposition and vaporization of some minerals, thus resulting in crystals including pyroxene, Fe–Ni metal needlewhiskers, and troilite deposited from shock-produced gaseous phases in the pores and cracks of meteorite. Olivine crystallized from gaseous components was not found in the Yanzhuang meteorite. Experiment results indicated that the equilibrium coefficients (K) between olivine (Mg2 SiO4 ) or pyroxene (MgSiO3 ) and their vaporized gaseous components Mg, SiO2 , and O2 are K = 10–48 and K = 10–30 , respectively, at 1100 °C (Olsen 1981), showing that under the same temperature conditions, vapor-grown pyroxene is more easily formed than vapor-grown olivine, which is consistent with the properties of vapor-grown crystals in the Yanzhuang meteorite. For example, there is coarsegrained vapor-grown low-Ca pyroxene in the pores of melt pockets, but no vaporgrown olivine has been found. The duration of high pressure induced by the impact between the chondritic parent bodies is usually as short as a few microseconds(French 1968), even though the duration would last for several seconds in the impact between big asteroids. The cooling of shock-heated meteorites was usually rapid (1–300 °C/s) (Scott 1982). Instantaneous high temperature induced by shock wave is not enough for vaporization and deposition of vapor-grown crystals in a great quantity in meteorite. Although the Yanzhuang meteorite was severely shock-metamorphosed, only a small amount of vapor-grown crystals have been found, hence indicating that the meteorite experienced a very short duration of high-pressure and high-temperature regime. Among the tens of cracks investigated in the Yanzhuang meteorite, we found vapor-grown metal needle-whiskers and troilite only in several cracks. The rapid cooling history of the Yanzhuang meteorite after impact event has been confirmed by the study of metallic dendrites (Chen et al. 1995) and by the study of radiogenic gases (Scott 1982). Vapor-grown low-Ca pyroxene only occurred in the melt pockets that experienced relatively low cooling rates (6–30 °C/s) and not in the melt veins that experienced rapid cooling rates (100–400 °C/s), although the peak shock temperature in the melt facies including veins and pockets was higher than 1500 °C.

6.4 Summary

111

6.4 Summary (1) Three types of vapor-grown crystals were found in the heavily shocked Yanzhuang chondrite. These crystals include troilite with well-developed rhombic crystal faces or dodecahedral form, Fe–Ni needle-whiskers, and euhedral to subhedral low-Ca pyroxene crystallites. (2) The shock-induced vapor-grown crystals in the Yanzhuang meteorite have shown the mineralogical characteristics of rapid crystallization from vapor phase, such as the abundant networks of microholes in troilite, the symmetrically distributed ring growth steps on the surface of FeNi needle-whiskers, and the occurrence of subhedral to euhedral low-Ca pyroxene crystals on the wall of the pores. (3) The occurrence of vapor-grown crystals in meteorites is indicative of that the severely shock-metamorphosed chondrites could result in the vaporization of chondritic minerals and the in situ deposition of vapor-grown crystals. The instantaneous high pressure and high temperature experienced by meteorites are not enough for the formation of vapor-grown crystals in a great quantity. Hence, species and quantity of mineral deposited from gaseous components should be very limited.

References Begemann F, Palme H, Spettel B, Weber HW (1992) On the thermal history of heavily shocked Yanzhuang H chondrite. Meteoritics 27:174–178 Chen M, Xie XD (1995) TEM microstructures of the metallic mendrites in the shock-induced melt pocket of the Yanzhuang meteorite. Neues Jahrb Mineral H.8:337–343 Chen M, Xie XD, El Goresy A (1995) Nonequilibrium solidification and microstructures of metal phases in the shock induced melt of the Yanzhuang (H6) chondrite. Meteoritics 30:28–32 French BM (1968) Shock metamorphism as a geological process. In: French BM, Short NM (eds) Shock metamorphism of natural minerals. Mono Book Corp., Baltimore, pp l–17 Gooding JL, Muenow DW (1977) Experimental vaporization of the Holbrook chondrite. Meteoritics 12:401 Kieffer SW (1975) From regolith to rock by shock. The Moon 13:301 McKey DS, Clanton US, Morrison DA et al (1972) Vapor phase crystallization in Appolo 14 breccia. In: King EA Jr. (ed) Proc. 3rd. Lunar Sci. Conf., MIT Press, New York, pp 739–752 Olsen E (1981) Vugs in ordinary chondrites. Meteoritics 16:45 Olsen EJ, Bunch TE (1984) Equilibration temperatures of the ordinary chondrites: a new evaluation. Geochim Cosmochim Acta 48:1363 Robie RA, Hemingway BS, Fisher JR (1978) Thermodynamic properties of minerals and related substances at 298.15 K and 1 bar (105 Pascals) pressure and at higher temperature. U. S. Geol Surv Bull, p 1452 Scott ERD (1982) Origin of rapidly solidified metal-troilite grains in chondrites and iron meteorites. Geochim Cosmochim Acta 46:813 Stöffler D, Bischoff A, Buchwald V et a1 (1988) Shock effects In Kerridge JF, Matthews MS (eds) Meteorites and the early solar system. Univ. of Arizona Press, Arizona, pp 165–202

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Xie XD, Chen M (1997) Shock-produced vapor-grown crystals in the Yanzhuang meteorite. Sci China D 40(2):113–119 Xie XD, Chen M. (2018) Yanzhuang meteorite: mineralogy and shock metamorphism. Guangdong Science & Technology Press, Guangzhou, p 202 (in Chinese with English abstract) Xie XD, Li ZH, Wang DD et al (1991) The new meteorite fall of Yanzhuang, a severely shocked H6 chondrite with black molten materials. Meteoritics 26:411 Xie XD, Li ZH, Wang DD et al (1994) The new meteorite fall of Yanzhuang, a severely shocked H6 chondrite with black molten materials. Chin J Geochem 13:39–46

Chapter 7

Morphology and Cooling Rates of FeNi Metal–Sulfide Eutectic Blobs

Abstract The metal-troilite blobs in the Yanzhuang shock-produced melt can be divided into three types: (i) blobs with dendritic texture, (ii) blobs with cellular texture, and (iii) irregular cellular blobs. All they are with textures of FeNi–FeS eutectic intergrowth aggregates. Based on the dendrite arm spacing or cell widths of blobs, the calculated cooling rates in melt pockets are 0.8–67.8 °C/s, while those in melt veins are 103–2935 °C/s. It was revealed that the local shock-induced melting took place at the surface of Yanzhuang parent body to form melt pockets of 4–35 mm in size. The melt veins formed by filling the fractures of shock melt are only 1–5 mm in width. This indicates that the Yanzhuang meteorite had subjected extremely heavy impact and in-situ melting event, and experienced very complicated rapid cooling process. Keywords Metal-troilite blobs · Dendrite · Cellular texture · Cooling rate

7.1 Introduction The impact process of meteorite parent body is a very common event in cosmic space. This impact process would cause marked change in internal constituent and structure of metal grains in the ordinary chondrites (Agrell et al. 1963; Jain and Lipschutz 1968; Bennett and McSween 1995). The FeNi metal-troilite blobs are shock-produced eutectic product of metal melt under high temperatures of 950– 1400 °C. In this process, the single liquid phase transformed into two solid phases: one is FeNi metal phase and the other is FeS phase (Begemann et al. 1992). The dendritic and cellular structures often occur in the fast cooling alloy melt. Such dendritic and cellular structures can be easily observed in the metal-troilite eutectic blobs/nodules in chondrites. From the measured width between dendrite arms and the size of cells, it was obtained that the fast cooling rate of metal melt in the range of 950–1400 °C is 10–1 –106 °C/s (Blau et al. 1973; Blau and Goldstein 1975; Miyake and Goldstein 1974), and the melt volume and the heat conduction cooling way could also be estimated (Scott 1982; Taylor and Heymann 1971). Yanzhuang meteorite is regarded as a most strongly shocked H-group ordinary chondrite ever recovered by human beings up to present time. The energy of shock © Guangdong Science & Technology Press Co., Ltd and Springer Nature Singapore Pte Ltd. 2020 X. Xie and M. Chen, Yanzhuang Meteorite: Mineralogy and Shock Metamorphism, https://doi.org/10.1007/978-981-15-0735-9_7

113

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7 Morphology and Cooling Rates of FeNi Metal–Sulfide Eutectic Blobs

wave function was very high, leading to the formation of large areas of blackened regions and shock-induced black melt veins and melt pockets. The melt veins and melt pockets are composed of silicate groundmass and metal–sulfide blobs. Their chemical compositions are the same as their chondritic host rock, implying that these veins and pockets are products of in-situ remelting of the Yanzhuang parent body subjected heavy impact. The cooling rates of the molten masses are 6–400 °C/s (Chen et al. 1995). It was indicated that the eutectic reaction took place during cooling of the Yanzhuang molten masses, leading to the formation of a lot of metal-troilite eutectic blobs. We have conducted a series of detailed study on the characteristics of occurrences of these metal-troilite eutectic blobs/nodules (Chen 1992; Xie et al. 1994; Chen et al. 1995; Li et al. 1995; Mao et al. 1999). We also studied the morphological features of metal phase in these metal-troilite blobs/nodules and calculated the cooling rates of metal melt in the Yanzhuang meteorite, and estimated the volume scale of sock-produce melt and the heat diffusion conditions during its cooling. All the results of studies are described in this chapter.

7.2 Morphological Features of FeNi Metal–Sulfide Eutectic Blobs The FeNi metal-troilite blobs are widely distributed in the Yanzhuang melt veins and melt pockets. They show the structure of eutectics and intergrowth aggregates and occur in the elliptic, bean-shaped, and bead-string-like forms, and show the structure of eutectics and intergrowth aggregates (Figs. 7.1, 7.2, 7.3, 7.4, and 7.5). In rare cases, they occur as large irregular eutectic nodules (Figs. 7.6, 7.7, and 7.8). The metal phase in the metal-troilite blobs/nodules is in the dendritic, cellular, and steak-like forms, while troilite occurs as a cement filling the interstices of metal phase. The size of metal-troilite blobs/nodules in the Yanzhuang meteorite is of 0.1– 11 mm, and it related mostly to the spherule occurrence (Table 7.1). The size of blobs in melt pockets generally is larger than 1 mm in diameter, and the largest nodules could reach 11 mm, while the size of blobs in melt veins is generally smaller than 1 mm. The fine melt veinlets and networks in the Yanzhuang meteorite are even thinner (0.1 mm). Similarly, the apparent dimensions of irregular metal-troilite blobs are also related to their occurrences. In melt pockets, they are of 0.5 mm × 0.35 mm in sizes, while those in melt veins are much smaller, and the smallest ones are only of 0.l mm × 0.05 mm. On the basis of micro-morphological characteristics of metal phase, the metal-troilite blobs can be divided into following three types: 1. Metal-troilite blobs with dendritic texture The size of metal-troilite blobs/nodules with dendritic texture in the Yanzhuang meteorite is in the range of 0.5–11 mm, with the average size of 6 mm. This type blobs show clear dendritic texture with some elongated and rounded cells in limited areas. The areas between dendrites and surrounding the cells are filled with troilite (Fig. 7.9). They are mainly distributed inside melt pockets or thick melt veins with distinct

7.2 Morphological Features of FeNi Metal–Sulfide Eutectic Blobs

115

Fig. 7.1 Micro-photograph of elliptic and rounded metal-troilite eutectic blobs in the Yanzhuang silicate melt showing dendritic and cellular texture of metal (reflected light)

boundaries with surrounding silicate glass. The development level of dendritic texture is related to the dimensions of metal-troilite blobs. Dendrites in the tiny blobs might be very small, but those in the melt pockets might be large enough and part of dendritic texture turns into cells, implying much slower cooling of these large grains and much longer time for their solidification. 2. Metal-troilite blobs with cellular texture These types of blobs are mainly distributed inside the melt veins. They are of 0.1– 5 mm in size and 0.3 mm in average. The distinct characteristic feature of this type blobs is the development of internal cellular texture (Fig. 7.10). In these blobs, a lot of small metal cells are cemented by troilite (Fig. 7.11a), or the whole blob becomes a large metal cell (Fig. 7.11b) and a lot of small dendrites or cells appeared in its margins. 3. Irregular cellular metal-troilite blobs This type of metal-troilite blobs can be observed in both melt pockets and melt veins. Blobs are of extremely irregular shape (Fig. 7.12). The maxim dimensions are of 0.5 mm × 0.4 mm, but generally they are of 0.3 mm × 0.2 mm. FeNi metal in this type blobs occurs as irregular cells. Most of the cells are connected with each other. Metal phase and troilite form compact eutectic intergrowth texture. This texture is produced in the Yanzhuang parent body subjected large-scale impact, melting, differentiation, and recrystallization. Comparing with the first two types, metal phase

116

7 Morphology and Cooling Rates of FeNi Metal–Sulfide Eutectic Blobs

Fig. 7.2 Photomicrograph of small rounded and subrounded metal-troilite eutectic blobs in the Yanzhuang silicate melt (reflected light)

of this type shows marked rounding and elongating features and much slower cooling and later-stage reheating effects. After etching with 2% nitric acid, the metal phase displays zoning structure composed of Ni-rich rim and martensitic inner core.

7.3 Determination of Dendrite Arm Spacing or Cell Widths and Calculation of Cooling Rates 7.3.1 Determination of Dendrite Arm Spacing or Cell Widths The dendrite arm spacing and the cell widths in the Yanzhuang metal-troilite blobs were obtained by Mao et al. (1999) using the grain analytical method shown in Figs. 7.9, 7.10, and 7.12. The CA 6300 grain analytical system of the Automation Institute, Chinese Academy of Sciences was used for this measurement. Firstly, the black-white images of FeNi–FeS eutectic intergrowth structure were collected by a microscope. In these images, metal and troilite phases have different gray-level values which were subjected linear enhancement, local contrast enhancement, and

7.3 Determination of Dendrite Arm Spacing or Cell Widths …

117

Fig. 7.3 BSE image showing metal-troilite eutectic blobs of different sizes in the Yanzhuang silicate melt

homogenization treatments using a computer system. After cutting apart of grains and boundary treatment, the granularity statistics of FeNi metal phase and cells was fulfilled by the software. This statistic method may cause some decrease of granularity since the grain boundaries subjected etching and rounding. Figure 7.13a, b are histograms of the cell widths in metal-troilite blobs with irregular metal cells in Yanzhuang melt pockets and melt veins, respectively. Cellular microstructures are very well developed in melt pockets, and the cell width varies in the range of 1–100 µm, in some cases it can be of 200 µm, but the majority is of 22–25 µm. The structures of FeNi eutectic intergrowth aggregates in melt veins are homogeneous with the cell widths concentrated in the range of 5–9 µm. It is seldom to observe cells larger than 17 µm or smaller than 3 µm in width. The results of measurements of dendrite arm spacing and cell widths indicate that the dimensions of shock-induced metal melt directly influence the development of dendrites inside metal-troilite nodules. The melt pockets in the Yanzhuang meteorite are the product of large-scale recrystallization of meteorite host rock, in which melting, mixing, and eutectic reactions of enormous FeNi metal and troilite took place, forming large enough metal-troilite blobs with well-developed dendrites and cells. The melt veins are the product of small-scale shock event with the shock-induced melt filling the broken fractures in the meteorite. Hence, the metal-troilite blobs have

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7 Morphology and Cooling Rates of FeNi Metal–Sulfide Eutectic Blobs

Fig. 7.4 Photomicrograph of metal-troilite eutectic blobs/nodules of different shapes in the Yanzhuang silicate melt (transmitted and reflected light)

much smaller sizes, and dendrites and cells are much less developed in the melt veins. All these features were used by Scott (1982) to study the cooling rates of metal melt. Scott (1982) observed FeNi metal-troilite blobs of 0.2–4 mm in size and showing dendritic and cellular texture in 12 chondrites. Using the secondary dendrite arm spacing and cell widths of metal in nodules, he calculated their cooling rates in the temperature range of 1400–950 °C are of 10–7 –104 °C/s (Fig. 7.14). Among these chondrites, eight are breccias containing normally cooled metal grains. Their cooling rates are of 1–300 °C/s and belong to heat radiation cooling type. He considers that these metal grains were contacted by hot silicate with only 6–40 mm in volume. Thus, these blobs were melted in the surface layer of a shocked parent body. But, the Ramsdorf, Rose City, and Shaw chondrites show that they were subjected strong reheating up to 1000 °C. The FeNi–FeS blobs in them are quite large and the cooling rates during solidification reached 10–1 –10–4 °C/s. Hence, Scott (1982) assumed that the metal grains in these three chondrites might be solidified in the hot silicate of 10–300 cm in volume, and in many iron meteorites and some chondrites, the shockproduced irregular fine metal-troilite eutectic blobs were formed through in-situ fast solidification at cooling rates of 105 °C/s. Though the Scott’s calculations are of semi-quantitative character, this method is still of some reference value and it is widely used by meteoritical society to calculate the cooling rates of metal up to now.

7.3 Determination of Dendrite Arm Spacing or Cell Widths …

119

Fig. 7.5 Photomicrograph of metal-troilite eutectic blobs in the Yanzhuang silicate melt showing the flow texture (reflected light)

7.3.2 Calculation of Cooling Rates In accordance with Scott’s approach, we calculated the cooling rates of metal grains during solidification in the interval of 140–950 °C after the Yanzhuang meteorite subjected shock heating. They are 103–393 °C/s for melt veins and 6.3–28 °C/s for melt pockets (Table 7.2). We consider that this is the result of shock-induced non-equilibrated physico-chemical reactions (Chen 1992). Afterwards, Mao et al. (1999) also calculated the cooling rates for metal grains in the Yanzhuang melt veins and melt pockets on the basis of metal dendrite arm spacing and cell widths. Their basis is the results of metallographic experiments for alloys (Blau et al. 1973; Miyake and Goldstein 1974) that is the cooling rates of metal melt [R/(°C/s)] at high temperature stage (950–1400 °C) has following relation with the dendrite arm spacing and cell dimension (D/µm) in metal spherules: R = 530000D−2.9

(7.1)

Mao et al. (1999) consider that this method may yield some errors which come from the following way: owing to the differences in geometric morphology of dendrite arms and cells and orientation problems in cutting, the difference in measured widths can reach 40%, which would lead to the calculated cooling rates having

120

7 Morphology and Cooling Rates of FeNi Metal–Sulfide Eutectic Blobs

Fig. 7.6 Photomicrograph of a large metal-troilite eutectic nodule of 2 mm size in the Yanzhuang silicate melt (reflected light)

relative errors in 3–5 times. Furthermore, in the metallographical experiments, the selection of alloy materials and some other factors could give indefinite results which would yield 9 times errors in calculated cooling rates. Besides, the friction of meteorite with atmosphere during its falling may cause reheating of molten materials which would widen the cell dimensions in FeNi+FeS blobs and lead to the errors in calculation of cooling rates. The results of our calculations of cooling rates for shock-induced metal melt in the Yanzhuang meteorite on the basis of formula (7.1) are listed in Table 7.2. The cooling rates calculated by Mao et al. (1999) using the dendrite arm spacing and cell widths are 0.8–67.8 °C/s for melt pockets and 103–2935 °C/s for melt veins. Their results are more and less closer to those of ours (Chen 1992), but variation range of cooling rates in melt veins is larger than ours. The melt veins in the Yanzhuang meteorite are extremely thin and the cooling process of these veins went very fast. Hence, the cooling rates of the Yanzhuang melt veins (103 °C/s) are larger than those of common chondrites (102 °C/s) in one order of magnitude. This may be caused by the large-scope movement of metal melt in the meteorite host rock. Considering the comprehensive factors of different errors, Mao et al. (1999) estimated the cooling rates of shock-induced metal melt in the temperature interval of 1400–950 °C are in the range of 0.8–2935 °C/s.

7.4 Cooling Conditions and Solidification History …

121

Fig. 7.7 Photomicrographs of irregular shaped metal-troilite eutectic nodules in the Yanzhuang silicate melt showing: a The occurrence of nodules 1, 2, and 3. b Enlarged image of nodule 1. c Enlarged image of nodule 2. d Enlarged image of nodule 3 (reflected light)

7.4 Cooling Conditions and Solidification History of Shock-Produced Melt The metallographic study of alloys indicates that the degree of development of metaltroilite blobs and dendrite texture is related to the cooling environment during fast solidification of metal melt. We have calculated the cooling rates of metal melt in melt veins of Suizhou, Sixiangkou, Peace River, and Mbale chondrites. The results indicate that the cooling rates of metal with dendrite arm spacing of 0.1–0.15 µm in width in the Suizhou very thin melt veins are 107 °C/s (Xie et al. 2011), while the cooling rates of metal with dendrite arm spacing of 2–3 mm in width in the Sixiangkou melt veins become 104 °C/s, those of 6–8 µm in width in the Peace River melt veins changes to 103 °C/s, and those of 10–12 µm in width in the Mbale melt veins further changes to 102 °C/s (Chen et al. 1998). On the basis of study of the Yanzhuang meteorite and that of predecessor’s data, Mao et al. (1999) made a drawing which shows the relationship between the apparent diameters of metal-troilite blobs in the fast cooling melt and the dendrite arm spacing and cell widths under different conditions (Fig. 7.15). The vertical axes on the right are cooling rates obtained by measuring the dendrite arm spacing or cell widths. The tree lines are ideal cooling rates obtained by metallographic experiments of alloys

122

7 Morphology and Cooling Rates of FeNi Metal–Sulfide Eutectic Blobs

Fig. 7.8 Photomicrographs of irregular-shaped metal-troilite eutectic nodules and numerous tiny metal particles in the Yanzhuang silicate melt (reflected light)

(Scott 1982; Blau and Goldstein 1975). Among them, the upper line represents the metal melt cooled in the heat radiation way under condition of 20 °C. The middle and lower lines reflect the cooling conditions of metal melt inside the cold silicate and metallic cover layers, respectively. According to the mutual relations shown in Fig. 7.15, Mao et al. (1999) assumed that the locations of projective spots of metal-troilite blobs in the Yanzhuang meteorite are quite different to those of lunar soil samples, iron meteorites, and other chondrites. They indicated that these differences are related to their cooling conditions. The cooling action of metal melt in the lunar soil is a mixed process of heat radiation during its splashing and heat conduction of silicates in lunar soil. Hence, the projective spots of metal grains are located above the heat radiation line and the heat conduction line of cold silicates, as well as in the area between these two lines. As for the iron meteorite Verkhen Dnieprovsk, its projective spot is located on the heat conduction line because the locally shock-produced metal melt in the meteorite was rapidly cooled to form dendrite texture through heat conduction action of surrounding metallic phase. Because different chondrites have different shock degrees, the cooling conditions of shock-induced metal melt are also quite different. The Ramsdorf and Rose City chondrites were subjected heavy impact. The large amount of formed molten masses in them were cooled slowly and solidified through heat radiation way, while the less shocked San Emigdio and Walters chondrites having only melt veins or melt networks penetrating in the host rock were cooled very fast,

7.4 Cooling Conditions and Solidification History …

123

Table 7.1 Apparent size of FeNi–FeS grains and cooling rate during solidification Apparent size of FeNi grain/mm

FeNi dendrite arm spacing or cell width/µm

Cooling rate °C/s

Melt pocket

0.5 × 0.35*

100

0.8

Melt pocket

0.4 × 0.2*

22

67.8

Melt pocket

11

37

15

Melt pocket

8

41

9

Melt pocket

7

50

6.3

Melt pocket

7

42

10

Melt pocket

6

45

8.5

Melt pocket

6

42

10

Melt pocket

3

45

8.5

Melt pocket

3

60

3.6

Melt pocket

1.1

44

9

Melt pocket

0.8

40

12

Melt pocket

1.4

42

10

Melt pocket

0.7*

60

3.7

Melt pocket

0.6

48

7

Melt pocket

0.5

40

12

Melt pocket

0.3

30

28

Melt pocket

0.2*

41

11

Melt pocket

0.8

35

17.5

Melt vein

0.3

17

143

Melt vein

1

16

170

Melt vein

0.4

15

206

Melt vein

1.1

18

121

Melt vein

0.4

14

251

Melt vein

0.2

12

393

Melt vein

0.3

17

143

Melt vein

0.2

14

251

Melt vein

0.8

17

143

Melt vein

2

18

121

Melt vein

0.4

18

121

Melt vein

0.5

19

103

Melt vein

0.8

18

121

Melt vein

4

18

121

Melt vein

3

7

121

Melt vein

1*

6

1877

Melt vein

0.1×0.1

18

2935

* After

Mao et al. (1999). All others are from Chen (1992)

124

7 Morphology and Cooling Rates of FeNi Metal–Sulfide Eutectic Blobs

Fig. 7.9 Reflected light photomicrograph of a rapidly solidified metal-troilite blob in the Yanzhuang chondrite. Metal dendrite, which are tree-like single crystal, and rounded cells are enclosed by troilite. The cooling rate during solidification which can be estimated from the spacing of secondary dendrite arms is 3.7 °C/s (after Mao et al. 1999)

thus, their projective spots are located in the vicinity of heat conduction line of cold silicates. The Yanzhuang metal-troilite blobs show rather large distribution area of their projective spots in Fig. 7.15. The positions of these spots are distributed in the area between those of Ramsdorf and Rose City chondrites and of Emigdio and Walters chondrites, as well as along the heat radiation line. This is because the Yanzhuang meteorite had subjected very strong shock and experienced large-scale remelting. The metal melt may not only be cooled through both heat radiation and heat conduction of cold silicates, but also through mixing of these two cooling actions. The complexity in cooling actions of shock-produced metal melt in the Yanzhuang meteorite is consistent with the characteristics of occurrence of abundant melt veins, melt pockets, and melt networks in this meteorite. In the figure showing the relationship between the apparent diameters of metaltroilite blobs and dendrite arm spacing or cell widths, the projective spots of metaltroilite blobs in Yanzhuang melt pockets are located on or above the heat radiation line, while those in melt veins are in the area between the heat radiation line and the heat conduction line of cold silicates with small amount of melt networks setting right

7.4 Cooling Conditions and Solidification History …

125

Fig. 7.10 Rapidly solidified metal-troilite (FeNi–FeS) grains in the Yanzhuang chondrite, showing cellular microstructure: rounded and elongated cells are sintered by troilite. The cooling rate during solidification which can be estimated from the width (D) of rounded cell is 11 °C/s (after Mao et al. 1999)

Fig. 7.11 Photomicrographs of metal-troilite spherules in the Yanzhuang meteorite. a Cellular texture. b Single perfect round metal cells

on the heat conduction line of cold silicates. The different distributions of projective spots of metal-troilite blobs in the Yanzhuang meteorite on the cooling line figure are consistent to the characteristics of their occurrences. The melt pockets in the Yanzhuang meteorite have quite large volume and they often occur in the surface of their parent bodies or the whole melt pockets were splashed to space and slowly cooled through heat radiation action. Hence, their projective spots are located in the upper part in the figure. The Ramsdorf and Rose

126

7 Morphology and Cooling Rates of FeNi Metal–Sulfide Eutectic Blobs

Fig. 7.12 Rapid solidified metal-troilite grain with irregular cell microstructures in the Yanzhuang chondrite. Metal cells (M) exhibit coarse globular texture. Troilite (S) fills in the holes among globular cells. The width (D) of globular cell can be obtained by grain analytical method. The cooling rate is 67.8 °C/s with an average cell width 22 µm (after Mao et al. 1999)

City chondrites show similar characters, because all they have common strongly shock-metamorphosed characteristics. Large amount of molten masses occur in these chondrites and many FeNi metal and troilite formed eutectic intergrowths in melt regions. Such shock process often occurs in the surface of chondrite parent body, the shock-produced melts were slowly cooled through heat radiation. The produced dendrites in blobs are very well developed and the cells are large enough. On the basis of measured cell widths, the cooling rate for metal melt in the Yanzhuang melt pockets is 0.8 °C/s, and that in the Ramdorf chondrite is 0.1 °C/s, and in the Rose City chondrite is 0.03 °C/s. The melt veins in the Yanzhuang meteorite are originated from melt pockets. The scales of melt vein are small and they are of allothigenic origin. These features are similar to those for melt veins in most weakly shocked ordinary chondrites. In the cooling line figure, the locations of metal-troilite blobs in the Yanzhuang meteorite are quite close to those of San Emigdio and Walters chondrites. In the cooling line figure, their projective spots are located in the area between the heat radiation line and the heat conduction line of cold silicates, or on the heat conduction line of cold silicates. These melt veins are very thin (only a few millimeters in thickness or even

7.4 Cooling Conditions and Solidification History …

127

Fig. 7.13 Histograms of the cell widths in metal–troilite grains with irregular metal cells. a In the Yanzhuang melt pockets. b In the Yanzhuang melt veins (after Mao et al. 1999)

Fig. 7.14 Plot of cooling rates of chondritic metallic melt calculated by dendrite arm spacing and cell diameter. The chondrites are: Di = Dimmit, MM = Meza-Madaras, Pu = Pulsora, Ra = Ramsdorf, Rc = Rose City, Se = San Emigdio, Te = Tell, Ti = Tysnes Island, Wa = Walters, We = Weston (after Scott 1982)

128

7 Morphology and Cooling Rates of FeNi Metal–Sulfide Eutectic Blobs

Table 7.2 Characteristics and cooling rates of FeNi–FeS grains in melt pockets and melt veins (1400–950 °C) No.

Occurrence

Grain size of FeNi intergrowth/mm

FeNi dendrite arm spacing or cell width/µm

Cooling rate °C/s

1

Melt pocket

11

37

15

2

Melt pocket

8

44

9

3

Melt pocket

7

50

6.3

4

Melt pocket

7

42

10

5

Melt pocket

6

45

8.5

6

Melt pocket

6

42

10

7

Melt pocket

3

45

8.5

8

Melt pocket

1.1

60

3.6

9

Melt pocket

0.8

44

9

10

Melt pocket

3

40

12

11

Melt pocket

1.4

42

10

12

Melt pocket

0.6

48

7

13

Melt pocket

0.5

40

12

14

Melt pocket

0.3

30

28

15

Melt pocket

0.8

35

17.6

16

Melt vein

0.8

18

121

17

Melt vein

0.3

17

143

18

Melt vein

1

16

170

19

Melt vein

0.4

15

206

20

Melt vein

1.1

18

121

21

Melt vein

0.4

14

251

22

Melt vein

0.2

12

303

23

Melt vein

0.3

17

143

24

Melt vein

0.2

14

251

25

Melt vein

0.8

17

143

26

Melt vein

2

18

121

27

Melt vein

0.4

18

121

28

Melt vein

0.5

19

103

29

Melt vein

4

18

121

30

Melt vein

3

18

121

7.4 Cooling Conditions and Solidification History …

129

Fig. 7.15 Plot of the spacing of secondary dendrite arms or cell widths against the diameters of the rapidly solidified metal spherules/blobs for different chondrites. Cooling rates during solidification, which are estimated from dendrite or cell size, are shown on right ordinate. Three straight lines give estimated cooling rates of metallic spherules cooled by radiation (top line), by conduction at the top of a layer of cold silica (middle), and at cold metal (bottom) (after Scott 1982 besides data of Yanzhuang). The meteorites are: VD = Verkhne Dnieprovsk, Wa = Walters, SE = San Emigdio, Ra = Ramsdorf, Rc = Rose City

thinner than 1 mm). They were cooled rapidly through mixed actions of both heat radiation and heat conduction. Based on the measured dendrite arm spacing and cell widths, the calculated cooling rates for Yanzhuang melt veins are of 103–2935 °C/s, and for San Emigdio and Walters melt veins are 300 °C/s and 5000 °C/s, respectively. Taylor and Heymann (1971) reported that their experimental study on the cooling rates of metal phase in the central part of melt indicates that the cooling rate of melt phase [°C/(°C/s)] has relations with the half thickness of slab-shaped melt (T /m). The approximate relation formula is as follows: lgT = −0.5 lgC − 1.6

(7.2)

When the melt has a ball shape, the relation of its apparent radius R with C will be: lgR = −0.5 lgC − 1.8

(7.3)

It was assumed that the Yanzhuang melt pocket can be approximately regarded as a ball, and the melt vein as a slab-shaped body. Thus, we can easily obtain the apparent dimension of shock-produced melt in the Yanzhuang meteorite and calculate

130

7 Morphology and Cooling Rates of FeNi Metal–Sulfide Eutectic Blobs

the cooling rates for metal-troilite blobs in the Yanzhuang melt pockets to be of 0.8– 67.8 °C/s, and the calculated diameter of melt pocket from formula (7.3) to be of 35–4 mm. The obtained cooling rates for metal-troilite spherules in the Yanzhuang melt veins are 103–2935 °C/s, and the calculated vein thickness from formula (7.2) is l–5 mm.

7.5 Summary (1) On the basis of micro-morphological characteristics of metal phase, the metaltroilite blobs in the Yanzhuang shock-induced melt can be divided into three types: (i) metal-troilite blobs with dendritic texture, (ii) metal-troilite blobs with cellular texture, and (iii) irregular cellular metal-troilite blobs. All they are with textures of FeNi–FeS eutectic intergrowth aggregates. (2) The apparent diameters of metal-troilite blobs are 0.1–11 mm. The blobs in melt pockets with very well developed dendritic textures generally are larger than 1 mm in diameter, and the largest one could reach 11 mm, while the metal-troilite blobs in melt veins are usually smaller than 1 mm. (3) The dendrite arm spacing or cell widths of metal-troilite blobs in melt pockets are 100–22 µm, and the calculated cooling rates are 0.8–67.8 °C/s, while those in melt veins are 19–6 µm and 103–2935 °C/s, respectively. Hence, the fast cooling rates of metal-troilite blobs in the Yanzhuang shock melt under the high-temperature conditions of 1400–950 °C are of 0.8–2935 °C/s. (4) The local shock-induced melting took place at the surface of Yanzhuang parent body to form melt pockets of 4–35 mm in size. These melt pockets then cooled down through heat radiation way with small variation range of cooling rates. The melt veins formed by filling the fractures of shock melt are only 1–5 mm in width. They were cooled rapidly through mixed actions of both heat radiation and heat conduction of cold silicates. Thus, the variation range of cooling rates for melt veins is much larger than that for melt pockets. (5) The Yanzhuang meteorite had subjected extremely heavy impact and in-situ melting event, and experienced very complicated rapid cooling process.

References Agrell SO, Long JVP, Oglive RE (1963) Nickel content of kamacite near the interface with taenite in iron meteorite. Nature 198:749–750 Begemann F, Palme H, Spettel B, Weber HW (1992) On the thermal history of heavily shocked Yanzhuang H chondrite. Meteoritics 27:174–178 Bennett ME, McSween HY (1995) Shock features in iron-nickel metal and troilite of L-group ordinary chondrites. Meteorit Planet Sci 31:255–264

References

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Blau PJ, Goldstein JI (1975) Investigation and simulation of metallic spherules from lunar soils. Geochim Cosmochim Aata 39:305–324 Blau PJ, Axon HJ, Goldstein JI (1973) Investigation of the Canyon C Diablo metallic spheroids and their relationship to the breakup of the Canyon Diablo meteorite. J Geophys Res 78:363–374 Chen M (1992) Micromineralogy and shock effects in Yanzhuang chondrite (H6). Ph.D. thesis, Guangzhou Branch of the Institute of Geochemistry, Chinese Academy of Sciences, pp 1–95 (in Chinese with English abstract) Chen M, Xie XD, El Coresy A (1995) Nonequilibrium solidification and microstructures of metal phases in the shock induced melt of the Yanzhuang (H6) chondrite. Meteoritics 30:28–32 Chen M, Xie XD, El Goresy A, Wopenka B, Sharp TG (1998) Cooling rates in the shock veins of chondrites: constraints on the (Mg, Fe)2 SiO4 polymorph transformations. Sci China D 41:522– 528 Jain VA, Lipschutz ME (1968) Implication of shock effects in iron meteorites. Nature 220:139–143 Li ZH, Xie XD, Zhang DT (1995) The spatiotemporal pattern of the metal FeNi melt solidification in space. Sci China D 38:457–465 Mao YH, Wang DD, Zhang MC, Chen YH (1999) A study on cooling rate and micro-shape characteristic of the metal phase in rapidly solidified metal-troilite grains in Yanzhuang chondrite. Acta Mineral Sin 19:222–230 Miyake GT, Goldstein JI (1974) Nedagolla, a remelted iron meteorite. Geochim Cosmochim Aata 38:747–755 Schmitt RT, Deutsch A, Stöffler D (1993) Shoeck effects in experimentally shocked sample of the H6 chondrite Kernouv (abstract). Meteoritics 28:431–432 Scott ERD (1982) Origin of rapidly solidified metal-troilite grains in chondrites and iron meteorites. Geochim Cosmochim Aata 46:813–823 Stöffler D, Kell K, Scott ERD (1991) Shock metamorphism of ordinary chondrites. Geochim Cosmochim Aata 55:3845–3867 Taylor GJ, Heymann D (1971) Postshock thermal histories of reheated chondrites. J Geophys Res 76:1879–1895 Willis J, Goldsten JI (1981) A revision of metallographic cooling rate curves for chondrites. Lunar Planet Sci 12:1135–1143 Xie XD, Chen M (2018) Yanzhuang meteorite: mineralogy and shock metamorphism. Guangdong Science &Technology Press, Guangzhou, p 202 (in Chinese with English abstract) Xie XD, Li ZH, Wang DD, Liu JF, Hu RY, Chen M (1994) The new meteorite fall of Yanzhuang, A severely shocked H6 chondrite with black molten materials. Chin J Geochem 12:39–46 Xie XD, Sun ZY, Chen M (2011) The distinct morphological and petrological features of shock melt veins in the Suizhou L6 chondrite. Meteorit Planet Sci 46:459–469

Chapter 8

Intra-microstructures of FeNi Metal in Eutectic Blobs

Abstract Our SEM and TEM studies revealed that the dendrites in FeNi–FeS eutectic nodules/blobs in both shock-produced melt veins and melt pockets of the Yanzhuang meteorite show zoning in their microstructures, which indicates nonequilibrium solidification of metal phase. In melt veins, three asymmetric microstructural and compositional zones: core, Ni-rich rim and martensite between the core and ring were discovered, while in melt pockets, a typical symmetric core-crust microstructure consisting of martensitic interiors and Ni-rich rim was revealed. It is suggested that the difference in cooling rates following shock-induced hightemperature melting might be an important factor in producing different dendritic microstructures in melt veins and melt pockets. The solidification environment might be considered as the second influence factor. Keywords FeNi metal · Eutectic blob · Dendrite · Zoning microstructure · Martensite

8.1 Introduction Under dynamic high-pressure and temperature, a serious of shock effects could take place in FeNi metal of chondrites (Xie 1973; Xie and Huang 1991; Stöffler et al. 1991). Under heavily shock compression such FeNi metal would be melted totally, and then recrystallized, solidified, and quenched to form microcrystallites, non-crystalline phases, as well as to martensite and austenite. Troilite also experienced pervasive melting and recrystallization. The Yanzhuang chondrite is a strongly shocked H-group chondrite. Shock-produced melt bodies are intensely developed in this meteorite. A big amount of metal-troilite eutectic spherules/nodules of different sizes occur in such melt phase. Hence, study of intra-microstructures and chemical compositions of FeNi metal in these spherules/nodules would provide important basis

© Guangdong Science & Technology Press Co., Ltd and Springer Nature Singapore Pte Ltd. 2020 X. Xie and M. Chen, Yanzhuang Meteorite: Mineralogy and Shock Metamorphism, https://doi.org/10.1007/978-981-15-0735-9_8

133

134

8 Intra-microstructures of FeNi Metal in Eutectic Blobs

for understanding of pressure–temperature history of this meteorite. This chapter is a summary of our research results in this aspect (Chen 1992; Chen et al. 1995) together with some newly obtained results (Xie and Chen 2018).

8.2 General Features of FeNi Metal in Melt Veins and Melt Pockets As it was described in Chap. 7 that under shock-induced high-pressure and temperature, metal and sulfide phases in the chondritic melt of the Yanzhuang meteorite were completely molten and after polyreaction, occur as rapidly solidified and recrystallized FeNi metal-troilite eutectic with dendritic or cellular texture, in which metallic dendrites were enclosed in a troilite groundmass. The FeNi metal-troilite eutectic nodules and blobs in melt veins show some differences to those in melt pockets. The sizes of eutectic nodules/blobs in melt veins are small, usually, smaller than 3 mm. All large FeNi–FeS eutectic nodules (>4 mm in size) occur only in melt pockets (Figs. 7.1 and 8.1). Although the melt veins and melt pockets of the Yanzhuang meteorite were produced in the same impact event (Chen 1992), differences in the metal dendritic microstructures between the melt veins and melt pockets are evident. Detailed descriptions about these differences are given in the following sections.

Fig. 8.1 Microphotograph showing the branch-shaped metal dendrites in a FeNi + FeS eutectic nodule in the Yanzhuang melt pocket

8.3 Intra-microstructures of FeNi Metal in Melt Veins

135

8.3 Intra-microstructures of FeNi Metal in Melt Veins Several metal-troilite nodules (1–2.5 mm in size) selected from one melt vein of ~10 mm in width were analyzed. On the etched with 1% Nital surface of section of metal-troilite nodules, almost all metal particles exhibit heterogeneous microstructures or three microstructure zones (Figs. 8.2, 8.3, and 8.4): the cores (Zone A), Ni-rich rims (Zone C), and martensite regions (Zone B) between the cores and rims. The asymmetrical distributions among the three zones in the dendrites are easily recognizable. (1) Zone A (or Outer belt): This zone usually occurs in the one side of metal grains in plate-shaped, subrounded, bead-like, and irregular forms, mainly in the major sterns of dendrites and bead- and needle-shaped dendrites (Fig. 8.2). It does not appear in the side arms of dendrites. The width of Zone A is about 20–40 μm, and it occupies 30–60% cut area of the metal grains. This zone has compact texture and could stand up to the 1% Nital etching for its surface maintains strong yellow-white metallic luster after etching. The transmission electron microscopic (TEM) study revealed that Zone A is composed of poorly crystallized metal crystallites of 1–3 μm in size and non-crystalline metal phase. The metal crystallites together with non-crystalline phase formed small spherules of 10– 300 μm in size (Figs. 8.5 and 8.6). The selected area electron diffraction (SAED) patterns show a few dim and disperse diffraction rings. The structure determination shows that they are closing to γ-FeNi phase. Table 8.1 gives the average chemical composition of Zone A: Fe = 91.72 ± 1.3%, Ni = 7.53 ± 1.1%. The average P content of this zone is 0.3 wt%.

Fig. 8.2 BSE image of rapidly solidified metal-troilite structure in the shock-produced melt vein showing the asymmetrical distributions and uniform spreading of the metal dendrites

136

8 Intra-microstructures of FeNi Metal in Eutectic Blobs

Fig. 8.3 An enlarged BSE image of the central part of Fig. 8.2 showing the three asymmetrical microstructural zones in metal dendrites, that is cores (Zone A), martensites (Zone B) between Zone A and Zone C, and Ni-rich rims (Zone C), and showing a multi-core structure in a dendrite at the upper-right part of the image

Fig. 8.4 BSE image showing the three zones (A, B, and C) constitute a subembodied and subclosed structure, and showing the compact Zone A, the martensitic Zone B, and the narrow discontinuous bright Zone C

Zone C

87.447

84.119

Average

89.031

Average

19

87.753

16

82.739

89.049

15

18

89.051

14

82.170

88.378

13

17

89.689

91.720

Average

91.528

92.471

8

12

90.509

7

11

91.523

6

89.348

91.401

5

10

92.651

4

87.451

90.923

3

9

90.923

2

Zone B

92.990

1

Zone A

Fe

No.

Zone

0.570

0.522

0.607

0.580

0.617

0.755

0.654

0.560

0.591

0.512

0.619

0.501

0.571

0.570

0.539

0.588

0.645

0.551

0.570

0.604

0.561

0.501

Co

14.133

11.812

13.656

16.900

9.814

11.200

9.341

9.825

9.201

9.209

8.666

9.871

11.375

7.529

7.701

7.535

7.453

7.283

6.491

7.797

8.211

7.762

Ni

Table 8.1 Composition of FeNi metal grains in Yanzhuang melt veins (wt%)

0.135

0.159

0.169

0.078

0.153

0.280

0.045

0.151

0.077

0.183

0.192

0.124

0.175

0.123

0.042

0.173

0.040

0.188

0.124

0.120

0.122

0.176

Al

0.135

0.159

0.186

0.018

0.020

0.011

0.017

0.020

0.024

0.010

0.019

0.028

0.028

0.010

0.000

0.033

0.000

0.000

0.010

0.010

0.090

0.018

Si

0.020

0.005

0.016

0.038

0.035

0.024

0.009

0.032

0.046

0.052

0.041

0.025

0.049

0.069

0.055

0.100

0.050

0.059

0.082

0.089

0.053

0.060

Cr

1.390

1.044

2.125

0.911

0.058

0.064

0.085

0.061

0.082

0.043

0.033

0.031

0.062

0.057

0.071

0.061

0.092

0.016

0.081

0.066

0.021

0.045

S

100.999

99.460

100.863

100.087

99.200

99.700

98.220

99.698

101.098

100.098

99.711

100.879

98.999

99.803

99.498

100.010

99.o79

99.900

101.533

Total

8.3 Intra-microstructures of FeNi Metal in Melt Veins 137

138

8 Intra-microstructures of FeNi Metal in Eutectic Blobs

Fig. 8.5 TEM image showing the small metal spherules in Zone A. The inset is the SAED image of the spherules

Fig. 8.6 TEM image of the Zone A showing the spherules are composed of metal microlites and non-crystalline metal phase. The inset is the SAED image of the FeNi metal in Zone A

8.3 Intra-microstructures of FeNi Metal in Melt Veins

139

Fig. 8.7 TEM image showing the Zone B is composed of martensite (Mar) and austenite (Aus) TEM image showing the Zone B is composed of martensite (Mar) and austenite (Aus) strips. The inset is the SAED image of two sets of diffraction spots from martensite (200 and 211) and austenite (111 and 222)

Fig. 8.8 TEM image showing the microstructure of Zone C which is formed through interweaving of two microcrystalline phases

140

8 Intra-microstructures of FeNi Metal in Eutectic Blobs

Fig. 8.9 BSE image of a cut surface of a FeNi particle and the Ni-Kα scan line across A, B, C zones of this particle showing the “step-type” distribution pattern of Ni content

(2) Zone B (or Middle belt): This zone is located in one side of Zone A and occurs in the long-strip-shaped and crescent-moon-like forms. The width of this zone is 20–50 μm and it occupies 20–60% cut area of the metal grains. Under optical and scanning electron microscopes, Zone B clearly shows martensite-type (α2 ) structure (Fig. 8.3). TEM study revealed that Zone B is composed of narrow plate-shaped martensite with the plates of 0.1–0.3 μm in width and 1–3 μm in length. Linear and network high density dislocations, faults and twins can be seen inside the long plates (Fig. 8.5). Besides martensite, some austenite (γ-FeNi) relics still remain in this zone. The austenite is rather homogeneous in structure and does not contain specific structure defects. Hence, two sets of electron diffraction spots from martensite and austenite can be seen on the SAED patterns of Zone B (Fig. 8.7). Table 8.1 gives the average chemical composition of Zone B: Fe = 89.03 ± 2.0% and Ni = 9.81 ± 1.6%. The average P content of this zone is 0.65 wt%. Zone B encloses or partially encloses Zone A. It is usually the major constituent of secondary dendrite arms (Fig. 8.3). (3) Zone C (or Inner belt): This zone occurs as a narrow Ni-rich rim surrounding both Zone A and Zone B, and it usually forms a crust layer. Under metallographic microscope and SEM, the light-etched Zone C appears as a discontinuous narrow bright rim (Figs. 8.3 and 8.4). Its thickness ranges from 0.2 to 2 μm, but some parts are thicker and have crescent-moon-like form. The Zone C only contacts the Zone B and located at one side of Zone B. TEM study shows that Zone C is formed through interweaving of two microcrystalline phases of 20–40 nm in width and 80–120 nm in length (Fig. 8.8). These two FeNi microcrystalline

8.3 Intra-microstructures of FeNi Metal in Melt Veins

141

phases are kamacite and taenite. They came from decomposition or transformation of preexisting austenite (γ-FeNi) (Budka 1988). The Ni content of Zone C is relatively high (Table 8.1). The average content of Ni is 14.1 ± 2.8%. Clear boundaries between the tree zones were revealed in etched metal particles (Fig. 8.3). The microprobe profiles of Ni and P concentrations across Zone A, Zone B, and Zone C of a FeNi metal particle are shown in Figs. 8.9 and 8.10. They show that both Ni and P concentrations increase from Zone A to Zone B, while only Ni steeply increases in Zone C alone with a decrease in P. It is interesting that Zone A and Zone B have relatively constant Ni and P contents. Two compositional steps are recognized in the Ni profile across Zone A and Zone B (Fig. 8.10). This distribution regularity in chemical composition is also confirmed by the distribution line of Ni content (Fig. 8.11) as well as the result of surface scanning pattern (Fig. 8.12). It is also consistent with the results of microprobe analyses (Table 8.1). Figure 8.11 indicates that the content of S is in the reverse relationship with that of Ni. This feature can be seen on the planar scanning pattern of S (Fig. 8.13). Based on the characteristics of compositions and microstructures of the three FeNi metal zones in the dendrites, we consider that the crystallization sequence was Zone A > Zone B > Zone C. Thus, it can be seen that three different microstructural and compositional zones and the new “step-type” distribution pattern of Ni content in FeNi metal dendrites in eutectic nodules/blobs were discovered. The asymmetrical distributions among the three zones in FeNi metal dendrites are revealed. It is also found that in the same melt vein the microstructure zoning of all metal particles is spreading out in the same direction (Fig. 8.2). We consider that such distinct microstructures of nonequilibrium solidification of metal particles were found for the first time in shocked meteorites (Chen 1992).

Fig. 8.10 The “step-type” distribution curve of Ni content across A, B, and C zones of a FeNi particle shown in Fig. 8.9

142

8 Intra-microstructures of FeNi Metal in Eutectic Blobs

Fig. 8.11 BSE image of a FeNi particle and the Ni-Kα and S-Kα scan lines across A, B, and C zones of this particle showing the distribution features of Ni and S contents in these three zones

Fig. 8.12 The Ni-Kα surface scan image of the metal particle shown in Fig. 8.11

8.4 Intra-microstructures of FeNi Metal in Melt Pockets

143

Fig. 8.13 The S-Kα surface scan image of the metal particle shown in Fig. 8.11

8.4 Intra-microstructures of FeNi Metal in Melt Pockets Several metal-troilite nodules of 0.5–3 mm in size from the big melt pockets were selected for our investigations. After 1% Nital etching, the FeNi metal particles in melt pockets show a typical core-crust microstructure (Fig. 8.14): the martensite zone and the Ni-rich rim. The detailed descriptions are as follows: (1) The martensite zone (corresponds to the Zone B): This zone occurs in the interior of metal particles. It occupies >80% cut area of the metal grains. The plate-shaped martensitic microstructures can be clearly observed in this zone (Fig. 8.15). The main difference of this zone with the Zone B in melt veins is the lack of austenite. TEM study revealed a great number of faults, linear and network dislocations, as well as deformed twins (Fig. 8.16), and they are the important microstructural characteristics of the martensite plates (Chen and Xie 1995). The average chemical compositions of this zone are: Fe = 89.13 ± 1.5%, Ni = 8.53 ± 0.9%. The average P content of this zone is 0.62 wt% (Table 8.2). Here, we can see that the Ni content of this zone is slightly lower than the Zone B in melt veins. (2) The Ni-rich crust rims (corresponds the Zone C in melt veins): All metal particles in melt pockets have continuous and close Ni-rich crust rims (Figs. 8.11 and 8.15). The width of these rims is 1–8 μm or even wider. In some part, the Ni-rich rims become the connecting medium of the neighboring metal particles. TEM study revealed that this crust rim is an accumulation of microspheroidal masses of 10–300 nm in size. These masses further consist of crystallites or nuclei of 12–3 nm in size (Fig. 8.17). Electron diffraction shows the crust layer

144

8 Intra-microstructures of FeNi Metal in Eutectic Blobs

Fig. 8.14 BSE image of the two-zone (Zone B and Zone C) microstructure of a FeNi metal particle in a Yanzhuang melt pocket showing the symmetric distribution of core and crust after etching

is a polycrystalline phase consisting of poorly crystallized taenite (Chen and Xie 1995). The average chemical compositions of these rims are: Fe = 78.09 ± 2.0%, Ni = 20.31 ± 1.8%. The average P content of this zone is 0.46 wt% (Table

Fig. 8.15 SEM secondary electron image of an etched dendrite showing the microstructures of the strip-shaped martensitic (Mar) interior and the lightly etched crust rim

8.4 Intra-microstructures of FeNi Metal in Melt Pockets

145

Fig. 8.16 TEM bright-field image of the martensite interior of dendrite showing abundant dislocations, faults and some deformed twins (TW)

8.2). The crust rims have higher Ni content than the martensite interior, and the average Ni content of these crust rims is markedly higher than that of the Zone C in melt veins. It has been identified that the crust layer is a polycrystalline phase consisting of poorly crystallized taenite Our detailed study has revealed that a few metal particles in melt pockets also show the microstructure similar to that of Zone A in melt veins (Fig. 8.18). Their chemical compositions are: Fe = 91.71 ± 1.0%, Ni = 6.86 ± 0.8%. Here, we can see that these compositions are close to those of Zone A in melt veins. The only difference here is in that such microstructure is much less developed than that in Zone A. The linear and surface studies were conducted to see the distribution feature of Ni in metal particles (Figs. 8.19, 8.20, and 8.21). The profiles of Ni concentration across the martensite zone and the Ni-rich crust rim of a metal particle show the typical “M-type” distribution pattern. It demonstrates that the Ni concentration of the crust rim markedly higher than the martensite interior. In contrast, the P content decreases from the interior to the rim. This characteristic is consistent with the result of electron microprobe analysis listed in Table 8.2. The typical core-crust microstructures of metal dendrites are rather common feature in the melt regions of the heavily shocked and partially melted ordinary chondrites. Begemann and Wlotzka (1969) firstly found such unique metal microstructure in the L-group Ramsdorf chondrite. Afterward, this microstructure in metal particles was investigated by Taylor and Heyman (1971), Smith and Goldstein (1977), and Wilkening (1978).

Zone C

78.19

89.13

Average

19

87.87

16

80.05

89.51

15

76.83

89.22

14

18

89.45

17

87.69

13

91.71

Average

12

92.65

8

87.91

90.08

7

11

91.58

6

89.37

91.68

5

90.05

91.41

4

10

91.35

3

9

91.91

2

Zone B

92.12

Zone A

Fe

No.

1

Zone

0.01

0.05

0.24

0.20

0.16

0.26

0.16

0.33

0.12

0.28

0.15

0.16

0.14

0.14

0.16

0.10

0.20

0.14

0.13

0.07

0.20

Co

20.79

18.80

20.68

8.53

7.93

8.39

8.51

8.41

9.39

8.25

8.25

7.98

6.86

6.08

6.74

7.30

7.20

7.00

6.99

6.95

6.62

Ni

Table 8.2 Composition of FeNi metal grains in Yanzhuang melt pockets (wt%) Cr

0.00

0.00

0.22

0.23

0.10

0.04

0.19

0.18

0.82

0.22

0.22

0.00

0.13

0.13

0.37

0.00

0.09

0.08

0.20

0.14

0.00

Si

0.22

0.45

0.73

0.53

0.70

0.66

0.43

0.36

0.33

0.56

0.69

0.50

0.38

0.44

0.69

0.35

0.22

0.47

0.42

0.04

0.41

S

0.03

0.01

0.23

0.22

0.26

0.07

0.27

0.20

0.18

0.12

0.12

0.27

0.15

0.17

0.26

0.00

0.07

0.28

0.18

0.20

0.04

P

0.76

0.85

0.97

1.17

0.98

1.06

1.22

1.07

1.46

1.28

1.20

1.062

0.63

0.40

0.79

0.67

0.54

0.62

0.72

0.68

0.58

Total

(continued)

100.00

100.00

100.00

100.00

100.00

100.00

100.00

100.00

100.00

100.00

100.00

100.00

100.00

100.00

100.00

100.00

100.00

100.00

100.00

146 8 Intra-microstructures of FeNi Metal in Eutectic Blobs

Zone

Fe

76.34

77.21

78.88

77.37

79.76

78.09

No.

20

21

22

23

24

Average

Table 8.2 (continued)

0.14

0.10

0.17

0.30

0.19

0.05

Co

20.31

18.82

21.37

18.88

21.11

22.04

Ni

Cr

0.06

0.00

0.00

0.13

0.00

0.13

Si

0.45

0.26

0.36

0.74

0.59

0.28

S

0.19

0.19

0.14

0.34

0.21

0.33

P

0.78

0.88

0.58

0.88

0.69

0.83

Total

100.00

100.00

100.00

100.00

100.00

8.4 Intra-microstructures of FeNi Metal in Melt Pockets 147

148

8 Intra-microstructures of FeNi Metal in Eutectic Blobs

Fig. 8.17 TEM bright-field image of the crust layer of dendrite showing an accumulation of microspheroidal masses of crystallites or nuclei of 1–3 nm in size

Fig. 8.18 BSE image showing the Zone A appearing in a few FeNi metal particles in Yanzhuang melt pockets

8.4 Intra-microstructures of FeNi Metal in Melt Pockets

149

Fig. 8.19 BSE image of a FeNi particle with two-zone-microstructure and the Ni-Kα and S-Kα scan lines across B and C zones of this particle showing the “M-type” distribution of Ni content

Fig. 8.20 The Ni-Kα scan image of the metal particle shown in Fig. 8.19

However, the results of our investigations indicate that there exist certain close relations in microstructure and chemical composition features between FeNi metal particles occurred in melt veins and melt pockets. There also present some differences

150

8 Intra-microstructures of FeNi Metal in Eutectic Blobs

Fig. 8.21 The S-Kα scan image of the metal particle shown in Fig. 8.19

between both of them. These differences are thought to be related to the physical and chemical conditions during the solidification and crystallization of metal particles.

8.5 Formation Mechanism of Intra-microstructures of FeNi Metal The Fe–Ni binary phase diagram can be divided to the upper peritectic part and the lower isomorphous part (Fig. 8.22), that is the contained liquid FeNi phase under certain temperature and compositional conditions (Ni < 4%) could react with the solid FeNi phase (δ-phase) to form the peritectic transition region of an another solid phase (γ-phase), as well as to the isomorphous region of the solid solution (γ-phase) crystallized from liquid FeNi phase (Kubaschewski 1982). The intra-microstructures of FeNi metal in Yanzhuang melt veins and pockets are similar to a kind of peritectic structure, but according to our analysis of the chemical composition and the Fe–Ni binary phase diagram, we consider that they do not belong to the metallographic peritectic structure. Besides, based on the compositional characteristics of different FeNi metal microstructure zones, it can also eliminate the possibility of formation of microstructure zoning in metal through coring segregation, and the peritectic transition in the lastly solidified liquid FeNi phase in the region where the peritectic transition does not occur. The intra-microstructural features of FeNi metal in melt veins and melt pockets in the Yanzhuang meteorite can be analyzed by the occurrence of isomorphous transition

8.5 Formation Mechanism of Intra-microstructures of FeNi Metal

151

Fig. 8.22 The Fe–Ni binary phase diagram (after Kubaschewski 1982)

of non-equilibrium crystallization of liquid FeNi phase under the condition of rapid cooling. Although FeNi and FeS usually form eutectic bodies, but from the Fe–S binary phase diagram (Kubaschewski 1982) and the result of phase equilibrium study of Fe– Ni–S system (Thomas 1975), it is clear that these eutectic bodies become unstable when the temperature goes to the range of 950–1400 °C, and segregation, solidification, and recrystallization of FeNi metal took place in eutectic bodies. The quicker the cooling rate in this temperature range, the smaller the metal dendrite size and the spacing between secondary dendrite arms (Blau and Goldstein 1975; Scott 1982). Based on the calculation formula given by Scott (1982), and on the development features of FeNi metal dendrites, we obtained the cooling rates during metal solidification in the temperature range of 1400–950 °C for the shocked Yanzhuang meteorite: 103–393 °C/s for melt veins and 6.3–28 °C/s for melt pockets (Table 7.1). This would be the results of non-equilibrium physic-chemical actions caused by the shock waves. The cooling of melt veins is rather fast, coring segregation takes place during the solidification of metal, the Ni-poor FeNi of Zone A would be first solidified from metal melt. Owing to the non-equilibrium solidification, crystallization of FeNi with higher Ni content (Zone B) takes place at the second stage, and finally, the solidification of FeNi of Zone C with high Ni content occurs. The FeNi in Zone A solidified at the stage of fast cooling. Although its free energy for driving crystallization is high enough, the diffusion of its atoms is rather difficult, it does not benefit to the formation and growth of dendrites. Hence, a part of

152

8 Intra-microstructures of FeNi Metal in Eutectic Blobs

FeNi solidified as non-crystalline phase. Afterward, the cooling rate lightly decreases, nucleation rate (the number of crystal nuclei/s mm3 ) becomes larger, much larger than growth rate (mm/s), FeNi solidified to the very tiny crystallites. This FeNi crystallite phase mixed with the non-crystalline FeNi phase to form the FeNi metal of Zone A. As the temperature further decreases to reach the equilibrium temperature of the meteorite and the cooling rate becomes slower, the solidified crystals turned coarser to form FeNi of Zone B. This part of FeNi crystals than transforms to martensite from γ-FeNi phase. The marked microstructure difference in Zone A and Zone B makes a clear boundary between these two zones. It should be pointed that the metallographic coring segregation in crystals is regarded as the difference in composition. Besides the difference in composition, coring segregation of metal in melt veins also includes the clear difference in structure. The cooling rates of melt pockets are less rapid. The small amount of FeNi in Zone A solidified from original liquid FeNi phase becomes unstable. It transforms to metal phase through further diffusion reaction with liquid FeNi phase. The composition and structure of this metal phase are similar to that of FeNi crystallized from liquid FeNi phase. Both they make the main constituent part of the Zone B. During the subsequent fast cooling, FeNi in Zone B transforms to martensite. After solidification of FeNi in Zone B, the small amount of remained Ni-rich liquid phase, which is difficult to conduct complete diffusional reaction with the already solidified metal, crystallized around the surface of solidified metal particles to form a Ni-rich outer crust layer (Zone C) (Begemann and Wlotzka 1969). The internal microstructures of metal in melt veins occur in asymmetrical and preferential distribution. This is because there exists an asymmetric and preferential fast heat radiation field within the shock reheated melt veins. The radiation of heat is rather fast in a specific direction which causes the crystallization and growth of all metal dendrites in FeNi–FeS eutectic nodules along the direction of relatively large negative thermal gradient. This might be the reason why all metal particles in FeNi + FeS eutectic nodules have their microstructures spread in one direction. The melt pockets in the Yanzhuang meteorite are much thicker and larger than melt veins. The cooling of melt pockets is relatively slower than the melt veins. So, the preferential fast heat radiation field appearing in melt veins is absent in melt pockets, so that the FeNi particles solidified from metal liquid have the symmetric distribution of core-crust microstructure zoning.

8.6 Summary (1) Dendrites in FeNi–FeS eutectic nodules/blobs in both shock-produced melt veins and melt pockets of the Yanzhuang meteorite show zoning in their microstructures, which indicates non-equilibrium solidification of metal phase. (2) Metal dendrites in melt veins have three asymmetric microstructural and compositional zones: (1) Core (6.4–7.3 wt% Ni); (2) martensite between the core and

8.6 Summary

153

rim (7.4–8.5 wt% Ni); (3) Ni-rich rim (12.8–21.4 wt% Ni), and a new “step-type” distribution pattern of Ni content across the three zones was discovered. (3) Metal dendrites in melt pockets have a typical symmetric core-crust microstructure consisting of martensitic interiors (7.5–8.1 wt% Ni) Ni-rich rim (12.5– 23.3 wt% Ni), and the “M-type” distribution pattern of Ni content across the two zones was revealed. (4) It is suggested that the difference in cooling rates following shock-induced high-temperature melting might be an important factor in producing different dendritic microstructures in melt veins and melt pockets. The solidification environment might be considered as the second influence factor, for the former was solidified in an asymmetric and preferential heat radiation field, while the latter solidified in the environment without such heat radiation field.

References Begemann F, Wlotzka F (1969) Shock induced thermal metamorphism and mechanical deformation in the Ramsdorf chondrite. Geochim Cosmochim Aata 33:1351–1370 Blau PJ, Goldstein JI (1975) Investigation and simulation of metallic spherules from lunar soils. Geochim Cosmochim Acta 39:305–324 Budka PZ (1988) Meteorites as specimens for microgravity research. Metall Trans A 19(A):343–358 Chen M (1992) Micromineralogy and shock effects in Yanzhuang chondrite (H6). Ph.D. thesis, The Institute of Geochemistry, Chinese Academy of Sciences, p 95 (in Chinese with English abstract) Chen M, Xie XD (1995) TEM microstructures of the metallic mendrites in the shock-induced melt pocket of the Yanzhuang meteorite. Neues Jahrbuch für Mineralogie 8:337–343 Chen M, Xie XD, El Coresy A (1995) Nonequilibrium solidification and microstructures of metal phases in the shock induced melt of the Yanzhuang (H6) chondrite. Meteoritics 30:28–32 Kubaschewski O (1982) Iron-binary phase diagrams. Springer-Verlag, Berlin Heidelberg New York, pp 73–182 Scott ERD (1982) Origin of rapidly solidified metal-troilite grains in chondrites and iron meteorites. Geochim Cosmochim Aata 46:813–823 Smith BA, Goldstein JF (1977) The metallic microstructures and thermal histories of severely reheated chondrites. Geochim Cosmochim Aata 41:1061–1072 Stöffler D, Keil K, Scott ERD (1991) Shock metamorphism of ordinary chondrites. Geochim Cosmochim Acta 55:3845–3867 Taylor GJ, Heymann D (1971) Postshock thermal histories of reheated chondrites. J Geophys Res 76:1879–1893 Thomas MO (1975) Experimental approach to the state of the Core Part 1. The liquidus relations of the Fe-rich portion of the Fe-Ni-S system from 30 to 100 Kb. Am J Sci 275:278–290 Wilkening LL (1978) Tysnes island: an unusual clast composed of solidified immiscible Fe-FeS and silicate melts. Meteoritics 13:1–9 Xie XD (1973) Brief introduction of shock metamorphism. Geol Geochem 1:4–6 Xie XD, Chen M (2018) Yanzhuang meteorite: mineralogy and shock metamorphism. Guangdong Science & Technology Press, Guangzhou, p 202 (in Chinese with English abstract) Xie XD, Huang WK (1991) Thermal and collision history of Jilin (H5) and Qingzhen (EH3) chondrites. Chin J Geochem 10:109–119

Chapter 9

Spatiotemporal Pattern of FeNi Metal Melt Solidification and Crystallization Mechanism in Space

Abstract The tetra-concentric-ring structure was found to be the fundamental spatiotemporal pattern of the FeNi metal solidification–crystallization in the microgravity environment in space. It was revealed that the single-layer tetra-concentric-ring structure is a fundamental unit involved in the growth of metal dendrites. Its repetition will result in a double-layer-branched dendrite, and its multi-repetition will result in a multi-layer-concentric-ring structure, thus giving rise to multi-layer-branched dendrites. The formation, propagation, and interaction of tetra-concentric-ring structures in the same layer are responsible for the growth of dendrite tip and stern, while dendrite sidebranches are grown up at the junction of interaction of coupled tetra-concentric-ring growth steps between the adjacent layers. Keywords Metal dendrite · Spatiotemporal pattern · Tetra-concentric-ring structure · Microgravity environment

9.1 Introduction The Yanzhuang meteorite is one of the most heavily shocked, most highly reheated, partially remelted and recrystallized H-group chondrite ever found. It consisted of remelt-recrystallized black phase and unmelted light-colored phase. It is known from Chap. 7 that the black melt phase contains round and elliptic FeNi-FeS eutectic blobs/nodules of 0.1–11 mm in size. The metal in blobs/nodules exhibits dendritic, bead-shaped and network structures (Xie et al. 1991, 1994; Xie and Chen 2018; Li et al. 1991; Begemann et al. 1992) due to rapid cooling, indicating that the product of remelt-recrystallization of the chondrite was formed under microgravity, high vacuum, and superlow temperature conditions (Li et al. 1992a, b). Hence, these remelted and recrystallized FeNi nodules/blobs are good samples for the study of the spatiotemporal pattern of FeNi metal melt solidification in the environment far from the thermodynamic equilibrium, as well as for study of metal dendrite growth and formation mechanism of non-magmatic octahedral iron meteorites. In collaboration with Zhaohui Li, the first author of this book studied the characteristics of spatiotemporal pattern of FeNi metal melt solidification in the Yanzhuang meteorite in the environment of space. Especially, the discovery of tetra-concentric-ring growth © Guangdong Science & Technology Press Co., Ltd and Springer Nature Singapore Pte Ltd. 2020 X. Xie and M. Chen, Yanzhuang Meteorite: Mineralogy and Shock Metamorphism, https://doi.org/10.1007/978-981-15-0735-9_9

155

156

9 Spatiotemporal Pattern of FeNi Metal Melt Solidification …

structure on the metal dendrite surface provided important evidence for explanation of growth mechanism of dendrites (Li et al. 1992a, b, 1995; Xie and Chen 2018). The research result in this aspect is introduced in this chapter.

9.2 Sample and Methods The FeNi-FeS eutectic blobs/nodules under instigation were selected from black remelted phase in Yanzhuang No. 3 fragment. The average chemical composition of the black melt phase is very close to the composition of the unmelted lightcolored phase (Table 2.1). In sampling, the black blobs were gently crushed by mechanical means, followed by the careful separation of FeNi-FeS grains using steel needle and tweezers. The internal microstructure and chemical composition of FeNi-FeS grains were studied by the following techniques: (i) SEM and EDS techniques were used to study the surface microstructures and determine the average chemical composition of selected FeNi-FeS grains. (ii) The same techniques were used to study the surface microstructures and determine the chemical composition of prepared FeNi-FeS polished slices. (iii) TEM and EDS techniques were used to study the surface microstructures and chemical composition of FeNi-FeS grains after Ar+ ion thinning.

9.3 Spatiotemporal Pattern of FeNi Metal Melt Solidification In Chap. 6, we mentioned the occurrence of tetra-concentric-ring growth lines on the head of vapor-grown FeNi needles. Such characteristic concentric growth lines were also observed on the FeNi grains recrystallized from liquid in the Yanzhuang meteorite. However, the growth microstructures on recrystallized FeNi grains are much more complicated than those of vapor-grown ones. The morphology of former FeNi metal skeletal crystals includes cross-shaped, network-shaped, and branchshaped (Fig. 9.1a). It has been revealed that the specific tetra-concentric-ring growth structure (TCRGS) is well developed on the head of the FeNi metal dendrites of various shapes (Li et al. 1992a, b, 1995). The typical features of TCRGS are shown in Fig. 9.1b. Our research indicates that the development and growth of such TCRGS are probably identical in three-dimensional directions (Fig. 9.2), and the TCRGS is characterized as being multilayered. The spatiotemporal pattern of FeNi metal solidification in space has the following forms: (1) Initial form Cross-shaped capillary grooves, measuring 3–5 µm in width with an average of 3 µm, appear in the melt. The distance from the center of one groove to that of

9.3 Spatiotemporal Pattern of FeNi Metal Melt Solidification

157

Fig. 9.1 Backscattered electron (BSE) images of FeNi metal dendrites. a The cross-shaped, network-shaped and branch-shaped dendrites, and petal-shaped at the points of intersection. The size of dendrites decreases from 150 µm at the points of intersection to 10 µm at the two tips. b The tetra-concentric-ring structure formed from FeNi metal melt after its solidification

Fig. 9.2 BSE images of FeNi metal dendrites showing a the petal-shaped FeNi dendrite. The numbers 1, 2, 3, 4 are four petals which are four sidebranches; b enlarged image of petal No. 2 showing the tetra-concentric-ring (TCR) structure on the surface of this petal

158

9 Spatiotemporal Pattern of FeNi Metal Melt Solidification …

Fig. 9.3 BSE images showing a initial stage of growth of TCR structure on an FeNi metal dendrite (on the upper right corner), and b the double-TCR structure on an FeNi metal dendrite, which is consisted of rhombic pagoda-shaped steps in the top layer, tetra-concentric-hexagonal steps growth up in normally crossed parabola-like capillary grooves in the middle layer, and TCR steps in the base layer

another is estimated at 13 µm. At the boundaries of capillary grooves are developed rhombic or polygonal structures and outside the grooves are growing arc-shaped steps (Fig. 9.3a). (2) Double-concentric-ring structure With the growth of FeNi metal dendrites, the cross-shaped capillary grooves have evolved into normally crossed parabola-like grooves. On the rhombic base plane at the boundaries of capillary grooves are developed upward-pointed rhombic pagodashaped steps. On the four sides of the step on the rhombic base plane are symmetrically growing concentric polygonal steps. In the meantime, growing, respectively, on the two contiguous planes of a octahedron constituted by the normally crossed parabola-like grooves is a set of concentric growth steps (the outer ring steps are ~5 µm in diameter), resulting in a double-concentric-ring structure (Figs. 9.3a and 9.4). (3) Single-layer tetra-concentric-ring structure The typical structure of its kind is shown in Fig. 9.3b. It consists of rhombic pagodashaped steps in the top layer, tetra-concentric-hexagonal steps in the middle layer and tetra-concentric steps in the bottom layer. Its formation mechanism is generally similar to that of the double-concentric-ring structure, except a difference which lies in that on the four planes of octahedroid are, respectively, grown up concentricring steps, resulting in a single-layer tetra-concentric-ring structure. The single-layer

9.3 Spatiotemporal Pattern of FeNi Metal Melt Solidification

159

Fig. 9.4 BSE image showing the double-TCR structure formed at the initial stage of the growth of an FeNi metal dendrite

tetra-concentric-ring structure is typical of the spatiotemporal pattern of FeNi metal melt solidification in space and is a fundamental unit involved in the growth of metal dendrites. Its growth is obviously of periodicity. (4) Double-layer concentric-ring structure The typical structure of this kind is shown in Fig. 9.5a. It consists of double-layer tetra-concentric growth steps, four “deformed rhombs” and four “arc bridge-shaped growth steps” and normally crossed parabola-like capillary grooves. The damage probably caused by sample preparation led to the absence of their similarity to the rhombic pagoda-shaped steps at the top of single-layer tetra-concentric-ring structure and the tetra-concentric-hexagonal step structure. The double-layer tetra-concentricring structure is the result of repetition of single-layer tetra-concentric-ring structure. Its simplified sketch is shown in Fig. 9.6. (5) Multi-layer tetra-concentric-ring structure The multi-repetition of a single-layer tetra-concentric-ring structure will result in a multi-layer concentric-ring structure, thus giving rise to multi-layer-branched dendrites (Fig. 9.5b).

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9 Spatiotemporal Pattern of FeNi Metal Melt Solidification …

Fig. 9.5 BSE images showing a the tetra-concentric-ring structure in an FeNi metal dendrite, and b arrayed FeNi metal dendrites formed as a result of the growth of multi-layer tetra-concentric-ring structures

Fig. 9.6 Sketch map showing the double-layer TCR structure of an FeNi metal dendrite. 1-lower layer; 2-upper layer, 3-concentric-ring growth steps; 4-arc bridge-shaped growth steps; 5-normally crossed parabola-like capillary grooves; 6-deformed rhombus; and 7-sidebranch growth area

9.4 Growth Mechanism of FeNi Metal Dendrites

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9.4 Growth Mechanism of FeNi Metal Dendrites The growth of dendrites involves two relatively independent but closely linked stages (Huang and Clicksman 1981a, b): (i) the growth of tip and stern, and (ii) the growth of sidebranches. Comprehensive studies of the surface structure, internal structure and chemical composition of shock-melted and recrystallized FeNi metal in the Yanzhuang chondrite have shown that the metal dendrites were formed as a result of the growth, propagation, and interaction of rhombic pagoda-shaped steps, tetraconcentric-hexagonal (polygonal) steps, and tetra-concentric-ring steps on the crystal faces differing in energy during the melt solidification under microgravity, high vacuum, and superlow temperature conditions (Li et al. 1992a, b, 1995). The normally crossed parabola-like capillary grooves and the macroscopic structures of crystals have control over the growth and preparation of various types of steps. The growth of FeNi metal dendrites is described as follows: (1) Nucleation To the accompaniment of the cooling of shock-induced melt till the solidification temperature of FeNi metal, the metal began to solidify and crystallize. At the initial stage of crystallization refractory elements such as Ir and other Pt-family, the elements will serve as the center of nucleation and Fe and Ni will rapidly form a cubic-like cell body. (2) The growth of dendrites Square pagoda-shaped steps are grown up on the (001) faces of a cuboid cell body, tetra-concentric square pagoda-shaped on the four sideplanes of the cell body neighboring the (001) and tetra-concentric-ring steps on the (111), (−111), (−1−11), and (1−11) faces of the cell body. The growth of the above three types of steps occurs relatively independently synchronously. However, owing to the difference in energy of the crystal faces on which they are grown up, their growth rates are different. It is probably because the {111} faces of a cell body are low in energy, so that the tetraconcentric-ring steps grow relatively rapidly, forming the tips and sterns of dendrites (the directions of growth are the same as those of sterns), and because the crystal faces on which the tetra-concentric square pagoda-shaped steps are grown up are high in energy that the steps grow relatively slowly, resulting in the formation of sidebranches of dendrites. The natural growth of the above-described three types of steps on the different crystal faces of cuboid cell bodies will result in the formation of a singlelayer tetra-concentric-ring structure consisting of square pagoda-shaped steps (in the upper layer), tetra-concentric square pagoda-shaped steps (in the middle layer), and tetra-concentric-ring steps (in the base layer). What has been actually observed is a single-layer tetra-concentric-ring structure consisting of rhombic pagoda-shaped steps, tetra-concentric hexagonal steps and tetra-concentric-ring steps. This difference may also be related to the difference in energy of the base plane faces on which different types of the symmetrical growth of concentric-ring steps on the four (111) faces of cell body.

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9 Spatiotemporal Pattern of FeNi Metal Melt Solidification …

The single-layer tetra-concentric-ring structure is the fundamental unit of the growth of FeNi metal dendrites. Its repetition will result in the double-layer tetraconcentric-ring-structured dendrites and its multi-repetition will produce multilayer tetra-concentric-ring-structured dendrites. “Deformed rhombi” and “arc bridgeshaped steps” are a link between the tetra-concentric-ring structure of two neighboring layers (Fig. 9.6), the former derived from the cancel (in the opposite direction) of the neighboring concentric-ring steps at the point where they intersect and the latter being the result of the local convergence of a large number of tetra-concentricring steps. After branching, the sidebranches of dendrites will reproduce tips, sterns, and secondary sidebranches in the same way as the tetra-concentric-ring structure is developed. According to the growth model described above, FeNi metal melt solidification will lead to formation of octahedral metal dendrites with multi-layer sidebranches. During the dendrite growth process, the growth propagation and mutual reaction of three type steps on different faces play control role over the growth direction and morphology of sterns and branches. The possible crystallographic model of FeNi metal crystallization in cooperation to the above-mentioned mechanism of the growth of dendrites is shown in Fig. 9.7. In this figure, the [001] direction indicates the direction of the growth of the tip and stern of a dendrite. The (111), (1−11), (−1−11), and (−111) octahedral faces represent the four base planes of the growth of tetra-concentric-ring structure. On the other hand, the [100], [010], [−100], and [0−10] directions represent the directions of the growth of four dendritic sidebranches which constitute a sidebranch layer.

Fig. 9.7 Crystallographic model for the TCR growth structure in FeNi metal dendrite. The left sketch is the crystallographic model for TCR growth structure (for the explanation, see the text). The right sketch shows a TCR structure in an FeNi metal dendrite. 1-normally crossed parabola-like capillary groove; 2-square pagoda-shaped step (the side view); 3-the cubic base plane; 4-the tip; 5-concentric-ring growth steps; 6-the arrows pointing the directions of sidebranch growth

9.5 Driving Forces for Formation of Tetra-Concentric-Ring Structure

163

9.5 Driving Forces for Formation of Tetra-Concentric-Ring Structure The source of driving force for the growth of highly regular tetra-concentric-ring structure from homogeneous FeNi metal melt in the state far from the thermodynamic equilibrium is a complicated problem. In the following, only a qualitative analysis will be made. As described above, the remelting and crystallization of FeNi metal in the Yanzhuang meteorite occurred in the environment of microgravity in space. So it can, to the first approximation, be consisted that FeNi metal melt solidification occurred in a diffusion–reaction controlling system under the condition of a small thermal gradient. The driving force for FeNi metal solidification–recrystallization capillary forces (or surface tension) plays an important role, except thermal diffusion metallogenesis (the formation of the core of kamacite dendrites and of the crust of taenite dendrites due to the diffusion of the component) and mineral phase transformation (the formation of martensite). The ideal is based on the following experimental data: (1) A initial concentric-ring structure is generally 5 µm in size and an initial tetraconcentric-ring structure is about 13 µm in size. (2) Cross-shaped (two-dimensional) or normally crossed parabola-like (threedimensional) capillary grooves measuring about 2–5 µm in width are always accompanied by the nucleation of FeNi metal melt and the growth of octahedral dendrites. During the growth of dendrites, the capillary grooves have control over the formation of tetra-concentric-ring structure and the propagation and interaction of growth steps. They are also the loci where dendrite sidebranches grow. (3) In the three-dimensional direction of FeNi metal dendrite growth, the development and growth of the tetra-concentric-ring structure are identical, displaying the characteristics of the FeNi metal dendrite growth under the condition of microgravity (Budka 1988). (4) In the FeNi metal dendrites of the Yanzhuang chondrite, we have not observed such a phenomenon that a tetra-concentric-ring is broken to form a spiral ring. The phenomenon that a concentric ring is broken to form a spiral ring is one that is commonly observed in chemical system in the Earth’s gravity environment (Epstein 1991; Bewesrdorff et al. 1991). (5) FeNi metal dendrites exhibit core-crust structure (Fig. 8.14). The average chemical composition is: core, Fe 90.67 ± 0.96, Ni 9.34 ± 0.95; crust (l µm in thickness), Fe 81.08 ± 3.35, Ni 18.9 ± 3.95 (Li et al. 1992a). Electron microprobe and electron diffraction analyses have confirmed that the dendrites contain kamacite, taenite, martensite, and plessite. Budka (1988) pointed out that in the microgravity environment, the octahedral Widmanstätten structure in FeNi meteorites can be formed directly from a peritectic phase transformation of high-temperature FeNi metal melt. Budka’s analyses and the ideal microstructure and mineralogy of FeNi meteorites as he proposed are similar

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9 Spatiotemporal Pattern of FeNi Metal Melt Solidification …

to those of remelt-recrystallized FeNi metal octahedral dendrites in the Yanzhuang chondrite, which provides important experimental grounds for the formation of some non-magmatic octahedrites from remelt-recrystallization of ordinary chondrites. It is also indicated that some non-magmatic FeNi meteorites can be formed from the direct solidification–recrystallization of FeNi metal melts under microgravity conditions. However, there are still some direct differences between them. For example: (i) The observed content of Ni in the core of kamacite dendrites (~9 ± 1%) is obviously in excess of that required for the peritectic phase transformation of δ-kamacite in the FeNi high-temperature phase diagram, and (ii) The plessite bands in the outer part of FeNi dendrite crust are extremely thin, probably much thinner than what is expected by Budka. Such differences may be attributed to the formation of dendrites at a rapid rate from shock-generated FeNi metal melt in the Yanzhuang chondrite under overcooling conditions because the phase boundary of the Ni-low field in the Fe–Ni phase diagram may be significantly changed as a result of the overcooling process (Budka 1988; Cech 1956). The average composition of the core and crust of FeNi metal dendrites in the Yanzhuang chondrite lend great support to the view point that FeNi metal dendrites were formed under overcooling conditions.

9.6 Summary (1) The tetra-concentric-ring structure is a fundamental spatiotemporal pattern of the FeNi metal solidification–crystallization in the environment of microgravity in space in the state far from the thermodynamic equilibrium. (2) The single-layer tetra-concentric-ring structure is a fundamental unit involved in the growth of metal dendrites. Its repetition will result in a double-layerbranched dendrite, and its multi-repetition will result in a multi-layer concentricring structure, thus giving rise to multi-layer-branched dendrites. (3) The formation, propagation, and interaction of tetra-concentric-ring structures in the same layer are responsible for the growth of dendrite tip and stern, while dendrite sidebranches are grown up at the junction of interaction of coupled tetraconcentric-ring growth steps between the adjacent layers. Once independent sidebranches are formed, their sterns, tips, and sidebranches will be developed in the same mechanism of the growth as tetra-concentric-ring structure. The repetition of the above-described process will result in the formation of an array of dendrites. (4) The mechanism of the growth of tetra-concentric-ring structures may be a fundamental way by which non-magmatic octahedrites formed from the recrystallization of FeNi metal melt under the microgravity conditions in space.

References

165

References Begemann F, Palme H, Spettel B et al (1992) On the thermal history of heavily shocked Yanzhuang H chondrite. Meteoritics 27:174–178 Bewesrdorff A, Borckmans P, Müller SC (1991) The pattern appeared in chemical systems. In: Walter HU (ed) Fluid sciences and materials science in space, Chap 8. China Science & Technology Press, Beijing, pp 204–228 Budka PZ (1988) Meteoritics as specimens for microgravity research. Metall Trans A 19(A):1919– 1923 Cech RE (1956) Evidence for solidification of a metastable phase in Fe-Ni alloys. Trans AIME 206:585–589 Epstein IR (1991) Spiral wave in chemistry and biology. Science 252:67 Huang SC, Clicksman ME (1981a) Fundamentals of dendritic solidification–I. Steady-state tip growth. Acta Metall 29:701–716 Huang SC, Clicksman ME (1981b) Fundamentals of dendritic solidification–II. Development of sidebranch structure. Acta Metall 29:717–734 Li ZH, Xie XD, Zhang DT (1991) The discovery of four concentric ring growth pattern of FeNi metal nucleation and crystallization in space. Meteoritics 26:364 Li ZH, Xie XD, Zhang DT (1992a) On the mechanism of FeNi metal crystallization from shock melt in space. Meteoritics 27:249–250 Li ZH, Xie XD, Zhang DT (1992b) The formation and evolution of the concentric ring growth pattern of FeNi metal crystallization from shock melt in space. Antarctic Meteorites 17:223–226 Li ZH, Xie XD, Zhang DT (1995) The spatiotemporal pattern of FeNi metal solidification in space and the mechanism of its crystallization. Sci China B 38:457–465 Xie XD, Chen M (2018) Yanzhuang meteorite: mineralogy and shock metamorphism. Guangdong Science & Technology Press, Guangzhou, p 202 (in Chinese with English abstract) Xie XD, Li ZH, Wang DD, Liu JF, Hu RY, Chen M (1991) The new meteorite fall of Yanzhuang, A severely shocked H6 chondrite with black molten materials. Meteoritics 26:411 Xie XD, Li ZH, Wang DD, Liu JF, Hu RY, Chen M (1994) The new meteorite fall of Yanzhuang, A severely shocked H6 chondrite with black molten materials. Chin J Geochem 12:39–46

Chapter 10

Fe–Mn–Na Phosphates and Al-Free Chromite in the Metal-Troilite Eutectic Nodule

Abstract An assemblage with metal, troilite, Fe–Mn–Na phosphates, and Alfree chromite was identified in the metal-troilite eutectic nodules in the shockproduced chondritic melt of the Yanzhuang H6 meteorite. Electron microprobe and Raman spectroscopic analyses show that a few phosphate globules have composition of Na-bearing graftonite (Fe, Mn,Na)3 (PO4 )2 , whereas most other correspond to Mn-bearing galileiite Na(Fe,Mn)4 (PO4 )3 and a possible new phosphate phase of Na2 (Fe,Mn)17 (PO4 )12 composition. The Yanzhuang meteorite was shocked to a peak pressure of some tens GPa and a peak temperature of ~2000 °C. All minerals were melted after pressure release to form a chondritic melt due to very high postshock heat that brought the chondrite material above its liquidus. The volatile elements P and Na released from whitlockite and plagioclase along with elements Cr and Mn released from chromite are concentrated into the shock-produced Fe–Ni–S–O melt at high temperatures. During cooling, microcrystalline olivine and pyroxene first crystallized from the chondritic melt, and the formed metal-troilite eutectic intergrowths and silicate glass finally solidified at about 950–1000 °C. On the other hand, P, Mn, and Na in the Fe–Ni–S–O melt combined with Fe and crystallized as Fe–Mn–Na phosphates within troilite, while Cr combined with Fe and crystallized as Al-free chromite also within troilite. Keywords Metal-troilite eutectic · Phosphate · Chromite · Shock melt · Yanzhuang chondrite

10.1 Introduction In the shock melt of the Yanzhuang meteorite, we observe a large FeNi metal plus troilite eutectic nodule. In the FeS matrix of this nodule, we found several tens of Fe–Mn–Na phosphate spherules and two Al-free chromite crystals. They formed an unusual and rare FeNi metal + troilite + Fe–Mn–Na phosphate + Al-free chromite assemblage (Xie et al. 2014; Xie and Chen 2018). In this chapter, we describe the occurrence, chemical composition, and Raman spectra of minerals in this assemblage. The Fe–Mn–Na phosphate spherules are too small for mineral determination. © Guangdong Science & Technology Press Co., Ltd and Springer Nature Singapore Pte Ltd. 2020 X. Xie and M. Chen, Yanzhuang Meteorite: Mineralogy and Shock Metamorphism, https://doi.org/10.1007/978-981-15-0735-9_10

167

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Synthesis of phosphates was conducted by solid-state reaction to compare and identify their natural counterparts in this nodule. The formation mechanism of this new assemblage has also been discussed in this chapter.

10.2 Fe–Mn–Na Phosphates in Chondritic Meteorites Shock-induced melt regions (melt veins or melt pockets) can be observed in many chondritic meteorites (e.g., Fredriksson et al. 1963; Rubin 1985; Stöffler et al. 1991), and melting temperatures of up to 1500 °C or more have been reported by Begemann and Wlotzka (1969), Smith and Goldstein (1977), Chen and Xie (1996), and Xie et al. (2014). The melt regions consist of recrystallizing silicate, oxide, iron–nickel metal, and sulfide, and, sometimes, small amounts of silicate melt glass (Price et al. 1979; Scott 1982; Stöffler et al. 1991; Chen and Xie 1996). Metal and sulfide phases in the chondritic melt were completely molten and occur as rapidly solidified metaltroilite eutectics with dendritic or cellular texture, in which metallic dendrites were enclosed in a troilite groundmass (Scott 1982; Rubin 1985; Chen et al. 1995a, b; Xie and Chen 2018). The investigation of textures, compositions, and microstructures in the metal-troilite eutectics can be used to construct the postshock thermal histories of chondritic meteorites (Begemann and Wlotzka 1969; Taylor and Heymann 1971; Smith and Goldstein 1977; Scott 1982; Rubin 1985; Chen and Xie 1995; Chen et al. 1995a, b; Leroux et al. 2000; Kong and Xie 2003; Xie and Chen 2018). The cooling rates deduced from the structures of metal dendrites in the shock-induced melts of chondrites are ranging from 0.1 to 5000 °Cs−1 (Scott 1982). Fe–Mn phosphates typically do not occur in ordinary chondrites (Fuchs 1969; Rubin 1997), but some Fe– Mn phosphates were reported in iron and pallasite meteorites (Olsen and Fredriksson 1966; Bild 1974). Olsen and Steele (1993) reported the discovery of Fe–Mn–Na phosphate (Na,K)2 (Fe,Mn)8 (PO4 )6 within troilite nodules of some IIIAB iron meteorites. They provisionally referred to this mineral as 2:8:6 phosphates due to the absence of X-ray data available for this phase. Chen and Xie (1996) reported a number of Na-bearing Fe–Mn phosphate inclusions within troilite in some metal-troilite eutectic nodules of the Yanzhuang chondrite. These phosphate inclusions occur as fine-grained spherules with 2–8 μm diameter, and containing 48–58 wt% FeO, 2–5 wt% MnO, 38–40 wt% P2 O5 , and up to 4.55 wt% Na2 O. On the basis of microprobe analyses, they indicated that only a few grains have the composition of graftonite (Fe2.95 Mn0.05 )3 (PO4 )2 , but the majority of the Fe–Mn–Na phosphate grains in Yanzhuang may correspond to the 2:8:6 phosphate (Na,K)2 (Fe,Mn)8 (PO4)6 as those found in IIIAB iron meteorites by Olsen and Steele (1993). Olsen and Steele (1997) reported a new Fe–Na phosphate mineral galileiite NaFe4 (PO4 )3 that occurs in troilite nodules in iron meteorites of the IIIA and IIIB groups. Grains of galileiite are very small, generally 10 μm or less. This new mineral

10.2 Fe–Mn–Na Phosphates in Chondritic Meteorites

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contains 49.0 wt% FeO, 3.98 wt% MnO, 40.2 wt% P2 O5 , and 5.87 wt% Na2 O. It is associated with Ca-free graftonite, chromite and, occasionally, schreibersite. Semeneko and Perron (2005) reported the presence of coarse glassy Fe-Na phosphate globules (up to approximately 20 μm in diameter) containing microcrystals of chromite (≤1 lm) in melt regions of the Krymka (LL3.1) chondrite. These glassy phosphate globules occur in troilite of some metal-troilite intergrowths. Energydispersive X-ray spectroscopic analyses give a composition for the phosphate of (in wt% oxides) 7–11 Na2 O, 40–50 FeO, 0.45 MnO, 37–40 P2 O5 , and some other minor components. They assumed that this composition is somewhat Na-poor compared to the sodium iron phosphate maricite (FeNaPO4 , 17.8 wt% Na2 O, 41.3 wt% FeO, and 40.8 wt% P2 O5 ). We consider that the ideal composition of maricite is Na2 O 17.8 wt%, FeO 41.3 wt%, P2 O5 40.8 wt%, but the Na-Fe phosphate in meteorite Krymka contains only 7–11 wt% of Na2 O, which is too low to compare with the Na2 O-content of maricite (17.8 wt%). Furthermore, no X-ray diffraction data were reported for this Krymka’s phosphate. Hence, identification of this phosphate as maricite still raises doubt.

10.3 Metal-Troilite Eutectic Nodule No. 1 in Yanzhuang Shock Melt A large metal-troilite eutectic nodule of about 3 × 2.5 mm in size was observed in a melt pocket of the Yanzhuang meteorite (Xie et al. 2014; Xie and Chen 2018). We labeled it as eutectic nodule No. 1. We prepared a thin slice of this meteorite containing the nodule No. 1. This nodule then was extracted from thin slice and used to make a polished section (Fig. 10.1). The enlarged image of this nodule is shown in Fig. 10.2. The FeNi metal-troilite nodule No. 1 was then investigated with an optical microscope in reflected light. Chemical compositions of minerals were Fig. 10.1 Photograph of the eutectic FeNi metal-troilite nodule No. 1 extracted from a shock melt pocket of the Yanzhuang meteorite

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Fig.10.2 Cut and polished surface of a large FeNi-FeS eutectic nodule (nodule No. 1) extracted from a melt pocket in the Yanzhuang meteorite. The rectangles are areas of investigation, and the capital letters are codes of areas of investigation

quantitatively determined with a JEOL JXA-8100 electron microprobe using the wavelength-dispersive technique at 15 kV accelerating voltage and beam current of 15 nA for silicates and chromite, and 10 nA for phosphates and feldspathic glass. Backscattered electron imaging techniques were used to reveal the morphological features of this metal-troilite nodule. Raman spectra of phosphate and oxide mineral in this nodule were recorded with a Renishaw R-2000 instrument. A microscope was used to focus the excitation beam (Ar+ laser, 514 nm line) to 2-lm-wide spots and to collect the Raman signal. Accumulations of the signal lasted 120–150 s. The laser power is 26.8 mW. From Fig. 10.2, we can see that metal dendrites in nodule No. 1 consist of primary trunks (up to 500 μm in length) with perpendicular secondary branches (up to 100 μm long), while the cellular metal crystals lack secondary dendrite arms. Troilite in this nodule occurs as groundmass filling the spaces between metal dendrites and cells. The metal dendrites and cells in nodule No. 1 have quite smooth surfaces, while the troilite grains in this nodule were cracked and fractured. The phosphates occurring in the shock-produced metal-troilite nodules of the Yanzhuang meteorite are too small for structure determination by X-ray diffraction. The only way to obtain their structural data is Raman spectroscopy. For this reason, synthesis of phosphates was conducted to compare Raman spectra to natural counterparts. Synthesis of NaFe4 (PO4 )2 and a Fe–Mn–Na phosphate compound was conducted by solid-state reaction. The identification of phosphate minerals in nodule

10.3 Metal-Troilite Eutectic Nodule No. 1 in Yanzhuang Shock Melt

171

No. 1 was achieved by comparison of their Raman spectra with those of synthesized products.

10.3.1 Mineral Composition of Shock Melt Containing Nodule No. 1 As it was described in Chap. 4, the shock-induced chondritic melt in the Yanzhuang chondrite consists of recrystallized olivine, pyroxene, silicate melt glass, and elliptic and rounded metal-troilite eutectic blobs of different sizes. Figure 10.3 displays two types of silicate minerals: the coarse-idiomorphic crystals of olivine and low-Ca pyroxene (up to 20–30 lm in length) with interstitial silicate melt glass, and the microcrystalline granular crystals of olivine and pyroxene (only a few lm in size) immersed in silicate melt glass. Tiny metal globules are mainly dispersed within silicate melt glass. Results of electron microprobe analyses of silicate minerals and melt glass in Yanzhuang melt regions are shown in Table 10.1. It should be pointed out that olivine in the chondritic melt pocket has the same composition as olivine of the chondritic host, but pyroxene in the chondritic melt pocket contains higher

Fig.10.3 BSE image showing the mineralogy of the chondritic melt pocket in Yanzhuang. Note the coarse-grained, idiomorphic crystals of low-Ca pyroxene (Pyx) and olivine (Ol) with interstitial silicate melt glass (Gl), and the microcrystalline granular crystals of olivine and pyroxene immersed in silicate melt glass. M = small metal globules (Xie et al. 2014)

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Table 10.1 Microprobe analyses of silicates and melt glass in Yanzhuang melt regions (wt%) Olivine

Pyroxene

Glass

Melt

Host

Melt

Host

1

2

3

(10)

(11)

(11)

(9)

(1)

(1)

(1)

Na2 O

n. d

n. d

0.06

0.02

0.48

1.27

3.16

K2 O

0.01

n. d

n. d

n. d

5.33

0.91

0.45

FeO

16.93

16.85

10.32

10.84

11.22

5.33

3.61

MgO

42.48

42.63

30.53

30.86

21.31

9.31

6.12

MnO

0.43

0.46

0.42

0.44

4.27

3.74

0.81

CaO

0.07

0.04

1.24

0.68

1.0

2.85

3.43

SiO2

38.91

38.82

54.88

56.16

53.47

60.35

65.43

TiO2

0.04

0.04

0. 10

0.11

0.21

0.20

0.24

Al2 O3

n. d

n. d

0.64

0.20

9.41

14.88

15.98

Cr2 O3

0.16

0.10

0.69

0.14

0.30

0.18

0.38

Total

99.07

98.94

99.39

99.45

98.36

99.08

99.65

The number in parentheses is the number of analyses. n. d. not detected.

contents of Al2 O3 , Cr2 O3 , CaO, and Na2 O in comparison with pyroxene in the chondritic host. The chondritic melt pocket of Yanzhuang contains about 5–6% melt glass occurring among the recrystallized silicate minerals (Fig. 10.3). EPMA measurement also shows that the melt glass has a heterogeneous composition (Table 10.1), corresponding to a quench glass phase. Iron–nickel metal and troilite inside the melt regions of the Yanzhuang meteorite were melted during a shock event and occur as eutectic metal-troilite nodules of different sizes (Fig. 10.4). These eutectic nodules display a quench texture composed of dendritic or cellular FeNi enclosed in troilite. The secondary dendrite arm spacing and cell widths of cellular crystals range from 30 to 60 μm. Results of electron microprobe analyses of FeNi metal and troilite in melt regions and chondritic host are shown in Table 10.2. It should be pointed out that FeNi metal in nodule No. 1 contains higher Ni and P and lower Fe and Cr in comparison with the FeNi metal in the chondritic host. On the other hand, the troilite in nodule No. 1 contains higher Ni and Cr and lower P, S, and Co in comparison with the troilite in the chondritic host (see Table 10.3). The FeNi metal in chondritic host contains extremely low content of P (0.001%), but the P-content of the metal dendrites in nodule No. 1 is relatively high (0.45–0.65%). In contrast to this, the troilite in the chondritic host contains 1.10% of P, while the troilite in nodule No. 1 does not contain any P. This phenomenon indicates that during shock melting, the P decomposed from melted phosphates may only concentrate in metal phase. We observed more than 60 Fe–Mn–Na phosphate globules and two chromite crystals in the troilite matrix of the above-mentioned large metal-troilite eutectic nodule No. 1 (Xie et al. 2014). Electron microprobe and Raman spectroscopic analyses show that a few phosphate globules have the composition of Na-bearing graftonite

10.3 Metal-Troilite Eutectic Nodule No. 1 in Yanzhuang Shock Melt

173

Fig.10.4 Backscattered electron (BSE) image showing the metal-troilite (M+S) eutectic blobs of different sizes in the Yanzhuang chondritic melt pocket

(Fe,Mn,Na)3 (PO4 )2 , whereas most others correspond to Mn-bearing galileiite Na(Fe,Mn)4 (PO4 )3 and a possible new phosphate phase of Na2 (Fe,Mn)17 (PO4 )12 composition. Two chromite crystals were found within troilite and between two neighboring FeNi dendrite arms in nodule No. 1. Their composition is different from that of chromite in the chondritic host. Both chromite crystals in nodule No. 1 do not contain any Al (Xie et al. 2014). Such unique metal + troilite + Fe–Mn–Na phosphates + Al-free chromite eutectic assemblage is very rare. Its formation mechanism needs to be investigated in detail. For understanding of the formation mechanism of above-mentioned Fe–Mn– Na phosphates and Al-free chromite in nodule No. 1, here we give some introduction about the occurrence and composition characteristics of phosphate minerals and chromite in the Yanzhuang chondritic host. As we know that merrillite, Ca9 NaMg(PO4 )7 , is the only phosphate mineral observed in the Yanzhuang unmelted chondritic host. It occurs as irregular grains in the interstices of silicate minerals. The grain size of merrillite ranges from 0.1 to 0.3 mm. The chemical composition of merrillite is (in wt% oxide): 45.59 CaO, 4.43 MgO, 2.87 Na2 O, 0.07 K2 O, 0.04 Cr2 O3, and 46.59 P2 O5 (Chen 1992). Raman spectroscopic analysis indicates that the spectrum of Yanzhuang merrillite shows five Raman bands at wavenumbers 976, 958, 1086, 602, and 448 cm−1 (Chen 1992). This spectrum is almost identical with that of whitlockite in other chondrites (Chen et al. 1995a, b; Xie et al. 2002).

91.62

0.55

0.03

0.02

0.09

n. d

n. d

100.81

Fe

P

S

Mn

Cr

V

Ti

Total

99.71

0.02

n. d

0.13

n. d

0.02

0.55

90.29

8.17

0.52

n. d

0.01

2

100.42

n. d

0.02

0.12

0.02

0.03

0.45

91.23

8.02

0.51

n. d

0.02

3

7.83

0.42

0.01

0.01

100.20

0.04

0.01

0.23

0.01

0.03

0.60

91.01

4

metal in the Yanzhuang chondritic host. n. d. not detected

8.11

Ni

a FeNi

0.01

0.37

Co

0.01

Si

Al

1

Sample number

99.01

0.04

n. d

0.10

n. d

0.01

0.52

89.87

7.96

0.50

n. d

0.01

5

98.89

0.01

n. d

0.16

n. d

0.01

0.50

89.81

7.98

0.40

0.01

0.01

6

Table 10.2 Microprobe analyses of FeNi metal in the Yanzhuang eutectic nodule No. 1 (wt%)

100.36

n. d

n. d

0.11

0.01

0.03

0.53

91.00

8.21

0.47

n. d

n. d

7

98.83

0. 06

0. 01

0.14

n. d

n. d

0.50

90.71

6.95

0.45

n. d

0.01

8

100.14

0.03

n. d

0.12

n. d

0.01

0.65

90.81

8.05

0.45

0.01

0.01

9

99.81

0.02

n. d

0.13

0.01

0.02

0.54

90.72

7.91

0.45

n. d

0.01

Ave (9)

99.12

n. d

n. d

0.32

0.03

0.10

0.01

92.18

6.21

0.66

n. d

n. d

FeNia (5)

174 10 Fe–Mn–Na Phosphates and Al-Free Chromite in the Metal-Troilite …

63.19

n. d

36.07

n. d

0.10

n. d

n. d

99.59

Fe

P

S

Mn

Cr

V

Ti

Total

99.45

0.02

n. d

0.16

n. d

36.27

n. d

62.87

0.03

0.09

0.01

0.01

2

98.57

0.04

n. d

0.20

0.05

36.57

n. d

61.9

0.07

0.06

n. d

0.02

3

in the Yanzhuang chondritic hos. n. d. not detected

0.16

Ni

a Troilite

n. d

0.07

Co

n. d

Si

Al

1

Sample number

98.70

n. d

0.01

0.10

0.01

36.13

0.01

62.24

0.10

0.08

0.01

0.01

4

98.60

n. d

0.01

0.12

0.02

36.95

n. d

62.28

0.14

0.06

n. d

0.01

5

Table 10.3 Microprobe analyses of troilite in the Yanzhuang eutectic nodule No. 1 (wt%)

98.63

0.01

n. d

0.16

0.02

36.93

0.01

61.35

0.06

0.06

n. d

0.03

6

99.04

n. d

n. d

0.14

0.02

36.25

n. d

62.32

0.13

0.09

0.02

0.02

7

98.67

n. d

n. d

0.16

0.01

36.60

0.01

61.59

0.19

0.08

0.02

0.02

8

99.03

0.01

n. d

0.14

0.02

36.48

n. d

62.18

0.11

0.07

0.01

0.01

Ave (8)

99.74

n. d

n. d

0.07

0.03

37.14

1.10

60.96

0.05

0.42

n. d

n. d

Troilitea (5)

10.3 Metal-Troilite Eutectic Nodule No. 1 in Yanzhuang Shock Melt 175

176

10 Fe–Mn–Na Phosphates and Al-Free Chromite in the Metal-Troilite …

Chromite occurs in the Yanzhuang unmelted chondritic rock as euhedral, subhedral, or irregular grains of 30–80 lm in size. These grains are cracked and fractured. It contains 28.10–29.62 wt% FeO, 2.97– 3.41 wt% MgO, 0.92–1.43 wt% MnO, 53.06– 55.75 wt% Cr2 O3 , and 7.27–8.18 wt% Al2 O3 (Chen 1992). Hence, the Al2 O3- content of chromite in the Yanzhuang meteorite is high enough. The above facts demonstrate that the originally occurred merrillite in the Yanzhuang meteorite no more exists in shock-induced melt. Phosphates appeared in shock melt are newly formed Fe–Mn–Na phosphates. The chromite originally occurred in Yanzhuang chondritic rock disappeared in shock melt. The chromite crystallized from shock melt is an Al-free one.

10.3.2 Occurrence of Fe–Mn–Na Phosphate Spherules We have reported a number of Na-bearing Fe–Mn phosphate inclusions within troilite in some metal-troilite eutectic nodules of the Yanzhuang chondrite (Chen and Xie 1996). These phosphate inclusions occur as fine-grained spherules with 2–8 μm diameters and contain 48–58 wt% FeO, 2–5 wt% MnO, 38–40 wt% P2 O5 , and up to 4.55 wt% Na2 O. On the basis of microprobe analysis of 21 spherules occurring within troilite in different eutectic nodules, we indicated that only a few grains have the composition of graftonite (Fe2.95 Mn0.05 )3 (PO4 )2 , but the majority of them are Fe–Mn–Na phosphate grains, Their Na2 O-content varies in the range from 1.32 to 4.55 wt. At that time, we considered that the Na-rich Fe–Mn phosphates (Na2 O ≥ 4 wt%) having composition of (Na,Ca,K)2 (Fe,Mn)8 (PO4 )6 might correspond to the 2:8:6 phosphate (Na,K)2 (Fe,Mn)8 (PO4 )6 found in IIIAB iron meteorites by Olsen and Steele (1993). The Na-bearing Fe–Mn phosphates (1.32–3.72 wt% of Na2 O) could represent a mixture of graftonite and a unknown Fe–Mn–Na phosphate. In our later studies, we observed more than 60 Fe–Mn–Na phosphate globules in the above-mentioned large metal-troilite eutectic nodule No. 1 (Xie et al. 2002, 2014; Xie and Chen 2018). Some of them are shown in Figs. 10.5, 10.6, 10.7, 10.8, 10.9, 10.10, 10.11 and 10.12 (arrows). Each globule has its own symbol, such as A1, A2, B1, B2, K1, K2, L1, and L2. The English capital letters from A to N represent the rectangle areas of investigation shown in Fig. 10.2, and the Arabic numerals are their sequence numbers in rectangle areas. For instance, A1 is the first globule in rectangle area A, and L2 is the second globule in rectangle area L. Our study revealed that the sizes of Fe–Mn–Na phosphate globules range from 2 to 13 μm in diameter. The globule A1 and globule A6 in Fig. 10.5b, globule B1 in Fig. 10.6, globules C1, C2, C5, and C6 in Fig. 10.7a, globule D1in Fig. 10.8, globule E2 in Fig. 10.9a, globule F1 in Fig. 10.9b, globule G1 in Fig. 10.10a, globule H1 in Fig. 10.10b, globule I1 in Fig. 10.11a, globule J1 in Fig. 10.11b, and globule K1 in Fig. 10.12a are larger than 10 μm in diameters. Microscopic study shows that the surface of globules is quite smooth, and no inclusions of other minerals were observed within the globules. Under reflected light, the phosphate globules in nodule

10.3 Metal-Troilite Eutectic Nodule No. 1 in Yanzhuang Shock Melt

177

Fig. 10.5 BSE images showing a the Fe–Mn–Na phosphate globules (arrows) within troilite in area A of nodule No. 1. b The enlarged image of the upper part of Fig. 10.5a

178

10 Fe–Mn–Na Phosphates and Al-Free Chromite in the Metal-Troilite …

Fig. 10.6 BSE images showing a the Fe–Mn–Na phosphate globules (arrows) within troilite in area B of the nodule No. 1. b The enlarged image of the upper part of Fig. 10.6a

10.3 Metal-Troilite Eutectic Nodule No. 1 in Yanzhuang Shock Melt

179

Fig. 10.7 BSE images of a the Fe–Mn–Na phosphate globules (arrows) within troilite in area C of the nodule No. 1. b The enlarged image of the middle part of Fig. 10.7a

180

10 Fe–Mn–Na Phosphates and Al-Free Chromite in the Metal-Troilite …

Fig. 10.8 BSE images of a the Fe–Mn–Na phosphate globules (arrows) within troilite in area D of the nodule No. 1. b The enlarged image of the upper-middle part of Fig. 10.8a

10.3 Metal-Troilite Eutectic Nodule No. 1 in Yanzhuang Shock Melt

181

Fig. 10.9 BSE images of a the Fe–Mn–Na phosphate globules (arrows) within troilite in area E and that in area F (b) of the nodule No. 1

182

10 Fe–Mn–Na Phosphates and Al-Free Chromite in the Metal-Troilite …

Fig. 10.10 BSE images of a the Fe–Mn–Na phosphate globule (arrows) within troilite in area G and that in area H (b) of the nodule No. 1

10.3 Metal-Troilite Eutectic Nodule No. 1 in Yanzhuang Shock Melt

183

Fig. 10.11 BSE images of a the Fe–Mn–Na phosphate globule (arrows) within troilite in area I and that in area J (b) of the nodule No. 1

184

10 Fe–Mn–Na Phosphates and Al-Free Chromite in the Metal-Troilite …

Fig. 10.12 BSE images of a the Fe–Mn–Na phosphate globules (arrows) within troilite in area K and that in area L (b) of the nodule No. 1

10.3 Metal-Troilite Eutectic Nodule No. 1 in Yanzhuang Shock Melt

185

No. 1 mainly show to have a completely round shape. In addition, globules with droplike semicircle, ellipsoid, and triangle shapes also occur in this nodule. Although all observed Fe–Mn–Na phosphate globules occur within troilite, their close relationship with FeNi metal in the nodule is noticeable. According to our statistics, among 45 phosphate globules observed in this nodule, 31 globules (70%) occur in direct contact with dendritic or cellular metal, and only 14 (30%) are completely embedded in troilite.

10.3.3 Composition of Fe–Mn–Na Phosphate Spherules A larger phosphate globule D1 in nodule No. 1 was selected for electron microprobe analysis to investigate the chemical homogeneity of globules (Fig. 10.8b). The result of analysis on three spots of this globule shows that not only the main components (FeO, MnO, Na2 O, and P2 O5 ), but also the minor components (MgO, CaO, SiO2 , and Cr2 O3 ) are very similar to each other (Table 10.4), indicating the globule has a homogeneous composition. Among the 45 phosphate globules observed in nodule No. 1, only 26 globules (larger than 6 lm in diameter) were measured by EPMA (Tables 10.5, 10.6, and 10.7). The measured composition of phosphates is similar in P2 O5 -content (39–40 wt%) and slightly variable in FeO + MnO-contents (52–58 wt%), with varying Na2 Ocontent (from 0.75 wt% for globule C2 in Table 10.5 up to 6.65 wt% for globule D2 in Table 10.6). The above-listed analytical data are similar to those for phosphate globules we obtained earlier in Yanzhuang eutectic nodules (Chen et al. 1996). But the variation Table 10.4 Microprobe analyses of phosphate globule D1 in nodule No. 1 (wt%) Points

1

2

3

Ave

Na2 O

5.03

5.32

5.16

5.17

K2 O

0.12

0.16

0.13

0.14

FeO

49.86

49.57

49.69

49.71

MgO

0.15

0.14

0.16

0.15

MnO

3.59

3.95

3.50

3.68

CaO

0.35

0.31

0.49

0.38

SiO2

0.47

0.36

0.32

0.38

TiO2

0.04

0.04

n. d

0.03

Al2 O3

0.06

0.02

0.02

0.03

Cr2 O3

0.86

0.83

0.82

0.84

P2 O5

40.00

40.18

40.52

40.23

Total

100.53

100.88

100.81

100.74

Na2 O

5.03

5.32

5.16

5.17

186

10 Fe–Mn–Na Phosphates and Al-Free Chromite in the Metal-Troilite …

Table 10.5 Microprobe analyses of graftonite in the Yanzhuang nodule No. 1 (wt%) Ave

Gfta

Gftb

Globule number

C2

C5

L1

E2

Na2 O

0.75

0.84

0.90

1.11

K2 O

0.17

0.13

0.17

0.21

0.17

FeO

54.52

54.71

55.93

53.63

54.70

58.0

27.75

MgO

0.13

0.13

0.05

0.05

0.09