The Opaque Minerals in Stony Meteorites [Reprint 2022 ed.] 9783112651025

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PAUL RAMDOHR The Opaque Minerals in Stony Meteorites

PAUL RAMDOHR

The Opaque Minerals in Stony Meteorites

With 306 Figures

AKADEMIE-VERLAG 19 7 3

.

BERLIN

Erschienen im Akademie-Verlag GmbH, 108 Berlin, Leipziger Straße 3—4 Copyright 1972 by Akademie-Verlag GmbH Lizenznummer: 202 • 100/465/73 Gesamtherstellung: VEB Druckerei „Thomas Müntzer", 582 Bad Langensalza Bestellnummer: 7613330 (5764) - E S 1 8 F 3 , 18 D4 Printed in German Democratic Republic

Table of contents Preface and Acknowledgements Summary of mineral composition Distribution of opaque minerals in stony meteorites The minerals and textures

7 10 14 16

A. Elements and intermetallic compounds

16

Kamacite Taenite Plessite Henderson Phase — Perryite Schreibersite Cohenite Graphite Native Copper Osbornite ("gold")

17 19 19 20 22 23 24 25 26

B. Sulfides Troilite "Troilite" in carbonaceous chondrites. . Chalcopyrrhotite Mackinawite Pentlandite Unknown layer-structure mineral Daubreelite Oldhamite Niningerite Alabandite Sphalerite Chalcopyrite Pyrite

C. Oxides Chromite Ilmenite Magnetite Spinel Rutile

28

. . . . . ' .

28 31 32 34 34 36 38 40 41 42 43 43 44

44 44 48 49 51 52

6

Preface D. Bare and insufficiently known minerals Minerals A, B Mineral C ( = Djerfisherite) Mineral D (Rutile) Mineral E Minerals F, G, H, I, K, L, M, N Layer structure minerals in enstatite achondrite

52 52 53 53 54 55 57

E. Fabric, Texture, and Structure

57

F. Weathering effects

64

6. Fusion crust formed in the atmosphere

67

E. Application of reflecting microscope in elucidation of textures of silicate minerals in meteorites

69

Summary

72

Zusammenfassung

73

Card File

74

Literature

76

Explanations and abbreviations

77

Meteorite list

80

Preface It may seem remarkable that, although the iron meteorites and some mesosiderites have received comprehensive microscopic examination, until recently only a few satisfactory investigations of opaque minerals have been made on the stony meteorites; the names of E D W A K D S , H E N T S C H E L , JASKOLSKI, P A U L Y , SZTROKAY, and YTJDIN should be mentioned in this connection. The neglect, however, is not so surprising as it might appear in view of the very much larger number of stony than of iron meteorites. Moreover, the standard methods of metallography have been available for the study of iron meteorites for about a century, whereas the stony meteorites present serious difficulties in preparation on account of their poor polishing qualities and the variety of unusual opaque minerals they contain. For example, native copper, which has been thought to occur very rarely in stony meteorites, actually is present in at least half of them — a fact that itself testifies to the inadequacy of our earlier knowledge. As long ago as 1931, V. M. GOLDSCHMIDT suggested that I should undertake the microscopic study of the opaque minerals in stony meteorites because of the significant problems they posed. For thirty years, more pressing tasks prevented me from acting upon his suggestion. The present investigation should by no means be considered exhaustive, limited as it has been by the time and material available. Nevertheless, at its conclusion, polished surfaces of some 400 meteorites had been examined. The polished surfaces were for the most part prepared by the expert hands of H . LAMMLER. All the preparations were imbedded and impregnated with a plastic that sets at a rather high temperature (120° —150°). Thus time was saved and many failures were averted. For the carbonaceous chondrites a plastic hardening at room temperature was used. Even though the polished surfaces were excellent, not all the original material for preparation could be considered satisfactory. For some meteorites only fragments of perhaps a few milligrams were available—not enough to be representative of the entire meteorite. I was convinced that these polished surfaces of meteorites, as significant described material, should be collected and preserved in a single depository, together with descriptions and documents concerning them. However, this seems not possible. Too much irreplaceable material has already been consumed by repeated analyses, age determinations, thin-section preparation, and similar investigations, often carried out on obviously unrepresentative material. My

8

Preface

earlier proposal, that the polished surfaces, descriptions, photographs, etc., be deposited in one place—for example, in the meteorite collection of the Smithsonian Institution, in Washington — unfortunately could not be followed through. Now I should like to suggest that the institutions that have all the material at their disposal make it available in a collective publication. Since my preliminary reports (1961)1) three more papers (1962, 1963a, b) have appeared, containing parts of the present, more comprehensive work. Some of the photomicrographs in them are reproduced in the present volume. The abundance of observations on the material examined since the earlier reports has resulted in much new knowledge and necessitated some corrections. The new findings and reinterpretations presented here are clearly indicated. Although the original plan was to restrict attention to the opaque-mineral content of stony meteorites, it sometimes seemed interesting and important to make comparisons with mesosiderites, pallasites, and the troilite, graphite, and silicate nodules of iron meteorites. Some very remarkable similarities, especially with enstatite chondrites and achondrites, are established. In this work I refrain from discussing the origin or history of meteorites, limiting myself to statements of what can be observed. To offer theories or hypotheses is not at all may purpose. Only where, more or less incidentally, it must be stated that a certain theory is supported by distinctly faulty observations or is based on insufficient material will it be mentioned. In general, one gains the impression that a good many theories in the recent literature on meteorites are rather unsatisfactory. Sometimes a single trivial observation may invalidate the whole line of reasoning. The present study follows about the same arrangement as an earlier report (1963 b), but it is more comprehensive throughout and contains some essential additions. It should be mentioned that the explanations of the figures are an essential part of this publication. Many peculiarities are not otherwise mentioned in the context. My gratitude is here expressed to all the institutions and persons who provided material for these investigations. Their names appear as contributors and in the table (page 80). I wish to acknowledge my special indebtedness to Dr. B R I A N MASON formerly of the American Museum of Natural History in New York, who translated part of the manuscript from the German; Dr. C L I F FORD FRONDEL of the Mineralogical Museum of Harvard University; Dr. E. P. HENDERSON of the Smithsonian Institution; Dr. A . S C H Ü L L E R , late director of my old institute in Heidelberg; and Dr. H. F R E U N D of the Leitz Company, who provided the facilities for some of the photographic work. Dr. G. KULLERTTD was always a helpful friend. Most of all, however, I should like to express my gratitude for the facilities I have enjoyed during three sojourns at the Geophysical Laboratory of the Carnegie Institution of Washington; likewise, I must thank See Literature, page 76.

Preface

9

the Max-Planck-Institut fur Kernphysik, and its director, Professor W . G E N T NEE, for much help. Many workers in the field have always been ready to offer aid, among them Drs. ZAHRINGER, OTTEMANN, F E C H T I G , and D U K E in determinations with the electron probe. One of my pupils, Dr. MOH, devoted much time to helping with photographic work. The appearance of this small book—actually ready for print in 1964—had to be postponed two or three times, mostly because of printing problems. In the meantime hundreds of meteorite papers have appeared, many of them containing microscopic data on one meteorite or of groups. It was impossibly to refer to all of them without considering the scope of each paper, which would obviously lead too far. Heidelberg — March 1 9 6 9 P A U L RAMDOHR

Summary of Mineral Composition To treat the structure and mineral composition of each meteorite individually would lead to unnecessary repetition. On the other hand, generalizations are dangerous, for every meteorite may have its unique features, which may be of genetic significance. Therefore the author has compromised by first describing the features and minerals that are the same or similar in most specimens, then discussing variants and peculiarities like grain size, crystal form, intergrowths, and other special characters. In the table (pages 80—104), specimens that display uncommon features are indicated by exclamation points. This was not done for some rare types of meteorites, however, since in such material it is not possible to say what is common and what is uncommon. In the tables and descriptions, the minerals are presented in the following order: elements and intermetallic compounds; sulfides; oxides; and rare minerals. Within these groups the arrangement is approximately in the order of abundance or in the order of genetic significance; thus, a rare but occasionally abundant mineral may be discussed before a more widespread but always minor constituent—copper, for instance. Quantitative estimates have been omitted deliberately. The relative abundance of minerals varies greatly over very short distances, not only in brecciated and polymict meteorites, as would be expected, but also in many that superficially appear quite uniform. Dr. K. K E I L (1962) published the laboriously made, and exceedingly valuable, measurements of the mineral contents over very large areas of many meteorites. He could not, of course, include the minor constituents, but even without them he could show that, in contrast to the experience with terrestrial rocks of about the same grain size, it is very hard to get a representative average, for analytical determination, for example. Because of this difficulty, many analyses, even though performed with superb technique and great accuracy, may be of little value, and often the classification may be wrong. That is especially true for the small sections of the present study. Some observations reported as the rule could be exceptions, and vice versa. The following opaque and semiopaque minerals, the natural objects for oremicroscopic investigation, to be discussed in detail later, have been identified: The most common are kamacite, ora-iron, with variable nickel content; taenite, or iron-nickel solid solution, with the structure of y-iron; and plessite, an intergrowth of the oc and y phases. Some unexplained complexities in the relative proportions of two phases, and in the type of intergrowth, are discussed in a later

'Summary of mineral composition

11

section; the distinction of the intergrowths from the individual components is not always clear. Cohenite, Fe3C, is normally absent in stony meteorites; when present it was usually in very small amount and restricted to transitional types. The expected correlation between its presence and the degree of reduction of the meteorite was commonly observed, though frequently it was not found where it might be expected—in Pillistfer, for example. Schreibersite may be present in very different types of stone meteorites, but it is very rare, small, and surely often a product of local disequilibria. I t is a frequent and partly abundant component in enstatite chondrites and achondrites. A completely new mineral:, originally a high temperature member of the Fe-Ni-Si mix-crystal series but having exceptional properties and probably containing phosphorus (referred to in the tables as Fe^Si,,), was found in only four meteorites — St. Marks, Indarch, Khairpur, and Grady 2 (1937) — quite abundantly in the first three. Graphite, which can be quite common, was seen in only about one-tenth of the specimens, in most, but by no means all, of those showing a high degree of reduction. Diamond is absent in stony meteorites besides the ureilites but may be present in iron meteorites. Native copper, though present in trace amounts only, was observed in more than half the specimens examined, especially in the typical chondrites. Gold was thought to be present in only one chondrule of an exceptional meteorite, but the interpretation was rendered doubtful when osbornite was reported in some other meteorites. The properties of the "gold" did not fully agree with either gold or osbornite. Troilite was present in all the specimens examined, in many of them being the most abundant opaque mineral. In the carbonaceous chondrites in which native iron is sometimes lacking it should be called pyrrhotite, Fe1_;I.S, especially in those with high sulfur content, such as Mighei. Chalcopyrrhotite, in the sense of H. BORCHERT (1934) and of DIE Erzmineralien (1960) as a cubic high-temperature solid solution (Fe, Cu, Ni, Zn)S, was surprisingly, found, sometimes abundantly, in at least a third of all the specimens. Because of its similarity to troilite, and because it readily escapes attention when present in small amount, it has often been overlooked. "Valleriite", which had also not been reported previously, was mentioned in the earlier papers of the writer (1962, 1963a, b). Actually, in most, perhaps all specimens it seems to be the recently described mackinawite, a tetragonal form of FeS with layer structure. I t occurs quite abundantly as a disintegration product of chalcopyrrhotite and an exsolution product of pentlandite. Pentlandite has only recently been recognized in meteorites; it was first identified in K a b a b y SZTROKAY (1960), and i n K a r o o n d a b y MASON and W I I K (1962).

Normally it is present in all sulfur-rich meteorites lacking a-Fe,. but suprisingly it is sometimes found accompanied by that mineral. I t may be present in an

12

Summary of mineral composition

eighth of all the meteorites examined, partly perhaps a breakdown product of chalcopyrrhotite. It is common, too, as a product of intermediate weathering. Oldhamite, CaS, with a very small content of iron, is limited to meteorites that are highly reduced and/or have a high sulfur content. Alabandite as MnS proper seems to be rare. Instead, a complex mix-crystal (Mg, Fe, Mn, Cr . . .)S with NaCl structure is common in highly reduced and sulfur-rich meteorites. Meanwhile it has been named "niningerite". A remarkable observation is the occurrence of a new mineral, with tetragonal or hexagonal layer structure, whose properties fall between those of graphite and valleriite. It is extremely soft, and is practically restricted to carbonaceous chondrites, in which it is abundant. It is surely an Fe-S compound, very probably containing carbon and perhaps hydrogen. Its extremely low hardness, and therefore poor polishing qualities, prevented its recognition earlier. Daubreelite, FeCr2S4, is easily recognized. It is normally, but not exclusively, found in sulfur-rich and highly reduced stone meteorites, such as Atlanta, Cumberland Falls, Hvittis, Norton Co., Pillistfer, and St. Marks. But it is present in Mt. Morris and abundant in Shallowater. Most meteorites containing daubreelite contain little or no chromite. Sphalerite, ZnS, occurs in only trace amounts, owing to the low zinc content of most meteorites. It can be rather easily recognized because of its characteristic occurrence and forms in strongly reduced and sulfur-rich meteorites (Khairpur, St. Marks, and in sulfide nodules in irons, for example Canyon Diablo). Chalcopyrite, CuFeS2, has been observed in a few meteorites. Primary pyrite has been observed in only one meteorite (Karoonda). In contrast to the numerous sulfides, the primary oxides are limited to chromite, magnetite, ilmenite, spinel, rutile, and cuprite. Chromite, FeCr a 0 4 , actually very often chromium-spinel or intermediate members, is one of the most widespread and variable minerals of meteorites. It is abundant in mesosiderites and in surely 90 percents of all stony meteorites, and is present in pallasites and some irons. Its brittleness and its inertness toward solution or recrystallization give it special genetic significance. Primary magnetite is much rarer than chromite, occurring in fewer than a tenth of the meteorites examined, and it appears to differ in its genetic relationships even in the same meteorite. It also varies in its properties; in some meteorites it evidently contains nickel, chromium, and possibly manganese as well. Ilmenite, FeTiOs, overlooked until about ten years ago except for a littlenoticed description by S T . J A S K O L S K X in 1 9 3 8 , and still considered a rarity in meteorites, was recognized in more than half of all the specimens examined in this study, occasionally in large grains, sometimes, though more rarely, as exsolution lamellae especially in chromite, and likewise in magnetite, mineral I (see below), and possibly hypersthene. Spinel, MgAl204, or, better, translucent minerals of the spinel group, was observed in perhaps 25 sections in this study but mostly in minute quantities.

Summary of mineral composition

13

Rutile will be described below under "mineral D " . In addition to these rather common arid in general well identified minerals, the following phases were observed in small amounts, often in a single polished section. They are gradually being investigated by electron-probe techniques. A number of them have been identified and described in some detail (1963b). Mineral A is strongly anisotropic, dark yellow-green but perhaps variable (St. Marks, Indarch). B is certainly an extraterrestrial mineral, occurring in thin lamellae as a decomposition product of A. G (meanwhile named djerfisherite) has a dark olive color (St. Marks, Cumberland Falls). D is a transparent mineral with very high refractive index, replacing ilmenite and chromite; it has been determined to be a colorless rutile. E is a dark brown, probably isotropic . mineral (Kandahar). J 1 is a white mineral, probably containing arsenic (Mokaia). G is light blue (Pesjanoe). H is yellow, almost metallic, similar to osbornite, but probably anisotropic (Tysnes). / is a colorless spinel-like mineral showing coarse exsolution of ilmenite. K is a very dark gray sulfide. L is strongly pleochroic, between bright shiny green and medium gray. M has metallic reflectivity; it is intergrown with copper and distinctly bluish. N is brownish gray; it is anisotropic but displays an octahedral inversion network. In addition to these, three layer-structure minerals, I, II, and III, were found, each of which probably contains chromium but which, in spite of rather different optical properties, could not be established as chemically distinct from one another or from minerals A and B. A number of minerals that are described in connection with their genetic associations, such as fusion crust and weathering under terrestrial conditions, will not be discussed in detail at this point. They include the extremely uniform skeletal"magnetite", which is present in practically every meteorite fusion crust; the wiistite sometimes formed in the same surroundings; the alteration products of chromite near the outermost crust; the pseudobrookite formed from ilmenite; products of the reaction between (Mg, Fe)S and troilite, etc.; and, finally, the smaller or larger masses of limonite, present in all meteorites not collected immediately after the fall. Actually, the "limonite" masses represent a rather complicated association: magnetite formed from iron, often in botryoidal forms; maghemite, in exceptionally large quantities; goethite, the main component; lepidocrocite, in a few meteorites only; secondary pentjandite, rather common, precipitated on troilite; gel-pyrite; and perhaps secondary pyrrhotite. Akaganeite, /9-FeOOH, described only recently, seems to be not uncommon. Graphite may be a weathering product of cohenite, but the distinction between weathering and breakdown is difficult. More precise investigations on more abundant material or on very unusual meteorites will certainly reveal additional opaque minerals, but the author believes that the present study is reasonably comprehensive. Neither an exhaustive description of the minerals identified nor the determination of all the newly recognized phases is the purpose of this volume. Years of work, and more — many more — young co-workers would be required for those objectives.

14

Distribution of opaque minerals

Distribution of Opaque Minerals in stony Meteorites With only a few exceptions stony meteorites display a much more uniform silicate mineralogy than that encountered in terrestrial rocks. These meteorites consist essentially of olivine and pyroxene, usually orthopyroxene, with minor amounts of plagioclase. Glass in small quantities is frequently present; it was the experience in the present study that more glass occurs in these meteorites than had previously been realized. Very rarely other silicates become essential components : serpentine in carbonaceous chondrites, and quartz, tridymite, and cristobalite in highly reduced types of stony meteorites. Some silicates are poorly described or perhaps not even firmly established as minerals in meteorites. Examples of such minerals are kosmochlor and Weinbergerite. Apatite is rather common, and farringtonite, merrihueite, krinovite, and some others have been observed only rarely (MASON a.o.). Of the opaque minerals some thirty species have been recognized, but only about eight are common and widespread. With the exception of nickel-iron, which only rarely occurs in terrestrial rocks, these minerals are commonty found in basic igneous rocks. In recent years renewed attention has been focused on PRIOR'S classification of chondrites. In this classification, based upon chemical composition, the meteorites are classified from highly oxidized carbonaceous to the common olivine pyroxene chondrites to the highly reduced enstatite chondrites. In the last the iron is present as free metal and the chromium as sulfide. Manganese and calcium occur wholly or partly in the sulfide form, and under extreme conditions titanium and silicon may be present in a metallic phase. In individual meteorites the relationships may be considerably more complex than would appear from earlier classifications. Each chondrule and often each zone within a single chondrule may represent local equilibrium. Many examples of mineralogical changes brought about on a very local basis by hot reactive gases have been observed; Grady 2 (1937) is one of them. The question of equilibrium is always problematic. It appears that equilibrium has been more generally established in reduced meteorites that have been exposed to high temperatures. Meteorites that have been exposed to reducing conditions as well as high temperature were greatly affected both in mineralogical composition and in grain size. The brecciated structures tend to be erased under such circumstances. However, heavy brecciation was also observed in some of the enstatite achondrites, for example Cumberland Falls. Some of the other achondrites, Kapoeta, for instance, were found to be extremely heterogeneous. In many mesosiderites graphite occurs in association with ilmenite and chromite. In some areas one finds that preexisting brecciated structures have been erased and in others the brecciated structures are unaltered. Commonly "spontaneous fusion" has produced special features in small aeras. Even the smallest fragments in carbonaceous chondrites carry droplets of nickel-iron, sulfides, and sometimes glass. These phases indicate a complex

Distribution of opaque minerals

15

genetic history. "Normal" types of the most common chondrites were observed to contain isolated inclusions of carbonaceous chondrite. Such inclusions display only slight alterations (Figs. H6—8). P R I O R ' S rule points out t h a t in any stony meteorite the content of free metal is richer in nickel the richer the siliconates are in ferrous iron. Essentially, it means t h a t nickel is more siderophile than iron. The principle can normally be applied to the mineral associations, but often it is necessary to also include a reference to the ratio between metal and sulfur. Apparently the relative amount of sulfur is rather constant. Therefore, the metal, the first kamacite, will disappear if the Fe + Ni content is low, disregarding iron in silicates and magnetite. Many chondrites having large amounts of troilite contain little or no iron in a metallic phase. A good example is Karoonda, which, from the characteristics of its magnetite (plentiful exsolutions of ilmenite), was, at some time in its history, at a high temperature. I t is certain t h a t in many meteorites sulfur has been introduced in some form, FeS then replacing iron; and kamacite, and later also taenite, often forming fine networks. Nickel behaves toward sulfur as a nobler (more "siderophile") element t h a n iron. Thus, with the introduction of sulfur the metal phase is enriched in nickel, and the amount of kamacite decreases. With continued introduction of sulfur, the metal phase disappears completely (except for relicts "armored" by olivine), and finally in some meteorites pentlandite appears. Thus, pentlandite may be expected as a primary phase only when the metal phase has been greatly lowered in amount. Further complications are produced by the presence and reactivity of hydrocarbons. If P R I O N S rule is correct, the paraffinoid hydrocarbons act to reduce the FeO, which is partly in magnetite and partly in the silicates, whereby H 2 0 and C0 2 are expelled as reaction products. Graphite may remain, either fine-grained or in quite coarse crystals. Only at greatly increased temperature does this mineral act as a reducing agent. The well founded distinction between low-iron (L) and high-iron (H) meteorites may become a matter of chance if we consider, for instance, t h a t a mesosiderite like Vaca Muerta contains large areas which, if found as isolated pieces, would lead to its classification as an achondrite exceptionally poor in iron. I n chondrites, also, similar differences in the distribution of the "opaques" are not too uncommon.

The Minerals and Textures A. Elements and Intermetallic Compounds The Iron Group1) It is appropriate to group the nickel-iron phases in stony meteorites as follows: a-iron ( 15% Ni (up to 50% and more) now and formerly "taenite"; and a more or less distinct interior part with decreasing Ni, often exsolved into a myrmekitic intergrowth of the taenite + kamacite (now "plessite") or with relic structures of acicular martensite. Plessite in the classical meaning, filling the interstices, mostly does not have "plessitic intergrowth" at all. 2 ) "Widmannstatter structure", as explained by L. J. SPENCER, is preferable to "Widmannstatten's" structure. One could correctly say "the structure described by the Baron von WIDMANNSTATTEN" (after whom it was named), but the correct abbreviated German form is Widmannstatter's Struktur; therefore, in English, the author prefers Widmannstatter structure. The "a", of course, could be "ae".

Elements and intermetallic compounds

17

tint. There is no doubt that a hardness tester indicates taenite to be harder than kamacite. In a polished section, however, the situation may be different. In sections made using water, taenite is harder; using oil (in the modern polishing machines), it is usually softer. The same holds for (harder) chalcopyrite and (softer) galena: oil as a lubricant reverses often the hardness relations (see p. 275 of reference 1960). The differences in hardness can readily be made visible ("light line" with diminished aperture-iris), but the beginner should make a blind test first. Even in fine-grained myrmekitic intergrowths the differentiation is resasonably easy after some experience (Figs. A2, 3). Schreibersite has somewhat lower reflectivity and is markedly rose-brown (in oil immersion); cohenite, very rare in stony meteorites, is noticeably harder and anisotropic. Both schreibersite and cohenite are anisotropic; a - F e and y-Fe, isotropic. But in the recognition of anisotropy it is necessary to be on the watch for "pseudoanisotropy" which is likely to be present at the boundary between grains of differing hardness in poorly polished specimens, such as those prepared by the older metallographic methods with fine abrasive paper. Also, closely spaced parallel scratches, which may be submicroscopic or not entirely removed by polishing, may produce a pseudoanisotropic effect. Only very rarely, when the specimen contains either kamacite or taenite alone, can •'•here be any doubt about the identification. Generally, the form of both aco

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152

Fig. B50

Shallowater, Texas

Mag. 70 X

Daubrielile, large grains filling interstices between silicate grains. The daubrielite is rather strongly weathered and shows rims of (?) goethite. The unattacked grain in the lower right is troilite, a rather strange association without unmixing

Fig. B51

Shallowater, Texas

Mag. 35 X

The same as the last picture

Fig. B52

Obernkirchen, Germany

Mag. 250 x , imm.

Nodule of troilite with unmixed daubrgelite. The lamellae are partly bent. Troilite has precipitated secondary pentlandite

Fig. B53

Obernkirchen, Germany

Mag. 770 X, imm.

Mixed crystal of I'"eCr,,S4 (preponderating) and FeS in lamellar incomplete unmixing. The oblique orientation of the lamellae is not understood. Normally they are parallel to the elongation

Fig. B54

Murray, Kentucky

Mag. 695 X, imm.

New mineral, probably an Fe-C-S compound, in confusedly oriented platy masses, pseudomorphous after an idiomorphic and probably cubic mineral

Fig. B55

Renazzo, Italy

Mag. 685 X, imm.

New mineral, probably an Fe-C-S compound, strongly zoned replacing kamaeite (white). Black areas are silicates

Fig. B50

Pig. B51

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Fig. B54

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Fig. B53

Fig. B55

Fig. B56a

Cold Bokkeveld, S. Afrik^,

Mag. 220 X, dry

New layer structure mineral, gray in various tinges

Fig. B56b

Cold Bokkeveld, S. Africa

Mag. 175 x , imm.

New layer structure mineral, gray in various tinges, replaces troilite, in very hard copy bright white. Silicates are black. Same as 56a b u t immersion 1

Fig. B57 a, b

Mag. 600 X, imm.

The new mineral of B54 — 56 in an aggregate of very fine plates. With one nicol b u t position in a and b 90° different

Fig. B58

Mighei, USSR

Mag. 685 X, imm.

New mineral, probably an F e - C - S compound, replacing kamaeite, while the lamellae of taenite remain almost unaffected. The smallest spherules of nickel-iron are "armored" by olivine and remain intact. This phenomenon is formally rather similar (in black-white photography) to the selective weathering of plessite, b u t because of the optical relationships of the replacing mineral it can be easily recognized as different

Fig. B59 "Graphite-like

Pauly's photograph 62 (in 1958b, pi. 13,2) from the basalt of Idglokunguak, 455 X, imm. mineral" as a " b e a r d " on chalcopyrite in siderite. The similarity with the "Fe-C-S mineral" described here is remarkable

Fig. B56a

Fig. B56b

Fig. B57a

Fig. B57b

Fig. B58

Fig. B59

156

Fig. B60

Khairpur, Pakistan

Mag. 720 X , imm.

Large grain of partly euhedral daubrteliU at the boundary of two troilite grains. Troilite contains partly regular, partly branched exsolutions of (iaubreelite

Fig. B61

Khairpur, Pakistan

Mag.520 X , imm.

Large grains of homogeneously oriented troilite containing lamellae and branches of daubrtelite. One large and partly euhedral grain of "alabandite", one thick "cigar" of sphalerite (very little darker than '-'alabandite")

Fig. B62

Khairpur, Pakistan

Mag. 500 X, imm.

Cigar-shaped inclusion of sphalerite, probably formed as wurtzite, in troilite. troilite grain shows exsolutions of daubrtelUe

Fig. B63

Khairpur, Pakistan

Another

Mag. 420 X, imm.

An isolated grain showing troilite (main part, brightest), daubrtelite (left corner, euhedral, nearly as bright as trolite), "alabandite" (dark gray), and sphalerite (darkest). The difference has been made visible by use of high contrast film (see text).

Fig. B64

Hvittis, Finland

Mag. 175 X, imm.

Broad lance-like lamella of sphalerite, dark gray, between the main mass of troilite, strongly fractured, and daubrtelite, a (111) section of an idiomorphic crystal; at the right a dilute lamella of this mineral; black are silicates

Fig. B60

Pig. B61

Fig. B63

Fig. B62

Fig. B64

158

Fig. Cl

Kandahar, Afghanistan

Mag. 85 x , imm.

Chromite (coarse chromite) idiomorphic against troilite (white) and xenomorphio against the silicates (black)

Pig. C2

Yaca Muerta, Chile

Mag. 50 X

The light gray mineral following the boundaries of the silicates is chromite, and the orientation of the exsolution lamellae of ilmenite shows that it is a single crystal over the whole extent of the photograph!

Fig. C3

Miller, Arkansas

Mag. 150 X

Odd-shaped chromite (coarse chromite), completely xenomorphic in silicates. Against the nickel-iron (kamacite, white) the chromite is idiomorphic. Besides troilite (lower center, very light gray) at least two silicates (gray) and cavities (black)

Fig. C3

Fig. C4

Vaca Muerta, Chile

Mag. 250 X, imm.

Chromite (coarse chromite) with exsolution of ilmenite in thin lamellae || (111) directions. Silicates, black, nickel-iron, white

Fig. C5

Juvinas, France

Mag. 300 x , imm.

A large grain of coarse chromite, locally intergrown with iron (white), outside a little troilite (grayish white). In the chromite very many excellent unmixings of ilmenite || (111). Some larger platelets of ilmenite, oriented 11 (111) likewise, might be primarily oriented overgrowths

Fig. C4

Fig. C5 11 Ramdohr: The Opaque Minerals

Kg. C6

Molong, Australia

Mag. 1000 x , imm.

Chromite (whole field) showing unmixing of a thin platy mineral of low reflectivity («of ilmenite, hematite, eskolaite, rutilel), including a rounded melting drop of troilite and iron.

Fig. C6a Enlarged part of G6 to show the kamacite-troilite relations

Fig. C7

Pantar, Philippine Islands

Mag. 200 X, imm.

Exceptionally rounded grain of chromiie included in troilite (extremest SW corner a little iron). Chromite contains exsolution lamellae of ilmeniie. {N. £ . with higher magnification a second generation of extremely many becomes visible.)

Fig. C8

Hermitage Plains, Australia

Mag. 175 X, imm.

Coarse chromite, extremely xenomorphic against partially euhedral silicates. Some kamacite, white, partly oxidized

Fig. C9

Nardoo, Australia

Mag. 100 X

Aggregate (heap)-cArowiiie, imbedded, as usual, in a silicate of low reflectivity. Chromite grains are partly baked together. White is kamacite, the silicate with higher reflectivity olivine

Fig. CIO

Narellan, Australia

Mag. 130 X

Coarse aggregate (heap)-chromite (light gray) of which a part of the grains are baking together. The immediate surrounding is a low-reflecting silicate ( ? plagioclase). Predominant silicate seems to be olivine. Trottile and iron, both pure white

Fig. Cll

jBroken Bow, Nebraska

Mag. 135 X

Aggregate chromite in a cube-like from. Not a partially replaced "coarse chromite", as shown by the difference of oriented internal reflections. Upper right a troilite grain — all enclosed in different silicates

Fig. C12

Farmington, Kansas

Mag. 225 X, imm.

Chromite (aggregate chromite), in part sintered ("baked together"). The silicates contain dustlike sulfide

Fig. C13

Vaca Muerta, Chile

Mag. 140 X., imm.

Chromite (aggregate chromite), sintered in an apparently rhythmic reaction. Also silicates (black), nickel-iron (white), troilite (light gray)

Fig. C8

Fig. CIO

Fig. C12

Fig. C9

Fig. C l l

Fig. C13

Fig. C14

Forest City, Iowa

Mag. 170 x , imm.

Chromite (aggregate chromite), angular, randomly distributed grains in silicate. Also some troilite

Fig. C15

Forest Vale, Australia

Mag. 170 X

Chromite with a porous external zone. Also normal chromite in contact with troilite (light gray) and iron (white). At least three silicates can be distinguished

Fig. C16

Gilgoin, Australia

Mag. 140 X

Chromite, idiomorphic with porous outermost zone, partly imbedded in iron, white, and silicates. Some troilite, grayish white

Fig. C17

Allegan, Michigan

Mag. 220 X, imm.

A chondrule containing considerable chromite which, in its properties and intergrowths, resembles "pseudomorphous chromite". I t was perhaps originally a fine-grained aggregate of kosmochlor

Fig. C18

Plainview, Texas

Mag. 135 X

A chondrule consisting of a mass of very coarse chromite crystals surrounded by silicate with an abundance of evenly dispersed chromite grains and surrounded by a nearly continuous rim of chromite. Outside of the chondrule occur silicates (probably olivine and plagioclase) with iron (white) and trolite (light gray) inclusions. The cracks crossing the chondrules pinch out immediately in the surrounding — the explanation of this behavior is rather enigmatic

Mag. 400 X

Fig. C19 Same as CI 8, enlarged

Fig. C14

Fig. C16

Fig. C18

Fig. C15

Fig. C17

Fig. C19

168

Fig. C20

Cavour, S. Dakota

Mag. 135 X

A very large aggregate of "heap ehromite" with the outer zone distinctly more compact than the interior. Two silicates, probably olivine and plagioclase. A little iron (with weathering crust) and troilite, both with the same color

Fig. C21

Kiel, Germany

Mag. 45 x

An aggregation of different types of ehromite. In the middle, an in situ breccia, probably formed from a single crystal. On the upper left a partly preserved grain, lower right a grain obviously including skeletons of a silicate. Lower middle and lower left large areas of "pseudumorphous ehromite"

Fig. C21a

Salisbury, S. Rhodesia

Mag. 125 X

A ehromite chondrule with skeleton crystals of ehromite (gray). Radiating "pseudomorphous ehromite", obviously originating from an acicular Cr-bearing silicate. Filling of interstices and boundary to surrounding is formed by plagioclase

Fig. C22

Cavour, S. Dakota

Mag. 140 X

A large idiomorphic ehromite, actually a Mg-rich member, as the frequent internal reflections show, together with a cloud of pseudomorphous ehromite included in a low-reflecting silicate

Fig. C21a

Pig. C22

Fig. C23

Grady (1937), N. Mexico

Mag. 210 X, imm.

A small chondrule, rich in aggreggate chromite, which has recrystallized into compact masses at the margins. In the interior the chromite has transformed into coarse crystals. Black is silicate, white nickel-iron and a little troilite. Fractures are filled with limonite

Fig. C24

Appley Bridge, England

Mag. 105 X

A silicate with lower reflectivity, probably albite, is filled with small grains of chromite (locally welded into a band), replacing the original silicates with high refractive index. Also a nickel-iron (white) and troilite (light gray)

Fig. C25

Estacado, Texas

Mag. 90 X

Chromite (disintegration or pseudomorphous chromite) formed from a highly refracting silicate around a large remarkable pore (black). At left troilite, surrounding an idiomorphic crystal of kamacite. The reflectivity of newly formed matrix silicate of the chromite is very distinctly lower than that of the original silicate

Fig. C26

Estacado, Texas a section from the the preceding figure

Mag. 315 x

Illustrates the breakdown of the silicate mineral with the formation of a large amount of chromite,possibly measurable plainmetrically. — I t is very rare, that the chromium-silicate is not entirely desintegrated

Fig. C25

Fig. C26

172

Fig. C27

Ensisheim, France

Mag. 50 x

A chondrule composed of a low reflecting silicate, probably plagioclase, with abundant chromite, locally baked to larger grains. (The print, made to show the difference between the three silicates, does not show distinctly the various reflectivity of chromite and troilite

Fig. C28

Bachmut, USSR

Mag. 200 x

A chondrule sized aggregate of radiating pseudomorphs after a chromium bearing silicate enclosed in a low reflecting silicate, probably albite (a glass of similar composition could be possible)

Fig. C29

Bluff, Texas

Mag. 70 x

Chromite (désintégration chromite) formed by the désintégration of the silicate component of a chondrule. The other is olivine (very contrasty print)

Fig. C30

Estacado, Texas

Mag. 320 x , dry

A nearly compact heap of "aggregate chromite". The outlines of the fine polygonal grains become visible by defocusing. Such a grain could be easily mistaken in a badly polished section for "coarse chromite". White is troilite, three different silicates, very dark gray to black

Fig. C27

Fig. C28

4 Fig. C29

Fig. C30 •

174

Fig. C31

Elsinora, Australia

Mag. 175 X, imm.

Olivine, very dark gray, the even distribution of small chromite grains shows t h a t they are a n exsolution product. Many cracks filled with terrestrial oxidation products, mostly goethite. Two large grains of troilite

Fig. C32

Locality 14, Saudi Arabia

Mag. 50 x

Chromite, in numerous parallel-oriented grains in a large crystal of olivine practically making u p the whole chondrule. The regular distribution suggests exsolution. Fractures filled with limonite

Fig. C33

Holbrook, Arizona

Mag. 35 X

A small chondrule containing about'25 per cent evenly distributed chromite. I n places the chromite appears to have been aggregated into larger masses by a later process. (See C34; corresponding areas are signed | 1)

Fig. C34

Holbrook, Arizona

Mag. 150 X, imm.

A section from the preceding picture. The small individual grains and the local aggregates are better shown

Fig. C33

Pig. C34

Kg. C35

Dalgety Downs, Australia

Mag. 340 X (dry !)

Chromite, bright tiny spots at edges of olivine fragments in a matrix of feldspar. Olivine shows some internal reflections and cracks, feldspar with minute inclusions of chromite

Fig. C36

Vaca Muerta, Chile

Mag. 220 X

Chromite, grayish white, myrmekitic in olivine (dark gray). In similar distribution, but in much smaller amount, ilmenite

Fig. C37

Grady, New Mexico (1937)

Mag. 170 X , imm.

Troilite grains are rimmed by chromite. The granular nature of the silicates is revealed by internal reflections

Fig. C38

Locality 13 a, Saudi Arabia

Mag. 160 x , imm.

Large chromite grains, especially abundant in this meteorite, are surrounded by porous troilite

Fig. C39

Rose City, Michigan

Mag. 200 x , imm.

Chromite (coarse chromite) penetrated and locally replaced by troilite, preferentially in the (100) direction. The large white area is nickel-iron, the black silicate

Fig. C36

Pig. C35

Pig. C38

Fig. C39 12 Itamdohr: The Opaque Minerals

Pig. C40

Beenham, New Mexico

Mag. 170 X, imm.

A large grain of ilmenite showing occasional twin lamellae and (1011) and (0001) cleavages, idiomorphic against troilite (the large face is (0001)), and xenomorphic against silicate

Fig. C41

L'Aigle, France

Mag. 170 x , imm., Nic.+

Exceptionally large grain of ilmenite. Strongly distorted and showing good twin Iamellation according to two directions of {1011}

Fig. C42

Adelie Land, Antarctic region Nicols partly crossed

Mag. 250 X, imm.

Xenomorphic ilmenite, one individual, showing excellent twinning. Included an egg-shaped troilite with different orientation; at the lower boundary iron. Dark gray with many internal reflections are silicates

Fig. C43

Locality 9, Saudi Arabia Nicols partly crossed

Mag. 440 X, imm.

A relatively large grain of ilmenite is broken into an aggregate of subparallel grains showing pressure lamellae

Fig. C44

Wickenburg, Arizona

Mag. 200 x

Ilmenite, a coarse aggregate. The color varies with the orientation. White is partly a loose leaching network of taenite, in p a r t (at right) iron with a matrix of troilite. The surrounding silicates are dotted by fine grains of iron and troilite

Fig. C45

Tadjera, Algeria Nicola partly crossed

Mag. 400 X, imm.

Ilmenite, a recrystallized aggregate from a former single crystal which was transected by twinning lamellae or undulating distortions. Small chains of iron droplets. Black areas are silicates

Fig. C46

Arcadia, Nebraska Nicols partly crossed

Mag. 150 X, imm.

A large grain of ilmeniut, showing pressure effects (same as C47), enclosed in silicate

Fig. C47

Arcadia, Nebraska

Mag. 250 x , imm.

The same as C46 a t higher magnification. The undulose extinction and the twin lamellae are better shown. The recrystallized p a r t is also distinct

Fig. C48

Pantar, Philippine Islands

Mag. 220 X, imm.

Two grains of magnetite (medium gray) confirmed by the microprobe, with ckromite (dark gray). White is iron with tiny troilite inclusions. The silicates are black

Fig. C48

182

Fig. C48a

Groenewald-Benoni, Transvaal

Mag. 700 X

A large grain of magnetite, medium gray, includes some smaller grains of chromite (r^)Troilite is light gray, kamacite white; silicates are black

Fig. C49

Karoonda, S. Austr.

Mag. 170 X , imm.

A large chondrule of subparallel olivine, with fine grains of magnetite (white). Also older magnetite which shows fine-grained exsolution of ilmenite, not visible in the photograph

Fig. C50

Essebi, Congo

Mag. 220 x , imm.

Fine globules of magnetite (gray) in silicates (black). A few spots of pyrrhotite

Fig. C51

Orgueil, France

Mag. 400 X , imm.

Tiny globules in fish-roe like aggregates of magnetite "swimming" in a matrix of serpentine (almost black in oil immersion I). With highest power magnification sometimes the radiated spherolitic nature of the globules is visible. — That is a very common occurrence of magnetite in carbonaceous chondrites

Fig. C51

184

Fig. C52

Vaca Muerta, Chile

Mag. 300 X, imm.

.Rutile, grains with different orientation showing bireflection. Penetrating bands are goethite, white inclusions troilite. Black are silicates

Fig. C53

Vaca Muerta, Chile Nicols nearly crossed

Mag. 300 x , imm.

Same as picture C52, anisotropism becomes more distinct

Fig. C54

Vaca Muerta, Chile

Mag. 400 x , imm.

Ilmenite, gray, strongly replaced by rulile, light gray, following the intense twin lamellation. The white component in the upper and lower middle is troilite, the white one with a rim of goethite is iron. Black are silicates

Fig. C55

Vaca Muerta, Chile

Mag. 340 X, imm.

Mineral D ( = rutile) in all stages of replacement of chromite (dark gray). Twin lamellae, in various shades of light gray, are visible in this mineral. The apparent patchiness is due to internal reflections

186

Pig. C56

Vaca Muerta, Chile

Mag. 500 X, imm.

A crystal of rutile cut || (001) imbedded in iron and upgrown onto silicates. In appearance it is very similar to cliftonite from the same meteorite (A36), thus it could be mistaken for diamond — microprobe confirmed it as rutile

Fig. C57

Sena, Spain

Mag. 570 X, imm.

Ilmenite grain, along the twin lamellation altered to rutile (higher reflectivity, distinct bireflection)

Pig. C58

Farmington, Kansas

Mag. 170 X, imm.

Granular ilmenite, in various shades of gray. One grain contains fine lamellae of D (rutile), certainly not twin lamellae. White is mostly nickel-iron (with a little troilite)

Pig-. C59

Miller, Arkansas

Mag. 400 X, imm.

Ilmenite, gray, with lamellae of rutile, together with chromite, dark gray, nickel-iron, white, and a little troilite, light gray. Black is silicate

Fig. C56

Fig. C57

Pig. C58

Pig. C59

Pig. C60

Forest Yale, Australia

Mag. 225 X

Large grain of spinel with very different reflectivity. The lowest reflectivity not much higher than the silicates, the highest surpassing chromite, which forms (lower middle) an aggregate with troilite and iron. Three different silicates

Fig. C61

Grady (1937), New Mexico

Mag. 360 X, dry!

Spinel-type mineral (I) with exsolution of ilmenite || (111). The lower side of the grain has been partly converted to magnetite by terrestrial weathering

Fig. C62

Grady (1937), New Mexico

Mag. 490 x , imm.

Same as C61. The internal reflections affect the photograph, but other features, especially the exsolution lamellae of ilmenite, are more prominent

Pig. C61

Pig. C62

Fig. D1

St. Marks, S. Africa

Mag. 700 X, imm.

Troilite, a thin layer at the top and left corner, and a broader layer below, everywhere marked by fractures. Daubrielite, not distinguishable in reflectivity, as a thin band between troilite and mineral A, which forms the main mass of material — characterized by the zig-zag contraction cracks and the strong anisotropism (not visible in the single crystal in this photograph). Mineral A is partly replaced by fine lamellae of mineral B. The thin white lamellae in A are daubrielite. The thin oblique ledge at the top is graphite with deformation lamellae. Black areas, in part with internal reflections, are silicates

Fig. D2

St. Marks, S. Africa

Mag. 265 X, dry!

Complicated mineral association! Dark gray in different shades — the silicates. Thin gray lamellae, partly a little bent, graphite. Brighter gray, partly rounded blebs, partly fillings between silicates, ( M n , Fe, Mg)S. Light gray, with cleavage cracks and some breakouts, is mineral A. About the same color, but without cracks is troilite, white is kamacite. The crossed ( x ) component is mineral C; it shows at the boundary a little unmixing of troilite

Fig. D3

St. Marks, S. Africa

Mag. 230 x , imm.

Similar to D2, but in oil with distinctly increased contrast. In mineral A fine lines of S become visible. At some places oldhamite becomes visible. C is now much more characteristic

192

Fig. D4

Pena Blanca Spring, Texas

Mag. 600 X, imm.

A complcx inclusion of sulfide in cnstalite, at least five different minerals besides troilite (c, b, I, m, x, n, (')• x is the Fe- ai