Schreibersite Growth and Its Influence on the Metallography of Coarse-Structured Iron Meteorites

Home , Brianite, Brian Mason (geochemist), Cohenite, Elbogen (meteorite), Haxonite, Hexahedrite

SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES • NUMBER 21

Roy S. Clarke, Jr. and Joseph I. Goldstein

SMITHSONIAN INSTITUTION PRESS City of Washington 1978 ABSTRACT Clarke, Roy S., Jr., and Joseph I. Goldstein. Schreibersite Growth and Its Influence on the Metallography of Coarse-Structured Iron Meteorites. Smithson- ian Contributions to the Earth Sciences, number 21, 80 pages, 28 figures, 20 tables, 1978. —The role that schreibersite growth played in the structural development process in coarse-structured iron meteorites has been examined. The availability of many large meteorite surfaces and an extensive collection of metallographic sections made it possible to undertake a comprehensive survey of schreibersite petrography. This study was the basis for the selection of samples for detailed electron microprobe analysis. Samples containing representative structures from eight chemical Groups I and IIAB meteorites were selected. Electron microprobe traverses were made across structures representative of the observed range of schreibersite associations. Particular emphasis was placed on schreibersite-kamacite interface compositions. An analysis of these data has led to a comprehensive description of the structural development process. Massive schreibersite, one of the four major types of schreibersite encountered, may be accounted for by equilibrium considerations. Subsolidus nucleation and growth with slow cooling from temperatures at least as high as 850° C, and probably much higher, explain the phase relationships that one sees in meteorite specimens. The retention of taenite in the octahedrites establishes that bulk equilibrium did not extend as low as 550° C. Schreibersite undoubtedly continued in equilibrium with its enclosing kamacite to lower temperatures. A second type of schreibersite to form is homogeneously nucleated rhabdite. It nucleated in kamacite in the 600° C temperature range, either as a consequence of low initial P level or after local P supersaturation developed following massive schreibersite growth. A third type of schreibersite is grain boundary and taenite border schreibersite. It formed at kamacite-taenite interfaces, absorbing residual taenite. Nuclea- tion took place successively along grain boundaries over a range of temperatures starting as high as 500° C or perhaps slightly higher. Grain boundary diffusion probably became an increasingly important factor in the growth of these schreibersites with decreasing temperature. The fourth type of schreibersite is microrhabdite. These schreibersites nucleated homogeneously in supersatuated kamacite at temperatures in the 400° C range or below. P diffusion controlled the growth rate of schreibersite. The Ni flux to a growing interface had to produce a growth rate equal to that established by the P flux. This was accomplished by tie line shifts that permitted a broad range of Ni growth rates, and these shifts account for the observed range of Ni concentrations in schreibersite. Equilibrium conditions pertained at growth interfaces to temperatures far below those available experimentally. Kinetic factors, however, restricted mass transfer to increasingly small volumes of material with decreasing temperature.

OFFICIAL PUBLICATION DATE is handstamped in a limited number of initial copies and is recorded in the Institution's annual report, Smithsonian Year. SERIES COVER DESIGN: Aerial view of Ulawun Volcano, New Britain.

Library of Congress Cataloging in Publication Data Clarke, Roy S., Jr. Schreibersite growth and its influence on the metallography of coarse-structured iron meteorites. (Smithsonian contributions to the earth sciences; no. 21) Bibliography: p. 1. Schreibersite. 2. Metallography. 3. Meteorites, Iron. I. Goldstein, Joseph I., joint author. II. Title. III. Series: Smithsonian Institution. Smithsonian contributions to the earth sciences; no.21. QEl.S227no. 21 [QE395] 550'.8s [552] 77-10840 Contents

Page Introduction 1 Acknowledgments 2 Historical Background 2 Introduction 2 19th-century Studies of Schreibersite 3 Early 20th-century Studies of Iron Meteorites 4 Modern Work on Metallic Phases of Iron Meteorites 5 Related Materials 8 Experimental 9 Results 11 Coahuila 13 Bellsbank 16 Ballinger 18 Santa Luzia 21 Lexington County 25 Bahjoi 29 Goose Lake 30 Balfour Downs 31 Discussion 37 Equilibrium Considerations of Phase Growth 37 Coahuila 40 Ballinger 42 Santa Luzia 43 Lexington County and Bahjoi 44 Goose Lake and Balfour Downs 45 Bellsbank 46 Low Temperature Phase Growth and the Equilibrium Diagram 48 Diffusion-Controlled Schreibersite Growth 54 Cooling Rate Variations 58 Interface Data and Schreibersite Distribution 59 Interface Data and the a/a + Ph Boundary 66 Schreibersite with Cohenite Borders 70 Summary and Conclusions 73 Appendix 75 Literature Cited 77 Schreibersite Growth and Its Influence on the Metallography of Coarse-Structured Iron Meteorites

Roy S. Clarke, Jr. and Joseph I. Goldstein

Introduction pared to terrestrial or lunar rocks. As a consequence, meteorites have a unique place in the Meteorites are currently understood to be the study of the development of the planetary system, oldest rocks available for scientific study, contain- yielding information that is available from no ing components and structures that span the pe- other source. riod from the final stages of solar nebula conden- Iron meteorites represent only a small fraction sation to the present (Anders, 1962, 1971; Gross- of the meteorites that have been observed to fall, man and Larimer, 1974; Wasson, 1974). They are about five percent. Fortunately, many specimens fragments of parent bodies that accreted from from ancient falls have been preserved under such preexisting aggregations of material during the conditions that deterioration has not been severe period of planetary system formation some 4.6 and are available for study (Buchwald, 1976). If billion years ago. These parent bodies were subse- this were not the case, our view of iron meteorite quently disrupted into smaller objects that then compositions and structures would be much nar- had independent existences in space. Individual rower than it is. Only one of the meteorites used fragments eventually intercepted the earth and in this study is an observed fall. landed as recoverable meteorites. Meteorite struc- The spectacular metallographic structures re- tures and compositions have undergone varying vealed on prepared surfaces of iron meteorites degrees of modification while resident in their have fascinated scientist and dilettante alike for parent bodies, as small bodies in space, on their more than 160 years (Perry, 1944). The Widman- passage through the atmosphere, and on landing statten pattern is the historical distinguishing char- on the earth's surface. Further changes result acteristic of the populous octahedrite classes of from long residence time on the ground and may iron meteorites (Goldstein and Axon, 1973). At an continue during storage in collections. Bearing early date this structure was understood to result evidences of these complex histories, meteorites from very slow cooling at relatively low tempera- are samples not only from a far distant place but tures in the meteorite parent body. The tempera- also from a far distant time, having been preserved ture range through which this structure develops in a remarkably gentle environment when com- is now believed to be from 700° to 350° C, temperatures well below those where silicate systems un- Roy S. Clarke, Jr., Department of Mineral Sciences, National Museum dergo change. Interpretation of metallographic of Natural History, Smithsonian Institution, Washington, D. C. 20560. Joseph I. Goldstein, Department of Metallurgy and Materials structures, therefore, gives information on a late, Sciences, Lehigh University, Bethlehem, Pennsylvania 18015. low temperature period in the history of a parent

1 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES body, information that is not readily available Office of Computer Services was responsible for from other types of meteorite studies. the preparation of data reduction programs. A Modern interpretations of iron meteorite struc- grant from the Secretary's Fluid Research Fund tures have been based on the Fe-Ni equilibrium supported a brief visit to Cambridge University, diagram (Goldstein and Olgivie, 1965a), as it was Cambridge, England, in the fall of 1974, where a the only applicable system known with sufficient serious beginning was made on the interpretation accuracy in the low temperature range required. of the data. The support of the Geochemistry The Widmanstatten pattern, therefore, resulted Program, Division of Earth Sciences, National Sci- simply from taenite transforming to kamacite with ence Foundation, through Grant No. DES 74- decreasing temperature as a solid state, diffusion 22518 is also acknowledged. The acronym USNM controlled reaction. Recently the iron-rich corner (for the former United States National Museum) of the Fe-Ni-P equilibrium system has been deter- is used for catalog numbers in the National Mu- mined experimentally down to 550° C, leading to seum of Natural History, Smithsonian Institution. new possibilities for investigating meteorite struc- This paper was presented to the George Wash- tures (Doan and Goldstein, 1970). In this study ington University, Washington, D.C., by R. S. the metallography of a group of schreibersite-con- Clarke, Jr. in partial fulfillment of the require- taining coarse-structured iron meteorites has been ments for the Ph.D. degree. examined using this low temperature Fe-Ni-P diagram. It will be shown that equilibrium growth at higher temperatures combines with diffusion-con- Historical Background trolled growth at lower temperatures to explain observed structure more comprehensively than INTRODUCTION was possible using the simpler system. Schreiber- Meteorites, with their dramatic arrivals on earth, site growth becomes an integral part of the overall exotic origins, and unusual structures, have at- structural development process, and schreibersite tracted much more than routine scientific interest may no longer be considered an "inclusion" that ever since they became respectable objects of study can be safely ignored when structural development at the end of the 18th century. Many of the leaders is discussed. in the development of 19th-century physical sci- ACKNOWLEDGMENTS ence made significant contributions to their study, and several of them will be mentioned below. The A number of colleagues and associates have older literature is extensive, but it is mainly obso- contributed invaluable assistance throughout the lete for other than descriptive and historical pur- course of this study. The support and encourage- poses. A critical review of this material was not ment of departmental chairman William G. Mel- attempted, but a survey of selected early develop- son and his predecessor Brian Mason are acknowl- ments will be given. Scientific interest in schreiber- edged with sincere thanks. Erik Randich provided site dates from near the beginning of meteorite penetrating discussion of many of the problems studies, and it is interesting to place modern work studied here, and his analysis of ternary diffusion in this perspective. as applied to schreiberate growth and meteorite The first major development in the metallogra- cooling rates was made available to us prior to phy of meteorites was the discovery of the phe- publication. A. D. Romig, Jr., of Lehigh Univer- nomenon that has become known as the Widman- sity made available to us data on the low tempera- statten pattern. This structure was first discovered ture Fe-Ni-C system prior to publication. Eugene by William Thomson, an Englishman living in Jarosewich and Charles R. Obermeyer III fur- exile in Naples. He published a French version of nished unstinting help with the electron micro- his findings along with a good illustration in a probe work. Grover C. Moreland and Richard Swiss journal in 1804. What appears to be an Johnson prepared many excellent polished sec- identical paper, only in Italian, dated 6 February tions. Ann Garlington and Kathy P. Porter were 1804, was published in Siena four years later (G. scrupulous in their attention to the details of Thomson, 1804, 1808). Modern writers on meteor- manuscript preparation. Dante Piacesi, Jr., of the itics have been unaware of the earlier paper by NUMBER 21

Thomson, and this has led to confusion as to plates forming parallel to the faces of an octahe- whether Thomson or Von Widmanstatten really dron, giving the name octahedrite to those meteor- had priority. The record seems to be unambigu- ites that displayed this pattern on polished and ously in favor of Thomson. Marjorie Hooker of etched surfaces. the U.S. Geological Survey retrieved this 1804 paper, and its existence was mentioned in a foot- 19TH-CENTURY STUDIES OF SCHREIBERSITE note in a biographic study of Thomson by C. D. Waterston (1965: 133). The writer ran across Wa- Schreibersite was first characterized in 1832 by terston's paper among unindexed material in the the great Swedish chemist J. J. Berzelius (1832a,b, F. A. Paneth meteorite literature collection that 1833, 1834). He isolated brittle, silver-white grains was deposited in the Smithsonian Institution in of a magnetic material from an acid insoluble 1974. residue of the B©humilitz, Bohemia, coarse octa- Alois von Widmanstatten, Director of the Impe- hedrite. Berzelius' elaborate analytical procedure rial "Fabrik-Produkten-Cabinett" (Industrial Prod- yielded a surprisingly good analysis for the time, ucts Collection), Vienna, discovered the phenom- demonstrating that his material was an iron-nickel- enon that now bears his name independently in phosphorus compound containing about 14 per- 1808. He devoted many years to its study and cent phosphorus and considerably more nickel circulated his observations privately, drawing the than the bulk of the meteorite. In 1834 he reported phenomenon to the attention of students of mete- similar analyses for schreibersite from the Elbo- orites. Von Widmanstatten's observations were fi- gen, Bohemia, medium octahedrite and the Kras- nally published in 1820 by his co-worker, Carl von nojarsk, Siberia, pallasite (the historic Pallas Iron). Schreibers (1820). The history of the discovery of It is also interesting to note that Berzelius' concept the Widmanstatten pattern has been reviewed by of the Widmanstatten pattern included the ele- Paneth (1960) and Smith (1960, 1962). Smith's ment phosphorus, as is indicated by the following review contains excellent reproductions of early quotation: prints and a lengthy translation of Schreibers' Es mochte wahrscheinlich seyn, dass die sogenannten Widman- description of Widmanstatten's work. These early stddtschen Figuren einer, der hier analysirten Schuppen analo- reproductions of iron meteorite structures by Wid- gen, Verbindung zwischen Eisen, Nickel und Phosphor, zuzu- manstatten not only represent an advance in the schreiben seyen, welches zu erforschen ich aus Mengel an science of meteoritics but also one in the art of Material verhindert bin. printing. Berzelius (1832b:297) During the 19th century descriptive studies of Early descriptions of schreibersite occurrences meteorites advanced with the broadening base of were confounded by inadequate separation tech- understanding in mineral chemistry and related niques, poor analytical methods, and complica- physical science. Many of the iron meteorites tions arising from both the varied morphology known today were already represented in collec- and the wide range in composition that is typical tions, and a number of them had been carefully of this mineral. C. U. Shepard (1846, 1853) ana- described in the literature. The major phases of lysed schreibersite and attempted to assign a min- iron meteorites had been characterized and given eral name to it. His suggestion, however, was mineral names: kamacite (low-nickel Ni-Fe, fer- supported by inadequate data. Credit for naming rite, a-iron, b.c.c), taenite (high-nickel Ni-Fe, schreibersite belongs to A. Patera, whose recom- austenite, y-iron, f.c.c), plessite (fine-grained in- mendation was reported by Haidinger (1847). The tergrowth of kamacite and taenite), troilite (FeS), name honors Professor Karl von Schreibers (1775- cohenite (Fe3C), and schreibersite ((Fe,Ni)3P) were 1852), director of the Imperial Cabinet, Vienna, a all known and their bulk compositions understood. pioneer worker on meteorites and colleague of Accessory minerals such as daubreelite (FeCr2S4), Von Widmanstatten. chromite (FeCr2O4), and graphite (C) were also The writings of J. Lawrence Smith, a distin- known. The Widmanstatten pattern was under- guished chemist of the middle part of the last stood to have formed by slow cooling under crys- century (Silliman, 1886; Phillips, 1965), include tallographicly controlled conditions, kamacite numerous examples of early work on meteorites. SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

His description of the Tazewell, Tennessee, finest The 19th-century work on meteoritic phos- octahedrite contains a lengthy discussion of his phides that culminated with Cohen's and Farring- observations (Smith, 1855). At this early date he ton's reviews was limited by the capabilities of recognized the name schreibersite and applied it classical descriptive mineralogy and classical ana- to the correct mineral. He reports having identi- lytical chemistry. A plateau had been reached in fied schreibersite by visual inspection in a number descriptive iron meteorite studies in general that of iron meteorites from "the Yale College Cabi- became an accepted norm that was exceeded by net," where it had previously been thought to be few workers during the first half of the 20th pyrite. Smith's low nickel value for Tazewell century. Most iron meteorite descriptions pub- schreibersite is suspect and his attempt at a for- lished during this period employed neither con- mula was unsuccessful, but he clearly recognized cepts nor techniques beyond those available to the species and understood which elements were Cohen. The descriptive literature grew in volume its major constituents. This work was undoubtedly and in quality, but new interpretations lagged. done, at least in part, in the chemical laboratory Powerful petrographic techniques were being de- of the Smithsonian Institution. This laboratory veloped and used by metallurgists and ore micro- was organized by Professor Smith in 1854, and scopists, but they were not much used by students Secretary Joseph Henry reported that during the of iron meteorites. The availability of good trans- year "he also made a series of analyses of meteor- mitted light microscopes in the hands of petrogra- ites, among which were fourteen specimens from phers who were accustomed to working with sili- the cabinet of James Smithson, the founder of the cate minerals resulted in stony meteorites being Institution" (Goode, 1897:614). preferred subjects of investigation. Confusion in nomenclature, however, persisted in the literature for many years. Von Reichenbach EARLY 20TH-CENTURY STUDIES OF (1861) discussed schreibersite under the descriptive IRON METEORITES term "Ganzeisen" and also used the name lamper- ite. Both of these names are now part of the The emergence of modern physical chemistry synonymy of schreibersite. Rose (1865) introduced and physical metallurgy during the decades adja- the synonym rhabdite into the literature, a name cent to the turn of the century had a marked derived from the Greek word for "rod." This effect on the future course of iron meteorite term is still in common use by meteoriticists, but investigations. Descriptive studies following the only when morphological distinctions are of inter- 19th-century pattern continued to comprise the est. Rhabdite is the term used for the small schrei- bulk of the published literature and, indeed, con- bersite crystals occurring in the kamacite of iron tinue to be published today as essential documen- meteorites. In polished sections these small schrei- tation. More modern experimental approaches, bersite crystals have rhombohedral cross sections however, became significant, resulting in a body that in some cases are elongated, suggesting rods of knowledge more readily interpretable in terms or needles. of origins and developmental histories of meteor- The early literature on meteoritic phosphides, ites than was previously possible. schreibersite and rhabdite, was reviewed in detail Actually, the roots of this work go back to a by Cohen (1894). Included were a number of much earlier period. Pioneering synthetic experi- analyses of schreibersite and rhabdite isolated ments were made by Michael Faraday as early as from various meteorites by Cohen and his co- 1820. He prepared metals of meteoritic composi- workers, as well as selected analyses from the tions in the course of an investigation to improve literature. This work established that schreibersite steel (Stodart and Faraday, 1820). At a later pe- and rhabdite are morphological variations of the riod, Daubree (1868) described synthetic experi- same mineral species, the rhabdite form having a ments related to meteorites. Sorby (1877, 1887), somewhat higher nickel concentration than that the father of metallography, made astute observa- observed in large schreibersite inclusions. A simi- tions, some of which were based on artificial alloys lar review was given a few years later by Farrington of meteoritic compositions. Unfortunately, Sorby's (1915), using much of the same data Cohen used pioneering efforts in the metallography of mete- but with several later analyses. orites were not pursued. The following quotation NUMBER 21 from his 1877 paper reflects his perceptive under- ously flawed. These studies, however, pointed the standing of the structures he observed: way for future workers. The introduction of X-ray diffraction analysis These facts clearly indicate that the Widmanstatt's Figuring is into metallurgical and mineralogical research in the result of such a complete separation of the constituents the 1920s had an important influence on meteorite and perfect crystallisation as can occur only when the process takes place slowly and gradually. . . . Difference in the rate of studies. Young (1926) published an early study cooling would serve well to explain the difference in the establishing that kamacite precipitates from tae- structure of some meteoritic iron which do not differ in nite, not the converse as many had previously chemical composition; ... we are quite at liberty to conclude thought. X-ray examination of the Widmanstatten that they may have been melted .... pattern continued for over a decade with deepen- ing understanding and some controversy (Mehl During the same period the well-known French and Derge, 1937; Derge and Komnel, 1937; Owen, mineralogist, Meunier (1880), reported the results 1938; Smith and Young, 1938, 1939). of his synthetic work on meteoritic systems. A Rudolf Vogel's contributions to metallurgical paper by Brezina (1906) is an excellent summariz- studies in meteorites deserves special mention, his ing statement of this early period. It contains a published papers having covered the period 1925 number of good photomicrographs of iron mete- to 1967. They treated a broad range of topics orites, an unusual feature for papers of the time, related to meteoritics, eight of them being of and it discusses the development of these struc- particular interest in this context (Vogel, 1927, tures in terms of exsolution phenomena (solid 1928, 1932, 1951, 1952, 1957, 1964; Vogel and state transformation). He also gave a succession Baur, 1931). These papers deal with the role of for the formation of the constituents of iron mete- phosphorus in the development of meteoritic orites, starting with olivine in pallasites: olivine, structures. His 1928 paper discusses schreibersite daubreelite, troilite, graphite, schreibersite, co- and rhabdite precipitation in terms of a hypothet- henite, chromite, swathing kamacite, kamacite ical ternary Fe-Ni-P system. His 1932 and 1952 bands, taenite, and plessite. This is a remarkably papers discuss the same material in terms of exper- informative listing for the time, demonstrating imentally derived ternary diagrams. careful petrographic observation and keen insight. A watershed publication that represents the cul- One of the perennial problems of meteorite mination of this earlier period of iron meteorite research has been that of drawing the attention of research is Perry's (1944) monograph on the met- those with the knowledge and facilities to make allography of meteoritic iron. Although many significant contributions to the meteorite speci- photographs of iron meteorites had been pub- mens and the important and interesting scientific lished in the older literature, most of these were problems they represent. Despite Sorby's enthusi- external views of complete specimens or macro- asm for meteorite studies, metallurgists showed photographs of meteorite slices prepared for ex- little interest and had to be attracted into the hibit purposes. Perry's publication was the first field. An example of this process is an invited comprehensive and systematic collection of iron paper read before the Vienna meeting of the Iron meteorite photomicrographs. The variety of me- and Steel Institute in 1907. Professor Frederick tallic structures in iron meteorites was presented Berwerth's (1907) paper, "Steel and Meteoritic to the scientific public in a way that has had Iron," was based on his knowledge of the meteor- significant impact up to the present. Sorby had ite collection of the Imperial Natural History Mu- pointed out the power of the metallographic ap- seum, Vienna. It was an open invitation to metal- proach in 1877, but it was not seriously pursued lurgists to become involved in this field of study. until E. P. Henderson drew the publisher and During the early decades of this century, syn- meteorite collector S. H. Perry into this field in thetic studies were combined with analyses based the 1930s (Henderson and Perry, 1958: ii). on equilibrium phase diagrams. The work of Os- mond and Cartaud (1904), Benedicks (1910), and Belaiew (1924) are examples. Unfortunately, the MODERN WORK ON METALLIC PHASES OF tools were not available at the time to determine IRON METEORITES sufficiently accurate equilibrium diagrams. As a Modern work on the metallic phases of iron result, many of the conclusions drawn were seri- meteorites includes studies that may be grouped SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

into the following categories for convenience: (1) interfaces. Both approaches gave comparable cool- interpretations of the Widmanstatten pattern in ing rates within reasonable limits of error. A terms of cooling histories of meteoritic parent number of other papers were published during bodies, (2) compositional studies utilizing trace this period that contributed significantly to our elements and structure as a means of identifying knowledge of the structural detail of meteoritic parent bodies, (3) schreibersite growth as a key to iron (Short and Anderson, 1965; Goldstein, 1965; structure development and cooling history, (4) Reed, 1965b; Axon and Boustead, 1967). Cooling descriptive studies and literature reviews. These rate and thermal history studies were pursued by studies are interrelated and provide essential back- Goldstein and co-workers (Short and Goldstein, ground for the work to be discussed below. Brief 1967; Goldstein and Short, 1967a, b; Fricker et al., mention of major contributions in these areas will 1970). Goldstein and Doan (1972) discussed the be given here. effect of phosphorus on the formation of the Progress in developing accurate low tempera- Widmanstatten pattern and reported the first lab- ture phase diagrams has played an essential role oratory production of an artificial Widmanstatten in modern iron meteorite studies. The Fe-Ni dia- pattern. They conclude that cooling rate calcula- gram of Goldstein and Ogilvie (1965a) has been tions are not seriously affected by phosphorus. A used by meteoriticists for over 10 years (Figure 1). comprehensive review of Widmanstatten pattern Electron microprobe techniques were employed studies has been published by Goldstein and Axon by these workers to improve upon earlier equilib- (1973). rium diagrams (Owen and Sully, 1939; Owen and Compositional studies of iron meteorites have Lui, 1949) and to reconcile the Fe-Ni diagram traditionally gone hand-in-hand with classification, with observations on meteorites (Agrell et al., bulk Ni values and metallographic structure being 1963). Buchwald (1966) studied the Fe-Ni-P system the primary considerations in assigning classifica- at low temperature using X-ray techniques and tion categories. Modern versions of this type of proposed a diagram that he used to discuss mete- classification have been given by Buchwald and orite structure development. More extensive work Munck (1965) and Goldstein (1969). The grouping on the Fe-Ni-P system based on longer cooling of iron meteorites on the basis of trace element periods and electron microprobe measurements content was introduced by Goldberg et al. (1951) was reported by Doan and Goldstein (1970), and and Lovering et al. (1957). Wasson and co-workers their diagram is basic to the interpretation of the in a series of papers (Wasson, 1974; Scott and experimental observations discussed below. Wasson, 1975, 1976) have extended this approach The modern literature on the growth of the to define genetic groups in a large number of iron Widmanstatten pattern is extensive. Massalski and meteorites, based on Ni, Ga, Ge, and Ir contents Park (1962) used the Fe-Ni equilibrium diagram to and structural considerations. Sixteen iron mete- calculate initial temperatures of kamacite precipi- orite groups have been characterized, represent- tation and final equilibrium temperatures, using ing 12 genetically related meteorite groups and a areal distribution of kamacite and taenite and a number of individual meteorite parent bodies. lever rule calculation. Wood (1964) and Goldstein iRecent work on schreibersite growth in iron and Ogilvie (1965b) considered the nonequilibrium meteorites may be dated from the observations of nature of iron meteorite structures, and combined Henderson and Perry (1958). Using conventional phase diagram and kinetic considerations to derive chemical analysis, they observed unusually low Ni cooling rates and to estimate parent body sizes. concentrations in swathing kamacite bordering the Both papers recognize Widmanstatten pattern large low-Ni (12%) schreibersite in the Tombigbee growth as diffusion controlled and not subject to River meteorite, a meteorite they also showed to bulk equilibrium consideration. Wood (1964) uses contain low-Ni (20%) rhabdite. They explained the relationship between Ni buildup at the centers their observations in part on the migration of Fe of taenite bands and kamacite band widths to and Ni atoms in the solid state and suggested that calculate cooling rates. The Goldstein and Ogilvie much of the phosphide may have separated from (1965b) approach used a detailed analysis of diffu- the liquid. Somewhat later, the first electron mi- sion gradients within taenite at taenite-kamacite croprobe measurements of schreibersite composi- NUMBER 21

900 1

800

700 r

600 - \ \ \ 500 0 \ 1 1 1 400 - 1 a+y 1 w 1 1 300 -

i i i i 1 1 1 1 1 1 1 i 1 10 20 30 40 50 ATOMIC % NICKEL FIGURE 1. —LOW temperature Fe-Ni phase diagram of Goldstein and Ogilvie (1965). tions were reported by Adler and Dwornik (1961) kinetics demonstrated that larger schreibersites on schreibersite and individual rhabdite crystals nucleated between 700° and 500° C, and that small from the Canyon Diablo meteorite. schreibersites (rhabdites) formed between 500° and Goldstein and Ogilvie (1963) reported electron 400° C. They also demonstrated that P solubility microprobe analyses of the composition of various- was greater in kamacite than in taenke. sized schreibersites in the Canyon Diablo, Breece Reed (1965a) emphasized the important role P and Grant meteorites. They observed a size-com- plays in the formation of iron meteorite structures. position correlation, the smaller the schreibersite He determined the range of Ni values in schreiber- the higher the Ni content, and showed the pres- sites and rhabdites in a large number of meteor- ence of a zone of Ni depletion (swathing zone) ites, finding Ni values as low as 14% in large even around very small schreibersites (rhabdites). schreibersites and as high as 50% in small taenite- They suggested that most schreibersite grew by border schreibersites. Ni depletion at kamacite- solid state precipitation; although massive schrei- schreibersite interfaces was also demonstrated. bersites showing no relation to the Widmanstatten The precipitate size and Ni content relationships pattern probably formed directly from the liquid were confirmed, and Reed pointed out that size- state. Size and composition of the schreibersites for-size rhabdite grains in the Canyon Diablo me- were explained as dependent upon nucleation teorite are 7% higher in Ni than those in the temperature and time available for growth of the Coahuila meteorite. The author recognized the precipitate. A growth analysis based on diffusion approximate nature of chemical analysis values 8 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

for total P, suggesting that in most cases the actual Modern reviews that are particularly important values are higher than the measured values. He to the investigations at hand have been mentioned discussed the sequence of phase formation in me- previously. The paper by Goldstein and Axon teorites based on Vogel and Baur's (1931) phase (1973) is a comprehensive review of the develop- diagram and stressed the need for more precise ment of the Widmanstatten pattern. The recent information on phase equilibria. book by Wasson (1974) is a broad treatment of In a subsequent paper, Reed (1967) measured P meteorites and includes an excellent summary of distribution in the Mount Edith meteorite, dem- the work on chemical groupings of iron meteorites onstrating the practicality of measuring Ni and P with extensive listings of individual iron meteorite distribution profiles in kamacite, taenite, and analyses and classification. The treatise on iron schreibersite. Later Reed (1969) discussed the role meteorites by Buchwald (1976) has been of great of P in relation to Widmanstatten pattern forma- help. Much of the descriptive material on individ- tion and cooling rate calculations, and reported ual meteorites had been available to us in manu- measurements of P content in the kamacite of 61 script, and Dr. Buchwald spent many hours teach- meteorites. ing the senior author the rudiments of iron mete- Doan and Goldstein (1969) discussed the forma- orite metallography during his two-year stay tion of phosphides in the various meteorite classi- (1968-1970) at the Division of Meteorites, Smith- fication groups on the basis of their newly deter- sonian Institution. mined Fe-Ni-P phase diagram. They estimated total P values from examination of the largest RELATED MATERIALS available polished sections of a number of meteor- Schreibersite occurrences are not restricted ites, and used them as the basis of a discussion of solely to iron meteorites. It is an accessory mineral schreibersite precipitation reactions in various me- in pallasites, mesosiderites, and enstatite chon- teorite composition ranges. They inferred thermal drites, and has been reported in several ordinary equilibrium in iron meteorites down to 650° C, chondrites, and a few carbonaceous chondrites with schreibersites forming at higher temperatures and achondrites (Ramdohr, 1973; Powell, 1971; while rhabdites form from kamacite at low temper- Malissa, 1974). Schreibersite is also known as a atures when Ni and P diffusion are severely lim- product of combustion in the coal mines of Com- ited. mentry and Cranzac, France (Palache et al., 1944: Comerford (1969) discussed schreibersite and 125), as a cavity mineral in iron slags (Spencer, cohenite occurrences in iron meteorites and 1916), and associated with metallic iron in the pointed out that rhabdites, schreibersites that nu- Disko Island, Greenland, basalts (Pauly, 1969). cleated within kamacite, may be more reliable for Schreibersite has been observed by a number of diffusion analysis than grain boundary schreiber- workers in metal particles in lunar soil and rocks. sites. Axon and co-workers have discussed schrei- Individual particles have been described in great bersite occurrences in a broad range of meteorites detail, and Ni/Co ratios have been used to identify (Axon and Faulkner, 1970; Axon and Waine, 1971, metal of meteoritic origin (Goldstein et al., 1970; 1972; Axon and Smith, 1972). The papers of Axon Goldstein and Yakowitz, 1971; Axon and Gold- and Waine (1971, 1972) discuss schreibersite mor- stein, 1972; Goldstein et al., 1972). Nickel concen- phology, mode of occurrence, and relationship to tration in metal and schreibersite in contact with other minerals in great detail. Hornbogen and each other have also been used to suggest final Kreye (1970) have reported detailed metallo- equilibrium temperatures and to imply cooling graphic studies on the Coahuila and Gibeon mete- histories. As lunar studies have continued, it has orites and have discussed Ni and P solid state become apparent that a greater proportion of reactions in some detail. Reed (1972) has reported schreibersite-containing lunar metal than had pre- analyses of schreibersite in the Oktibbeha County viously been thought has been derived from proc- meteorite, the highest Ni schreibersite known in esses other than simple disruption of impacting meteorites (65.1%), and De Laeter et :al. (1973) meteorites (Axon and Goldstein, 1973; El Goresy have reported on schreibersite in the Redfields et al., 1973; Brown et al., 1973; Carter and Pado- meteorites, the lowest Ni schreibersite on record vani, 1973; McKay et al., 1973; Goldstein and (7.0%). Axon, 1973; Gooley et al., 1973). NUMBER 21 9

A second phosphide mineral, barringerite procedures for obtaining these values were devel- (Fe,Ni,Co)2P, has been reported from the Ollague oped in the electron microprobe laboratory of the pallasite (Buseck, 1969). This material is probably Department of Mineral Sciences, National Mu- a secondary product of some type and not an en- seum of Natural History, Smithsonian Institution. dogenous meteoritic mineral (Buchwald, 1976: Reed (1967, 1969) had reported traverse data on 105). the Mount Edith meteorite and had demonstrated The phosphide mineral, schreibersite, is the the practicality of measuring the low P levels in only phosphorus mineral that is observed as a kamacite with the microprobe. Doan and Gold- minor phase in most iron meteorites, and it is the stein's (1970) experience in their phase diagram only phosphorus-containing mineral that will be studies was also encouraging. Ideally, one re- considered in this paper. There are, however, quired step-by-step traverses over many meteorite nine phosphate minerals known in meteorites, structures, each traverse consisting of a large num- and seven of these have been identified as trace ber of individual analyses for P and Ni. Ni concen- constituents in iron meteorites (Fuchs, 1969; Bild, trations were in a range that presented no unusual 1974). They are chlorapatite, Ca5(PO4)3Cl; grafton- analytical problems, but the expected low levels of ite, (Fe,Mn)3(PO4)2; sarcopside, (Fe,Mn)3(PO4)2; P required special care. whitlockite, Ca3(PO4)2; brianite, Na2MgCa(PO4)2; An A.R.L. EMX electron microprobe (Applied panethite, Na2Mg2(PO4)2; and farringtonite, Research Laboratory, Sunland, California) was Mg3(PO4)2. These minerals are observed in a vari- used in this work. Simultaneous measurements ety of associations: as inclusions? in graphite-troi- were made for Ni Ka (LiF crystal) and P Ka (ADP lite-silicate nodules, in simple troilite nodules or crystal) radiation at an X-ray takeoff angle of in troilite-chromite nodules, as inclusions in or in 52.5°. The instrument was operated at an acceler- contact with schreibersite, and associated with ating voltage of 20 KV, with an approximately 0.1 other minor minerals or isolated in metal. The fia sample current on 100% Fe and a beam size of phosphate-phosphide association has been used to approximately 1 /xm. Average count rates mea- calculate equilibrium oxygen fugacity in iron me- sured on 50% Ni and calculated to 100% Ni were teorites and pallasites (Olsen and Fredriksson, 30,000 cts/sec, with a peak to background ratio of 1966; Olsen and Fuchs, 1967). In the iron meteor- 230. Count rates for 100% P measured on schrei- ites considered in this paper, phosphate minerals bersite were 25,000 cts/sec with a peak to back- are either absent or isolated within inclusions that ground ratio of 915. Standard statistical assump- represent a higher temperature stage in the devel- tions suggest that for the 20 second counting opment of the meteorite structure. No evidence periods employed, a detectability limit for P of has been developed in this work to indicate that 150 ppm for a single determination would be the observed phosphide-metal equilibria have been achieved, within a satisfactory range for the prob- affected by the presence of phosphates. lem at hand (Ziebold, 1967). Neighboring P determinations within a given traverse generally agreed Experimental within ±50 ppm. The standards used were 100% Fe, 100% Ni, a Two types of data are needed in order to inter- series of Fe-Ni alloys of known composition, and a pret iron meteorite structural development as a fragment of a large schreibersite from the Canyon consequence of cooling through the subsolidus Diablo meteorite. The 100% Fe was used for deter- Fe-Ni-P system. Accurate bulk compositions are mining both Ni and P background counts. The the first requirement, and good bulk Ni values Fe-Ni standards were prepared by R. E. Ogilvie are available from the literature. The problem of and obtained from J. I. Goldstein. They were obtaining good bulk P values has not been solved used to derive an empirical curve that was incor- satisfactorily, and estimated values were found to porated into a data reduction program to correct be useful. This aspect of the problem will be Ni determinations. The schreibersite standard discussed in detail below. The second type of data contained 12.9% Ni, 0.26% Co, and.was assumed required are P and Ni gradients within kamacite to contain 15.5% P. A linear relationship between and schreibersite, and particularly P and Ni values P counts in schreibersite and concentration in an at kamacite-schreibersite interfaces. Experimental unknown was assumed. This was thought to cause 10 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES no serious error in low level P measurements, but kamacite and crossed a very thin, elongated phos- it does introduce errors in P determination in phide (4 /mm wide) at the edge of a taenite lamella. schreibersite. In this study, P values for schreiber- It passed into kamacite and after traveling 275 /Am site were only used to identify the mineral, and entered a rhombohedral phosphide (rhabdite) 30 for this purpose corrections seemed unnecessary. /u,m across and then passed into kamacite again. Sections were prepared for study by standard The two unusually high P values in the kamacite metallographic procedures. Pieces of meteorite are undoubtedly due to unseen phosphides that were mounted in one-inch diameter Bakelite, were included in the excitation volume of the ground on a graded series of sandpapers, and microprobe beam. Figure 3 is a similar plot for polished with 3 /xm diamond paste and finally the Lexington County Meteorite. In this case only Linde B alumina compound. The sections were a portion of the complete traverse is reproduced then lightly etched with 0.5% nital (0.5 ml nitric (700 out of 1600 fim). The traverse started in acid in 100 ml of 95% ethanol). Light etching was kamacite, entered a phosphide at a taenite border, essential to reveal structures that were examined passed into kamacite, a second phosphide, and a and to permit their being relocated and analyzed second taenite lamella. Interface concentration in the microprobe. It would have been impossible values and concentration gradient shapes and to obtain the mass of data required using unetched lengths were read from these plots. sections. While deep etching is known to cause Data obtained in this way are subject to several errors in microprobe measurements at phase inter- sources of error beyond those that apply to an faces, the light etching used here did not seem to individual point analysis in a homogeneous mate- cause a problem. A schreibersite-kamacite-taenite rial. Stability of the microprobe over the long structure in the Carleton meteorite was selected to periods required to obtain lengthy traverses was a test this point. As nearly identical traverses as problem. Due to the survey nature of this investi- possible, considering that the section had to be gation, compromises were made in favor of large removed from the microprobe, were made before numbers of traverses and for lengthy traverses. and after etching. The results were identical within The microprobe beam current was monitored and reasonable experimental error. Areas selected for kept within narrow limits, but this does not neces- microprobe analysis were photographed in detail, sarily prevent all instrumental drift. Data were permitting the recording of the exact location of rejected when drift became an obvious problem, microprobe traverses. but normally measurements on standards or re- The electron microprobe data was accumulated peated measurements on the same structures gave using a step-scan procedure. Ni and P counts identical results within reasonable limits of error were measured simultaneously and recorded on (± 0.005% P, ±0.1% Ni). punch cards along with step length. The data The schreibersite-kamacite interface measure- were computer processed, resulting in a second ments are subject to errors due to both finite set of cards containing calculated P values, empir- beam size and to interface geometry. The finite ically corrected Ni values, and step length. These beam produces measured interface profiles that cards in turn were used to obtain computer gen- are less sharp than the actual profile. It is unlikely erated plots. Photographs of the data plots as that this is a serious error in determining the received from the computer are given in Figures 2 interface value of P in kamacite, as this is only and 3. The only additions to these diagrams are read to the nearest 0.01%. The interface value of the vertical lines that have been added to help Ni in kamacite is probably increased slightly by identify the structural elements. Figure 2 is a the proximity of high Ni in the adjoining schrei- traverse across a structure in the Ballinger, Texas, bersite. More serious errors may result from inter- meteorite, catalog number USNM 824. The label- face geometry, as there is no reason to assume ing indicates that this is the fifth traverse made that the interfaces measured were perpendicular over structures in that meteorite, the data having to the surface of the section. The small size of been taken on 8 September 1972. Two symbols most of the structures measured, combined with are used to indicate P concentration, the ordinate their large number, made it impractical to attempt scale being zero to 0.7% P for low values and zero measuring slopes and applying corrections. Repro- to 70% P for high values. The traverse started in ducibility of measurements on the same structure NUMBER 21 11

BRLLINGER 824 05 8 SEPT 72

ELEMENT rn = P (0-.7) © = P (0-70) * = NI (0-70)

B 5 a

me a

50 100 150 200 250 300 350 400 450 500 550

DISTflNCE (MICRONS) FIGURE 2. —Computer plotted Ni-P traverse across structure in Ballinger meteorite. and on similar structures within the same meteor- in Ni and P contents. The data in Table 1 were ite indicated that this is not too serious a problem taken from Wasson (1974) and Buchwald (1976). for the study at hand. Buchwald's values combine, where appropriate, chemical analytical values with the results of plan- Results ometric analyses of large surfaces. Contributions of Ni and P due to large schreibersite inclusions Eight coarse-structured iron meteorites covering are included in this approach, resulting in esti- a range of bulk Ni and bulk P contents were mates more representative of the meteorite as a selected for Ni and P measurements at kamacite- whole than uncorrected chemical values. Wasson's schreibersite interfaces (Table 1). Four structural Ni values are chemical values determined on small and five chemical classification categories are rep- samples taken for trace element analysis. A com- resented in the group. The Bellsbank meteorite plete review of the literature of each of these contains very low total Ni, while Balfour Downs is meteorites and a discussion of their detailed met- one of the highest Ni members of Group IA. allography has been given by Buchwald (1976). Coahuila contains only about 0.3 wt. %P, while Microscopic examination of metallographic sec- Bellsbank contains exceptionally high P. The other tions of these eight meteorites led to the selection meteorites in the group range between these limits of representative schreibersite-containing struc- 12 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

LEXINGTON COUNTY 3334 03 30 SEPT 72

ELEMENT m = P (0-.7) O = P (0-70) A = NI t0-70)

HO O a

100 ISO 200 250 300 360 400 450 600 550

DISTRNCE (MICRONS) FIGURE 3. —Computer plotted Ni-P traverse across structure in Lexington County meteorite. tures for detailed study by the electron microprobe morphologies of schreibersite, a for kamacite and step-scan procedure described above. Several mi- y for taenite. The following entries for a given croprobe traverses were made for each meteorite, traverse identify the specific schreibersites mea- and the data obtained on Ni and P values at sured and give the length of schreibersite tra- kamacite-schreibersite interfaces and on Ni and P versed. The next column, headed %NiSch, gives gradients in kamacite are summarized below. A the weight percent Ni in schreibersite either at a separate table accompanies the description of trav- kamacite-schreibersite interface or within a large erses for a given meteorite, the format used being schreibersite. Where an interface measurement the same in each case (Tables 2-9). The first two has been made, the schreibersite Ni value will be columns give the specimen number, which in- followed in the next two columns by the weight cludes the USNM catalog number, and the percent Ni in kamacite at the interface (%Niot) and traverse number. Column three contains two types the weight percent P at the interface (%Pa). The of information. The first entry for each traverse is next two sets of two columns each indicate the enclosed in square brackets and gives the sequence length of observed Ni and P gradients away from of phases traversed and the length of the traverse. the interface and the Ni and P values at which a The abbreviations used here are Ph for the various gradient was no longer distinguishable. Factors NUMBER 21 13

TABLE 1. — Composition and classification of selected meteorites

Weight Percent Chemical Structural Meteorite %P* Classification** Classification* Coahuila 0.3 5.6 5.49 IIA H Bellsbank 2 5.3 4.13 IRANOM H

BaTMnger 0.4 6.5 6.19 IA-AN Og

Santa Luzia 0.9 6.6 6.3 I IB Ogg

Lexington County .... 0.3 -7.0 6.69 IA 0g

Bahjol 7.7 7.95 IA-AN 0g

Goose Lake 0.4 8.3 8.00 IA-AN 0m

Balfour Downs 0.3 8.4 8.39 IA Og

•Buchwald (1976) **Wasson (1974) such as impingement and termination of a partic- below and specific measurements are listed in ular traverse influence these final observed Ni Table 2. and P values. In the final three columns the Ni and P values from columns three through six have 3298, traverse 1: crossed a 150 /xm wide schreibersite bordering been converted to atomic percent for use later. a 2 x 2 mm troilite-daubreelite inclusion and then extended into the surrounding kamacite for nearly 1 mm. 3298, traverse 2: crossed a 40 yum wide schreibersite bordering a 0.5 x 0.5 mm troilite-daubreelite inclusion and then ex- COAHUILA tended 350 (im into the surrounding kamacite. 3298, traverse 3: crossed a 100 /u.m wide schreibersite bordering The Coahuila, Mexico, meteorite is an example the 5x5 mm troilite-daubreelite inclusion and then ex- of an ordinary hexahedrite (IIA), similar in com- tended for 350 /xm into surrounding kamacite. This traverse position and structure to a number of other hexa- is reproduced at the upper left in Figure 4. hedrites. It consists of single crystal kamacite con- Section 1641(1) primarily contains regions of taining profuse small schreibersites generally of kamacite with evenly distributed rhabdite, the in- rhabdite morphology, and occasional larger 2 dividual rhabdites averaging 200 fim in cross- schreibersites. The larger schreibersites sometimes sectional area. Also present in areas of clear kam- border troilite or troilite-daubreelite inclusions, acite are several large schreibersite inclusions and and in rare instances are associated with cohenite. schreibersite-cohenite inclusions. The largest Section 3298 is dominated by six troilite-dau- schreibersite inclusion has an area of 0.25 mm2. A breelite inclusions, ranging in size from 5 to 0.3 0.1 x 0.6 mm schreibersite is bordered by cohen- mm in diameter. Half to three-quarters of the ite, and an adjacent, slightly smaller schreibersite length of the borders of these sulfides are rimmed is partially surrounded by cohenite. One cohenite with schreibersite. One small sulfide is bordered area, 0.2 x 0.3 mm, contains six small schreiber- for most of its circumference with schreibersite, sites. A 0.4 x 1.4 mm cohenite area contains a the remaining border being rimmed with cohenite. number of small schreibersites and surrounds a Kamacite in the vicinity of these inclusions contains central schreibersite measuring 0.04 x 0.6 mm. many subgrain boundaries and is free of schreiber- An 80 X 150 fim daubreelite is surrounded by site. Microrhabdites measuring less than a few schreibersite. Two lamellar schreibersites, each microns in maximum length and generally consid- approximately 0.02 X 1.0 mm, are present, and erably smaller are present in profusion away from one of these is partially bordered with cohenite. these inclusions. Somewhat larger rhabdites are present in several areas aligned along Neumann 1641(1), traverse 4: crossed a large skeletal schreibersite enclos- bands. Several lamellar schreibersites, 5 to 10 /am ing a kamacite area. The traverse first crossed 450 fjun of kamacite, then 120 /urn of schreibersite, 70 jxm of included wide and extending to lengths that are a major kamacite, a second 70 (im schreibersite crossing, and termi- part of a millimeter or more, are also present. A nated after passing through 360 /tm of kamacite. The schrei- summary of the traverses of this section is given bersite was of uniform composition, and the measurements 14 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

TABLE 2. —Coahuila schreibersite-kamacite interface measurements

Weight Percent Ni Gradient P Gradient Atomic Percent Specimen Traverse Length Length Ni 3SNi %P Ni %N1 %P No. No. Structure traversed & schreibersites measured * Sch a a (ym) Wt.%Ni (um) Wt.%P * Sch a a

3298 1 [Ph border of troilite-daubreelite inciusion-a, 1.1 mm] Schreibersite, 150 pm wide 23.5 3.4 0.07 600 5.5 800 0.16 20.1 3.2 0.13

3298 2 [Ph border of troilite-daubreelite inclusion-a, 400 ym] Schreibersite, 40 ym wide 25.0 3.6 0.07 250 5.3 350 0.15 21.4 3.4 0.13

3298 3 [Ph border of troilite-daubreelite inclusion-a, 450 ym] Schreibersite, 100 um wide 25.0 3.7 0.08 250 5.3 300 0.15 21.4 3.5 0.14

1641(1) 4 [a-Ph-a-Ph-a, 1 mm] Schreibersite 260x500 ym with 60x170 um included kamacite 120 ym traverse 23.5 3.5 0.08 200 5.3 50 0.10 20.1 3.3 0.14 70 ym traverse 23.5 3.5 0.08 250 5.3 200 0.10 20.1 3.3 0.14

1641(1) 5 [a-Ph-a, 350 um] Rhabdite, 25 ym traverse Enter 34.5 4.9 0.06 50 5.3 Flat 0.06 29.7 4.7 0.11 Exit 34.5 4.9 0.06 50 5.3 100 0.07 29.7 4.7 0.11

1641 6 [a-Ph-a, 750 um] Rhabdite (45x75 um), 45 um traverse 27.5 3.7 0.07 175 5.3 100 0.08 23.6 3.5 0.13

1641 7 [a-Ph-a, 800 um] Rhabdite (50x70 um), 70 ym traverse Exit 27.0 4.0 0.07 100 5.3 Flat 0.07 23.1 3.8 0.13

1641 8 [o-Ph-a-Ph-a, 850 ym] Rhabdite (50x70 ym), 50 um traverse 27.0 4.1 0.07 150 5.3 Flat 0.07 23.1 3.9 0.13 Rhabdite (25x30 ym), 25 ym traverse 29.0 3.7 0.06 200 5.3 100 0.08 24.9 3.5 0.11

1641 9 [a-Ph-a, 200 ym] Rhabdite (20x40 um), 25 ym traverse 33.0 4.5 0.05 50 5.3 Flat 0.05 28.4 4.3 0.09

in Table 2 are for the initial and final schreibersite-kamacite fim of rhabdite and then 0.5 mm of kamacite. Only the interfaces. measurements on leaving the rhabdite were usable. 1641(1), traverse 5: crossed 100 fim of kamacite, a 25 fim wide 1641, traverse 8: crossed 370 fim of kamacite, 50 fim of rhabdite, rhabdite and terminated after another 200 fim of kamacite. 25 fim of kamacite, 25 fim of rhabdite, and 350 fim of This traverse is reproduced at the lower left in Figure 4. kamacite. The large rhabdite traversed here is the same one traversed in number 7 but from a near perpendicular direc- Section 1641 is a neighboring specimen to tion. Values reported in Table 2 are averages of two similar 1641(1) and is similar in metallography. Evenly interface measurements. distributed rhabdites averaging about 200 jam2 in 1641, traverse 9: crossed a smaller rhabdite approximately 1 mm from traverses 6 through 8. 90 fim of kamacite were area are the primary feature. Random larger crossed, followed by 25 fim of rhabdite and 80 fim of schreibersites, schreibersite-daubreelite, and kamacite. Average interface values are listed in Table 2. schreibersite-cohenite inclusions similar to those This traverse is reproduced at the lower right in Figure 4. described above are present, although they are Ni and P concentration-distance profiles typical neither as numerous nor as large as the largest of the Coahuila data outlined above are given in mentioned above. Two unusually large rhabdites Figure 4. These diagrams were prepared from and two smaller ones were selected for measure- tracings of the computer plotted step-scan data. ment. Ni concentrations are indicated by the solid lines, 1641, traverse 6: crossed 400 fim of kamacite, a 45 fim wide and P concentrations multiplied by 100 by dashed rhabdite, and 300 fim of kamacite. Values reported in Table lines. The Ni profiles are normally readily repro- 2 are an average of the two similar interface measurements. This traverse is reproduced upper right in Figure 4. duced from the data charts without ambiguity. 1641, traverse 7: crossed 230 fim of kamacite, entered the The low level P data contains more scatter; draw- rhabdite from an area of disturbed kamacite, traversed 70 ing the appropriate line, and particularly selecting -NUMBER 21 15

45-

40-

o m UJ 0.

£ 3O O 28 UJ S 25 25 100 (Jm

20-

15- P x 100

0.08 0.07 Ni

3.7 3.7

3296 1641 400 MICRONS

0.05,, N4.9 4.5

300 MICRONS FIGURE 4. —Ni and P profiles at kamacite-schreibersite interfaces in Coahuila: upper left, traverse 3; upper right, 6; lower left, 5; lower right, 9. 16 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

the kamacite interface P level, is more subjective. 33% Ni schreibersite has an essentially flat P con- A smoothing procedure relying on eyeball exami- centration at 0.05%. For these four cases, the nation was used. Ni values for schreibersite and length and steepness of the P gradient decreases Ni and P concentrations in kamacite at the kama- with decreasing P at the interface. cite-schreibersite interface are indicated at an appropriate place on the diagrams. The bulk Ni BELLSBANK content of the meteorite is also given and indicated by a short dashed line. The Bellsbank, South Africa, meteorite is a The four profiles (Figure 4) illustrate the ob- phorphorus-rich hexahedrite. Massive and large served range of Ni in Coahuila schreibersite and skeletal schreibersite crystals, with individual faces the shape and level of the Ni and P gradients in over a centimeter in length and areas larger than the surrounding kamacite. They are given from 2 cm2, dominate polished surfaces and appear to upper left to lower right in order of decreasing P be distributed throughout this meteorite (photo- concentration at the kamacite-schreibersite inter- graph in Henderson, 1965). The kamacite matrix face . Three of the profiles are of large rhabdites contains a network of lamellar schreibersite. These (traverses 6, 5, 9, Table 2), while the one at the rarely intersecting planar crystals may exceed a upper left is of a 100 /xm wide schreibersite border- centimeter in length, but are normally only 5 to 10 ing a large troilite-daubreelite inclusion (traverse /u,m thick. Occasional large rhabdites and subgrain 3, Table 2). The high P interface value and the boundary schreibersites are present. Microrhab- high level in the surrounding kamacite are of dites are present in profusion in kamacite areas particular interest. The three rhabdites appear to away from the other forms of schreibersite. The increase in Ni, both with decreasing cross-sectional Bellsbank meteorite has experienced severe terres- area and with increasing interface Ni in kamacite trial weathering. Many of the lamellar schreiber- value. The interface P values do not seem to sites have been replaced with corrosion products, correlate with either Ni in schreibersite or with and corrosion along kamacite-schreibersite inter- interface Ni in kamacite values. Ni and P gradients faces is common. extend for several hundred microns around the Section 2162 contains parts of two massive larger schreibersites but appear to be much more schreibersites surrounded by schreibersite-free restricted in extent around the smaller ones. The kamacite areas. Two lengthy subgrain boundaries

TABLE 3. —Bellsbank schreibersite-kamacite interface measurements

Weight Percent Ni Gradient P Gradient Atomic Percent Specimen Traverse Length Length Ni SNi XP N1 XN1 %P No. No. Structure traversed & schreibersites measured * Sch (um) Wt.SSNI (um) Wt.JSP * Sch a a a a 2162 1 [a-Ph, 3.6 mm] Massive schreibersite (7x2 mm), 1 mm traverse 12.4 1.6 0.09 900 3.9 1000 0.20 10.6 1.5 0.16

2162 2 [a-Ph-a, 900 um] Large rhabdite along sub-grain boundary, 40x50 um, diagonally across body. 22.0 3.2 0.09 20 4.5 50 0.18 18.8 3.1 0.16

2162 3 [a-Ph-a, 200 um] Same rhabdite as traverse 2, parallel to 50 pm direction Enter 22.0 3.3 0.09 40 4.2 Flat 0.09 18.8 3.1 0.16 Exit 22.0 2.7 0.05 40 4.2 60 0.12 18.8 2.5 0.09

2162 4 [a-Ph-a, 300 um] Rhabdite along sub-grain boundary, 15x120 um 22.6 2.6 0.05 80 4.3 100 0.20 19.3 2.5 0.09

2162 5 [a-Ph-a, 300 um] Rhabdite along sub-grain boundary, 15x120 um, repeat of traverse 2162-4 22.8 2.6 0.05 90 4.3 100 0.18 19.5 2.5 0.09

2162 6 [a-Ph-a, 130 ym] Large rhabdite Isolated in micro-rhabdite area of a, 30x60 pm 18.6 2.8 0.09 40 4.2 30 0.20 15.9 2.7 0.16 NUMBER 21 17

MICRO-RHABOITES

400 MICRONS

Px 100 \

400 MICRONS FIGURE 5. —Ni and P profiles at kamacite-schreibersite interfaces in Bellsbank: top, traverse 1; lower left, 6; lower middle, 3; lower right, 4 and 5. (Dashed line indicates bulk Ni value.) 18 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES cross the kamacite and are sites of small lamellar, the one on the left, but the P interface value is irregular-shaped, and rhabdite-shaped schreiber- considerably lower. The middle profile has uni- sites. The kamacite matrix contains profuse micro- form Ni in the schreibersite but has two different rhabdites. Oxidation products are present at sites sets of P and Ni kamacite interface values, the previously occupied by lamellar schreibersite, and only such observation made in this study. along some Neumann bands and possibly cleavage planes. BALLINGER 2162, traverse 1: crossed 2.6 mm of kamacite and entered a large schreibersite and continued for 1 mm. The traverse The Ballinger, Texas, meteorite is a low-Ni was perpendicular to a straight kamacite-schreibersite inter- coarse octahedrite. Figure 6 is a photograph of face, and the Ni content of the schreibersite was uniform for the slice studied, USNM 824. The area marked PS the length of the measurement (reproduced at top of Figure 5). Numerical data for this and succeeding Bellsbank trav- shows where material was taken for section prepa- erses are given in Table 3. ration. Section 824 is from the back of this surface 2762, traverse 2: crossed 650 fim of kamacite, a rhabdite at a and contains a large schreibersite similar in size subgrain boundary and continued into surrounding kamacite and shape to the one at the lower right in Figure for 200 /urn. Average values for the two interface values are 6. Section 824(1) is the outlined surface after repol- given in Table 3. 2162, traverse 3: crossed 70 fim of kamacite, the same rhabdite ishing. The overall structure consists of large measured above, and passed into 70 fim of kamacite. kamacite grains and three areas of heiroglyphic Traverse 2 crossed diagonally, while this one passed through schreibersite. A Widmanstatten pattern is sug- the middle parallel to the 50 ^im edge. Two different gested in some areas of the structure, but it is interface values for Ni and P in kamacite were obtained, certainly not well developed. The large schreiber- perhaps influenced by the presence of the subgrain boundary (Table 3 and lower middle of Figure 5). site on the left is completely surrounded by rims 2162, traverse 4: crossed 170 fim of kamacite, the narrow of partially decomposed cohenite. The center direction of an elongated rhabdite situated along a subgrain group of schreibersite crystals is partly bordered boundary, and 100 fim of kamacite (lower right in Figure 5). by partially decomposed cohenite, while the schrei- 2162, traverse 5: a remeasurement of traverse 4. bersite on the right is completely free of cohenite. 2762, traverse 6: crossed a large rhabdite surrounded by a region of clear kamacite within a microrhabdite area (lower Areas of this surface containing the best Widman- left in Figure 5). statten pattern development are well away from the large schreibersite inclusions. It is interesting An attempt was made to determine Ni in other to note that the lower left-hand edge of this slice small schreibersites in Bellsbank. A number of contains a rim of kamacite heat altered to a2, a measurements were made on schreibersites rang- remnant from ablation heating. This structure has ing down to a minimum of a few microns in not been previously observed in the Ballinger width. Twelve measurements were obtained where meteorite. the P value indicated that the microprobe beam Section 824 contains a large schreibersite 0.8 had been completely within a schreibersite. The mm long and averaging 1 to 2 mm in width, Ni values ranged from 22% to a maximum of surrounded by a narrow area of schreibersite-free 32%. Interface measurements were not attempted kamacite. The kamacite matrix contains a profu- on these small bodies. sion of both Neumann bands and rhabdites. Neu- The four profiles in Figure 5 represent the mann band bending, particularly at the interfaces traverses described above and illustrate the shapes with the large schreibersite, suggests mild mechan- and levels of the Ni and P gradients in the sur- ical distortion in Ballinger. The kamacite contains rounding kamacite. The profile at the top is the numerous grain boundaries that are the sites of interface portion of traverse 1. The low value for grain boundary schreibersites. Terrestrial oxida- Ni in the schreibersite combines with an unusually tion has penetrated areas of this section. low interface value for Ni and a high interface value for P in kamacite. The rhabdite profile at 824, traverse 1: crossed three areas of the large schreibersite. the lower left has the same P interface value, but 500 fim of kamacite were crossed, followed by 350 fim of schreibersite, 800 fim of kamacite, 400 fim of schreibersite, higher Ni in both kamacite and schreibersite. The 450 fim of kamacite, 670 fim of schreibersite, and finally 500 P gradient in this case is particularly severe. The fim of kamacite. The three schreibersite areas traversed are rhabdite at the lower right has similar Ni values to essentially free of Ni gradients and they all have approxi- NUMBER 21 19

1 cm I 1 FIGURE 6. —Macrostructure of a slice of Ballinger meteorite (large dark inclusions are schreibersite, the one on the left enclosed in partially decomposed cohenite; area marked PS was used for microstructure examination [824(1)]; and edge marked a2 contains a rim of ablation recrystallized kamacite).

mately the same Ni value. Numerical data for this and in Figure 6. The kamacite matrix contains Neu- succeeding Ballinger traverses are given in Table 4. mann bands and rhabdites in a wide range of 824, traverse 2: crossed two rhabdites along a grain boundary 2 mm away from the massive schreibersite. 400 fim of kamacite sizes. Kamacite lamellae are separated by grain were crossed, followed by two 40 fim rhabdites separated by boundaries that are alternately sites of grain 25 fim of kamacite, and finally 300 fim of kamacite. Because boundary schreibersites and taenite-plessite bands. of impingement effects, the interface values in Table 4 are an average of the two similar external measurements. 824(1), traverse 4: crossed 230 fim of kamacite and entered a 824, traverse 3: crossed two areas of a 1 x 0.4 mm skeletal small schreibersite embedded in a taenite border, passed schreibersite 3.5 mm from the massive schreibersite. The through a narrow band of taenite, and then 220 fim of values given in Table 4 are averages of the two external kamacite. interface values. 824(1), traverse 5: crossed 150 fim of kamacite, a schreibersite embedded in taenite, 20 fim of taenite, 270 fim of kamacite, Section 824(1) is from a typical Widmanstatten a large rhabdite, and finally 120 /AITI of kamacite (see Figure area of the Ballinger meteorite, the area outlined 2 in experimental section). 20 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

TABLE 4. —Ballinger schreibersite-kamacite interface measurements

Weight Percent N1 Gradient P Gradient Atomic Percent Specimen Traverse Length Length *N1Sch %P Ni XN1 XP No. No. Structure traversed & schreibersites measured a (ym) Wt.%N1 (um) Wt.SP * Sch a a a 824 1 [a-Ph-a-Ph-a-Ph-a, 3.7 mm] Massive schreibersite, 350 ym Enter 19.5 2.9 0.06 500 5.2 150 0.10 16.6 2.8 0.11 Exit 19.5 3.0 0.06 300 4.3 200 0.09 16.6 2.9 o.n Massive schreibersite, 400 ym Enter 19.5 2.6 0.06 200 4.3 200 0.08 16.6 2.5 o.n Exit 19.3 2.6 0.06 150 4.0 150 0.08 16.5 2.5 o.n Massive schreibersite, 670 ym Enter 19.7 ... 0.06 150 4.0 100 0.08 16.8 2.5 o.n Exit 19.5 2.6 0.06 500 5.0 150 0.08 16.6 2.5 0.11

824 2 [a-Ph-o-Ph-a, 850 ym] Two large rhabdites, 40x40 um each, separated by 25 um. Values recorded are averages of similar values. 34.5 4.2 0.04 200 6.0 100 0.09 29.7 4.0 0.07

824 3 [a-Ph-a-Ph-o, 700 um] Skeletal schreibersite 1x0.4 mm. Values recorded are average of similar values from the two exterior Interfaces. 35.8 4.8 0.05 150 6.8 200 0.08 30.8 4.6 0.09

824(1) 4 [a-Ph-Y-o» 480 um] Rhabdite embedded in taenite border, 15x20 um 49.8 6.0 0.03 50 7.2 50 0.05 43.1 5.7 0.05

824(1) 5 [a-Ph-Y-a-Ph-a, 650 ym] Rhabdite embedded In taenite border, 5 um wide 49.5 6.0 0.03 50 7.2 50 0.05 42.9 5.7 0.05

Large rhabdite (35x25 um), 250 ym into a 41.5 5.2 0.05 50 7.2 50 0.07 35.8 5.0 0.09 from above

824(1) 6 [ct-Ph-Y-a, 300 um] Rhabdite embedded in taenite border, 10x15 um 51.0 6.3 0.03 50 7.2 50 0.06 44.2 6.0 0.05

824(1) 7 [a-Ph-a-Ph-a-Ph-a-Ph-a, 1.4 mm] Grain boundary schreibersite in sequence with taenite, 20x15 ym 47.0 5.7 0.03 50 7.2 50 0.06 40.7 5.4 0.05 Rhabdite 50 ym from above, 40x50 ym 44.5 5.7 0.04 100 7.2 100 0.07 38.4 5.4 0.07 Rhabdite 400 ym from above, 40x40 ym 40.0 5.0 0.05 100 7.2 200 0.09 34.5 4.8 0.09 Rhabdite 600 ym from above, 30x50 ym 40.0 5.0 0.05 100 7.2 200 0.09 34.5 4.8 0.09

824(1) 8 [a-Ph-a, 1.0 mm] Grain boundary schreibersite in sequence with taenite, 15x140 ym 48.8 6.0 0.03 100 7.0 300 0.08 42.3 5.7 0.05

824(1), traverse 6: crossed 80 fj.m of kamacite, a taenite border Large rhabdites in Ballinger contain more Ni than schreibersite, 15 fim of taenite, and 150 fim of kamacite. similar ones in Coahuila or Bellsbank, and their 824(1), traverse 7: crossed 140 /u,m of kamacite, a grain boundary interface values are generally higher in Ni and schreibersite in sequence with taenite, 50 fim of kamacite, a large rhabdite, 300 (im of kamacite, a large rhabdite, 620 lower in P. Grain boundary schreibersite tends to jum of kamacite, a large rhabdite, and finally 80 /um of contain more Ni than kamacite matrix rnabdites, kamacite. and taenite border schreibersite contains the high- 824(1), traverse 8: crossed 90 jxm of kamacite, a grain boundary est Ni values observed. As the Ni values in the schreibersite in sequence with taenite, and 1 mm of kamacite. schreibersite go up, interface values in kamacite tend to go up for Ni and down for P. Ni and P The massive schreibersite in Ballinger contains gradients extend over relatively large distances higher Ni than similar sized schreibersite in Bells- around the massive schreibersite and over much bank, and the Ni in kamacite interface values are shorter distances in the higher Ni forms of schrei- higher while the P interface values are lower. bersite . NUMBER 21 21

SANTA LUZIA Luzia traverses are given in Table 5. 1618P, traverse 2: crossed 1.0 mm of kamacite, 0.4 mm of massive schreibersite, 1.9 mm of kamacite, 0.8 mm of massive The Santa Luzia, Brazil, meteorite is a P-rich schreibersite, 0.9 mm of kamacite, and 0.2 mm of massive coarse octahedrite, a chemical Group IIB meteor- schreibersite. 1618P, traverse 3: crossed 0.2 mm of massive schreibersite, 1.0 ite. The structure of a typical section is illustrated mm of kamacite, and 0.2 mm of massive schreibersite. in Figure 7. Large, hieroglyphic schreibersites with broad areas of swathing kamacite dominate the Section 1618PS contains troilite, massive schrei- structure. Two such schreibersite areas are shown bersite and areas of swathing kamacite (Figure 8). in the figure, the central one bordering a large Its metallographic characteristics are similar to troilite. The second schreibersite area could also those of section 1618P. be associated with a lens-shaped troilite nucleus that lies outside the plane of this section. The 1618PS, traverse 4: crossed 0.2 mm of troilite, 2.9 mm of areas of structure between the troilite-schreiber- kamacite, 140 /tm of massive schreibersite, 0.6 mm of kamacite, 2.9 mm of massive schreibersite, and 0.5 mm of kama- site-swathing kamacite areas contain a coarse Wid- cite. manstatten pattern, particularly well developed in 1618PS, traverse 5: crossed 0.4 mm of troilite, 0.7 mm of the top of Figure 7. These Widmanstatten areas massive schreibersite, 4.6 mm of kamacite, 2.5 mm of massive are reminiscent of the schreibersite-free areas of schreibersite, and 1.8 mm of kamacite. A representative the Ballinger meteroite. Weathering has pene- interface profile for this traverse is reproduced in Figure 9. 1618PS, traverse 6: crossed 0.6 mm of massive schreibersite and trated deeply into Santa Luzia, particularly along 1.9 mm of kamacite. the major grain boundaries separating the swathing kamacite areas from the areas of Widmanstat- Section 1618PS(1) contains an outer edge of the ten pattern. massive schreibersite and spans the swathing kam- Figure 8 is a sketch of the section of the photo- acite zone, containing a segment of its exterior graph (Figure 7), indicating how this slice was boundary. A detailed electron microprobe traverse sampled for detailed metallographic and electron was not made on this sample, but sufficient mea- microprobe analysis. Four metallographic sections surements were taken to establish the Ni and P were prepared from the areas indicated by dashed concentration patterns for the swathing zone. In lines in the sketch. Section 1618 is from an area of the direction away from the schreibersite and Widmanstatten pattern, 1618P from the schreiber- toward the exterior grain boundary, the Ni in site-swathing kamacite area, and 1618PS and kamacite increased from slightly more than 2% at 1618PS(1) from the sulfide-schreibersite-swathing the schreibersite interface to approximately 5% kamacite area. The numbered arrows in the sketch within the first 0.5 mm. Over the next 7 mm the indicate the location of the traverses described Ni concentration increased smoothly to approxi- below. Lengthy traverses were made in each of mately 7%. It is in the first 0.2 mm of kamacite the two hieroglyphic phosphide areas, with short surrounding the schreibersite that the Ni concen- ones being made in the Widmanstatten pattern tration increases most rapidly, and it is this zone area. that is free of microrhabdites. The P concentration Section 1618P contains massive schreibersite and increases to approximately 0.1% within this same swathing kamacite (Figure 8). Clear kamacite sur- region of approximately 0.2 mm bordering the rounds the large schreibersite, grading into kama- interface. Upon entering the zone of microrhab- cite containing a profusion of microrhabdites. dite precipitation, P concentration falls off in kam- Neumann bands are most obvious in the transition acite to a uniform level of approximately 0.05% areas between clear kamacite and microrhabdite for the remainder of the swathing zone. Late- areas. Occasional subgrain boundaries are present stage microrhabdite precipitation seems to have within the kamacite, and small schreibersites of effectively removed the P gradient that must have various morphologies are associated with them. been produced within the swathing zone during the massive schreibersite growth, while apparently 1618P, traverse 1: crossed 1.3 mm of massive schreibersite, 2.0 mm of kamacite, 0.7 mm of massive schreibersite, 0.5 mm of only slightly modifying the Ni gradient. kamacite, 0.2 mm of massive schreibersite, and 2.6 mm of The clear kamacite surrounding the central kamacite. Numerical data for this and succeeding Santa schreibersite within the swathing zone is 100 to 22 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

FIGURE 7. —Etched surface of Santa Luzia meteorite, USNM 1618: center, troilite surrounded by schreibersite and swathing kamacite; left, schreibersite area in swathing kamacite.

FIGURE 8. —Sketch of Santa Luzia meteorite slice in Figure 7, (coarse pattern, troilite; fine pattern, schreibersite; lines enclose areas of swathing kamacite; broken lines, locations of metallographic sections; numbered arrows, positions of electron microprobe traverses; stars, positions of residual taenite areas).

200 /xm wide. This clear kamacite grades into and it is in this transitional area that Neumann kamacite containing profusion of microrhabdites, bands are frequently best developed. Microrhab- NUMBER 21 23

TABLE 5. — Santa Luzia schreibersite-kamacite interface measurements

Weight Percent Ni Gradient P Gradient Atomic Percent Specimen Traverse Length Length vn N1 %P No. No. Structure traversed & schreibersites measured * Sch a a (ym) Wt.«N1 (ym) Wt.*P %N1Sch a #r a 1618P 1 [Ph-a-Ph-a-Ph-a, 7.3 mm] Massive schreibersite, 1.3 mm traverse Start 16.0 13.6 Exit 16.5 2.4 0.07 500 4.9 500 0.15 14.1 2.3 0.13 Massive schreibersite, 0.7 mm traverse Enter 16.5 2.3 0.06 600 4.9 400 0.15 14.1 2.2 0.11 Exit 16.5 2.4 0.07 200 3.8 100 0.08 14.1 2.3 0.13 Massive schreibersite, 250 ym traverse Enter 17.9 2.4 0.07 200 3.8 Flat 0.07 15.3 2.3 0.13 Exit 17.9 2.4 0.07 1000 5.3 800 0.16 15.3 2.3 0.13 1618P 2 [a-Ph-a-Ph-a-Ph, 5.3 mm] Massive schreibersite, 430 ym traverse Enter 16.0 2.3 0.07 900 4.0 500 0.09 13.6 2.2 0.13 Exit 16.0 2.3 0.07 1000 3.7 300 0.09 13.6 2.2 0.13 Massive schreibersite, 800 ym traverse Enter 16.2 2.2 0.07 600 3.8 400 0.09 13.8 2.1 0.13 Exit 16.2 2.4 0.07 400 4.2 300 0.09 13.8 2.3 0.13 Massive schreibersite, 200 ym traverse Enter 16.1 2.3 0.07 400 4.2 200 0.09 13.7 2.2 0.13 Stop 16.1 13.7 1618P 3 [Ph-a-Ph, 1.4 mm] Massive schreibersite, 220 ym traverse Start 16.0 13.6 Exit 16.0 2.3 0.07 200 4.0 300 0.11 13.6 2.2 0.13 Massive schreibersite, 200 ym traverse Enter 16.2 2.3 0.07 300 4.0 300 0.11 13.8 2.2 0.13 Stop 16.2 13.8 1618PS 4 [Troilite-a-Ph-a-Ph-a, 7.4 mm] Massive schreibersite, 140 ym traverse Enter 17.3 2.6 0.08 600 4.8 800 0.16 14.8 2.5 0.14 Exit 17.3 2.3 0.06 300 3.5 300 0.08 14.8 2.2 0.11 Massive schreibersite, 2.9 mm traverse Enter 16.6 2.4 0.07 300 3.5 200 0.08 14.4 2.3 0.13 Exit 17.1 2.5 0.08 500 5.0 250 0.14 14.6 2.4 0.14 1618PS 5 [Troll1te-Ph-a-Ph-a, 1 cm] Massive schreibersite, 0.7 mm traverse Start 16.6 14.2 Exit 16.6 2.5 0.07 900 4.6 700 0.14 14.2 2.4 0.13 Massive schreibersite, 2.5 mm traverse Enter 16.4 2.5 0.07 500 4.6 800 0.14 14.0 2.4 0.13 Exit 16.6 2.5 0.07 600 5.2 800 0.18 14.2 2.4 0.13

1618PS 6 [Ph-a, 2.5 nin] Massive schreibersite, 0.6 mm traverse Start 16.6 14.2 Exit 16.6 2.3 0.07 600 4.8 500 0.13 14.2 2.2 0.13

1618 7 [a-Ph-a, 350 ym] Traverses across grain boundary schreibersite, 12x170 ym Tip 45.5 5.8 0.03 50 7.3 100 0.10 39.4 5.5 0.05 Center 44.7 6.0 0.04 100 7.4 100 0.10 38.6 5.7 0.07 Tip 45.7 6.2 0.04 50 7.4 100 0.10 39.6 5.9 0.07 1618 8 [a-Y-Ph-a, 400 ym] Taenite border schreibersite, 10x20 ym 48.0 6.4 0.03 100 7.4 150 0.10 41.5 6.1 0.05

1618 9 [a-Y-Ph-a, 350 ym] Taenite border schreibersite, 15x40 ym 48.5 6.5 0.03 70 7.5 150 0.10 42.0 6.2 0.05 24 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

TABLE 5—continued

Weight Percent N1 Gradient P Gradient Atomic Percent Specimen Traverse Length Length N1 %N1 %P N1 SN1 No. No. Structure traversed & schreibersites measured * Sch a a (um) Wt.%N1 (um) Wt.%P * Sch a 772 10 [a-Ph-a-Ph-a, 3.6 Itm] Massive schreibersite, 650 um traverse Enter 19.3 2.8 0.07 800 5.8 500 0.14 16.5 2.7 0.13 Exit 19.3 2.8 0.07 250 4.7 250 0.10 16.5 2.7 0.13 Massive schreibersite, 600 um traverse Enter 19.7 2.8 0.07 250 4.7 250 0.10 16.8 2.7 0.13 Exit 19.7 2.8 0.07 80 5.8 600 0.15 16.8 2.7 0.13 772 11 [a-Ph-a, 2.3 mm] Grain boundary schreibersite, 400x450 um Enter 28.0 3.7 0.06 1400 6.9 1000 0.11 24.0 3.5 0.11 Exit 28.0 3.9 0.06 500 6.3 300 0.12 24.0 3.7 0.11

772 12 [a-Ph, 750 um] Grain boundary schreibersite, 70 um traverse Enter 36.2 4.6 0.05 500 7.2 500 0.09 31.2 4.4 0.09 Stop 36.2 31.2 772 13 [a-Ph-Y-a, 1.4 mm] Taenite border schreibersite, 15x20 um a-border 47.0 6.0 0.03 200 7.4 200 0.07 40.7 5.7 0.05 Y-border 47.5 41.1

dites appear to increase in number per unit area Section 1618 is from an area of the large section for the first few hundred microns away from the containing coarse-structured Widmanstatten pat- clear kamacite. As distance increases further, there tern (Figure 8). Parts of four kamacite grains 4 to appears to be a slight increase in both concentra- 5 mm wide are present. The general metallogra- tion and coarseness of microrhabdites. Coarser phy of the section is similar to that described for microrhabdites are occasionally associated with Ballinger, with the exception that only microrhab- subgrain boundaries within the kamacite. There dites are present within kamacite areas. Measure- are also occasional aligned microrhabdites, as if ments described below were made along a 1.5 mm they had precipitated along Neumann bands that grain boundary containing taenite and taenite- are no longer present. Rare lamellar schreibersites plessite areas in sequence with grain boundary 2 fxm or less in width and as long as 200 ju,m are schreibersite. present. Near the outer edge of the swathing zone 1618, traverse 7: combines data from three short (350 fim) occasional segments of grain boundary type schrei- traverses perpendicular to the long direction of a single bersite protrude into the swathing kamacite or are grain boundary schreibersite. It measured 170 fim in length observed enclosed within it. Four areas that con- and was separated from taenite by 10 to 20 fim at either end. Measurements were made near the two ends and in the tain residual taenite as well as associated schreiber- middle of the schreibersite. The interface profile for the site were observed within the two swathing zones center traverse is given in Figure 9. (Figure 8), and close to their exterior borders. 1618, traverse 8: crossed 200 fim of kamacite, 10 fim of schrei- The swathing zone boundary has suffered se- bersite, 15 fim of taenite, and 170 fim of kamacite. This vere terrestrial weathering and now contains sec- taenite border schreibersite was embedded in the taenite lamella at one end of the schreibersite in traverse 7, approx- ondary iron oxides that have penetrated into the imately 200 fim away. The schreibersite interface profile is adjoining kamacite. Remnant grain boundary given in Figure 9. schreibersite is present, but it is encased in oxides 1618, traverse 9: crossed 200 fim of kamacite, 15 fim of schrei- precluding interface measurements. The amount bersite, 25 fim of taenite, and 120 fim of kamacite. The of remnant schreibersite suggests that originally location was 200 fim farther along the same taenite measured as much as 50% of the length of the grain bound- in traverse 8. ary may have been occupied by schreibersite. A Section 772 is from a separate small specimen of Ni determination on one of these schreibersites the Santa Luzia meteorite. It combines both the gave a value of 29%. swathing kamacite and Widmanstatten pattern NUMBER 21 25

400 MICRONS FIGURE 9. —Ni and P profiles at kamacite-schreibersite interfaces in Santa Luzia meteorite, USNM 1618: traverses 5, 7, and 8 as indicated. areas measuYed above on separate sections in one 772, traverse 12: crossed 0.7 mm of kamacite and 50 fim of microprobe section of 2 cm2. Figure 10 is a scale schreibersite. The Ni and P profiles are reproduced in Figure 11. drawing indicating the areas of interest. A massive 772, traverse 13: crossed a taenite boundary schreibersite (see schreibersite approximately 7 mm in length is inset in Figure 10). The major features of this traverse are surrounded by clear kamacite that in turn is en- given in Figure 11. closed in microrhabdite-containing kamacite. A major grain boundary containing grain boundary The data on the Santa Luzia meteorite may be schreibersite is within 3 mm of one end of the of particular significance from the standpoint of massive schreibersite. Leading off of that grain classification considerations. The schreibersite re- boundary is another that contains taenite as well lationships observed for the large swathing zones as grain boundary schreibersite, within 6 mm of and their included schreibersite are in many ways the massive schreibersite. Measurements have similar to the Bellsbank meteorite. The schreiber- been made in these areas as indicated in Figure site relationships in the Widmanstatten areas of 10. Santa Luzia are similar to those observed in Ballin- ger. Could the differences in these two structures 772, traverse 10: crossed 0.9 mm of kamacite, 0.6 mm of massive (Figures 6, 7) be due primarily to differences in schreibersite, 0.6 mm of kamacite, 0.6 mm of massive schrei- initial P concentration? bersite, and 0.9 mm of kamacite. Part of this traverse including the first kamacite-schreibersite interface is given in Figure 11. 772, traverse 11: crossed 1.4 mm of kamacite, 0.4 mm of grain LEXINGTON COUNTY boundary schreibersite, and 0.5 mm of kamacite. The kamacite-schreibersite interface on leaving the schreibersite is The Lexington County, South Carolina, mete- given in Figure 11. orite is a coarse octahedrite of intermediate Ni 26 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES r

1-2

772 FIGURE 10. —Scale drawing of schreibersites in an area of Santa Luzia meteorite section, USNM 772 (location of electron microprobe traverses indicated; traverse 13 is of a taenite border schreibersite, indicated in more detail in the inset). NUMBER 21 27

0.06

3.8

400 MICRONS

50-

45-

40- 1

36 Z iJ 35- - E

L r 30- s> u S

25-

20- -

15- — -

10- PxlO

0.05 6.0 5' 4.6 - N 0.0 772 *I2 772 * 13

FIGURE 11. — Ni and P profiles at kamacite-schreibersite interfaces in Santa Luzia meteorite, USNM 772: traverses 10 through 13 as indicated. (Dashed line indicates bulk Ni value.) 28 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCEES1,

3334 FIGURE 12. —Sketch of Lexington County section, USNM 3334, prepared from photomosaic of area studied (arrows, electron microprobe traverses; large structure at left, massive schreibersite surrounded by cohenite; subgrain boundaries, grain boundary schreibersites, taenite, and taenite border schreibersite are indicated). content. Polished and etched surfaces reveal large 3334, traverse 2: crossed 1.0 mm of kamacite, 50 fim of a large areas of well-developed Widmanstatten pattern. grain boundary schreibersite, and 1.5 mm of kamacite. • 3334, traverse 3: crossed 0.8 mm of kamacite, a 15 fim wide The kamacite of these areas contains rhabdites of taenite border schreibersite, 25 ^im of taenite, 0.3 mm of various sizes, grain boundary schreibersites, grain kamacite, a 15 fim wide schreibersite in sequence with boundary and residual taenite and plessite, and taenite, 10 fim of kamacite, a 40 fim taenite, and 0.4 mm of Neumann bands. Oxidation has severely pene- kamacite. The data for an abbreviated section of this traverse trated the exterior surface of the specimen and are given in Figure 3. 3334, traverse 4: crossed 0.1 mm of kamacite, a 10 fim wide invaded the interior of the meteorite along major taenite border schreibersite, 25 fim of taenite, and 50 fim of grain boundaries. Occasional skeletal schreiber- kamacite. sites in the millimeter size range are present, 3334, traverse 5: crossed 50 fim of kamacite, a 5 fim wide normally completely surrounded by cohenite. taenite-border schreibersite, 20 /xm of taenite, and 60 fim of These large schreibersites tend to be surrounded kamacite. by areas of swathing kamacite that interrupt the Figure 12 is a tracing of a photomosaic of the regularity of the Widmanstatten pattern. Isolated studied area in Lexington County, section 3334. patches of cohenite are also present. The path of the traverses, the phases crossed, and Section 3334 contains a typical Lexington County the nature of the structure are indicated diagram- Widmanstatten pattern area and a large schreiber- matically. This section illustrates a general prob- site surrounded by cohenite (6 x 0.3 mm). Edges lem encountered in obtaining kamacite-schreiber- of the section and major grain boundaries have site interface data in the more carbon-rich coarse- been invaded by oxidation, but the areas selected structured octahedrites. The large, low-Ni schrei- for microprobe analysis appear fresh. The paths bersites in these meteorites are frequently com- of the traverses listed below are indicated in Figure pletely surrounded by cohenite. Their Ni values 12 and numerical data are given in Table 6. may be determined, but kamacite interfaces are 3334, traverse 1: crossed 1.4 mm of kamacite, a 30 fim schreiber- not present. Traverse 1, however, illustrates an site partially embedded in cohenite at a cohenite-kamacite interesting schreibersite-cohenite relationship that interface, 60 fim of cohenite, 90 fim of schreibersite embed- will be observed later in other meteorites. Cohenite ded in cohenite, 0.3 mm of cohenite, 0.2 mm of massive borders around large schreibersites occasionally schreibersite, 0.1 mm of cohenite, 0.2 mm of kamacite, 50 fim of cohenite, 0.2 mm of massive schreibersite, 80 fim of include small schreibersites, and more frequently cohenite, and 0.8 mm of kamacite. have small schreibersites at cohenite-kamacite in- NUMBER 21 29

TABLE 6. —Lexington County schreibersite-kamacite interface measurements

Weight Percent Ni Gradient P Gradient Atomic Percent Specimen Traverse Length Length Ni XN1 % Ni %N1 XP No. No. Structure traversed & schreibersites measured * Sch a (ym) Wt.XNi (ym) Wt.%P * Sch a a 3334 1 [a-Ph-cohen1te-Ph-coheni te-Ph-coheni te-a- cohen1te-Ph-cohen1te-a, 3.5 mm] Schreibersite embedded in cohenite at a- Interface, 25x120 ym 34.8 4.4 0.04 150 6.0 200 0.07 29.9 4.2 0.07 Schreibersite embedded in cohenite, 140x280 ym

Enter 33.0 28.3 Exit 32.5 27.9 Massive schreibersite surrounded by cohenite (6x0.3 mm), 200 vm traverse

Enter 25.0 21.4 Exit 24.6 21.1 Massive schreibersite surrounded by cohenite (6x0.3 mm), 170 ym traverse Enter 24.1 20.6 Exit 24.1 20.6

3334 2 [a-Ph-a, 2.5 mm] Grain boundary schreibersite, 500x40 ym 39.3 5.0 0.04 300 . 7.0 200 0.06 33.9 4.8 0.07

3334 3 [a-Ph-Y-a-Ph-a-Y-a, 1.6 mm] Taenite border schreibersite, 15x40 ym 50.0 6.0 0 .03 50 7.2 150 0.06 43.3 5.7 0.05 Schreibersite near taenite, 10x20 ym 52.5 6.2 0 .03 50 7.4 100 0.05 45.6 5.9 0.05

3334 4 [a-Ph-Y-a, 200 ym] Taenite border schreibersite, 5x10 ym 50.5 6.2 0.03 50 7.3 50 0.05 43.6 5.9 0.05

3334 5 [a-Ph-Y-a, 140 ym]

Taenite border schreibersite, 5x10 ym 51.0 6.3 0.03 30 7.4 30 0.05 44.2 6.0 0.05 terfaces. The Ni values in these schreibersites of the carbide haxonite was observed within a increase away from the large schreibersite. In this pearlitic plessite area. case, the large schreibersite contains approxi- Section 1807(Sil) contains part of a complex mately 25% Ni, the small one completely embed- silicate-containing inclusion surrounded by schrei- ded in cohenite has an apparent Ni gradient from bersite and cohenite, and small areas of kamacite. 32.5% to 33.0% increasing in the direction toward 1807(Sil), traverse 1: started within the complex inclusion, cross- kamacite, and the kamacite interface schreibersite ing first an island of kamacite surrounded by cohenite. The contains 35'% Ni. The measurements in Table 6 phases in sequence were: 0.1 mm of cohenite, 0.2 mm of are otherwise comparable to those reported above kamacite, 0.1 mm of cohenite, 1.5 mm of schreibersite, 0.5 for similar structures. mm of cohenite, 0.1 mm of schreibersite containing a slight Ni gradient, and finally 1.0 mm of kamacite. The central part of this traverse is illustrated in Figure 13 and the data is BAHJOI given in Table 7. Section 1807 is of a typical Widmanstatten area The Bahjoi, India, meteorite is an observed fall of Bahjoi, free of large inclusions. Rhabdites, of 1934, the only one included in this study. It is a grain boundary schreibersites, and taenite border typical Group I meteorite of above average Ni schreibersites are present in abundance. Many content. Bahjoi contains complex silicate-troilite- graphite-chromite nodules surrounded by rims of taenite-plessite areas are present in a variety of schreibersite, and, in turn, cohenite. Grain bound- morphologies. One of these martensitic areas con- ary schreibersites, rhabdites of various sizes, and tains a 0.1 x 0.1 mm area of haxonite. taenite border schreibersites are present. Taenite 1807, traverse 2: crossed 30 /xm of grain boundary schreibersite, and plessite are present in greater abundance than 0.7 mm of kamacite, a 15 fim wide taenite border schreiber- in the previously examined meteorites, and pearl- site, 20 /xm of taenite, and 10 fim of kamacite. itic and martensitic forms are common. An area The observations made on the Bahjoi meteorite 30 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

TABLE 7. —Bahjoi schreibersite-kamacite interface measurements

Weight Percent Ni Gradient P Gradient Atomic Percent Specimen Traverse Length Length N1 %M %P %Ni,. . XN1 %P No. No. Structure traversed & schreibersites measured * Sch ( m) Wt.XNI ( m) Wt.SP Sch a a a 1807(Sil) 1 [Cohenite-a-cohenite-Ph-cohenite-Ph-a, 3.5 mm] Massive schreibersite, 10x4 mm, surrounded by cohenite and containing -30% silicate Enter 18.8 16.0 Exit 18.9 16.1 Schreibersite 100 ym wide embedded in cohenite at a-border Enter 32.3 27.7 Exit 33.5 4.2 0.04 600 6.5 200 0.06 28.8

1807 2 [Ph-a-Ph-a, 750 um] Grain-boundary schreibersite, 35x250 um 38.8 5.2 0.04 150 7.2 150 0.06 33.4 5.0 0.07 Taenite-border schreibersite, 20x60 wm 49.0 6.2 0.03 30 6.2 100 0.06 42.4 5.9 0.05 are mainly similar in character to examples previ- structures for distances up to 2 mm. Beyond this, ously described. Figure 13 is a composition profile normal Widmanstatten areas are present. based on data from a part of traverse 1. Part of 1332, traverse 1: began at the border of a taenite area, crossed the massive schreibersite and its cohenite border 2.4 mm of kamacite, and entered a 30 /xm wide grain are included. Ni values are low in cohenite and boundary schreibersite near its contact with a taenite border increase slightly upon moving away from the large of a plessite area. Numerical data for this and succeeding schreibersite. The small exterior schreibersite is Goose Lake traverses are given in Table 8. enclosed on three sides by cohenite and contains a 1332, traverse 2: crossed 430 /xm of kamacite, 50 fim of schreibersite embedded in a cohenite rim of a large schreibersite, measurable Ni gradient. This type of association 90 fim of cohenite, 0.2 mm of schreibersite, 50 pm of is common in high C Group I meteorites. cohenite, and 0.9 mm of kamacite. 1332, traverse 3: began by crossing a 40 fim wide schreibersite associated with a taenite-plessite area, crossed 2.5 mm of GOOSE LAKE kamacite, entered a 40 /xm wide schreibersite embayed in cohenite, 150 fim of cohenite, and 0.3 mm of massive The Goose Lake, California, meteorite is a high schreibersite. Ni member of Group I with a medium octahedrite 1332, traverse 4: started near the initial point of traverse 3 structure. Polished and etched surfaces reveal within the taenite border, crossed 0.2 mm of kamacite, a 50 many large cohenite bordered, skeletal schreiber- fim wide grain boundary schreibersite, and 0.7 mm of site inclusions, and occasional complex silicate - kamacite. troilite-graphite-schreibersite-cohenite associa- Section 1332(2) is similar to number 1332 in that tions. The cohenite has undergone partial decom- it also contains a 5 X 10 mm skeletal schreibersite- position to kamacite and graphite. Grain boundary cohenite area. This section contains an unusually schreibersites, rhabdites in a range of sizes, and large scrfreibersite that is only partially surrounded taenite border schreibersites are present. Taenite- by cohenite, thus permitting kamacite-schreiber- plessite areas are more abundant than in the site interface measurements. Its longest dimension meteorites examined previously and are present is 1.4 mm and has a maximum thickness of 0.6 in a variety of forms. Weathering has invaded the mm. Morphology suggests that it is an unusually structure along grain boundaries and is present in large grain boundary schreibersite rather than a cracks within the large schreibersites. massive schreibersite of the skeletal variety. The Section 1332 is dominated by two areas of cohen- areas away from these inclusions contained normal ite bordered schreibersite of approximately 5 X 10 Widmanstatten pattern. mm each (photograph in Doan and Goldstein, 1332(2), traverse 5: crossed 0.1 mm of kamacite, 0.1 mm cohen- 1969:765). One smaller cohenite-schreibersite in- ite, 1.8 mm of massive schreibersite, 170 fxm cohenite, 2.1 clusion is also present. Kamacite areas around mm of kamacite, 20 /tin of cohenite, 0.1 mm of large grain these inclusions are comparatively free of other boundary schreibersite, and 0.8 mm kamacite. NUMBER 21 31

4.2 0.04

400 MICRONS FIGURE 13. —Ni and P profile for Bahjoi meteorite, USNM 1807: left, massive schreibersite bordered by cohenite; right, small schreibersite partially enclosed in cohenite, but also with an edge in contact with kamacite. 1332(2), traverse 6: crossed 0.4 mm of kamacite, 25 fim of other meteorites. The schreibersite precipitates in cohenite, 1.2 mm of schreibersite containing a small Ni this case, however, exist in an environment that is gradient over part of its distance, and 0.5 mm of kamacite. This traverse crosses the length of the large schreibersite in more Ni-rich. number 5. 1332(2), traverse 7: crossed 40 /*m tip of large schreibersite, 1.2 BALFOUR DOWNS mm of kamacite, 0.2 mm of second tip of same schreibersite, and 0.4 mm of kamacite. This is the same large schreibersite The Balfour Downs, Western Australia, mete- traversed in numbers 5 and 6. 1332(2), traverse 8: crossed 0.5 mm of kamacite, 150 (im of the orite is one of the highest Ni members of Group third tip of the large schreibersite measured in traverses 5 I. The first prepared surface of this specimen was through 7, and 0.5 mm of kamacite. perpendicular to the one shown in Figure 14, 1332(2), traverse 9: crossed 0.4 mm of kamacite, 150 /xm of a along its bottom edge. This area of approximately small schreibersite near the large schreibersite in traverses 5 12 cm2 was free of large inclusions, having the through 8, and 0.3 mm of schreibersite. 1332(2), traverse 10: crossed 0.2 mm of kamacite, 30^im of a regular structure of the upper right-hand area in grain boundary schreibersite, and 0.2 mm of kamacite. Figure 14. On the basis of this casual observation, 1332(2), traverse 11: crossed 0.3 mm of massive schreibersite Balfour Downs was assumed to be a low P medium close to traverse number 5, 0.1 mm of cohenite, a 20 ^im octahedrite. Actually, it contains localized massive wide schreibersite embedded in cohenite, 0.1 mm of kama- schreibersite enclosing significant quantities of sil- cite, and 50 fim of cohenite. icates. These schreibersites are normally enclosed The associations measured in Goose Lake are in cohenite and surrounded by swathing kamacite similar in character to those described above for areas that interrupt the regular Widmanstatten 32 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

TABLE 8. —Goose Lake schreibersite-kamacite interface measurements

Weight Percent N1 Gradient P Gradient Atomic Percent Specimen Length Length Traverse %P %M %P No. Structure traversed & schreibersftes measured *N1Sch a a (ym) Wt.iSP N1 a No. (ym) Wt.*N1 * Sch a 1332 1 [y-a-Ph-y, 2.4 mm] Schreibersite at y-border, 25 ym wide 52.0 6.0 0.03 25 7.2 100 0.07 45.1 5.7 0.05

1332 2 [a-Ph-cohenite-Ph-cohenite-a, 1.6 mm] Schreibersite embaying cohenite, 55 ym traverse Enter 35.0 4.1 0.05 100 6.4 50 0.06 30.1 3.9 0.09 Exit 34.0 29.2 Massive schreibersite enclosed in cohenite (3.4x0.2 mm), 260 ym traverse 25.0 21.4

1332 3 [Ph-a-Ph-cohenite-Ph, 3.1 mm] Schreibersite near taenite, 40x70 ym 46.5 6.0 0.04 50 7.0 100 0.07 40.2 5.7 0.07 Schreibersite embaying cohenite, 30x50 ym 36.5 4.3 0.05 150 5.7 200 0.08 31.4 4.1 0.09 Massive schreibersite (6x2 mm), 300 ym traverse 18.5 15.8

1332 4 [y-a-Ph-a, 950 um] Grain boundary schreibersite (60x250 ym), 60 ym traverse 41.0 5.1 0.05 100 7.2 Flat 0.05 35.4 4.9 0.09

1332(2) 5 [a-cohen1te-Ph-cohen i te-a-cohen1te-Ph-a, 5.3 mm] Cohenite bordered massive schreibersite (2x6 mm), 1.8 mm traverse Enter 18.7 15.9 Middle 18.2 15.5 Exit 18.5 15.8

Large schreibersite with partial cohenite border (1.4x0.6 mm), 100 um traverse 31.3 3.8 0.06 200 5.7 100 0.08 26.9 3.6 o.n

1332(2) 6 [a-cohenite-Ph-a, 2.2 mm] Large schreibersite with partial cohenite border (1.4x0.6 mm), 1.2 mm traverse Enter 29.3 25.1 Middle 27.3 23.4 Exit 27.3 3.6 0.07 500 5.8 250 0.09 23.4 3.4 0.13

1332(2) 7 [Ph-a-Ph-a, 1.8 mm] Large schreibersite of traverses 5 and 6 40 um traverse 31.0 3.9 0.06 400 6.0 200 0.08 26.6 3.7 0.11 200 ym traverse Enter 27.0 3.3 0.06 600 6.2 150 0.08 23.1 3.1 o.n Exit 27.0 3.5 0.06 300 6.8 150 0.09 23.1 3.3 0.11

1332(2) 8 [a-Ph-a, 1.2 mm] Large schreibersite of traverses 5-7, 150 ym traverse Enter 29.1 3.7 0.06 500 6.5 150 0.09 25.0 3.5 o.n Exit 29.1 3.8 0.06 400 6.5 150 0.08 25.0 3.6 o.n

1332(2) 9 [a-Ph-a, 0.9 mm] Small schreibersite near traverses 5-8 (250x400 ym), 150 ym Enter 27.2 3.1 0.06 300 6.0 100 0.07 23.3 3.0 0.13 Exit 27.2 3.6 0.06 250 5.5 100 0.07 23.3 3.4 0.13

1332(2) 10 [a-Ph-a, 0.5 mm] Grain boundary schreibersite (0.03x1.6 mm) associated with plessite, 30 ym traverse 44.0 5.8 0.05 100 7.4 200 0.06 38.0 5.5 0.09

1332(2) 11 [Ph-cohenite-Ph-a-cohenite, 0.7 mm] Massive schreibersite (same as traverse 5), 0.4 mm traverse Start 18.3 15.6 Exit 18.3 15.6 Schreibersite (20x60 ym) embedded in cohenite-a border 37.3 4.5 0.06 150 6.0 100 0.09 32.1 4.3 0.11 NUMBER 21 33

FIGURE 14. —Etched surface of Balfour Downs meteorite, USNM 3202: left-hand side of the slice, large schreibersites, several containing silicates; (outlined area indicates polished section that was prepared for detailed study). 34 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

TABLE 9. —Balfour Downs schreibersite-kamacite interface measurements

Weight Percent Ni Gradient P Gradient Atomic Percent Specimen Traverse Length Length *N1 %P %P No. No. Structure traversed & schreibersites measured *N1Sch a a (ym) Wt.%N1 (ym) Wt.fcP *N1Sch a a

3202 1 [Ph-a-Ph-a-Ph-Y, 3.3mm] Massive schreibersite, 1550 ym traverse Start 20.5 17.5 Exit 21.0 2.9 0.05 1150 6.7 200 0.09 17.9 2.8 0.09 Residual schreibersite 30x800 ym, associated with taenite, crossed at two locations 41.0 5.3 0.04 100 0 100 0.09 35.4 5.1 0.07 41.0 5.5 0.03 50 35.4 5.2 0.05

3202 2 [Ph-a-Ph-a, 3.1 mm] Massive schreibersite, 600 ym traverse Start 21.3 18.2 Exit 21.5 2.6 0.05 300 4.8 300 0.09 18.4 2.5 0.09 Massive schreibersite, 500 ym traverse Enter 21.5 2.8 0.05 250 4.8 200 0.09 18.4 2.7 0.09 Exit 21.5 3.0 0.05 1500 7.0 300 0.11 18.4 2.9 0.09

3202 3 [o-Ph-o-cohen1te-Ph, 2.3 n Massive schreibersite, 400 ym traverse Enter 21.0 2.9 0.05 250 5.3 200 0.10 17.9 2.7 0.09 Exit 21.0 2.8 0.05 150 4.8 150 0.08 17.9 2.7 0.09 Massive schreibersite, 1350 ym traverse Enter 20.0 17.1 End 20.0 17.1

3202 4 [Ph-cohenite-a-cohenite-PI ,2.1 mm] Massive schreibersite. 800 ym traverse Start 19.5 16.7 Exit 19.5 16.7 Massive schreibersite, 250 ym traverse Enter 20.5 17.5 Stop 21.0 17.9

pattern (center left in Figure 14). Grain boundary mm of kamacite, 0.5 mm of massive schreibersite, and 1.5 schreibersite, schreibersite associated with taenite- mm of kamacite. 3202, traverse 3: crossed 0.3 mm of kamacite, 0.4 mm of massive plessite areas, and rhabdites in a variety of sizes schreibersite, 0.2 mm of kamacite, 80 jxm of cohenite, and are present. Taenite-plessite areas are present in a 1.3 mm of massive schreibersite. The path of this traverse is variety of forms and Neumann bands are com- indicated in Figures 15 and 16 and the electron microprobe mon. profile is shown in Figure 17. Section 3202 was prepared from the outlined 3202, traverse 4: crossed 0.8 mm of massive schreibersite, 0.1 mm of cohenite, 0.8 mm of kamacite, 0.1 of cohenite, and area in Figure 14. It was selected particularly for 0.2 mm of massive schreibersite. the presence of massive schreibersite that was partially free of cohenite borders. Figure 15 is a Also indicated in Figure 16 are Ni concentrations photograph of a photomosaic of a portion of this in various areas of the two massive schreibersites. section. The paths of the four traverses described One is tempted to conclude that a slight Ni gra- below are indicated in Figure 16, a sketch based dient exists, with the lowest Ni values being adja- on Figure 15. cent to cohenite. The differences, however, are small, and this should undoubtedly be looked at more systematically in the future. The Ni and P 3202, traverse 1: crossed 1.5 mm of massive schreibersite, 4.5 profile reproduced in Figure 17 illustrates several mm of kamacite, a 40 fim wide schreibersite surrounding a interesting features. The Ni concentration in kam- taenite area, 0.1 mm of kamacite, recrossed a 30 fim width acite is low close to these large schreibersites, as of the same schreibersite previously crossed, 50 /urn of kamacite, and 50 fim of taenite. Numerical data for this and are the Ni interface values. There also appears to succeeding Balfour Downs traverses are given in Table 9. be a Ni gradient in cohenite, with Ni increasing 3202, traverse 2: crossed 0.6 mm of massive schreibersite, 0.6 away from the massive schreibersite. NUMBER 21 35

FIGURE 15. —Photograph of photomosaic of area of Balfour Downs meteorite examined, USNM 3202: large schreibersite inclusions in kamacite, partially bordered by cohenite. (Line indicates position of traverse 3; see Figure 17.) 36 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

I mm

3202

FIGURE 16. —Sketch based on Balfour Downs section illustrated in Figure 15 (arrrows indicate location of traverses; numbers give schreibersite Ni concentrations in weight percent) NUMBER 21 37

0.05 2.9

600 MICRONS FIGURE 17. —Ni and P profiles in Balfour Downs meteorite, USNM 3202, traverse 3 in Figure 16 (schreibersite inclusion at the right enclosed in cohenite).

Discussion equilibrium is maintained down to 500° C for EQUILIBRIUM CONSIDERATIONS OF PHASE GROWTH diffusion distances as great as 1000 fim. Previous work, therefore, suggests that many of the schrei- The total amount of P present in an iron mete- bersite precipitates observed in iron meteorites orite has a marked effect on the structural devel- nucleated and grew under equilibrium conditions opment process, and consequently on the final during a significant part of their cooling history. structure observed in prepared sections. Studies It is the purpose of this section to examine schrei- of Widmanstatten pattern growth (Wood, 1964; bersite growth and related structural development Goldstein and Ogilvie, 1965b; Goldstein and Axon, in the selected group of coarse-structured iron 1973) have demonstrated that octahedrite meteor- meteorites in terms of the equilibrium Fe-Ni-P ites cooled from high temperatures and were in diagram of Doan and Goldstein (1970). How far equilibrium down to 700° C. Schreibersite growth can equilibrium considerations take us in explain- studies (Goldstein and Ogilvie, 1963; Doan and ing observed structures? Goldstein, 1969) led to similar conclusions concern- Before proceeding, however, brief mention ing equilibrium temperatures for iron meteorites must be made of the effects of S, C, and Co on in general. In a discussion of the effects of P on the Fe-Ni-P system. These elements are present in the development of the Widmanstatten pattern, iron meteorites in quantities that may be compa- Goldstein and Doan (1972) estimate that equil- rable to or in excess of P. The role of S has been brium in iron meteorites was maintained down to discussed recently by Goldstein and Axon (1973) 650° C, and probably to 600° C. Recent work by and Wai (1974). Goldstein and Axon (1973) point Randich (1975) suggests the schreibersite-kamacite out that S may be important in lowering the 38 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

temperature of metal melts, thereby mobilizing not available, and it has been assumed that it can metal without melting silicates. This effect, how- be ignored here except in cases where its physical ever, would only extend down to temperatures in presence in the form of cohenite or other carbides the range of 1000° C. At lower temperatures, is important. This assumption may well require material of Fe-FeS eutectic composition becomes reevaluation,when appropriate experimental data enclosed in solidifying metal, and the solubility of becomes available. S in the metallic phases becomes negligibly small. Another simplifying assumption made in this Sulfur is removed from the metal by this process, study is to ignore the presence of Co, an element becoming isolated in complex sulfide nodules (El present in iron meteorites in the range of 0.3% to Goresy, 1965). 0.7% (Moore et al., 1969). Co distributes itself The situation with respect to carbon is much between the three phases of interest, following Fe more complex. Graphite or graphite-troilite nod- and seemingly having little effect on the equilibria ules present in many meteorites represent high involved. temperature segregations of C as well as S. The In the following sections, individual meteorites petrography of these complex associations has will be discussed in terms of structural develop- been described by El Goresy (1965). Comparatively ment on equilibrium cooling through the low tem- large amounts of C, however, remain soluble in perature portion (995° to 550° C) of the Fe-Ni-P the metal to lower temperatures, temperatures system. The Doan and Goldstein (1970) isothermal within the schreibersite growth range. Brett (1967) sections were recalculated on an atom percent has discussed the occurrence of cohenite ((Fe,Ni)3C) basis, and four of these sections plotted on a in meteorites and described its formation in terms triangular coordinate system are given in Figure of phase equilibria using an extrapolated Fe-Ni-C 18. diagram. He proposed that cohenite formed Equilibrium in the system at hand may be de- within a narrow range of bulk Ni values in the fined as (1) the phases present are homogeneous, temperature range of 650° to 610° C. Scott (1971a) that is to say, free of measurable concentration has discussed carbides in iron meteorites in great gradients, and (2) the compositions of individual detail from a petrographic point of view. He has phases are related to the bulk composition by the also reported two new carbides, haxonite phase diagram. In the case at hand, this means ((Fe,Ni,Co)23C6) (Scott, 1971b), and a phase desig- that (a) the bulk composition is the composition of nated W-carbide ((Fe,Ni,Co)2.5C), both of which the single phase present when it lies within the contain considerably more Ni and Co in their kamacite (a) or taenite (y) field of the diagram; structure than is observed in cohenite. Scott (b) the bulk composition is on a tie line joining the (1971a) suggests that cohenite may have formed at compositions of the two phases present in the somewhat lower temperatures than those sug- kamacite + schreibersite (a + Ph), the kamacite + gested by Brett (1967). Both Scott (1971a) and taenite (a + y), and the taenite + schreibersite (y Buchwald (1976) point out that cohenite is much + Ph) fields of the diagram; and (c) the bulk more widely distributed in meteorites than previ- composition is related to the three phases present ously realized. Cohenite and schreibersite are fre- by the compositions at the corners of the triangular quently observed in intimate association (Drake, kamacite + taenite + schreibersite (a + y + Ph) 1970; Clarke, 1969; Buchwald, 1976), with cohenite field of the diagram. The proportions of the normally encapsulating large schreibersite inclu- phases present in the two- and three-phase fields sions in the high C Group I meteorites. These may be calculated using lever rule principles. Cal- schreibersite-bordering cohenites normally enclose culations of this type were carried out for each of occasional small later-formed schreibersites as has the meteorite compositions considered and are been illustrated above (Figure 13). This morphol- presented in a series of tables to follow (Tables ogy suggests the possibility of concurrent schrei- 10-15). Temperatures, the proportion of phases bersite-cohenite growth after major amounts of present, and their Ni contents are given. These schreibersite have formed. Carbon, therefore, is data will be discussed for each meteorite in terms seen to be a component of the system of direct of observed structures. Selected compositions rep- concern here. Experimental data on carbon's ef- resenting these meteorites are plotted on a series fect on the Fe-Ni-P system at low temperatures is of isothermal sections of the iron-rich corner of NUMBER 21 39

OS? 40 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

TABLE 10. —Equilibrium cooling of composi- nite transforms almost completely to massive single tions appropriate to the Coahuila meteorite crystal kamacite. The last of the taenite transforms (percentage of phases present at indicated to kamacite below 600° C, in the temperature temperatures with their Ni contents) range where schreibersite first nucleates. This se- 0.5 Atomic i P, 5.3 Atomic t N1 quence accounts for kamacite containing homoge-

°c %a *Y SPh %N 2Ni neously nucleated rhabdite, the most prominent a Y Ph feature of Caohuila. This low temperature of 995 100 5.3 975 100 5 .3 initial schreibersite nucleation and the small 925 100 5.3 875 100 5 .3 amount formed, however, make it difficult to 850 100 5 .3 750 60 40 3.3 6 .5 understand the presence of the occasional larger 700 75 25 4.0 9.2 650 93 7 4.4 12 schreibersites mentioned above. 600 99 1 5.2 14 550 99.7 0.3 5.2 14 If a somewhat higher P content is assumed, significant differences are revealed. With an assumed content of 1.1 atomic % P (Table 10),

1.1 Atomic 55 P, 5.3 Atomic 3. N1 transformation of taenite to kamacite starts more than 50° C higher, extends over a slightly larger °C %a %y SPh %H Ni a Y * Ph temperature range, and is complete at a higher 995 100 5.3 975 100 5 .3 temperature than in the previous case. Schreiber- 925 100 5 .3 875 100 5.3 site precipitates, while more than 5% taenite re- 850 12 88 3.5 5.6 750 50 50 3.8 6 .9 mains in the structure, and a total of 2% schreiber- 700 82 18 4.5 9 .1 site is present at 600° C. In this case, one would 650 94 5 1 5.0 11 9.0 600 98 2 5.2 13 expect schreibersite to nucleate at taenite-kamacite 550 98 2 5.2 13 boundaries, but no direct evidence for this has been developed. The actual equilibrium P value the Fe-Ni-P system from 850° to 550° C in Figure may well lie between the two selected values, 19. perhaps 0.8 atomic %. This amount of P would COAHUILA. —The Coahuila hexahedrite is a me- produce upon cooling a somewhat smaller quantity teorite of both low Ni (5.6 weight %, 5.3 atomic of schreibersite, initially nucleating at a tempera- %) and low P (0.3 weight %, 0.5 atomic %). These ture below 650° C, and consuming any remaining bulk composition values are well established in the residual taenite. Both sufficient P and adequate literature and are generally consistent with the time at higher temperatures would be available to major features of the observed structure. The permit growth of the isolated larger schreibersites presence of occasional large schreibersites as de- observed, and the bulk of the meteorite would scribed above suggests, however, that the P value appear to have formed under conditions of lower may be*somewhat low. For this reason the calcula- total P. tions given in Table 10 were made using two P The 5.3 atomic % Ni and 0.8 atomic % P com- values, the literature value (0.5 atomic %) and one positions are plotted on a series of isothermal approximately twice as high (1.1 atomic %). The sections in Figure 19. At 850° C, Coahuila's com- equilibrium phases present, their amounts in mole position is just inside the kamacite-taenite field of percent, and their Ni contents in atomic percent the diagram, yielding a structure containing a are listed in Table 10 for 10 temperatures between small amount of kamacite within a taenite matrix. 995° to 550° C. The field moves in relation to this composition Due to Caohuila's low P (0.5 atomic %), the with decreasing temperature, until at 650° C only predicted structural development sequence using a small amount of taenite remains in equilibrium the Fe-Ni-P diagram is essentially the same as that with a kamacite matrix. At 600° C, the composition derived using the Fe-Ni diagram. At 995° C (Table is within the kamacite-schreibersite field, perhaps 10), taenite of the bulk meteorite composition is having passed through the corner of the kamacite- the only phase present, and this situation contin- taenite-schreibersite field. By 550° C, the composi- ues until the meteorite cools to below 850° C. As tion lies well within the kamacite-schreibersite field the temperature drops an additional 200° C, tae- of the diagram, with approximately 98% kamacite NUMBER 21 41

850°C %P ABELLSBANK 5.0 3.6 • COAHUILA 5.3 0.8 • SANTA LUZIA 6.2 2.7 S-BALLINGER 6.2 1.4 • LEXINGTON COUNTY - BAHJOI 7.1 1.8 0 AGOOSE LAKE -BALFOUR DOWNS 7.8 2.7

0 20 100% Fe 10 15 ATOMIC %Ni FIGURE 19. — Meteorite bulk compositions plotted on series of isothermal sections of Fe-Ni-P system. 42 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES containing 5.1 atomic % Ni and 0.5 atomic % P, TABLE 11. —Equilibrium cooling of composi- being in equilibrium with 2% schreibersite contain- tions appropriate to the Ballinger meteorite (percentage of phases present at indicated ing 13 atomic % Ni and 25 atomic % P. temperatures with their Ni contents) The development sequence described above accounts for the major features of Coahuila struc- 0.7 Atomic % P, 6.2 Atomic * N1 "C %a *Y %Ph *N1 XN1 *N1 ture. Equilibrium cooling of this bulk composition a y Ph produced large single crystals of kamacite contain- 995 100 6.2 975 100 6.2 ing minor inclusions. The earliest-formed schrei- 925 100 6.2 875 100 6.2 bersite nucleated and grew on preexisting sulfide 850 100 6.2 750 15 85 3.7 6.6 inclusions where available, or perhaps at kamacite- 700 60 40 4.0 9.1 650 78 22 4.9 11 taenite interfaces. The great majority of the indi- 600 89 10 1 5.2 14 13 vidual schreibersite crystals, however, may be as- 550 99 1 6.0 15 sumed to have nucleated homogeneously within kamacite. If this process had stopped at 550° C, the meteorite would contain kamacite of 5.1 atomic 1.4 Atomic % P, 6.2 Atomic * N1

% Ni and 0.5 atomic % P, and schreibersite of 13 °C %a %y %Ph XN1 %N1 XN1 y Ph atomic % Ni. Phase composition measurements a 995 100 6.2 from the literature and those reported above indi- 975 100 6.2 925 100 6.2 cate that diffusion controlled growth continued to 875 5 95 3.9 6.3 850 4 95 1 4.0 6.3 4.4 lower temperatures, modifying phase composi- 750 30 68 2 4.0 7.2 5.3 700 62 36 2 4.5 9.1 7.1 tions significantly, while producing only subtle 650 77 20 3 5.0 11 9.0 600 88 8 4 5.2 14 13 changes in the gross structural features. These 550 96 4 5.7 15 changes in phase compositions and subtle differences in structure will be discussed in some detail later in this paper. tion of the structure. Small amounts of kamacite BALLINGER. —The Ballinger meteorite is a low form in taenite around 900° C, and by 850° C the Ni octahedrite probably containing more P than taenite matrix contains 4% kamacite and 1% the analytical value given in Table 1 (0.4 weight % schreibersite, a situation that might be expected to P, 0.7 atomic % P). The Ballinger photograph lead to the growth of several rather large schrei- (Figure 6) shows three areas containing large bersites on subsequent cooling rather than a num- schreibersite inclusions, perhaps suggesting that ber of small ones. By 750° C, one-third of the the amount of P should be doubled. The calcula- meteorite is kamacite, undoubtedly the swathing tions given in Table 11, therefore, were made kamacite surrounding the large schreibersites that using 6.2 atomic % Ni (6.5 weight %) and both 0.7 are present. As the temperature falls to 650° C, atomic % and 1.4 atomic % P. the meteorite transforms to 77% kamacite, and During equilibrium cooling in the low P case the schreibersite doubles its Ni content to 9.0 (Table 11), taenite is the only phase present until atomic %, while increasing its amount by one-half. kamacite appears around 800° C. At 600° C, the As the temperature drops to 600° C, kamacite equilibrium structure contains 89% kamacite, 10% increases in volume, schreibersite increases in both taenite, and only 1% schreibersite. Upon cooling amount and in Ni content, and taenite shrinks to to 500° C, all of the taenite would be expected to 8%. The higher P content seems to be more transform, producing an equilibrium structure of consistent with the observed structure and could 99% kamacite and 1% schreibersite. Both the pres- actually be a little low. The presence of taenite ence of the massive schreibersites and residual and high Ni schreibersite in the actual structure taenite in the actual structure argue against this (see above) demonstrates that equilibrium cooling interpretation. The growth of such large schrei- did not persist down to 550° C, and that nonequi- bersites requires more P, and the presence of librium phase growth continued to lower temper- taenite demonstrates that equilibrium cooling was atures. As in the case with Coahuila, however, the not maintained down to 550° C. gross features of the structure are accounted for The 1.4 atomic % P calculation for Ballinger by equilibrium cooling of this particular composi- (Table 11 ) leads to a more reasonable interpretation (Figure 19). NUMBER 21 43

SANTA LUZIA. —The Santa Luzia meteorite TABLE 12. —Equilibrium cooling of composi- structure is dominated by schreibersite and schrei- tions appropriate to the Santa Luzia meteorite bersite-troilite inclusions surrounded by broad (percentage of phases present at indicated temperatures with their Ni contents) areas of swathing kamacite (Figure 7). The literature values for its bulk composition, 6.3 atomic % 1.6 Atomic t p, 6.3 Atomic i Ni

Ni and 1.6 atomic % P (6.6 weight % Ni and 0.9 °c %a %y %Ph %Ni SN1 %Ni weight % P, Table 1), are very close to those a Y Ph 995 100 6.3 assumed for the high P Ballinger composition 975 100 6.3 925 10 90 4.5 6.4 (Table 11), only a 0.1% increase in Ni and 0.2% in 875 3 96 1 4.0 6.4 4.4 850 98 2 6.3 4.5 P. This apparently minor change, however, has 750 26 71 3 4.0 7.2 5.3 700 60 37 3 4.5 9.1 7.1 important ramifications for the structural devel- 650 76 21 3 5.0 11 9.0 600 87 8 5 5.2 14 13 opment process in Santa Luzia. In Ballinger the 550 95 5 5.8 15 expected sequence of phases with falling temperature,

y —> a + y —» a + y + Ph, 2 .7 Atomic 3i P, 6.2 Atomic 3i Ni °C %a *Y SPh *N1a XN1 *NiPh is followed. In Santa Luzia this sequence is inter- Y 995 43 57 5.3 6.8 rupted around 850° C, where over a range of 975 50 50 5.4 7.1 925 35 65 4.9 7.0 temperature taenite is in equilibrium with schrei- 875 4 90 6 4.0 6.4 4.4 850 93 7 6.3 4.5 bersite, the sequence of phases being 750 27 66 7 4.0 7.2 5.3 700 60 32 8 4.5 9.1 7.1 650 76 16 8 5.0 11 9.0 Ph Ph. 600 88 3 9 5.2 14 13 550 91 9 5.4 14 The growth of significant amounts of schreibersite within taenite subsequent to the disappearance of formed at the relatively high temperature of 850° preexisting kamacite produced conditions that re- C. Upon further cooling Ni and P continued to sulted in the development of large volumes of diffuse into the central schreibersite areas, P con- swathing kamacite and the unusual schreibersite tributing to additional growth and Ni contributing morphology observed in the meteorite. to both growth and Ni enrichment. As a conse- The actual bulk P value for Santa Luzia may be quence of this localized growth of schreibersite, a somewhat larger than the 1.6 atomic % value given large surrounding volume of low Ni metal was in Table 1. The calculations summarized in Table produced, the swathing kamacite zone of the final 12 indicate, however, that the same sequence of structure. This proposed growth sequence ex- phases would be expected whether 1.6 atomic % plains the morphology of these large sc(hreibersite or 2.7 atomic % P is assumed. In either case, structures without the necessity of invoking precip- kamacite and taenite would have been in equilib- itation from the liquid, a process that would re- rium at 925° C. It is reasonable to assume that this quire a bulk composition of more than 4.9 atomic kamacite nucleated within taenite at the preexist- % P at high temperatures. ing troilite-taenite grain boundaries, with kamacite The broad swathing zone developed as the me- lamellae radiating away from the troilite. Upon teorite cooled from 850° C down into the 700° to cooling to 875° C, the proportion of kamacite was 650° C range (Figure 7). This transformation takes markedly reduced at both P levels. This small place easily at these temperatures as only a rela- amount of kamacite presumably remained local- tively small amount of Ni movement is required ized near the troilite nodules. The taenite inter- for both schreibersite growth and enrichment, faces with troilite and the kamacite-taenite inter- and for production of the taenite border at the faces acted as nucleation sites for schreibersite outer edge of the zone. This expanding taenite upon further cooling, producing a skeletal struc- border acts as a barrier to isolate the large central ture that sets the pattern for subsequent schreiber- schreibersites from the bulk material beyond the site growth. Nearly half in one case, and more swathing zone. As the temperature drops and than half in the other, of the schreibersite that diffusion becomes increasingly restricted, condi- formed upon cooling the meteorite to 500° C tions are met for the nucleation of schreibersite at 44 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

this taenite border. By 550° C the taenite border TABLE 13. —Equilibrium cooling of composi- has been transformed and replaced with a contin- tions appropriate to the Lexington County and Bahjoi meteorites (percentages of phases uous grain boundary, approximately half of which present at indicated temperatures with their is occupied by grain boundary schreibersite. This Ni contents) is amply supported by examination of Santa Luzia 0. specimens. The swathing zone boundary contains 5 Atomic t P, 7.1 Atomic S N1 °c %a %y %Ph %M %M SUM a large amount of schreibersite and is, with a few a y Ph minor exceptions, free of residual taenite. The 995 100 7.1 975 100 7.1 swathing zone itself is also free of large schreiber- 925 100 7.1 875 100 7.1 sites and residual taenite, with the exception of 850 100 7.1 750 100 7.1 several small areas very close to the swathing zone 700 40 60 4.1 9.2 650 70 30 4.9 12 border. These appear to have been small areas of 600 79 20 0.3 5.2 14 13 residual taenite that became trapped within the 550 99 1 6.9 16 swathing zone at a very late stage and may have been part of the taenite border system at higher temperatures. Competition for P from neighbor- 1.8 Atomic 3i p, 7.1 Atomic i Ni ing schreibersites appears to be the reason that °C %a %y %N1 %N1 %MPh a y these areas were not converted completely to 995 100 7.1 975 5 95 5.5 7.2 schreibersite. 925 10 90 0.3 5.3 7.3 5.2 875 98 2 7.1 5.0 Schreibersite equilibration with taenite in the 850 97 3 7.2 5.3 850° C temperature range results in a redistribu- 750 Tr 96 4 (4.2) 7.2 5.3 700 42 54 4 4.5 9.1 7.1 tion of P in the meteorite leading to the develop- 650 60 35 5 5.0 11 9.0 600 78 16 5 5.2 14 13 ment of areas of coarse Widmanstatten pattern 550 94 6 6.5 15 (Figure 7). At these high temperatures P diffuses with ease, lowering its level even in the distant bulk metal to approximately 1.0 atomic %. Subse- quent to this, the swathing zone development bulk Ni. The P values of Lexington County (0.5 described above took place, resulting in the isola- atomic % P, 0.3 weight % P) are too low to be tion of that part of the meteorite from the areas reconciled with the amount of schreibersite ob- where the coarse Widmanstatten pattern formed. served in prepared sections. The Lexington The areas beyond the swathing zones developed County bulk Ni value is also probably somewhat an effective bulk composition similar to that of the higher than 7.0 weight % Ni when the contribution low P Ballinger case described above. Widmanstat- of large schreibersites is taken into consideration. ten pattern nucleated in these areas at tempera- Much of the kamacite in this meteorite contains tures below 750° C, and grain boundary schreiber- 7.0 weight % Ni, and some of it contains slightly site probably began nucleating at kamacite-taenite more Ni. Bahjoi also contains isolated large schrei- interfaces around 600° C. The observed metallog- bersites, requiring its effective bulk P to be greater raphy of these areas is consistent with this expla- than 0.3 weight % P. For these reasons, the equilib- nation. rium phase calculations in Table 13 were made at The bulk composition of Santa Luzia is plotted the intermediate Ni value of 7.1 atomic % (7.5 on the diagrams in Figure 19. As in the previous weight % Ni), and at 0.5 atomic % P and 1.8 cases, it is obvious from the observations reported atomic % P (1.0 weight % P). These calculations in the results section that bulk equilibrium was not suggest that the actual P value probably lies be- maintained down to 550° C, and that phase modi- tween the two, probably greater than 1 atomic % P. fication persisted to considerably lower tempera- Structure development in the cooling meteorite tures. does not start until below 750° C in the low P case LEXINGTON COUNTY AND BAHJOI. —The bulk (Table 13), taenite being the only phase present at compositions of the Lexington County and Bahjoi higher temperatures. Schreibersite precipitation meteorites appear to be poorly established (Table does not begin until below 650° C, a temperature 1). They are structurally similar, carbon- and sul- at which more than 70% of the structure has fur-rich members of Group I with intermediate transformed to kamacite. At 600° C, only 0.3% NUMBER 21 45

schreibersite is present. Under these conditions P for continued growth. Ni gradients within the schreibersite would have precipitated at kamacite- cohenite borders (Figure 13) suggest that Ni may taenite interfaces throughout a well-established continue to be supplied to the schreibersite Widmanstatten pattern. P could not move through through the cohenite. Under these circumstances, the Widmanstatten pattern to precipitate as schrei- a distinct swathing zone morphology is not devel- bersite borders at isolated sulfide interfaces. oped, but the surrounding kamacite does retain a Schreibersite in these meteorites must have precip- low Ni level and is comparatively free of later- itated initially prior to extensive Widmanstatten stage schreibersite precipitation. pattern development, requiring a greater bulk P Within the bulk metal surrounding these even- content. tual cohenite-schreibersite inclusions, initial Wid- The 1.8 atomic % P value used in Table 13 is manstatten pattern development started above somewhat higher than necessary to account for 700° C, following its normal sequence. Bulk Ni is the observed structure. With this amount of P in low enough that a coarse-structured octahedrite the system, kamacite would precipitate by 975° C pattern formed. Both of these meteorites have and schreibersite would be present by 925° C. If schreibersite morphologies and distributions simi- the P content were reduced to 1.4 atomic %, lar to a meteorite like Ballinger, but they contain neither kamacite nor schreibersite would be pres- more grain boundary taenite and residual taenite- ent in the system at 925° C or above. At 875° C, plessite areas, particularly Bahjoi. however, schreibersite would be present growing GOOSE LAKE AND BALFOUR DOWNS. —The Goose within taenite. Kamacite would not nucleate until Lake and Balfour Downs meteorites are two of below 750° C. The important consideration here is the highest Ni members of Group I (Table 1), and that within a reasonable range of P contents, they are meteorites that contain significant schreibersite nucleates within taenite, undoubtedly amounts of C and S. The analytical P values from at preexisting taenite interfaces with troilite or the literature again appear to be too low to explain troilite-silicate-graphite inclusions if they are avail- the observed metallography. A planometric esti- able. Major quantities of schreibersite grow under mate of 0.9 weight % P (1.6 atomic % P) for Goose these conditions prior to kamacite nucleation and Lake was given by Doan and Goldstein (1969), and the onset of Widmanstatten pattern development. this figure appears to be consistent with the ob- The 1.8 atomic % P value is used in Figure 19, served structure. Phase calculations are given in and it can be seen there that reasonable reduction Table 14 using a pair of compositions to represent in the amount of P changes only the amounts of the two meteorites, 7.9 atomic % Ni (8.3 weight % the phases present. Nucleation temperatures for Ni), and 1.4 and 2.7 atomic % P (0.75 and 1.5 both schreibersite and kamacite would be lowered weight % P), bracketing what was undoubtedly somewhat, and smaller amounts of P would pro- their effective bulk compositions. duce smaller quantities of schreibersite. Neither When 1.4 atomic % P is assumed (Table 14), of these meteorites would appear to contain as schreibersite nucleates within taenite at a temper- much as 6% schreibersite, but certainly they both ature above 850° C. If 2.7 atomic % P is assumed, contain more than 1 %. schreibersite nucleates above 975° C in the pres- The structural development sequence in Lexing- ence of 20% kamacite. No structural evidence for ton County and Bahjoi is similar to that of Santa initial precipitation at kamacite-taenite interfaces Luzia. With falling temperature a few massive has been observed, supporting the suggestion that schreibersites grow within taenite, followed by the actual P level was in the neighborhood of 1.6 nucleation and growth of a swathing kamacite atomic % P. This means that schreibersite nu- zone. The Ni values in these massive schreibersites cleated around 900° C, undoubtedly at taenite- (16 atomic % Ni for Bahjoi, 21 atomic % Ni for troilite boundaries, were they available. The result Lexington County) require that Ni enrichment was growth of a large amount of schreibersite in continued to below 550° C. At some temperature taenite, with nucleation of a kamacite swathing below 650° C, however, massive schreibersite zone following at a temperature somewhat above growth is stopped by precipitation of cohenite. 700° C. As the temperature fell, a large supply of Cohenite contains no detectable P and, therefore, Ni was required to keep these massive schreiber- isolates the massive schreibersite from a source of sites in equilibrium, a process that seems to have 46 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

TABLE 14. —Equilibrium cooling of composi- ate P level proved to be the most reasonable in tions appropriate to the Goose Lake and Bal- terms of explaining the observed structure and four Downs meteorites (percentage of phases present at indicated temperatures with their will be considered first. Ni contents) For the composition 5.0 atomic % Ni and 3.6 atomic % P (Table 15), kamacite is the only equilib- 1.4 Atomic % P, 7.9 Atomic % N1 rium phase present from 1100° C to below 975° C. %a XPh XN1 XN1 °c %y *NiPh a Y At a temperature somewhat below 975° C taenite 995 100 7.9 nucleates, and a small amount of it is in equilib- 975 100 7.9 925 100 7.9 rium with kamacite at 925° C. By 875° C, taenite 875 100 7.9 850 99 1 7.9 6.6 has grown to be nearly as abundant as kamacite 750 98 2 8.0 7.0 700 25 72 3 4.5 9.1 7.1 and the structure contains 6% schreibersite, un- 650 47 50 3 5.0 11 9.0 600 70 26 4 5.2 14 13 doubtedly having nucleated at preexisting kama- 550 92 4 4 7.1 18 16 cite-taenite grain boundaries as well as around sulfide inclusions. At 700° C most of the taenite has disappeared, and at 650° C, 90% kamacite is 2.7 Atomic % P, 7.8 Atomic % N1 in equilibrium with 10% schreibersite. The schrei- °C %a *Y %Ph %N1 %N1 %N1 bersite continues to increase in Ni as it cools, a Y Ph 995 20 80 6.2 8.2 reaching a value of 11 atomic % Ni at 600° C and 975 23 75 2 6.3 8.3 6.0 925 96 4 7.8 5.7 below, with a Ni content in the kamacite of 4.2 875 95 5 7.9 6.0 atomic percent. Both of these Ni values are in 850 93 7 7.9 6.2 750 92 8 7.9 6.6 general agreement with observed values, perhaps 700 25 67 8 4.5 9.1 7.1 650 47 44 9 5.0 11 9.0 a little on the high side. 600 70 21 9 5.2 14 13 550 91 9 7.0 16 If the 1.8 atomic % P value is assumed, kamacite and taenite are the phases present in the equilib- continued down to below 550° C (17 to 24 atomic rium structure from 1100° C down to below 850° % Ni in cohenite-enclosed massive schreibersite). C. Schreibersite first appears somewhat above 750° The size of the massive schreibersites requires that C, and is present in the amount of 2% when that the cohenite borders precipitated at temperatures temperature is reached. Nucleation under these below 750° C. Within this same 700° to 550° C circumstances would be expected to distribute temperature range, the bulk of the metal away schreibersite along kamacite-taenite borders, these from the large schreibersites transformed to nor- phases being present in approximately equal mal Widmanstatten pattern. Grain boundary amounts. Taenite disappears from the structure schreibersite, rhabdites, and taenite border schrei- below 700° C, and by 600° C, 3% schreibersite is in bersites were formed during this part of the se- equilibrium with kamacite. The final Ni contents quence. of both kamacite and schreibersite are somewhat In Figure 19 the 2.7 atomic % P value is used. It higher than in the 3.6 atomic % P case, the higher is obvious here that the same sequence of phases P situation agreeing better with actual measure- is obtained if the P value is somewhat reduced. ments on the meteorite. The higher Ni is the significant structural devel- When 5.3 atomic % P is assumed, the bulk opment factor in this case. composition lies within the kamacite + liquid field BELLSBANK. —The Bellsbank meteorite is anom- of the phase diagram from 1100° C to somewhat alous, unusually low in Ni and high in P. The above 1010° C. At 995° C, 5% schreibersite is in gross segregation of schreibersite in its structure equilibrium with kamacite, and this schreibersite makes it unusually difficult to arrive at a satisfac- remains constant in amount down to 925° C. Dur- tory bulk P content. For this reason the literature ing the next 50° drop in temperature one-third of values of 5.3 weight % Ni and 2 weight % P were the structure transforms to taenite and the amount bracketed in Table 15 by assumed compositions of of schreibersite more than doubles to 13%. The 5.3 weight % Ni and both 1 and 3 weight % P (5.0 taenite decreases in volume with further cooling atomic % Ni and 1.8 and 3.6 atomic % P, and 4.9 and disappears below 750° C. By 700° C the equilib- atomic % Ni and 5.3 atomic % P). The intermedi- rium phases are again kamacite and schreibersite, NUMBER 21 47

TABLE 15. —Equilibrium cooling of composi- The phase growth calculations that have been tions appropriate to the Bellsbank meteorite discussed above account for the major structural (percentage of phases present at indicated temperatures with their Ni contents) features of the selected group of meteorites. Ap- propriate compositions cooled under equilibrium 1.8 Atomic X P, 5 .0 Atomic % Ni conditions to approximately 600° C would produce °c %a %y %Ph %N1 %N1 «H1 a Y ph the observed proportion of phases and account 995 10 90 4.0 5.1 for the general character of the structures. Coa- 975 30 70 4.2 5.3 925 30 70 3.8 5.5 huila is single crystal kamacite containing a few 875 35 65 3.5 5.8 850 55 45 4.0 6.3 isolated moderate-size schreibersite inclusions. 750 68 30 2 4.0 7.2 5.3 700 88 9 3 4.5 9.1 7.1 Ballinger has a coarse Widmanstatten pattern in- 650 97 3 4.9 8.9 600 96 4 4.7 12 terrupted by large schreibersites that may be sur- 555 95 5 4.6 13 rounded by cohenite. Santa Luzia contains massive schreibersite areas surrounded by giant swathing 3.6 Atomic % P, 5.0 Atomic % N1 kamacite, separating areas of coarse Widmanstat- °C %a %Ph SN1 *N1 %Y *NiPh a Y ten pattern. The higher Ni members of the group 995 100 5.0 975 100 5.0 have a finer Widmanstatten pattern with massive 925 85 15 4.7 6.6 schreibersite morphology explainable in terms of 875 53 41 6 4.0 6.4 4.4 850 50 42 8 4.0 6.3 4.4 the amounts of P and C present. Bellsbank con- 750 63 28 9 4.0 7.2 5.3 700 85 5 10 4.5 9.1 7.1 tains massive schreibersite and low Ni kamacite 650 90 10 4.6 8.0 600 88 12 4.2 11 and has similarities to the giant swathing kamacite 550 88 12 4.2 11 areas in Santa Luzia.

5.3 Atomic X P, 4 .9 Atomic % Ni It is interesting at this point to summarize equi-

XPh %Ni SN1 %Niph librium data given above and compare it with °C U %y a Y what would be expected in terms of Widmanstat- 995 95 5 4.8 6.4 975 95 5 4.8 6.0 ten pattern growth from the binary Fe-Ni system. 925 95 5 4.8 5.2 875 52 35 13 4.0 6.4 4.4 Here Widmanstatten pattern growth is construed 850 49 36 15 4.0 6.3 4.4 750 62 22 16 4.0 7.2 5.3 in its conventional usage of simply taenite trans- 700 83 17 4.4 7.0 650 83 17 4.4 7.5 forming to kamacite, ignoring the presence of P 600 81 19 3.8 10 550 80 20 3.8 10 either in solution or in schreibersite. The compar- ison is made in Table 16, where the first two columns give the compositions of the meteorites schreibersite being present in the amount of 17%. as plotted in Figure 19, the compositions that best With further cooling, schreibersite increases in fit the structural arguments given above. These quantity and in Ni content, with a rather marked columns are followed by sets of data for the decrease in kamacite Ni content. The final Ni ternary system and for the binary system (Figure content of the schreibersite is possibly slightly 1). In each set the first column gives equilibrium lower than the measured value, the quantity of temperatures at which Widmanstatten pattern de- schreibersite seems too high, and the kamacite Ni velopment starts (y —>a + y) if undercooling values too low. The 3.6 atomic % P figure, the effects are ignored, probably a reasonable assump- one plotted in Figure 19, seems to be the best tion when P is present. The second column gives choice of the three. A somewhat higher P value, the expected temperatures at which taenite would perhaps 4 atomic %, would probably be even be completely transformed to kamacite (a + y —» better, however. This would mean that schreiber- a) if equilibrium were maintained down to these site would initially nucleate in the presence of less low temperatures. The third column gives the taenite, and that final massive schreibersite and expected temperature range for Widmanstatten kamacite would have slightly smaller Ni values. pattern growth (ATWPG). The last two columns The observed low temperature schreibersite mor- show the effect of P on increasing the tempera- phology suggests that an extensive Widmanstatten tures of kamacite nucleation (TaNuc) and taenite pattern was not present when schreibersite nu- disappearance (TyDis) over what would be ex- cleated . pected from the binary system. 48 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES*

TABLE 16. —Temperatures (° C) for Widmanstatten pattern development in the ternary system as compared to the binary system

Atomic % Fe-NI-P System Fe-N1 System AT due to P Meteorite Ni P c+Y-a Y-a+Y <»+Y+* AT ATWGP WGP Sue TYD1S

Bellsbank 5.0 3.6 (-930°)"* 690° 240° 775° 610° 165° (155°)** 80° 5.3 0.8 860° 610° 250° 765° 580° 185° 95° 30° 6.2 1.4 880° 580° 300° 755° 570° 185° 125° 10° 6.2 2.7 840° 580° 260° 755° 570° 185° 85° 10° Lexington County & Bahjoi 7.1 1.8 750° 565° 185° 740° 480° 260° 10° 85° Goose Lake & Balfour Downs • • 7.8 2.7 730° 555° 175° 725° 5°

* For this composition WPG starts with the reaction ** Nucleation of Y within a.

The presence of P in the coarsest structured Low TEMPERATURE PHASE GROWTH AND THE meteorites, Bellsbank through Santa Luzia, signifi- EQUILIBRIUM DIAGRAM cantly increases the temperature at which Wid- The preceding section has shown that the major manstatten pattern growth starts, increases the structural features of the selected meteorites may temperature at which transformation of taenite be explained on the basis of nucleation and growth would be expected to be complete, and also signifi- of phases in a system responding to changing cantly increases the temperature range over which equilibrium conditions with decreasing tempera- growth takes place. These three factors all work ture. Bulk equilibrium may have pertained down in the direction of producing larger kamacite crys- to 550° C or below for Bellsbank and Coahuila, tals than would be expected from the binary dia- but the other six meteorites were not in complete gram for the same cooling rate. In the coarse- equilibrium at that temperature (Figure 19). Had structured meteorites, Lexington County through they been, their structures would consist only of Belfour Downs, those with a well-developed Wid- kamacite and schreibersite, and the taenite that manstatten pattern, the amount of P has a less they contain would have transformed. All eight of dramatic effect. The important point here is that the meteorites also contain Ni and P concentration kamacite nucleation takes place at significantly gradients in kamacite adjacent to schreibersite (Ta- lower temperatures than is the case for meteorites bles 2-9), although the recent work of Randich with less Ni. The nucleation temperature is also (1975) suggests that these gradients formed at very close to that which would be expected from much lower temperatures. Some of the massive the binary diagram. This leads to much more and all of the smaller schreibersites have Ni con- restricted conditions for kamacite growth and the centrations that are greatly in excess of those retention of larger amounts of taenite and plessite permitted for schreibersite in equilibrium with in the structure, thereby producing the finer and kamacite at 550° C (Figures 18, 25). These obser- more regular Widmanstatten pattern that is ob- vations are explained by the fact that as tempera- served in these meteorites. ture drops the atomic mobility necessary to main- Details of these various structures as given in tain bulk equilibrium is lost. Elemental diffusion the results section, however, are not explained becomes slower and compositional changes take satisfactorily by this discussion. Taenite and ples- place over much shorter distances, resulting in site are retained in significant amounts in most of composition gradients. Kinetic factors must now these meteorites, demonstrating that equilibrium be considered as well as equilibrium states. Upon cooling did not extend to 550° C. Diffusion con- cooling under these restrictive.conditions, schrei- trolled growth, modifying the equilibrium struc- bersite continues to grow in volume, may increase ture in significant ways continued to lower temper- in Ni content, and late-formed schreibersites nu- atures. It is this aspect of meteorite structure that cleate and grow. Taenite shrinks in volume and will be discussed below. increases in Ni, while kamacite decreases in Ni. NUMBER 21 49

Phase interfaces are still in equilibrium in the compositions, but incidental measurements (Fig- sense that interface compositions represent tie ures 9, 11) may be used to bracket the position of lines within the Fe-Ni-P system (see below). Schrei- point C. The Ni value in taenite associated with bersite-kamacite interface measurements on mete- schreibersite is less than the schreibersite Ni value, orites, therefore, give equilibrium information rel- perhaps by about 5 atomic %. This suggestion is evant to the Fe-Ni-P systems at temperatures well supported also by Reed's (1972) measurements on below those that are available experimentally. the Octibbeha County meteorite and by similar Combining this low temperature equilibrium data measurements by the authors on this meteorite with both an analysis of the kinetic record present (60 atomic % Ni in schreibersite in contact with 55 in the form of diffusion gradients and a detailed atomic % Ni in taenite). Point C moves to the knowledge of specimen petrography may be ex- right, but trails behind point D. The P values in pected to provide new possibilities for the elucida- taenite at these interfaces are also not accurately tion of the structures at hand. known, but sufficient data is available to say that With these ideas in mind, it will be helpful first they are a fraction of the P value in kamacite to examine the Fe-Ni-P diagram asking what associate with the same schreibersite (see discus- changes would be expected with decreasing tem- sion below and Figure 22). The Ni and P contents perature. Meteorite data (Reed 1965a, 1967, 1969, for point B may be estimated from the kamacite- and Tables 2-9 above) indicate major changes in schreibersite interface values of these taenite- the diagram on cooling to low temperatures. The embedded schreibersites. They range from 5.7 to top part of Figure 20 may be used as a starting 6.2 atomic % Ni, with constant P values of 0.05 point for a discussion of this transition. It is a atomic %. In summary the diagram changes as schematic isothermal section of the ternary system follows with decreasing temperature: point D at a temperature slightly below 550° C. The scale moves to the right to approximately 45 atomic % on the Fe-P axis has been interrupted so that the Ni with point C moving to approximately 40 atomic lower part of the diagram could be enlarged for % Ni and to a P value that is much smaller than clarity. The low P concentrations that are observed 0.05 atomic %. As a consequence the a + Ph and in kamacite and taenite make it impossible to show a + y + Ph fields greatly expand in area mainly the three fields along the Fe-Ni axis and the three at the expense of the y + Ph field. The three schreibersite-containing fields using the same small fields along the Ni axis also lose a large scale. proportion of their areas, but they still retain Compositional data on schreibersite-kamacite considerable importance for the interpretation of and schreibersite-taenite associations suggest the meteorite structures. following trends in the ternary diagram with de- The preceding illustrates the direction and ex- creasing temperature. The constant P content of tent of change in the ternary system with decreas- schreibersite requires that the schreibersite coming temperature, but it says nothing about what position corner of the three-phase field (point D that temperature range might be. Do meteorite in Figure 20) lie on the 25 atomic % P line regard- structures continue to develop to very low temper- less of temperature. Ni values for schreibersite atures or is growth effectively concluded at tem- embedded in taenite borders, an association that peratures not far below 550° C? The bottom section approximates the schreibersite corner of the three- of Figure 20 suggests an approach that may be phase field, range from 41 to 45 atomic %. Point useful here. We know that D' lies on the 25 atomic D, therefore, must move at least that far to the % P line and can assume that it moves continuously right with decreasing temperature. Higher Ni con- to the right with decreasing temperature. The tents in schreibersite have been measured (Reed, points on the Fe-Ni axis can be obtained from the 1972), but these represent compositions in the estimated portion of the Goldstein and Ogilvie taenite-schreibersite field, while lower Ni contents (1965) binary Fe-Ni diagram down to about 350° represent compositions in the kamacite-schreiber- C. If A', B', and C could be estimated as a site field. Because of the steepness of the Ni function of temperature, we would have much of concentration gradients, no serious attempt has the information needed for extrapolated isother- been made to obtain taenite-schreibersite interface mal sections. 50 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

0 20 30 40 100 %Fe ATOMIC %Ni

20 30 40 ATOMIC % Ni FIGURE 20. —Fe-Ni-P system below 550°C: top, schematic isothermal section; bottom, parameters of use in extrapolating to lower temperatures. NUMBER 21 51

TABLE 17. —Experimental values of P concentration in TABLE 18. —Temperature versus P saturation concentra- kamacite and taenite in equilibrium with schreibersite tion in kamacite and taenite taken from Figure 22

Fe-P System* Fe-Ni-P System** T°C Atomic % Pa Atomic X PY Weight % Pa T°C 1/T ("KxlO"4) Atonic % Pa Atomic % Pa Atomic * PY 540 0.40 0.14 Q.ZZ 527 0.35 0.12 0.19 508 0.30 0.10 0.17 1010 1283 7.79 4.8 488 0.25 0.080 0.14 995 1268 7.89 4.1 473 0.22 0.070 0.12 975 1248 8.01 4.1 1.9 465 0.20 0.062 0.11 925 1198 8.35 3.7 1.6 460 0.19 0.058 0.11 875 1148 8.71 2.9 3.0 1.3 454 0.18 0.054 0.10 850 1123 8.90 2.8 1.2 449 0.17 0.052 0.09 750 1023 9.78 1.7 1.8 0.68 444 0.16 0.048 0.09 700 973 10.3 1.3 1.2 0.49 436 0.15 0.046 0.08 650 923 10.8 0.99 0.9 0.27 431 0.14 0.041 0.08 600 873 11.5 0.59 0.57 0.22 424 0.13 0.038 0.07 550 823 12.1 0.45 0.47 0.15 417 0.12 0.034 0.07 410 0.11 0.032 0.06 * Doan and Goldstein (1970), table IV. 403 0.10 0.028 0.06 ** Doan and Goldstein (1970), table III. 394 0.090 0.025 0.05 383 0.080 0.022 0.04 372 0.070 0.019 0.04 362 0.060 0.015 0.03 348 0.050 0.013 0.03 A', B', and C represent corners of composition 333 0.040 0.009 0.02 315 0.030 0.007 0.02 fields, and these particular compositions are very 298 0.020 0.005 0.01 dilute solutions of P in Fe or in Fe-Ni. Data for 252 0.010 0.002 0.006 these points at a number of temperatures within the experimental range are given by Doan and Goldstein (1970), and their values converted to that tie line belonged. The implication of this atomic % P are given in Table 17. This type of would be that schreibersite growth effectively data is frequently used to extrapolate concentra- stopped over a sequence of temperatures within a tion-temperature relationships to lower tempera- given meteorite. Numerical values for P concentra- tures and is plotted accordingly in Figures 21 and tion variation with temperature for both kamacite 22. Atomic % P on a logarithmic scale is plotted and taenite were read from Figure 22 and are against 1/T. Figure 21 uses P saturated kamacite listed in Table 18. values from the Fe-P system, and Figure 22 uses Although a flat line A'B' has several interesting both P saturated kamacite and taenite values from implications, it is necessary to consider as an alter- the Fe-Ni-P system. Straight lines were drawn with nate possibility that it actually slopes downward ease through the three sets of data, suggesting from left to right. The experimental technique that the extrapolations may give useful low tem- employed by Doan and Goldstein (1970) was perature composition estimates. The thermody- stretched to its limit in obtaining the lower temper- namic rationale for this procedure is given in ature data, resulting in the possibility of significant "Appendix." error for the problem at hand. There is also A particularly interesting observation based on ambiguity in their paper concerning one critical Figures 21 and 22 is that the kamacite line in the value. The weight % P in kamacite value for the plot of the Fe-Ni-P system data is identical to the Fe-P system at 550° C taken from their table IV is kamacite line in the plot of Fe-P system. There- 0.25 ±0.03. The value used on the 550° C isother- fore , both sets of data yield the same extrapolated mal section (their figure 8) is 0.4 weight % P. This P-temperature relationship for kamacite. This difference probably represents confusion as to means that line A'B' is parallel to the Fe-Ni axis, which value is best rather than a simple misprint and that the saturation value of P in kamacite is in the diagram, a difficulty that can best be re- independent of Ni concentration. Should this ac- solved by more careful experimental work. If this tually prove to be the case, it would have interest- 0.4 weight % P value had been used in the extrap- ing consequences for the interpretation of mete- olation, the line in Figure 21 would have lain to orite structures. Kamacite-schreibersite interface the right of the kamacite line in Figure 22. This values would not only represent tie lines in an a + would mean a sloping line A'B' with P decreasing Ph field of the ternary diagram, but the P value in as Ni increases. This situation could imply that all kamacite at a specific interface would represent schreibersite within a given meteorite grew down the temperature of the isothermal section to which to the same final temperature. Before attempting 52 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

.001 7 8 9 10 II 12 13 14 15 16 17 18 19 20 21 I/T (°KxlO~4) FIGURE 21. —Extrapolation of P saturation values in kamacite in Fe-P system to lower temperatures. NUMBER 21 53

001 10 II 12 13 14 15 16 17 18 19 20 21

FIGURE 22. —Extrapolation of P saturation values in kamacite and taenite in Fe-Ni-P system to lower temperatures. 54 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES to resoive this and related problems in terms of described by the mathematical treatment of Ran- meteorite measurements, however, it will be help- dich and Goldstein (1975). Diffusion in the Fe-Ni- ful if we first look at the diffusion process as it P system may be described using an extension of applies to schreibersite growth. Fick's first law. The P and Ni fluxes in terms of concentration gradients in kamacite and schreiber- DIFFUSION-CONTROLLED SCHREIBERSITE GROWTH site are as follows. a T = — (la) The data presented above on Ni and P gradients JP pp PNi demonstrates that schreibersite grew by drawing dX dX these elements from increasingly restricted vol- Ta — _ umes of kamacite with decreasing temperature. NiNi (lb) Petrographic considerations combined with an un- dX ax derstanding of the Fe-Ni-P system suggest the broad outlines of the nucleation and growth proc- Ph ,ph aCp T = _r>Fe,Ph (lc) ess. With this background in mind, a set of con- NiNi ax straints and assumptions can be developed that ax allow the qualitative application of the nonisothermal diffusion-controlled growth theory for ternary Jp"h = o (Id) systems recently developed by Randich and Gold- stein (1975) to be applied to schreibersite growth in these meteorites (see also Randich, 1975). Separate sets of equations are needed for each Figure 23 may be used to relate schreibersite phase. The P flux in schreibersite is zero due to the constant atomic proportion of this element in growth both to the Fe-Ni-P diagram and to growth a equations. The top part of the diagram is a sche- schreibersite. The symbols D£p and D£fN? are matic isothermal section at some temperature be- diffusion coefficients, measures of the influence of the concentration gradients of these two ele- low 550° C. The scale for P is broken so that very a a low levels of P can be indicated. The tie line ments on their own flux, where as D£Ni and D^iP represents schreibersite growing within kamacite. reflect cross effects usually referred to as the Its ends give interface P and Ni compositions for ternary diffusional interaction. The latter coefficients have been found to be so small in this both kamacite (CP, CNi) and schreibersite (Cp, C^i). The theory of Randich and Goldstein (1975) system that the terms containing them may be requires that equilibrium be maintained to the dropped from the above equations (Heyward and extent that Ni and P concentrations in the two Goldstein, 1973). These equations may then be phases are related by a tie line in the ternary simplified by neglecting the diffusional interaction system at the temperature that pertains. For dif- terms and the P flux in schreibersite. fusion-controlled conditions, this tie line need not a — be the equilibrium tie line (ETL), the one that J P - (2a) passes through the bulk composition of the system. Hx The lower part of Figure 23 indicates kinetic (2b) factors that are important in the growth process. A schreibersite (Ph) of half width £p" equal to £m is growing within kamacite in the X direction. The P (2c) and Ni values in schreibersite at the interface are ax CP and CNi, and the equivalent values in kamacite Fick's second law is applied in order to consider are CP and CNi . The P and Ni fluxes from kamacite the time dependence of the fluxes. It is assumed + + into the growing schreibersite are J£ f and J$U( > that diffusion coefficients are independent of com- and the Ni flux from the interface into the schrei- position. bersite is Sm.f • The concentration gradients in P and Ni are dCP/dX and aCNi/aX. (3a) The diffusion controlled growth process may be at ax pp ax2 NUMBER 21 55

ATOMIC %Ni

20 Ph o

o Ni 10

+ JP t aCp

x^ H FIGURE 23. —Relation of schreibersite growth both to Fe-Ni-P diagram and to growth equations: top, schematic low temperature ternary with tie line indicating schreibersite growing in a + Ph field; bottom, a + Ph interface compositions, composition gradients, and associated P and Ni fluxes. 56 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

schreibersite for the final stages of growth. Cool- (3b) dt ax NiNi ing is assumed to be uninterrupted, with schreibersite growth continuing down to very low tempera- JNl tures. Measurements with the electron micro- NiNi (3c) at ax ax2 probe, however, do not reveal changes that take place within less than 1 /urn of an interface. There- The resulting mass balance equations for P and fore, as a consequence of this limitation of the Ni at a schreibersite-kamacite interface are as fol- measurement technique, growth apparently stops lows. when subsequent changes become unresolvable. Randich's (1975) calculations indicate a lower limit (4a) dt of 250° C for detectable change. Equation 6a is a key to the understanding of (4b) schreibersite growth under the conditions that pertained in the meteorites under study. It demonstrates that the P flux controlled the schreiber- The two mass balances are related to each other site growth rate. The quantity Cp in the equation as the rate of movement of the interface, d£/dt, is the P content of schreibersite at the interface must be the same for both elements. (Figure 23), a constant at 25 atomic %. The quantity Cp is the interface P content in kamacite. It is (5) dt dt dt a small number, 0.2 to 0.05 atomic %, in the temperature range of formation of the observed A schreibersite growth rate may then be expressed interface relationships. The quantity (CP — CP), in terms of interface Ni and P concentrations, the therefore, may be considered to be independent flux of P in kamacite, and the fluxes of Ni in both of temperature even though Cp is not. With this kamacite and schreibersite. simplifying assumption, the rate of growth of a given interface, d£/dt, is seen to be dependent 1 + 7(-JPV) (6a) only on the P flux, J£f . The P flux, however, dt Cp — Cp will be dependent upon CP at any given temperature. The value of Cp combines with the P level in (6b) dt the surrounding kamacite to establish the P gradient, aCp/aX, the variable that determines the It is beyond the scope of this study to attempt flux (equation 2a). the difficult numerical analysis required to solve The system responds to the constraint of the P these growth equations. Randich (1975) has devel- flux determining schreibersite growth rates by oped a model for this purpose and successfully adjusting individual interface Ni concentrations so applied it to the growth of schreibersite in hexa- that the growth rate established by P (equation 6a) hedrites. He compiled a computer program that is matched by the growth rate due to Ni (equation treats diffusion controlled phase growth in ternary 6b). This permits individual schreibersites contain- systems when the requisite portions of the phase ing markedly different Ni levels to continue to diagram are known. The model accomodates ter- grow simultaneously within the cooling meteorite. nary interactions, nonisothermal transformations Kamacite and schreibersite interface Ni and P (variable cooling rates), and impingement (over- concentrations must continue to be related by a tie lapping diffusion fields). It is based on a one- line relationship, a requirement that is met by tie dimensional space grid and is rigorous only for lines shifting from the equilibrium tie line. The lamellar schreibersite growing within kamacite. nature of these tie line shifts and their effects on The concepts that this model employs, however, growth rates of individual schreibersites is illus- are valid for meteorites more complex than the trated in Figure 24. hexahedrites, and they will be used here in a Tie line shifts and their effects are illustrated qualitative interpretation of the data. using hypothetical ternary diagrams in Figure 24. Equations 6a and 6b relate P and Ni interface The symbols for Ni and P are retained as a matter concentrations to their gradients in kamacite and of convenience, although scale problems preclude NUMBER 21 57

\ V 1 1 1

^ 1 LJ 1

H es e OJ u li 3 1

d -10 IN % DIW01V !N % OIW01V

2 "° aC c - "I3

C a, •2 E

"rt OH u '3 + •S « 8.S !i •* .y " .S o a a 58 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

+ use of actual values. The tie line indicated at the kamacite, ]xU( , become larger and the Ni flux in higher temperature Ti, ETL(TX), is assumed to schreibersite, J^,f~. becomes smaller. All three of represent bulk equilibrium in the a + Ph region these changes increase the growth rate over that of the diagram. It passes through the bulk com- of ETL(T2). Ni is supplied to the growing schrei- position indicated by the small circle (5.5 atomic % bersite in a way that size is increased at the expense Ni and 6.0 atomic % P). Upon cooling to tempera- of Ni enrichment. ture T2 the a + Ph field expands along the a + Moving the tie line to the right of ETL(T2) has Ph/Ph boundary, and the a/a + Ph boundary the effect of slowing the growth rate due to Ni. moves closer to the Fe-Ni axis. It is assumed that The quantity 1/(CNI ~ C^i) and the Ni flux in + in this lower temperature range mass transfer is kamacite, ]yiti , become smaller, and the Ni flux insufficient to achieve bulk equilibrium. Growth in schreibersite, J^.f » becomes larger. All three becomes diffusion controlled and gradients de- of these changes decrease the growth rate over velop within the phases. Tie lines may therefore that for ETL(T2). Ni is supplied to the growing shift from the equilibrium tie line so that the Ni schreibersite in a way that it becomes enriched in growth rate can match the P growth rate. Three Ni rather than growing in size. A theoretically possible tie lines are indicated at T2, the equilib- infinite range of growth rates may be obtained by rium tie line for the same composition used above, these tie line shifts. One of these possible tie lines ETL(T2), and tie lines shifted to the right and to will give a unique growth rate that satisfies both the left of ETL(T2), equations 6a and 6b. The interface relationships for these four tie lines are illustrated at the right in Figure 24. The COOLING RATE VARIATIONS upper diagram shows interface Ni and P compositions for ETL(Tj). The bulk Ni value for the Cooling rates of meteorite parent bodies are system is indicated by Co1- The bulk P value is 0.5 known to vary over several orders of magnitude atomic % higher and was omitted for clarity. Bulk (Goldstein and Short, 1967b). These variations will equilibrium has been achieved at temperature Tj affect the growth of schreibersite, particularly in as is indicated by the absence of compositional the lower temperature ranges. Compositions and gradients in both kamacite and schreibersite. associations we observe in meteorite specimens Interface relationships for diffusion controlled will depend in part on specific cooling histories. growth at temperature T2 are illustrated in the Measurements of schreibersite, therefore, would lower right side of Figure 24. The P interface be expected to be interpretable in terms of cooling values are assumed to be identical in the three rates. Randich (1975) has recently demonstrated interface situations shown. The consequence is the validity of this assumption by calculating cool- similar P fluxes feeding schreibersite growth, ing rates for hexahedrites. In the course of his ]P,(+, and the establishment of similar growth work important observations were made that will rates, d£/dt, for the interfaces through equation help in the interpretation of more complex mete- 6a. Tie line shifts from ETL(T2) induced by con- orite structures. Several aspects of this work will ditions of limited mass transfer produce a contin- be reviewed here. uously varying range of growth rates that permits Calculations using the Randich and Goldstein Ni flow to match that required by P. (1975) schreibersite growth model were made us- The equilibrium tie line at the lower tempera- ing a bulk composition of 2.1 weight % P, 4.1 ture, ETL(T2), passes through the bulk composi- weight % Ni, and 93.8 weight % Fe. Cooling rates tion. Its interface Ni and P values are those that of 5 X 10~3, 5 x 10~4, and 5 X 10~50 C per second would be observed if the system were at complete were employed over a cooling interval of 900° C to equilibrium. The presence of compositional gra- 685° C. Decreasing the cooling rate produced dients in both kamacite and schreibersite, how- schreibersites of predicted greater widths and ever, show that complete equilibrium has not been higher Ni contents. P gradients were predicted in reached. kamacite and Ni gradients were predicted for both Shifting the tie line to the left results in an kamacite and schreibersite. The calculated Ni in- increase in the growth rate due to Ni (equation terface values were significantly lower than those 6b). The quantity 1/(CNI — CNI) and the Ni flux in for the equilibrium tie line for this composition. NUMBER 21 59

Tie lines had shifted to the left to increase the Fe-Ni-P system. This approach can be extended growth rate for Ni. An experimental validation of to much lower temperatures with the aid of con- these calculations was carried out using this same cepts developed by Randich (1975) in his analysis composition and the 5 x 10~4° C per second (43.2° of schreibersite growth in hexahedrites. The appli- C per day) cooling rate. Calculated and observed cation of diffusion theory permits the interpreta- compositions agreed within the limits of electron tion of schreibersite-kamacite interface data in a microprobe measurements. way that yields a more detailed understanding of Additional cooling rate calculations were made schreibersite distribution and composition than by Randich (1975) in a study of schreibersite nu- has been possible previously. cleation temperatures. Cooling rates in the mete- Phases and compositions resulting from equilib- orite range of 1°, 10°, and 100° C per million years rium cooling of the meteorite compositions of were used with the Coahuila bulk composition of interest have been summarized in Tables 10 5.49 weight % Ni and 0.49 weight % P. The through 15, and these same compositions have results were similar to those given above, the been plotted on the iron-rich corner of isothermal slower cooling rates producing slightly larger sections in Figure 19. At the top of Figure 25 schreibersite containing slightly higher Ni values. these compositions have also been plotted on the The calculated Ni values were within the range of 550°C isothermal section with their respective tie observed Ni values in hexahedrites, establishing lines. This is the lowest temperature for which an the validity of the model at cooling rates within experimentally based ternary section is available. the meteoritic range. Ni gradients were predicted Table 19 lists the equilibrium percentages of kam- for the schreibersites, contrary to observations on acite and schreibersite with their Ni contents as meteorites. This is probably explained by the fact taken from Figure 25. This plot shows that had that the ternary diffusion constant for Ni in schrei- complete equilibrium been reached at this temper- bersite is not known in this temperature range. ature for these compositions, taenite would have These calculations led to a conclusion that is disappeared and the observed structures would significant for our purpose. It was shown that if consist of kamacite containing inclusions of schrei- nucleation occurs above 500° C the amount of bersite. undercooling prior to nucleation in unimportant. The bottom section of Figure 25 contains an The kamacite-schreibersite assemblage at 500° C estimated isothermal section at approximately 300° will be in equilibrium, assuming a diffusion field C. The schreibersite corner of the three phase of 1000 [im or less. Randich (1975) also points out field is plotted at 45 atomic % Ni, a value that is that this situation leads to the conclusion that probably close to the real value. The kamacite Reed's (1965b) theory that lamellar schreibersite corner of the three phase field is plotted at 5.5 and rhabdite nucleated simultaneously must be atomic % Ni, and this value is considerably less correct. Had the plate schreibersite grown signifi- certain. The compositions used above are again cantly prior to nucleation of rhabdite, the rhabdite plotted with their tie lines. Although the diagram would have been prevented from nucleating. Both may not be accurate, it does tell us something of types are observed together over very short dis- what would be expected if equilibrium cooling tances, requiring that nucleation temperatures extended to this very low temperature. Propor- were very close. Under these circumstances, im- tions of phases and the expected compositions pingment of P and cooling rate are the two factors taken from this diagram are also listed in Table that control schreibersite growth for a given bulk 19. composition. The bulk compositions plotted in the lower section of Figure 25 would also consist of kamacite INTERFACE DATA AND SCHREIBERSITE and schreibersite only, and these two phases would DISTRIBUTION be uniform in composition. The Bellsbank schreibersite would contain much more Ni than was In a previous section we have seen that the observed for its massive schreibersite, and its kam- main petrographic features of coarse-structured acite would be considerably poorer in Ni than was iron meteorites can be understood in terms of observed. The massive schreibersite in Santa Luzia equilibrium cooling of specific compositions in the would be greatly enriched in Ni, its small schrei- 60 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

550° C

ABELLSBANK • COAHUILA

y +• Ph • SANTA LUZIA • BALLINGER • LEXINGTON COUNTY - BAHJOI A GOOSE LAKE - BALFOUR DOWNS

too %FE 20 30 40 50 ATOMIC % Ni

10 20 30 40 50 ATOMIC % Ni FIGURE 25. —Schreibersite compositions resulting from equilibrium cooling of meteorite compositions: top, 550° C isothermal section with meteorite compositions and their respective equilibrium tie lines; bottom, similar plot at approximately 300° C. bersite would contain less Ni, and its kamacite site. The Lexington County-Bahjoi composition would be poorer in Ni. The Coahulia, Ballinger, would produce the most Ni rich schreibersite. The and Goose Lake-Balfour Downs compositions all structures suggested by this diagram, therefore, lie on the same tie line, but would consist of are markedly different than those actually ob- different proportions of kamacite and schreiber- served in the meteorites under study. At what NUMBER 21 61

TABLE 19. —Equilibrium compositions at 550° C (Tables 2, 5, and 8) are plotted on isothermal and -300° C taken from Figure 25 sections in Figures 26. Average values were used 550°C where more than one set of interface measurements gave similar values. Any given tie line, Meteorite % a % Ph % Ni % Ni a ph therefore, may represent more than one set of

Beilsbank 87.0 13.0 4.1 11.9 measurements. The scale used made it impossible Coahuila 99.3 0.7 5.2 13.3 Santa Luzia 91.3 8.7 5.4 14.0 to indicate the differing low P values in kamacite, 96.3 3.7 5.8 15.0 so all of these compositions are indicated by points Lexington County & Bahjoi 94.8 5.2 6.6 16.2 resting on the Fe-Ni axis. This treatment of the Goose Lake & Balfour Downs 91.2 8.8 6.9 17.0 data may well be an over simplification as various tie lines within a given meteorite may actually belong on different isothermal sections. A range -300°C of temperatures may be involved, and this point

Meteorite % a. % Ph % Nia % Niph will be returned to in a later section. The three meteorites selected are representative of the group Bellsbank 86.4 13.6 2.5 20.8 of eight meteorites studied. Coahuila 97.2 2.8 4.3 38.0 Santa Luzia 90.2 9.8 3.6 30.5 Typical tie lines for the Coahuila meteorite are 94.5 5.5 4.3 38.0 Lexington County & given in the top section of Figure 26. They fan Bahjoi 93.3 6.7 4.7 41.2 Goose Lake & out through a narrow composition range of 3.2 to Balfour Downs • • • • 90.0 10.0 4.3 38.0 4.7 atomic % Ni in kamacite and 20 to 30 atomic % Ni in schreibersite. Kamacite P values at the interfaces range from 0.14 to 0.09 atomic %, a temperatures do these observed differences be- significant point that will be dealt with later. The come introduced into the structures? structural associations related to compositions have Taenite is retained in the form of the Widman- been discussed above, but it should be noted here statten pattern in the six octahedrites studied. that the tie line on the left represents early-formed This shows that mass transfer required by the structures (schreibersite bordering troilite-dau- taenite-kamacite transformation is insufficient at breelite inclusions and a large schreibersite), and somewhat above 550° C to achieve bulk equilibrium the three on the right are later-formed structures in the cooling rate range pertaining. The presence (large rhabdites). The estimated bulk composition of the Widmanstatten pattern may also affect sub- of Coahuila is indicated on the diagram as a large sequent P movement through the structure. Ran- dot. The point apparently falls on the right-hand dich's (1975) calculations show that mass transfer tie line. All the measured schreibersite composi- through 1000 /xm diffusion fields in kamacite is tions in Coahuila then appear to be to the left of sufficient for kamacite-schreibersite equilibrium to the ETL or on it, as was to be expected on the be maintained down to 500° C in the same range basis of Randich's (1975) analysis. Reed (1965a) of cooling rates. Kamacite-taenite transformations has reported Coahuila schreibersite with 34 atomic become restricted at higher temperatures than % Ni. It is not clear how this value would plot on kamacite-schreibersite transformations. There- Figure 26 as the necessary Ni value in kamacite is fore, it is reasonable to assume that as tempera- not available. tures drop below 600° C, localized compositional The relationship of the equilibrium tie line to changes become increasingly more important and observed meteorite tie lines is actually somewhat bulk compositional effects become increasingly less more complex then Randich (1975) has implied. important. As mass flow becomes more restricted, This can be concluded from consideration of Coa- individual structural units maintain interface equi- huila measurements, and it will become clearer librium and respond to compositional gradients in when measurements on the octahedrites are con- increasingly small volumes of material. These sidered. At temperatures above the final growth processes are responsible for the interface and stages represented by the Coahuila tie lines in gradient data that have been reported above. Figure 26, mass transfer had become inhibited Tie lines representing interface data from the except for very short-range interactions. Schrei- Coahuila, Santa Luzia, and Goose Lake meteorites bersite continues to grow as long as there is a P 62 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

COAHUILA 5.3%Ni, 08%P

SANTA LUZIA 6.2 %Ni, 2.7%P

1ST ATOMIC % Ni FIGURE 26. —Representative tie lines from Coahuila, Santa Luzia, and Goose Lake meteorites plotted on isothermal sections. NUMBER 21 63 flow to the interface. Growth stops when P imping- % Ni. Equilibrium extending to the 300° C temper- ment reduces the P flux to zero, or, for our ature range would have resulted in kamacite of purpose, when we can no longer measure interface 3.6 atomic % Ni and schreibersite of 31 atomic % relationship changes. Although a theoretical pos- Ni (Table 19, Figure 25). A very different situation sibility, there is no evidence that any of the schrei- is indicated by the tie lines in Figure 26. These bersites studied were in the process of dissolving. represent interfaces from a range of schreibersite As the P flux decreases with decreasing tempera- morphologies from massive schreibersite sur- ture, a given interface tie line would be expected rounded by large volumes of swathing kamacite to to shift to the right to conform to the change in taenite border schreibersite. shape of the a + Ph field of the phase diagram. It The explanation of these differences lies in the would also be expected to move farther to the fact that under conditions of diffusion-controlled right in order to slow the Ni growth rate as the P growth, P can be drawn from much greater dis- supply becomes more limited. The largest schrei- tances than Ni within a given time period. Com- bersites undoubtedly nucleated first and have plete equilibrium was not reached in the 600° to grown at comparatively high rates, drawing P 500° C temperature range. Massive schreibersites from large volumes of metal. They have the lowest had nucleated at much higher temperatures and Ni concentrations. The smaller schreibersites that grown to near their final size. This process had grew at slower rates due to a more limited P reduced the level of P in the complete volume of supply have higher Ni levels. The smallest schrei- metal to near the equilibrium level. Even the bersites would be expected to approach a Ni con- volumes of metal most remote from the large tent given by the tie line whose Ni value in kama- schreibersites probably only contained about 0.6 cite equals the Ni value of the kamacite in which atomic % P at 550° C (the equilibrium value is 0.4 they are growing. Kamacite in the Coahuila mete- atomic % P). Massive schreibersite growth may be orite has a higher Ni value than it would have had viewed as a process that equalizes P levels in if complete equilibrium had been reached. It is meteoritic metal in the 500° to 600° C temperature therefore possible that microrhabdites in Coahuila range regardless of initial P concentrations. Differ- contain more Ni than would be expected from the ences in initial P level are reflected in the amount low temperature equilibrium tie line. Unfortu- of schreibersite produced. nately, the necessary measurements needed to While P diffusion in this temperature range confirm this were beyond the capability of the resulted in a meteorite with a modest P gradient, techniques available. the slower Ni diffusion resulted in an important Tie lines for the Santa Luzia meteorite are segregation of this element. Ni could not be moved shown on a single temperature plot in the center from a sufficiently large distance to maintain bulk section of Figure 26. Kamacite Ni values range equilibrium in the times available. The massive from 2.2 to 6.2 atomic % with the corresponding schreibersites had grown to near their final size, Ni values in schreibersite of 14 to 42 atomic % but their Ni contents had to have been significantly covering a much broader range when compared below the equilibrium value. Their iriterface tie to Coahuila. Kamacite P values at the interfaces lines, therefore, must have been to the left of the range from 0.13 to 0.05 atomic %. The three tie equilibrium tie line. The Ni gradient that probably lines at the left represent the early formed massive already existed within the swathing kamacite zone schreibersites, the three to their right are from became more pronounced. Beyond the swathing grain boundary schreibersite, and the one on the zone, however, sufficient Ni was maintained to extreme right is from a late-stage structure, a yield the observed Wildmanstatten pattern. These small schreibersite partially embedded in taenite. areas of the meteorite have a higher average Ni The Santa Luzia bulk composition lies in about content than the initial bulk Ni content of the the center of the spread of tie lines, demonstrating meteorite. As a matter of fact, some areas of that interface tie lines depend upon local structure kamacite within the Widmanstatten pattern con- and may fall on either side of the ETL. tain more Ni than the accepted bulk Ni value. Had complete equilibrium been achieved for The consequence of massive schreibersite Santa Luzia at 550° C, kamacite would have con- growth under these circumstances of composition tained 5.4 atomic % Ni and schreibersite 14 atomic and cooling is to establish two distinctly different 64 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES structural areas within the meteorite, swathing rhabdites. The rhabdites and grain boundary kamacite areas containing massive schreibersites schreibersites develop analogously to the similar and areas of Widmanstatten pattern (Figure 7). morphologies described above for Coahuila, react- Both types of areas continue to change with de- ing to specific local conditions to achieve appropri- creasing temperature, but distances involved be- ate interface values. Nucleation of grain boundary come increasingly more restricted. The massive schreibersites under these circumstances may take schreibersites continue to grow, their growth rates place at lower temperatures, undoubtedly leading being determined by the flux of P from the swath- to smaller schreibersites with higher initial Ni ing kamacite. Interface Ni values shift to produce content that continues to increase upon further a tie line with a matching growth rate. The shift is cooling. Small rhabdites would also be expected to undoubtedly to the right, resulting in a slower reach high Ni values because they are growing in growth rate due to Ni and Ni enrichment of the a relatively Ni rich kamacite. schreibersite. The final Ni values of these schrei- A final type of schreibersite morphology ob- bersites approximates the equilibrium value at 550° served in Santa Luzia is represented by the tie line C. at the far right in Figure 26. It represents the The outer edge of the swathing zone develops taenite border schreibersite illustrated in Figures as a kamacite-taenite boundary that recedes from 10 and 11. Schreibersite of this type appears to the massive schreibersite with decreasing temper- nucleate within taenite at a taenite-kamacite inter- ature. The final stage in this progression is the face at a very late stage in the structural develop- replacement of the boundary in part by grain ment process. P is undoubtedly supplied mainly boundary schreibersite. P is supplied to the bound- from the kamacite, with most of the Ni coming ary initially by the swathing kamacite, and the from the taenite. Associations of this type yield taenite boundary supplies the initial Ni. Nuclea- the highest Ni schreibersites to be observed in tion of this type results in concentrations that are coarse octahedrites. The schreibersites may also required by the three-phase field of the ternary contain a slight Ni gradient, undoubtedly due to diagram. The initial schreibersite-kamacite tie line the fact that they are associated with kamacite on will be the a + Ph/a + y + Ph boundary for the one side and taenite on the other. If this type of given temperature. With decreasing temperature schreibersite nucleates earlier or grows more rap- the taenite border is replaced with a grain bound- idly due to a particularly high P gradient, it isolates ary that contains grain boundary schreibersite over itself from the enclosing taenite by converting it to about half of its distance. Nucleation may take kamacite. place over a range of temperatures as the taenite Goose Lake meteorite tie lines are shown in the recedes along the forming grain boundary, result- bottom isothermal section in Figure 26. Differ- ing in different initial Ni contents in the schreiber- ences from the Santa Luzia pattern result from site and different initial growth rates. As growth higher bulk concentrations of both carbon and continues at the individual interfaces, Ni concen- Ni. Only the upper portions of the two tie lines on trations adjust to match the P controlled growth the left can be indicated. At some point in the rate. The general tendency will be for tie lines to development of these schreibersites, cohenite pre- move to the right as the P supply dwindles. cipitated in the adjacent metal, transforming the Simultaneously with continued growth of the interface to a cohenite-schreibersite interface and massive schreibersites and development of the preventing further schreibersite growth. The par- exterior swathing zone grain boundary, Widman- tial range of Ni values in kamacite, therefore, is statten pattern development is continuing in the 3.2 to 5.7 atomic %, with Ni values in schreibersite remaining areas of the meteorite. The Ni level is ranging from 16 to 45 atomic %. The value of 45 high enough in these areas so that a coarse Wid- atomic % Ni is the highest observed for a schrei- manstatten pattern forms, containing large kama- bersite in contact with kamacite. Kamacite P values cite lamellae and occasional plessite areas and at the interfaces cover the same range as observed grain boundary taenite. Grain boundaries contain in Santa Luzia, 0.13 to 0.05 atomic %. This range occasional grain boundary schreibersite, and the in P values may indicate that these tie lines do not kamacite lamellae contain rhabdites and micro- all belong on the same isothermal section, a point NUMBER 21 65

that will be discussed in a later section. The bulk A third major type of schreibersite is grain composition again lies in the center of the spread boundary schreibersite. Its petrography suggests of tie lines. that it replaces taenite at late stages in structural The general outline of the schreibersite growth development. Heterogeneous nucleation probably process may be summarized on the basis of this takes place at kamacite-taenite interfaces, resulting discussion and that given in previous sections. In in initial concentrations being controlled by the the meteorites under study schreibersite petrogra- three-phase a + y + Ph field of the ternary phy has been strongly affected by sequential nu- diagram. P is supplied to the nucleation site mainly cleation. Initial schreibersite nucleation is a func- from the bordering kamacite and Ni comes mainly tion of temperature and bulk composition. It took from the taenite. Equation 6 does not describe place at greater than 850° C for all of the meteor- initial growth rates but may well apply to kamacite- ites containing massive schreibersite. The low P schreibersite interfaces after they become isolated Coahuila meteorite is the only exception. These from the influence of taenite. Nucleation temper- massive schreibersites grew rapidly under equilib- atures probably depend upon specific local condi- rium or near equilibrium conditions, at least as tions and may extend to considerably lower tem- far as P is concerned, for most of their growth peratures than those appropriate for rhabdite for- period. Randich (1975) has shown that meteorite mation. cooling rates are sufficiently slow to allow this. It There are three types of associations that may is important to note that massive schreibersite has be included under the term grain boundary schrei- achieved the major portion of its growth by the bersite. They appear to represent a sequence of time it has cooled to 600° C. On subsequent cool- stages in structural development. The first type is ing, Ni is enriched but size does not increase the isolated grain boundary schreibersite. These appreciably. The massive schreibersite growth are elongated schreibersites that lie along kama- process, therefore, has the effect of lowering the cite-kamacite grain boundaries. They frequently P level in the bulk metal and isolating it, as occupy grain boundary junctions and branch ac- schreibersite enclosed in zones of swathing kama- cordingly. Their morphology suggests that they cite, from areas of developing Widmanstatten pat- replaced residual taenite as it receded along the tern. grain boundary. Their Ni values decrease with By 600° C the Widmanstatten pattern is well distance away from the nearest taenite ribbon or established in these meteorites (Table 16). Kama- taenite-plessite area, suggesting that the lowest Ni cite lamellae have grown to near their final size, grain boundary schreibersite nucleated first. An and taenite bands have shrunk to near theirs. alternate possibility is essentially simultaneous nu- Both taenite and kamacite will continue to increase cleation with faster growth accounting for lower in Ni with cooling, but compositional changes will Ni. These schreibersites are frequently somewhat take place over increasingly short distances. It is larger than the higher Ni ones closer to taenite. in this environment, one that contains P in kama- There is no obvious reason, however, why P im- cite at the 0.5 to 0.8 atomic % level, that rhabdite pingment should be more critical at one site than nucleation must take place. They nucleate homo- at another. geneously in kamacite within a narrow range of The second type of grain boundary schreibersite temperatures and grow under conditions of equi- is found in close association with taenite and tae- librium combined with severe P impingment. It is nite-plessite areas. It appears to have formed sim- the limited amount of P and the competition for it ilarly to the isolated grain boundary schreibersites, from closely spaced schreibersites that controls but it either nucleated later or grew more slowly. their growth rates and consequently their eventual It separated itself from taenite but was not able to size. The larger the volume of metal from which a completely absorb it. The final stage in this se- schreibersite draws P, the faster it grows and the quence is the formation of taenite boundary larger it becomes. Tie line shifts permit Ni levels schreibersites. These are the small, highest Ni to vary depending upon growth rates. The larger schreibersites that appear to be embedded in tae- faster-grown rhabdites achieve lower Ni levels than nite at taenite-kamacite borders. These would the smaller slower-grown schreibersites. seem to be the final schreibersites to form. 66 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

INTERFACE DATA AND THE a/a + PH BOUNDARY scale to the right of the diagram is the temperature equivalent to the P concentration at the left, using In the earlier discussion of the extrapolation of the data from Table 18. The dashed line is the a/ experimental data to low temperatures the slope a + y boundary plotted in relation to temperature of the a/a + Ph boundary was mentioned. The and Ni concentration. It is obvious that interface extrapolation shows that the boundary moves to- values for individual meteorites, as is the case for ward the Fe-Ni axis with decreasing temperature the group as a whole, cover a range of both P and and suggests that it is parallel to it (Figures 21, Ni levels. A meteorite has interface values at sev- 22). This led to the simplifying assumption that P eral P levels, and a range of Ni values may be saturation values in kamacite are independent of covered at any given P level. All of the interface Ni and Fe concentrations and may be equated to values are to the left of the a/a + y boundary, the specific temperatures (Table 18). This in turn higher Ni values being closest to it. leads to a possible interpretation of the measured There are at least three factors that must be P in kamacite values at kamacite-schreibersite in- considered as possible contributors to the observed terfaces as being equivalent to a final growth distribution in Figure 27. These are (1) cooling temperature, a temperature below which subse- rates, (2) the possibility of a slopping a/a + Ph quent interface changes would be undetectable by boundary, and (3) the possibility of low tempera- the technique employed. ture nucleation at a/y boundaries and subsequent Observed P interface values cover a range of grain boundary diffusion playing a role in the concentrations from 0.16 to 0.05 atomic %. A growth of grain boundary and taenite border factor of three is involved, indicating that small schreibersite. The available data is insufficient to differences in measured concentrations may have sort out these influences in a quantitative way, but more than trivial significance. The higher P values their expected effects can be outlined and sugges- are associated with massive schreibersite at inter- tive observations mentioned. faces with low Ni kamacite, and the lower P values Cooling rates have only been reported for four are associated with taenite border schreibersite at of the meteorites under consideration here. Values interfaces with high Ni kamacite. Assuming that of 2° C per million years for Lexington County these values represent solubility limits, we can and Goose Lake and 3.5° C per million years for equate them to a temperature range of 445° to 350° Bahjoi were given by Goldstein and Short (1967b). C (Table 18). Measurements on the Santa Luzia The recent measurements of Randich (1975) gave meteorite alone covered most of this range, 0.14 a somewhat higher value of 5° C per million years to 0.05 atomic % P, equivalent to a temperature for Coahuila, and there is no reported value for difference of 430° to 350° C. Is it reasonable to Bellsbank. Kamacite band widths for the other assume that large schreibersites stopped detectable three meteorites were taken from Buchwald growth before the later formed schreibersites, or (1976), and cooling rates were estimated using the does the a/a + Ph boundary really have a down- Goldstein and Short (1967b) cooling rate curves. ward slope with increasing Ni? Are there other Ballinger and Balfour Downs gave cooling rates factors that may make a clear choice between these of 2° C per million years and Santa Luzia gave an two alternatives impossible on the basis of the estimated 0.8° C per million years. These numbers available data? indicate that cooling rate values for this group of The P and Ni kamacite interface data is summa- meteorites cluster in a rather narrow range center- rized in Figure 27. The points represent individual ing around 2° C per million years. interface Ni and P concentrations in kamacite and The effect of cooling rate variations on schrei- are plotted using the axes at the left and at the bersite-kamacite interface values has been dis- bottom of the diagram. The symbols indicate the cussed above. It is to be expected that slower meteorite from which the data were taken. There cooling rates would produce lower detectable in- is a general trend of decreasing P concentration in terface values of P in kamacite and lower levels of kamacite with increasing Ni concentration. The P in kamacite generally. This would be equivalent relationship of these values to the estimated a/a to detectable growth continuing to lower tempera- + y boundary of the Fe-Ni binary diagram of tures. This type of variation may be present on a Goldstein and Olgivie (1965) is also indicated. The minor scale in the data considered here. Coahuila NUMBER 21 67

A a,BELLSBANK

• b, COAHUILA

• C, SANTA LUZIA

d, BALLIN6ER

• e, LEXINGTON CO.

0.30 O f, BAHJOI 500 A Q, 600SE LAKE 490 O25 D h, BALFOUR DOWNS a + y 480 0.22 470 0.20 0.19 460 0.18 0.17 450 0.16 AA 440 0.15 0.14 430 420 a* o.i2 SJO.II 410 | 0.10 400 (A CCE A A|»^ Jft. < 0.09 390 0.08 380

0.07 *co 370

0.06 360 350 0.05- 340 0.04 330 320 0.03 310 300 29O 0.02 280

I 23456789

ATOMIC %Nia FIGURE 27. —P in kamacite at kamacite-schreibersite interfaces plotted against Ni in kamacite (temperature scale at right relates P saturation values to a/a + y boundary of Fe-Ni system). 68 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES does have the highest reported cooling rate of 5° or plotting on the wrong isothermal section. We C per million years, and its interface P values and saw in Figure 26 that all of the measured tie lines P levels in kamacite do seem to be somewhat for three of the meteorites can be plotted without higher than for some of the other meteorites obvious cases of overlap. This could mean they all studied. Bellsbank also seems to have unusually belong to the same isothermal section, but it could high levels of P in kamacite, and this could be the also be fortuitous. If interface data for all of the consequence of a comparatively high cooling rate. meteorites could be plotted as tie lines on a single The points representing both Coahuila and Bells- intelligible drawing, several cases of overlap would bank interfaces are in the upper part of Figure be observed. When tie lines are separated on the 27, perhaps meaning that detectable change ceased basis of P level as in Figure 28, no serious cases of at higher temperatures for these two meteorites overlap are encountered. The two tie lines at the due to higher cooling rates. left on the 0.09 atomic % P section do overlap An a/a + Ph boundary (AB in Figure 20) that slightly, but a small shift in Ni values could do slopes downward from the Fe-P axis toward the away with it. The surprising point is how little Fe-Ni axis would contribute to the type of data overlap is seen when the data are taken directly distribution observed in Figure 27. At any given from the tables given earlier and plotted in this temperature lower Ni schreibersites would have a way. higher P interface value in kamacite than higher Had all of the tie lines in Figure 28 been plotted Ni schreibersites. The range of the data, however, on a single isothermal section, there would have seems to require much too steep a slope to be been several cases of overlap. The most serious reasonable in terms of the extrapolation given offender would be a Bellsbank tie line, the second above. An Fe-P axis intercept value of approxi- one from the left on the 0.09 atomic % P isother- mately 0.2 atomic % would be indicated. This mal section. If it were plotted on the top section would mean either an unreasonably high P satu- of Figure 28, there would have been obvious ration temperature in the neighborhood of 465° C overlap with another Bellsbank tie line. All three (Table 18), or that the actual curve bends sharply of the tie lines in this top isothermal section are to the right of the indicated extrapolation (Figure from the Bellsbank meteorite, and there is addi- 21). It also seems unlikely that error in locating tional evidence to be discussed below that suggests the data points is a major factor. It was pointed that two temperatures are involved. out above that reproducibility of P determinations Plotting tie lines as has been done in Figure 28 was ±0.005 weight % and for Ni was ±0.1 weight supports the idea that a temperature effect is real. %. This means that the P levels shown are distin- That is to say that late formed schreibersites con- guishable from their nearest neighbors with rea- tinued significant growth to temperatures below sonable certainty and that the Ni variations are those of early formed schreibersites. Unfortu- meaningful. Ni variations at a given P level range nately, these meteorites do not supply us with a from 0.8 to 3.0 atomic %. Variations for a specific full range of tie lines at any one temperature. meteorite at a given P level are not so wide, but More work will be required before the slope of they do range up to 1.5 atomic %. This range in the a/a + Ph boundary within this low tempera- Ni values for a single meteorite at a given P level ture range can be established with certainty. suggests that factors other than simply a sloping A third possibly complicating process that could a/a + Ph boundary are involved. The boundary be of importance here has been discussed by may well have a gentle slope, but other factors Comerford (1969). He pointed out that grain must also contribute to the observed distribution. boundary diffusion might play an important role Tie lines representing three of the P levels in in the development of grain boundary schreiber- Figure 27 are plotted on isothermal sections in sites. He suggested that grain boundary diffusion Figure 28. The symbols used in Figure 27 are would supply Ni to schreibersites growing at low used at the schreibersite ends of the tie lines to temperatures more readily than lattice diffusion. identify the meteorite represented. Tie lines ac- The discussion above has pointed out that P is the tually belonging to a given isothermal section will controlling element rather than Ni, but his sugges- fan out across the field without overlap. When tion would be equally valid for P. overlap occurs it implies either error in the data The measurements on grain boundary schrei- NUMBER 21 69

0.09 ATOMIC %P 395° C

4.4

0 10 20 30 40 100 % Fe ATOMIC %Ni FIGURE 28. —Tie lines from various meteorites separated according to interface P saturation values in kamacite for plotting on isothermal sections. 70 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES bersite in the Santa Luzia meteorite are represent- small schreibersite size and lower temperature of ative of many such measurements reported above growth. (Figures 10, 11). Petrographic relationships sug- Grain boundary diffusion may also help explain gest that this type of schreibersite nucleated at the Bellsbank interface measurements summa- kamacite-taenite interfaces absorbing residual tae- rized in Figure 5. Four of the seven interfaces nite. Nucleation temperatures were probably low shown have a P level in kamacite of 0.09 weight % compared to those for homogeneously nucleated P (0.16 atomic %), and the other three are at 0.05 rhabdite, but the data does not allow a closer weight % P (0.09 atomic %). The schreibersite temperature estimate. It would also appear that interface illustrated at the top of the diagram is a nucleation occurred over a range of temperature, typical one for massive schreibersite. The schrei- nucleation of the lowest Ni schreibersite farthest bersite illustrated at the lower left is a large rhab- along the grain boundary away from taenite hav- dite (Table 3, traverse 6) surrounded by clear ing taken place first (Figure 11, traverse 11). The kamacite within an area of microrhabdites. There Ni concentration of schreibersite nucleated at a were no grain boundaries or subgrain boundaries kamacite-taenite border is that of the schreibersite observed. The schreibersite illustrated at the lower corner of the three-phase field at the nucleation right is another large elongated rhabdite (Table 3, temperature. The presence of the phase boundary traverses 4 and 5) that was observed to be along a probably serves both as a site for nucleation and definite subgrain boundary. Both of these rhab- as a source of P through the process of grain dites have extremely large P gradients, and their boundary diffusion. As the schreibersite becomes P interface values differ by 0.04 weight % P (0.07 larger, however, the contribution of grain bound- atomic %). The center schreibersite in the lower ary diffusion becomes increasingly less important. section of the diagram is a large rhabdite (Table The schreibersite grows rapidly, with its Ni level 3, traverses 2 and 3) along a subgrain boundary increasing only slightly. Its interface with kamacite with different P and Ni interface values in kama- represents a tie line that probably tends to shift to cite on its opposite sides. Tie lines representing the left with decreasing temperature, as P is drawn these interfaces are given in Figure 28. The three from a relatively large diffusion field. high P tie lines are in the top isothermal section Along the grain boundary at a location closer to and the fourth, representing the low P interfaces, the final residual taenite a second grain boundary is at the left in the middle section. A possible schreibersite nucleates at a lower temperature (Fig- rationale for these observations is that Bellsbank ure 11, traverse 12). Its initial Ni is higher, and cooled more rapidly than the other meteorites the P diffusion rate is lower. Grain boundary studied. As a result, growth apparently stopped at diffusion again makes an important contribution higher temperatures than has been the case for during initial growth, but it probably becomes other rhabdite-containing meteorites. The pres- increasingly less important as the schreibersite ence of grain boundaries, however, permitted P increases in size. flow to continue to significantly lower tempera- The observed taenite border schreibersite un- tures for the two schreibersites with the lower doubtedly nucleated at a still lower temperature interface levels. A faster type of diffusion permit- (Figure 11, traverse 13). It formed at the taenite- ted detectable interface changes to extend to lower kamacite border and had a high initial Ni concen- temperatures. It would be interesting to find simi- tration. Growth was insufficient to separate the lar situations in other meteorites. schreibersite completely from taenite. A small schreibersite that has a Ni gradient due to the SCHREIBERSITE WITH COHENITE BORDERS fact that it is in contact with kamacite at one end and taenite at the other resulted. This type of A prominent feature of the meteorites selected schreibersite has both the highest observed Ni for study is the presence of 100-to-200-/Lim-wide concentrations and the lowest observed P interface cohenite borders separating massive schreibersite values in kamacite. A reasonable explanation of from kamacite. It is a feature that can be recog- these observations is that grain boundary diffusion nized with the unaided eye on many well-prepared plays an increasingly important role both with exhibit sections of Group I meteorites, but it has NUMBER 21 71 been little noted in the literature until relatively minimum temperature at which cohenite could recently. Cohenite borders may partially or com- have formed is at or below the temperature at pletely enclose schreibersite bordering troilite in- which the schreibersite achieved its final Ni conclusions or massive schreibersite isolated within centration. According to the Fe-Ni-P diagram this kamacite. This type of association has been re- would be at 550° C or below. In this low tempera- ported above for the Ballinger (Figure 6), Lexing- ture range schreibersite would be surrounded by ton County (Figure 12), Bahjoi (Figure 13), Goose large volumes of kamacite. Is kamacite a reasona- Lake, and Balfour Downs (Figures 15, 16, 17) ble source for the large amount of carbon required meteorites. The Ni concentrations in these cohen- for cohenite growth? The work of Brett (1967) ite enclosed schreibersites range from 16 to 21 and A. D. Romig, Jr. (1977, pers. comm.) on the atomic %, comparable to those observed for simi- Fe-Ni-C system seems to require an alternate ex- lar unenclosed schreibersites within the same me- planation. teorite or within similar meteorites. The cohenite Consideration of the Goose Lake meteorite borders contain small amounts of Ni and no de- structure in terms of both its cooling history within tected P. Their Ni profiles show a gradient varying the Fe-Ni-P system (Table 14) and Romig's Fe-Ni- from as much as 1.7 weight % Ni at the cohenite- C diagram may be instructive at this point. Goose kamacite interface to as low as 0.8 weight % Ni at Lake is a meteorite that has large amounts of the cohenite-schreibersite interface (Figures 13, massive schreibersite. Surface areas totaling more 17). than 1000 cm2 were examined and found to con- Similar Ni gradients in cohenite borders have tain more than 20 areas of clustered massive schrei- been reported previously by Drake (1970) and bersites. All of these schreibersites appeared to be Scott (1971a). Drake (1970) noted that Ni diffuses enclosed in cohenite, although poor condition of through cohenite, increasing the Ni content of surfaces left some ambiguity in one or two cases. enclosed kamacite and taenite grains with decreas- In any event, the ubiquitous nature of cohenite ing temperature, but he felt that cohenite shells bordering massive schreibersite in Goose Lake was around schreibersite prevented equilibration of established. This suggested that Goose Lake might schreibersite with the enclosing metal. Scott be expected to yield a more straight-forward inter- (1971a) interpreted the presence of Ni gradients pretation of cohenite border growth than would in cohenite borders as implying equilibration of be possible using meteorites where the association Ni to low temperatures. The observed Ni concen- was more sporadic. trations in cohenite-enclosed schreibersite sup- Equilibrium compositions of phases derived ports Scott's interpretation. from the Fe-Ni-P system for a range of decreasing Massive schreibersite growth is apparently temperatures are given in the lower section of blocked by enclosure in cohenite. The absence of Table 14 for the assumed Goose Lake meteorite detectable P in, these cohenites indicates that P bulk composition. At 925° C schreibersite is in solubility is very low. It seems reasonable to as- equilibrium with 96% taenite. On cooling to 750° sume, therefore, that cohenite borders isolate C the schreibersite doubles in amount to 8% and schreibersite from a source of additional P. The increases in Ni from 5.7 to 6.6 atomic %. Taenite situation with Ni, however, is different. The co- is slightly reduced in volume during this process, henite borders contain appreciable Ni, and the but its Ni concentration remains essentially con- measured Ni gradients suggest a flow of Ni to the stant at 7.9 atomic %. Cooling through this 175° C cohenite-schreibersite interfaces. Under these cir- range has resulted primarily in the localization of cumstances the size of the schreibersite indicates P. Massive schreibersites have grown within taenite the maximum temperature at which cohenite and have achieved most of their eventual size. could have nucleated. The temperature must have In the 750° to 650° C temperature range major been low enough to permit schreibersite to reach structural changes take place. Taenite increases in its full size. At this maximum temperature, or at Ni from 7.9 to 11 atomic % and decreases in some lower temperature, cohenite encases the volume by one-half. Schreibersite increases in Ni schreibersite. Growth is stopped but Ni enrich- from 6.6 to 9.0 atomic % and increases in volume ment continues with decreasing temperature. The by approximately 10%. The shrinking taenite is 72 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES converted to kamacite of 5.0 atomic % Ni, which TABLE 20. —Carbon saturation values for taenite in makes up nearly half the structure at 650° C. With equilibrium with cohenite from the Fe-Ni-C diagram of A. D. Romig, Jr. (taenite Ni values are further decreasing temperature, kamacite ex- equilibrium values using the Goose Lake bulk com- pands in volume, taenite shrinks, and schreibersite position, Table 14). holds its own. All three phases continue to increase in Ni. °c Weight % C Atomic % Ni What happens if carbon is also present in the 730 0.75 system? Romig's Fe-Ni-C phase diagram gives car- 650 0.45 n bon saturation values for taenite in equilibrium 600 0.36 14 with cohenite. These are given in Table 20 for selected taenite compositions for the range of 500 0.18 •v25 temperatures covered. The taenite Ni values given are those of taenite in equilibrium with schreiber- final size, perhaps in the 650° to 700° C tempera- site at the indicated temperature and the Goose ture range, assuming the Goose Lake bulk com- Lake bulk composition using the Fe-Ni-P system position. These schreibersites are in contact with (Table 14). Carbon has appreciable solubility in taenite that has become saturated in carbon. With taenite throughout this temperature range, but its further cooling both taenite and schreibersite re- solubility in kamacite is negligible. This solubility quire more Ni, and taenite requires a repository of carbon in taenite means that a meteorite such for its carbon. Under these circumstances it is as Goose Lake with a structure consisting of 92% reasonable to assume the following nucleation re- taenite at 750° C might well contain enough dis- action taking place at taenite-schreibersite inter- solved carbon to produce significant amounts of faces: cohenite on subsequent cooling. The important OKQat) + Ph = y + cohenite + Ph. factor is that the amount of taenite present is drastically reduced with decreasing temperature. A cohenite border at the preexisting taenite-schrei- This results in carbon concentration increasing in bersite interface results. Cohenite becomes a re- the ever decreasing amount of taenite. Supersatu- pository for carbon and the Ni that previously ration must result at some temperature if sufficient resided there is absorbed by taenite and/or schrei- carbon were present to start with. bersite. As the structure develops, a dynamic equi- Brett (1967) assumed that P could be neglected librium involving two interfaces is established; car- in his discussion of cohenite formation in meteor- bon saturated taenite-cohenite and cohenite- ites. That may be true for some types of cohenite schreibersite. Cohenite grows as long as carbon associations, but it may well not hold for cohenite saturated taenite is present. Taenite shrinks in bordering massive schreibersite. It is difficult to volume, cohenite fills in behind the receding tae- understand these associations using the Fe-Ni-C nite, and schreibersite becomes enriched in Ni. At system alone. Brett (1967) shows that in this system some point kamacite precipitates at the cohenite- cohenite grows by the following reaction: taenite interface and then kamacite becomes the source of Ni for transport to schreibersite. The y = cohenite + a. cohenite border is established and subsequent He also points out that when the Ni content is changes involve diffusion of Ni and Fe only. high, kamacite and graphite form with falling The postulated reaction sequence would permit temperature. Cohenite is what we see, however, cohenite to grow over a broader temperature not significant amounts of graphite in kamacite. range than was suggested by Brett (1967). The If we assume that carbon is present in taenite, maximum temperature can be estimated from the then we are actually dealing with the Fe-Ni-P-C size of the cohenite-enclosed schreibersite. What system. The more complex system permits the is the highest temperature that could have pro- postulation of the following sequence of events to duced schreibersite of the observed size starting account for massive schreibersite with cohenite with a given bulk composition? In a meteorite like borders. Massive schreibersite nucleates at high Goose Lake the temperature might be as high as temperature and grows upon cooling to near its 750° C. The minimum temperature of nucleation NUMBER 21 73 would be established by the disappearance of suf- has led to a more comprehensive description of ficient taenite to supply the required carbon super- the structural development process in these indi- saturation. It seems unlikely for the meteorites vidual meteorites than has been available previ- that have been studied here that this would be ously. below 600° C. This postulated growth sequence Massive schreibersite, one of the four major could extend Brett's (1967) temperature range of types of schreibersite encountered, may be ac- 610° to 650° C upward, but it is unlikely that it counted for by equilibrium considerations. The would be lowered appreciably. If Brett's (1967) Fe-Ni-P equilibrium diagram and estimated bulk estimated rates of cohenite decomposition with compositions for individual meteorites were used temperature are correct, nucleation within the to explain the basic structures present. Subsolidus lower part of the possible temperature range nucleation and growth with slow cooling from would favor preservation of cohenite. Confirma- temperatures at least as high as 850° C, and prob- tion of this proposed sequence of events will have ably much higher, explain the phase relationships to await the results of detailed experimentation. that one sees in meteorite specimens. No need was found to invoke eutectic liquids to account for Summary and Conclusions any of the observed massive schreibersite morphologies. After nucleation, schreibersite and The role that schreibersite growth played in the kamacite grew in volume and increased in Ni structural development process in coarse-struc- content while taenite receded and increased in Ni tured iron meteorites has been examined. The content under equilibrium or near equilibrium availability to the writer of many large meteorite conditions into the 600° C temperature range. The surfaces and an extensive collection of metallo- retention of taenite in the octahedrites establishes graphic sections made it possible to undertake a that bulk equilibrium did not extend as low as 550° comprehensive survey of schreibersite petrogra- C. Schreibersite undoubtedly continued in equilib- phy. This study was the basis for the selection of rium with its enclosing kamacite to lower temper- samples for detailed electron microprobe analysis. atures. The growth of massive schreibersite from Samples containing representative structures from high temperatures into the 600° C temperature eight chemical Groups I and IIAB meteorites were range served both to localize P by reducing its selected. All of these meteorites contain schreiber- level throughout the bulk metal and to produce site, ranging in amount from easily detectable to the large swathing kamacite zones that are ob- abundant, and they contain additional P in solu- served. tion in kamacite and taenite. Their detailed metal- The second type of schreibersite to form is lography indicates that they are comparatively free homogeneously nucleated rhabdite. It nucleated of secondary effects such as shock and reheating in kamacite in the 600° C temperature range, that might mask or distort relationships of interest. either as a consequence of low initial P level or The range of bulk compositions covered is one after local P supersaturation developed following where small changes in P-Ni concentration rela- massive schreibersite growth. Growth of these tionships may be more important to the structural schreibersites was also under equilibrium condi- development process than would be the case at tions to below 500° C. They drew their source of P higher Ni levels. from limited diffusion fields when compared to Lengthy electron microprobe traverses were the massive schreibersites. made across structures representative of the ob- The third type of schreibersite is grain boundary served range of schreibersite associations, with the and taenite border schreibersite. It formed at exception of microrhabdites. Particular emphasis kamacite-taenite interfaces, absorbing residual tae- was placed on schreibersite-kamacite interface nite. Nucleation took place successively along grain compositions. Ni and P interface concentrations boundaries over a range of temperatures starting in kamacite and Ni interface values in schreibersite as high as 500° C or perhaps slightly higher. With were determined, along with associated Ni and P decreasing temperature taenite receded and be- concentration gradients in kamacite and Ni gra- came richer in Ni and the schreibersite that nu- dients in schreibersite. An analysis of these data cleated at these interfaces had higher Ni. The 74 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES highest Ni schreibersites are the small ones that on meteoritic schreibersites, therefore, yield a nucleated at low temperatures and are observed range of tie lines in the low temperature Fe-Ni-P still in contact with taenite. Grain boundary diffu- system. Equilibrium conditions pertained at sion probably became an increasingly important growth interfaces to temperatures far below those factor in the growth of these schreibersites with available experimentally. Kinetic factors, however, decreasing temperature. restricted mass transfer to increasingly small vol- The fourth type of schreibersite is microrhab- umes of material with decreasing temperature. dite. These schreibersites nucleated homogene- Schreibersite with cohenite borders is a common ously in supersaturated kamacite at temperatures occurrence in many meteorites. It is suggested in the 400° C range or below. that it formed as a reaction in the Fe-Ni-P-C P diffusion controlled the growth rate of schrei- system in the 750° to 600° C temperature range. bersite. The Ni flux to a growing interface had to Carbon saturated taenite reacted with schreibersite produce a growth rate equal to that established by to form cohenite. Cohenite then grew in equilib- the P flux. This was accomplished by tie line shifts rium with both taenite and schreibersite. that permitted a broad range of Ni growth rates Berzelius' prescient comment of 1832 on the and accounts for the observed range of Ni concen- role of P in the development of the Widmanstatten trations in schreibersite. Interface measurements pattern has been confirmed and expanded upon. Appendix

In the discussion of the low-temperature ternary In order to derive a relationship for the solvus diagram, experimental values for P saturation con- line (kamacite saturated with P) as a function of centrations in kamacite and taenite were extrapo- temperature (T), we must consider a situation lated to lower temperatures (Table 17, Figures 21, where AG for the reaction is not zero. This may 22). This is a commonly used procedure when be done by assuming a P concentration in kama- dealing with dilute solutions and is a variation on cite, Xg, such that Xp" is within the kamacite field a method used for the determination of partial of the equilibrium diagram molal properties of substances. It is applicable to a solid state transformations in dilute metallic sys- X P -P(sat)- tems and may be justified by the following ther- modynamic reasoning adapted from Swalin Kamacite of this concentration would react with (1972:168-171). schreibersite to form P-saturated kamacite,

At a given temperature (Tx) the end points of a aP + Ph «* «P(Sat) AGX. kamacite-schreibersite tie line in the kamacite- schreibersite field of the equilibrium diagram rep- The following equation may be written relating resent equilibrium concentrations of the two the Gibbs free energy ( AG2) for the above reaction, phases. The relationship between P-saturated the chemical potentials of the reacting species, kamacite (ap(Sat)) and schreibersite (Ph) may be and the activities of these species, expressed as follows: ph ttp(sat) ^ Ph + a AG = 0. AGX = n% - At°P = RT In -^ • Since the reaction represents equilibrium the change in the Gibbs free energy (AG) is zero. for a pure substance, Therefore, the chemical potentials (fi) of P in the two phases are equal, aPh = 1

f^P M'P(sat) for a dilute solution following Henry's law as are their activities (a), a? = yAX® •Ph _ „<*

aP — aP(sat). Since schreibersite is a pure substance with respect and from equation given above toP AP -•Ph _ aP — •(sat-m)) = ^-PCsat) requiring that

a Substituting these values of a and a in the P(sat) — 1 • Ph P expression for AGX given above we obtain If the saturation concentration of P in kamacite is sufficiently small, P may be assumed to follow fi$ - n° ph = RT In Xg - RT In Xp\ - RT In (1). Henry's law and the activity coefficient of P in P sat) kamacite, y?, may be evaluated. We know that the enthalpy of a component i, H,, is related to the temperature and chemical poten- aP(sat) — 7P AP(sat) tial by 1 = Hi.

75 76 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

Substituting, we obtain 3(1/'T) = - 3(ln R

- Hph -AH In Xp( = F Sat) R d(RT In Xg/T) a(RT In Therefore, when Xp\sat) is plotted on a log scale against 1/T, a straight line with slope — AHP/R should result. Straight lines with negative slopes 3(1/1) are observed in our case, and it is reasonable to assume on the basis of experience in other systems Xp and (1) are constants, dropping out these two that AHPa is independent of temperature over a terms, reasonable temperature range. It would seem then that an extrapolation over a reasonable tempera- d(lnX ) H a - H = - R Hsat) ture range is justified. A similar treatment would P Ph a(VT) apply to P-saturated taenite data. Literature Cited

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