SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES • NUMBER 21

Schreibersite Growth and Its

Influence on the Metallography

of Coarse-Structured

Iron Meteorites

Roy S. Clarke, Jr.

and Joseph I. Goldstein

SMITHSONIAN INSTITUTION PRESS

City of Washington

1978
ABSTRAC T

Clarke, Roy S., Jr., and Joseph I. Goldstein. Schreibersite Growth and Its

Influence on the Metallography of Coarse-Structured Iron Meteorites. Smithsonian 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. Nucleation 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

Meteorites are currently understood to be the

oldest rocks available for scientific study, containing components and structures that span the period from the final stages of solar nebula condensation to the present (Anders, 1962, 1971; Grossman and Larimer, 1974; Wasson, 1974). They are

fragments of parent bodies that accreted from

preexisting aggregations of material during the

period of planetary system formation some 4.6

billion years ago. These parent bodies were subsequently disrupted into smaller objects that then

had independent existences in space. Individual

fragments eventually intercepted the earth and

landed as recoverable meteorites. Meteorite structures and compositions have undergone varying

degrees of modification while resident in their

parent bodies, as small bodies in space, on their

passage through the atmosphere, and on landing

on the earth's surface. Further changes result

from long residence time on the ground and may

continue during storage in collections. Bearing

evidences of these complex histories, meteorites

are samples not only from a far distant place but

also from a far distant time, having been preserved

in a remarkably gentle environment when comRoy S. Clarke, Jr., Department of Mineral Sciences, National Museum

of Natural History, Smithsonian Institution, Washington, D. C.

20560. Joseph I. Goldstein, Department of Metallurgy and Materials

Sciences, Lehigh University, Bethlehem, Pennsylvania 18015.

pared to terrestrial or lunar rocks. As a consequence, meteorites have a unique place in the

study of the development of the planetary system,

yielding information that is available from no

other source.

Iron meteorites represent only a small fraction

of the meteorites that have been observed to fall,

about five percent. Fortunately, many specimens

from ancient falls have been preserved under such

conditions that deterioration has not been severe

and are available for study (Buchwald, 1976). If

this were not the case, our view of iron meteorite

compositions and structures would be much narrower than it is. Only one of the meteorites used

in this study is an observed fall.

The spectacular metallographic structures revealed on prepared surfaces of iron meteorites

have fascinated scientist and dilettante alike for

more than 160 years (Perry, 1944). The Widmanstatten pattern is the historical distinguishing characteristic of the populous octahedrite classes of

iron meteorites (Goldstein and Axon, 1973). At an

early date this structure was understood to result

from very slow cooling at relatively low temperatures in the meteorite parent body. The temperature range through which this structure develops

is now believed to be from 700° to 350° C, temperatures well below those where silicate systems undergo change. Interpretation of metallographic

structures, therefore, gives information on a late,

low temperature period in the history of a parent

1
SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

body, information that is not readily available

from other types of meteorite studies.

Modern interpretations of iron meteorite structures have been based on the Fe-Ni equilibrium

diagram (Goldstein and Olgivie, 1965a), as it was

the only applicable system known with sufficient

accuracy in the low temperature range required.

The Widmanstatten pattern, therefore, resulted

simply from taenite transforming to kamacite with

decreasing temperature as a solid state, diffusion

controlled reaction. Recently the iron-rich corner

of the Fe-Ni-P equilibrium system has been determined experimentally down to 550° C, leading to

new possibilities for investigating meteorite structures (Doan and Goldstein, 1970). In this study

the metallography of a group of schreibersite-containing coarse-structured iron meteorites has been

examined using this low temperature Fe-Ni-P diagram. It will be shown that equilibrium growth at

higher temperatures combines with diffusion-controlled growth at lower temperatures to explain

observed structure more comprehensively than

was possible using the simpler system. Schreibersite growth becomes an integral part of the overall

structural development process, and schreibersite

may no longer be considered an "inclusion" that

can be safely ignored when structural development

is discussed.

ACKNOWLEDGMENTS

A number of colleagues and associates have

contributed invaluable assistance throughout the

course of this study. The support and encouragement of departmental chairman William G. Melson and his predecessor Brian Mason are acknowledged with sincere thanks. Erik Randich provided

penetrating discussion of many of the problems

studied here, and his analysis of ternary diffusion

as applied to schreiberate growth and meteorite

cooling rates was made available to us prior to

publication. A. D. Romig, Jr., of Lehigh University made available to us data on the low temperature Fe-Ni-C system prior to publication. Eugene

Jarosewich and Charles R. Obermeyer III furnished unstinting help with the electron microprobe work. Grover C. Moreland and Richard

Johnson prepared many excellent polished sections. Ann Garlington and Kathy P. Porter were

scrupulous in their attention to the details of

manuscript preparation. Dante Piacesi, Jr., of the

Office of Computer Services was responsible for

the preparation of data reduction programs. A

grant from the Secretary's Fluid Research Fund

supported a brief visit to Cambridge University,

Cambridge, England, in the fall of 1974, where a

serious beginning was made on the interpretation

of the data. The support of the Geochemistry

Program, Division of Earth Sciences, National Science Foundation, through Grant No. DES 74-

22518 is also acknowledged. The acronym USNM

(for the former United States National Museum)

is used for catalog numbers in the National Museum of Natural History, Smithsonian Institution.

This paper was presented to the George Washington University, Washington, D.C., by R. S.

Clarke, Jr. in partial fulfillment of the requirements for the Ph.D. degree.

Historical Background

INTRODUCTION

Meteorites, with their dramatic arrivals on earth,

exotic origins, and unusual structures, have attracted much more than routine scientific interest

ever since they became respectable objects of study

at the end of the 18th century. Many of the leaders

in the development of 19th-century physical science made significant contributions to their study,

and several of them will be mentioned below. The

older literature is extensive, but it is mainly obsolete for other than descriptive and historical purposes. A critical review of this material was not

attempted, but a survey of selected early developments will be given. Scientific interest in schreibersite dates from near the beginning of meteorite

studies, and it is interesting to place modern work

in this perspective.

The first major development in the metallography of meteorites was the discovery of the phenomenon that has become known as the Widmanstatten pattern. This structure was first discovered

by William Thomson, an Englishman living in

exile in Naples. He published a French version of

his findings along with a good illustration in a

Swiss journal in 1804. What appears to be an

identical paper, only in Italian, dated 6 February

1804, was published in Siena four years later (G.

Thomson, 1804, 1808). Modern writers on meteoritics have been unaware of the earlier paper by
NUMBER 21

Thomson, and this has led to confusion as to

whether Thomson or Von Widmanstatten really

had priority. The record seems to be unambiguously in favor of Thomson. Marjorie Hooker of

the U.S. Geological Survey retrieved this 1804

paper, and its existence was mentioned in a footnote in a biographic study of Thomson by C. D.

Waterston (1965: 133). The writer ran across Waterston's paper among unindexed material in the

F. A. Paneth meteorite literature collection that

was deposited in the Smithsonian Institution in

1974.

Alois von Widmanstatten, Director of the Imperial "Fabrik-Produkten-Cabinett" (Industrial Products Collection), Vienna, discovered the phenomenon that now bears his name independently in

1808. He devoted many years to its study and

circulated his observations privately, drawing the

phenomenon to the attention of students of meteorites. Von Widmanstatten's observations were finally published in 1820 by his co-worker, Carl von

Schreibers (1820). The history of the discovery of

the Widmanstatten pattern has been reviewed by

Paneth (1960) and Smith (1960, 1962). Smith's

review contains excellent reproductions of early

prints and a lengthy translation of Schreibers'

description of Widmanstatten's work. These early

reproductions of iron meteorite structures by Widmanstatten not only represent an advance in the

science of meteoritics but also one in the art of

printing.

During the 19th century descriptive studies of

meteorites advanced with the broadening base of

understanding in mineral chemistry and related

physical science. Many of the iron meteorites

known today were already represented in collections, and a number of them had been carefully

described in the literature. The major phases of

iron meteorites had been characterized and given

mineral names: kamacite (low-nickel Ni-Fe, ferrite, a-iron, b.c.c), taenite (high-nickel Ni-Fe,

austenite, y-iron, f.c.c), plessite (fine-grained intergrowth of kamacite and taenite), troilite (FeS),

cohenite (Fe3C), and schreibersite ((Fe,Ni)3P) were

all known and their bulk compositions understood.

Accessory minerals such as daubreelite (FeCr2S4),

chromite (FeCr2O4), and graphite (C) were also

known. The Widmanstatten pattern was understood to have formed by slow cooling under crystallographicly controlled conditions, kamacite

plates forming parallel to the faces of an octahedron, giving the name octahedrite to those meteorites that displayed this pattern on polished and

etched surfaces.

19TH-CENTURY STUDIES OF SCHREIBERSITE

Schreibersite was first characterized in 1832 by

the great Swedish chemist J. J. Berzelius (1832a,b,

1833, 1834). He isolated brittle, silver-white grains

of a magnetic material from an acid insoluble

residue of the B©humilitz, Bohemia, coarse octahedrite. Berzelius' elaborate analytical procedure

yielded a surprisingly good analysis for the time,

demonstrating that his material was an iron-nickelphosphorus compound containing about 14 percent phosphorus and considerably more nickel

than the bulk of the meteorite. In 1834 he reported

similar analyses for schreibersite from the Elbogen, Bohemia, medium octahedrite and the Krasnojarsk, Siberia, pallasite (the historic Pallas Iron).

It is also interesting to note that Berzelius' concept

of the Widmanstatten pattern included the element phosphorus, as is indicated by the following

quotation:

Es mochte wahrscheinlich seyn, dass die sogenannten Widmanstddtschen Figuren einer, der hier analysirten Schuppen analogen, Verbindung zwischen Eisen, Nickel und Phosphor, zuzuschreiben seyen, welches zu erforschen ich aus Mengel an

Material verhindert bin.

Berzelius (1832b:297)

Early descriptions of schreibersite occurrences

were confounded by inadequate separation techniques, poor analytical methods, and complications arising from both the varied morphology

and the wide range in composition that is typical

of this mineral. C. U. Shepard (1846, 1853) analysed schreibersite and attempted to assign a mineral name to it. His suggestion, however, was

supported by inadequate data. Credit for naming

schreibersite belongs to A. Patera, whose recommendation was reported by Haidinger (1847). The

name honors Professor Karl von Schreibers (1775-

1852), director of the Imperial Cabinet, Vienna, a

pioneer worker on meteorites and colleague of

Von Widmanstatten.

The writings of J. Lawrence Smith, a distinguished chemist of the middle part of the last

century (Silliman, 1886; Phillips, 1965), include

numerous examples of early work on meteorites.
SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

His description of the Tazewell, Tennessee, finest

octahedrite contains a lengthy discussion of his

observations (Smith, 1855). At this early date he

recognized the name schreibersite and applied it

to the correct mineral. He reports having identified schreibersite by visual inspection in a number

of iron meteorites from "the Yale College Cabinet," where it had previously been thought to be

pyrite. Smith's low nickel value for Tazewell

schreibersite is suspect and his attempt at a formula was unsuccessful, but he clearly recognized

the species and understood which elements were

its major constituents. This work was undoubtedly

done, at least in part, in the chemical laboratory

of the Smithsonian Institution. This laboratory

was organized by Professor Smith in 1854, and

Secretary Joseph Henry reported that during the

year "he also made a series of analyses of meteorites, among which were fourteen specimens from

the cabinet of James Smithson, the founder of the

Institution" (Goode, 1897:614).

Confusion in nomenclature, however, persisted

in the literature for many years. Von Reichenbach

(1861) discussed schreibersite under the descriptive

term "Ganzeisen" and also used the name lamperite. Both of these names are now part of the

synonymy of schreibersite. Rose (1865) introduced

the synonym rhabdite into the literature, a name

derived from the Greek word for "rod." This

term is still in common use by meteoriticists, but

only when morphological distinctions are of interest. Rhabdite is the term used for the small schreibersite crystals occurring in the kamacite of iron

meteorites. In polished sections these small schreibersite crystals have rhombohedral cross sections

that in some cases are elongated, suggesting rods

or needles.

The early literature on meteoritic phosphides,

schreibersite and rhabdite, was reviewed in detail

by Cohen (1894). Included were a number of

analyses of schreibersite and rhabdite isolated

from various meteorites by Cohen and his coworkers, as well as selected analyses from the

literature. This work established that schreibersite

and rhabdite are morphological variations of the

same mineral species, the rhabdite form having a

somewhat higher nickel concentration than that

observed in large schreibersite inclusions. A similar review was given a few years later by Farrington

(1915), using much of the same data Cohen used

but with several later analyses.

The 19th-century work on meteoritic phosphides that culminated with Cohen's and Farrington's reviews was limited by the capabilities of

classical descriptive mineralogy and classical analytical chemistry. A plateau had been reached in

descriptive iron meteorite studies in general that

became an accepted norm that was exceeded by

few workers during the first half of the 20th

century. Most iron meteorite descriptions published during this period employed neither concepts nor techniques beyond those available to

Cohen. The descriptive literature grew in volume

and in quality, but new interpretations lagged.

Powerful petrographic techniques were being developed and used by metallurgists and ore microscopists, but they were not much used by students

of iron meteorites. The availability of good transmitted light microscopes in the hands of petrographers who were accustomed to working with silicate minerals resulted in stony meteorites being

preferred subjects of investigation.

EARLY 20TH-CENTURY STUDIES OF

IRON METEORITES

The emergence of modern physical chemistry

and physical metallurgy during the decades adjacent to the turn of the century had a marked

effect on the future course of iron meteorite

investigations. Descriptive studies following the

19th-century pattern continued to comprise the

bulk of the published literature and, indeed, continue to be published today as essential documentation. More modern experimental approaches,

however, became significant, resulting in a body

of knowledge more readily interpretable in terms

of origins and developmental histories of meteorites than was previously possible.

Actually, the roots of this work go back to a

much earlier period. Pioneering synthetic experiments were made by Michael Faraday as early as

1820. He prepared metals of meteoritic compositions in the course of an investigation to improve

steel (Stodart and Faraday, 1820). At a later period, Daubree (1868) described synthetic experiments related to meteorites. Sorby (1877, 1887),

the father of metallography, made astute observations, some of which were based on artificial alloys

of meteoritic compositions. Unfortunately, Sorby's

pioneering efforts in the metallography of meteorites were not pursued. The following quotation
NUMBER 21

from his 1877 paper reflects his perceptive understanding of the structures he observed:

These facts clearly indicate that the Widmanstatt's Figuring is

the result of such a complete separation of the constituents

and perfect crystallisation as can occur only when the process

takes place slowly and gradually. . . . Difference in the rate of

cooling would serve well to explain the difference in the

structure of some meteoritic iron which do not differ in

chemical composition; .. . we are quite at liberty to conclude

that they may have been melted ... .

During the same period the well-known French

mineralogist, Meunier (1880), reported the results

of his synthetic work on meteoritic systems. A

paper by Brezina (1906) is an excellent summarizing statement of this early period. It contains a

number of good photomicrographs of iron meteorites, an unusual feature for papers of the time,

and it discusses the development of these structures in terms of exsolution phenomena (solid

state transformation). He also gave a succession

for the formation of the constituents of iron meteorites, starting with olivine in pallasites: olivine,

daubreelite, troilite, graphite, schreibersite, cohenite, chromite, swathing kamacite, kamacite

bands, taenite, and plessite. This is a remarkably

informative listing for the time, demonstrating

careful petrographic observation and keen insight.

One of the perennial problems of meteorite

research has been that of drawing the attention of

those with the knowledge and facilities to make

significant contributions to the meteorite specimens and the important and interesting scientific

problems they represent. Despite Sorby's enthusiasm for meteorite studies, metallurgists showed

little interest and had to be attracted into the

field. An example of this process is an invited

paper read before the Vienna meeting of the Iron

and Steel Institute in 1907. Professor Frederick

Berwerth's (1907) paper, "Steel and Meteoritic

Iron," was based on his knowledge of the meteorite collection of the Imperial Natural History Museum, Vienna. It was an open invitation to metallurgists to become involved in this field of study.

During the early decades of this century, synthetic studies were combined with analyses based

on equilibrium phase diagrams. The work of Osmond and Cartaud (1904), Benedicks (1910), and

Belaiew (1924) are examples. Unfortunately, the

tools were not available at the time to determine

sufficiently accurate equilibrium diagrams. As a

result, many of the conclusions drawn were seriously flawed. These studies, however, pointed the

way for future workers.

The introduction of X-ray diffraction analysis

into metallurgical and mineralogical research in

the 1920s had an important influence on meteorite

studies. Young (1926) published an early study

establishing that kamacite precipitates from taenite, not the converse as many had previously

thought. X-ray examination of the Widmanstatten

pattern continued for over a decade with deepening understanding and some controversy (Mehl

and Derge, 1937; Derge and Komnel, 1937; Owen,

1938; Smith and Young, 1938, 1939).

Rudolf Vogel's contributions to metallurgical

studies in meteorites deserves special mention, his

published papers having covered the period 1925

to 1967. They treated a broad range of topics

related to meteoritics, eight of them being of

particular interest in this context (Vogel, 1927,

1928, 1932, 1951, 1952, 1957, 1964; Vogel and

Baur, 1931). These papers deal with the role of

phosphorus in the development of meteoritic

structures. His 1928 paper discusses schreibersite

and rhabdite precipitation in terms of a hypothetical ternary Fe-Ni-P system. His 1932 and 1952

papers discuss the same material in terms of experimentally derived ternary diagrams.

A watershed publication that represents the culmination of this earlier period of iron meteorite

research is Perry's (1944) monograph on the metallography of meteoritic iron. Although many

photographs of iron meteorites had been published in the older literature, most of these were

external views of complete specimens or macrophotographs of meteorite slices prepared for exhibit purposes. Perry's publication was the first

comprehensive and systematic collection of iron

meteorite photomicrographs. The variety of metallic structures in iron meteorites was presented

to the scientific public in a way that has had

significant impact up to the present. Sorby had

pointed out the power of the metallographic approach in 1877, but it was not seriously pursued

until E. P. Henderson drew the publisher and

meteorite collector S. H. Perry into this field in

the 1930s (Henderson and Perry, 1958: ii).

MODERN WORK ON METALLIC PHASES OF

IRON METEORITES

Modern work on the metallic phases of iron

meteorites includes studies that may be grouped
SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

into the following categories for convenience: (1)

interpretations of the Widmanstatten pattern in

terms of cooling histories of meteoritic parent

bodies, (2) compositional studies utilizing trace

elements and structure as a means of identifying

parent bodies, (3) schreibersite growth as a key to

structure development and cooling history, (4)

descriptive studies and literature reviews. These

studies are interrelated and provide essential background for the work to be discussed below. Brief

mention of major contributions in these areas will

be given here.

Progress in developing accurate low temperature phase diagrams has played an essential role

in modern iron meteorite studies. The Fe-Ni diagram of Goldstein and Ogilvie (1965a) has been

used by meteoriticists for over 10 years (Figure 1).

Electron microprobe techniques were employed

by these workers to improve upon earlier equilibrium diagrams (Owen and Sully, 1939; Owen and

Lui, 1949) and to reconcile the Fe-Ni diagram

with observations on meteorites (Agrell et al.,

1963). Buchwald (1966) studied the Fe-Ni-P system

at low temperature using X-ray techniques and

proposed a diagram that he used to discuss meteorite structure development. More extensive work

on the Fe-Ni-P system based on longer cooling

periods and electron microprobe measurements

was reported by Doan and Goldstein (1970), and

their diagram is basic to the interpretation of the

experimental observations discussed below.

The modern literature on the growth of the

Widmanstatten pattern is extensive. Massalski and

Park (1962) used the Fe-Ni equilibrium diagram to

calculate initial temperatures of kamacite precipitation and final equilibrium temperatures, using

areal distribution of kamacite and taenite and a

lever rule calculation. Wood (1964) and Goldstein

and Ogilvie (1965b) considered the nonequilibrium

nature of iron meteorite structures, and combined

phase diagram and kinetic considerations to derive

cooling rates and to estimate parent body sizes.

Both papers recognize Widmanstatten pattern

growth as diffusion controlled and not subject to

bulk equilibrium consideration. Wood (1964) uses

the relationship between Ni buildup at the centers

of taenite bands and kamacite band widths to

calculate cooling rates. The Goldstein and Ogilvie

(1965b) approach used a detailed analysis of diffusion gradients within taenite at taenite-kamacite

interfaces. Both approaches gave comparable cooling rates within reasonable limits of error. A

number of other papers were published during

this period that contributed significantly to our

knowledge of the structural detail of meteoritic

iron (Short and Anderson, 1965; Goldstein, 1965;

Reed, 1965b; Axon and Boustead, 1967). Cooling

rate and thermal history studies were pursued by

Goldstein and co-workers (Short and Goldstein,

1967; Goldstein and Short, 1967a, b; Fricker et al.,

1970). Goldstein and Doan (1972) discussed the

effect of phosphorus on the formation of the

Widmanstatten pattern and reported the first laboratory production of an artificial Widmanstatten

pattern. They conclude that cooling rate calculations are not seriously affected by phosphorus. A

comprehensive review of Widmanstatten pattern

studies has been published by Goldstein and Axon

(1973).

Compositional studies of iron meteorites have

traditionally gone hand-in-hand with classification,

bulk Ni values and metallographic structure being

the primary considerations in assigning classification categories. Modern versions of this type of

classification have been given by Buchwald and

Munck (1965) and Goldstein (1969). The grouping

of iron meteorites on the basis of trace element

content was introduced by Goldberg et al. (1951)

and Lovering et al. (1957). Wasson and co-workers

in a series of papers (Wasson, 1974; Scott and

Wasson, 1975, 1976) have extended this approach

to define genetic groups in a large number of iron

meteorites, based on Ni, Ga, Ge, and Ir contents

and structural considerations. Sixteen iron meteorite groups have been characterized, representing 12 genetically related meteorite groups and a

number of individual meteorite parent bodies.

iRecent work on schreibersite growth in iron

meteorites may be dated from the observations of

Henderson and Perry (1958). Using conventional

chemical analysis, they observed unusually low Ni

concentrations in swathing kamacite bordering the

large low-Ni (12%) schreibersite in the Tombigbee

River meteorite, a meteorite they also showed to

contain low-Ni (20%) rhabdite. They explained

their observations in part on the migration of Fe

and Ni atoms in the solid state and suggested that

much of the phosphide may have separated from

the liquid. Somewhat later, the first electron microprobe measurements of schreibersite composi-
NUMBER 21

900

 1

800

700

600

500

400

300

-

0

-

-

i i i i

\

\

\

\

1

1

1

1

1w

1

1

1 1 1 1 1 1

a+y

1

r

i 1

10 20 30 40

ATOMIC % NICKEL

FIGURE 1. —LOW temperature Fe-Ni phase diagram of Goldstein and Ogilvie (1965).

50

tions were reported by Adler and Dwornik (1961)

on schreibersite and individual rhabdite crystals

from the Canyon Diablo meteorite.

Goldstein and Ogilvie (1963) reported electron

microprobe analyses of the composition of varioussized schreibersites in the Canyon Diablo, Breece

and Grant meteorites. They observed a size-composition correlation, the smaller the schreibersite

the higher the Ni content, and showed the presence of a zone of Ni depletion (swathing zone)

even around very small schreibersites (rhabdites).

They suggested that most schreibersite grew by

solid state precipitation; although massive schreibersites showing no relation to the Widmanstatten

pattern probably formed directly from the liquid

state. Size and composition of the schreibersites

were explained as dependent upon nucleation

temperature and time available for growth of the

precipitate. A growth analysis based on diffusion

kinetics demonstrated that larger schreibersites

nucleated between 700° and 500° C, and that small

schreibersites (rhabdites) formed between 500° and

400° C. They also demonstrated that P solubility

was greater in kamacite than in taenke.

Reed (1965a) emphasized the important role P

plays in the formation of iron meteorite structures.

He determined the range of Ni values in schreibersites and rhabdites in a large number of meteorites, finding Ni values as low as 14% in large

schreibersites and as high as 50% in small taeniteborder schreibersites. Ni depletion at kamaciteschreibersite interfaces was also demonstrated.

The precipitate size and Ni content relationships

were confirmed, and Reed pointed out that sizefor-size rhabdite grains in the Canyon Diablo meteorite are 7% higher in Ni than those in the

Coahuila meteorite. The author recognized the

approximate nature of chemical analysis values
8 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

for total P, suggesting that in most cases the actual

values are higher than the measured values. He

discussed the sequence of phase formation in meteorites based on Vogel and Baur's (1931) phase

diagram and stressed the need for more precise

information on phase equilibria.

In a subsequent paper, Reed (1967) measured P

distribution in the Mount Edith meteorite, demonstrating the practicality of measuring Ni and P

distribution profiles in kamacite, taenite, and

schreibersite. Later Reed (1969) discussed the role

of P in relation to Widmanstatten pattern formation and cooling rate calculations, and reported

measurements of P content in the kamacite of 61

meteorites.

Doan and Goldstein (1969) discussed the formation of phosphides in the various meteorite classification groups on the basis of their newly determined Fe-Ni-P phase diagram. They estimated

total P values from examination of the largest

available polished sections of a number of meteorites, and used them as the basis of a discussion of

schreibersite precipitation reactions in various meteorite composition ranges. They inferred thermal

equilibrium in iron meteorites down to 650° C,

with schreibersites forming at higher temperatures

while rhabdites form from kamacite at low temperatures when Ni and P diffusion are severely limited.

Comerford (1969) discussed schreibersite and

cohenite occurrences in iron meteorites and

pointed out that rhabdites, schreibersites that nucleated within kamacite, may be more reliable for

diffusion analysis than grain boundary schreibersites. Axon and co-workers have discussed schreibersite occurrences in a broad range of meteorites

(Axon and Faulkner, 1970; Axon and Waine, 1971,

1972; Axon and Smith, 1972). The papers of Axon

and Waine (1971, 1972) discuss schreibersite morphology, mode of occurrence, and relationship to

other minerals in great detail. Hornbogen and

Kreye (1970) have reported detailed metallographic studies on the Coahuila and Gibeon meteorites and have discussed Ni and P solid state

reactions in some detail. Reed (1972) has reported

analyses of schreibersite in the Oktibbeha County

meteorite, the highest Ni schreibersite known in

meteorites (65.1%), and De Laeter et :al. (1973)

have reported on schreibersite in the Redfields

meteorites, the lowest Ni schreibersite on record

(7.0%).

Modern reviews that are particularly important

to the investigations at hand have been mentioned

previously. The paper by Goldstein and Axon

(1973) is a comprehensive review of the development of the Widmanstatten pattern. The recent

book by Wasson (1974) is a broad treatment of

meteorites and includes an excellent summary of

the work on chemical groupings of iron meteorites

with extensive listings of individual iron meteorite

analyses and classification. The treatise on iron

meteorites by Buchwald (1976) has been of great

help. Much of the descriptive material on individual meteorites had been available to us in manuscript, and Dr. Buchwald spent many hours teaching the senior author the rudiments of iron meteorite metallography during his two-year stay

(1968-1970) at the Division of Meteorites, Smithsonian Institution.

RELATED MATERIALS

Schreibersite occurrences are not restricted

solely to iron meteorites. It is an accessory mineral

in pallasites, mesosiderites, and enstatite chondrites, and has been reported in several ordinary

chondrites, and a few carbonaceous chondrites

and achondrites (Ramdohr, 1973; Powell, 1971;

Malissa, 1974). Schreibersite is also known as a

product of combustion in the coal mines of Commentry and Cranzac, France (Palache et al., 1944:

125), as a cavity mineral in iron slags (Spencer,

1916), and associated with metallic iron in the

Disko Island, Greenland, basalts (Pauly, 1969).

Schreibersite has been observed by a number of

workers in metal particles in lunar soil and rocks.

Individual particles have been described in great

detail, and Ni/Co ratios have been used to identify

metal of meteoritic origin (Goldstein et al., 1970;

Goldstein and Yakowitz, 1971; Axon and Goldstein, 1972; Goldstein et al., 1972). Nickel concentration in metal and schreibersite in contact with

each other have also been used to suggest final

equilibrium temperatures and to imply cooling

histories. As lunar studies have continued, it has

become apparent that a greater proportion of

schreibersite-containing lunar metal than had previously been thought has been derived from processes other than simple disruption of impacting

meteorites (Axon and Goldstein, 1973; El Goresy

et al., 1973; Brown et al., 1973; Carter and Padovani, 1973; McKay et al., 1973; Goldstein and

Axon, 1973; Gooley et al., 1973).
NUMBER 21 9

A second phosphide mineral, barringerite

(Fe,Ni,Co)2P, has been reported from the Ollague

pallasite (Buseck, 1969). This material is probably

a secondary product of some type and not an endogenous meteoritic mineral (Buchwald, 1976:

105).

The phosphide mineral, schreibersite, is the

only phosphorus mineral that is observed as a

minor phase in most iron meteorites, and it is the

only phosphorus-containing mineral that will be

considered in this paper. There are, however,

nine phosphate minerals known in meteorites,

and seven of these have been identified as trace

constituents in iron meteorites (Fuchs, 1969; Bild,

1974). They are chlorapatite, Ca5(PO4)3Cl; graftonite, (Fe,Mn)3(PO4)2; sarcopside, (Fe,Mn)3(PO4)2;

whitlockite, Ca3(PO4)2; brianite, Na2MgCa(PO4)2;

panethite, Na2Mg2(PO4)2; and farringtonite,

Mg3(PO4)2. These minerals are observed in a variety of associations: as inclusions? in graphite-troilite-silicate nodules, in simple troilite nodules or

in troilite-chromite nodules, as inclusions in or in

contact with schreibersite, and associated with

other minor minerals or isolated in metal. The

phosphate-phosphide association has been used to

calculate equilibrium oxygen fugacity in iron meteorites and pallasites (Olsen and Fredriksson,

1966; Olsen and Fuchs, 1967). In the iron meteorites considered in this paper, phosphate minerals

are either absent or isolated within inclusions that

represent a higher temperature stage in the development of the meteorite structure. No evidence

has been developed in this work to indicate that

the observed phosphide-metal equilibria have been

affected by the presence of phosphates.

Experimental

Two types of data are needed in order to interpret iron meteorite structural development as a

consequence of cooling through the subsolidus

Fe-Ni-P system. Accurate bulk compositions are

the first requirement, and good bulk Ni values

are available from the literature. The problem of

obtaining good bulk P values has not been solved

satisfactorily, and estimated values were found to

be useful. This aspect of the problem will be

discussed in detail below. The second type of data

required are P and Ni gradients within kamacite

and schreibersite, and particularly P and Ni values

at kamacite-schreibersite interfaces. Experimental

procedures for obtaining these values were developed in the electron microprobe laboratory of the

Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution.

Reed (1967, 1969) had reported traverse data on

the Mount Edith meteorite and had demonstrated

the practicality of measuring the low P levels in

kamacite with the microprobe. Doan and Goldstein's (1970) experience in their phase diagram

studies was also encouraging. Ideally, one required step-by-step traverses over many meteorite

structures, each traverse consisting of a large number of individual analyses for P and Ni. Ni concentrations were in a range that presented no unusual

analytical problems, but the expected low levels of

P required special care.

An A.R.L. EMX electron microprobe (Applied

Research Laboratory, Sunland, California) was

used in this work. Simultaneous measurements

were made for Ni Ka (LiF crystal) and P Ka (ADP

crystal) radiation at an X-ray takeoff angle of

52.5°. The instrument was operated at an accelerating voltage of 20 KV, with an approximately 0.1

fia sample current on 100% Fe and a beam size of

approximately 1 /xm. Average count rates measured on 50% Ni and calculated to 100% Ni were

30,000 cts/sec, with a peak to background ratio of

230. Count rates for 100% P measured on schreibersite were 25,000 cts/sec with a peak to background ratio of 915. Standard statistical assumptions suggest that for the 20 second counting

periods employed, a detectability limit for P of

150 ppm for a single determination would be

achieved, within a satisfactory range for the problem at hand (Ziebold, 1967). Neighboring P determinations within a given traverse generally agreed

within ±50 ppm.

The standards used were 100% Fe, 100% Ni, a

series of Fe-Ni alloys of known composition, and a

fragment of a large schreibersite from the Canyon

Diablo meteorite. The 100% Fe was used for determining both Ni and P background counts. The

Fe-Ni standards were prepared by R. E. Ogilvie

and obtained from J. I. Goldstein. They were

used to derive an empirical curve that was incorporated into a data reduction program to correct

Ni determinations. The schreibersite standard

contained 12.9% Ni, 0.26% Co, and.was assumed

to contain 15.5% P. A linear relationship between

P counts in schreibersite and concentration in an

unknown was assumed. This was thought to cause
10 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

no serious error in low level P measurements, but

it does introduce errors in P determination in

schreibersite. In this study, P values for schreibersite were only used to identify the mineral, and

for this purpose corrections seemed unnecessary.

Sections were prepared for study by standard

metallographic procedures. Pieces of meteorite

were mounted in one-inch diameter Bakelite,

ground on a graded series of sandpapers, and

polished with 3 /xm diamond paste and finally

Linde B alumina compound. The sections were

then lightly etched with 0.5% nital (0.5 ml nitric

acid in 100 ml of 95% ethanol). Light etching was

essential to reveal structures that were examined

and to permit their being relocated and analyzed

in the microprobe. It would have been impossible

to obtain the mass of data required using unetched

sections. While deep etching is known to cause

errors in microprobe measurements at phase interfaces, the light etching used here did not seem to

cause a problem. A schreibersite-kamacite-taenite

structure in the Carleton meteorite was selected to

test this point. As nearly identical traverses as

possible, considering that the section had to be

removed from the microprobe, were made before

and after etching. The results were identical within

reasonable experimental error. Areas selected for

microprobe analysis were photographed in detail,

permitting the recording of the exact location of

microprobe traverses.

The electron microprobe data was accumulated

using a step-scan procedure. Ni and P counts

were measured simultaneously and recorded on

punch cards along with step length. The data

were computer processed, resulting in a second

set of cards containing calculated P values, empirically corrected Ni values, and step length. These

cards in turn were used to obtain computer generated plots. Photographs of the data plots as

received from the computer are given in Figures 2

and 3. The only additions to these diagrams are

the vertical lines that have been added to help

identify the structural elements. Figure 2 is a

traverse across a structure in the Ballinger, Texas,

meteorite, catalog number USNM 824. The labeling indicates that this is the fifth traverse made

over structures in that meteorite, the data having

been taken on 8 September 1972. Two symbols

are used to indicate P concentration, the ordinate

scale being zero to 0.7% P for low values and zero

to 70% P for high values. The traverse started in

kamacite and crossed a very thin, elongated phosphide (4 /mm wide) at the edge of a taenite lamella.

It passed into kamacite and after traveling 275 /Am

entered a rhombohedral phosphide (rhabdite) 30

/u,m across and then passed into kamacite again.

The two unusually high P values in the kamacite

are undoubtedly due to unseen phosphides that

were included in the excitation volume of the

microprobe beam. Figure 3 is a similar plot for

the Lexington County Meteorite. In this case only

a portion of the complete traverse is reproduced

(700 out of 1600 fim). The traverse started in

kamacite, entered a phosphide at a taenite border,

passed into kamacite, a second phosphide, and a

second taenite lamella. Interface concentration

values and concentration gradient shapes and

lengths were read from these plots.

Data obtained in this way are subject to several

sources of error beyond those that apply to an

individual point analysis in a homogeneous material. Stability of the microprobe over the long

periods required to obtain lengthy traverses was a

problem. Due to the survey nature of this investigation, compromises were made in favor of large

numbers of traverses and for lengthy traverses.

The microprobe beam current was monitored and

kept within narrow limits, but this does not necessarily prevent all instrumental drift. Data were

rejected when drift became an obvious problem,

but normally measurements on standards or repeated measurements on the same structures gave

identical results within reasonable limits of error

(± 0.005% P, ±0.1% Ni).

The schreibersite-kamacite interface measurements are subject to errors due to both finite

beam size and to interface geometry. The finite

beam produces measured interface profiles that

are less sharp than the actual profile. It is unlikely

that this is a serious error in determining the

interface value of P in kamacite, as this is only

read to the nearest 0.01%. The interface value of

Ni in kamacite is probably increased slightly by

the proximity of high Ni in the adjoining schreibersite. More serious errors may result from interface geometry, as there is no reason to assume

that the interfaces measured were perpendicular

to the surface of the section. The small size of

most of the structures measured, combined with

their large number, made it impractical to attempt

measuring slopes and applying corrections. Reproducibility 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

m

e 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 meteorite indicated that this is not too serious a problem

for the study at hand.

Results

Eight coarse-structured iron meteorites covering

a range of bulk Ni and bulk P contents were

selected for Ni and P measurements at kamaciteschreibersite interfaces (Table 1). Four structural

and five chemical classification categories are represented in the group. The Bellsbank meteorite

contains very low total Ni, while Balfour Downs is

one of the highest Ni members of Group IA.

Coahuila contains only about 0.3 wt. %P, while

Bellsbank contains exceptionally high P. The other

meteorites in the group range between these limits

in Ni and P contents. The data in Table 1 were

taken from Wasson (1974) and Buchwald (1976).

Buchwald's values combine, where appropriate,

chemical analytical values with the results of planometric analyses of large surfaces. Contributions

of Ni and P due to large schreibersite inclusions

are included in this approach, resulting in estimates more representative of the meteorite as a

whole than uncorrected chemical values. Wasson's

Ni values are chemical values determined on small

samples taken for trace element analysis. A complete review of the literature of each of these

meteorites and a discussion of their detailed metallography has been given by Buchwald (1976).

Microscopic examination of metallographic sections of these eight meteorites led to the selection

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

step-scan procedure described above. Several microprobe traverses were made for each meteorite,

and the data obtained on Ni and P values at

kamacite-schreibersite interfaces and on Ni and P

gradients in kamacite are summarized below. A

separate table accompanies the description of traverses for a given meteorite, the format used being

the same in each case (Tables 2-9). The first two

columns give the specimen number, which includes the USNM catalog number, and the

traverse number. Column three contains two types

of information. The first entry for each traverse is

enclosed in square brackets and gives the sequence

of phases traversed and the length of the traverse.

The abbreviations used here are Ph for the various

morphologies of schreibersite, a for kamacite and

y for taenite. The following entries for a given

traverse identify the specific schreibersites measured and give the length of schreibersite traversed. The next column, headed %NiSch, gives

the weight percent Ni in schreibersite either at a

kamacite-schreibersite interface or within a large

schreibersite. Where an interface measurement

has been made, the schreibersite Ni value will be

followed in the next two columns by the weight

percent Ni in kamacite at the interface (%Niot) and

the weight percent P at the interface (%Pa). The

next two sets of two columns each indicate the

length of observed Ni and P gradients away from

the interface and the Ni and P values at which a

gradient was no longer distinguishable. Factors
NUMBER 21 13

TABLE 1. — Composition and classification of selected meteorites

Meteorite

Coahuila

Bellsbank

BaTMnger

Santa Luzia

Lexington County ... .

Bahjol

Goose Lake

Balfour Downs

•Buchwald (1976)

**Wasson (1974)

Weight Percent

%P*

Chemical

Classification**

Structural

Classification*

0.3

2

0.4

0.9

0.3

0.4

0.3

5.6

5.3

6.5

6.6

-7.0

7.7

8.3

8.4

5.49

4.13

6.19

6.3

6.69

7.95

8.00

8.39

IIA

IRANOM

IA-AN

I IB

IA

IA-AN

IA-AN

IA

H

H

Og

Ogg

0g

0g

0m

Og

such as impingement and termination of a particular traverse influence these final observed Ni

and P values. In the final three columns the Ni

and P values from columns three through six have

been converted to atomic percent for use later.

COAHUILA

The Coahuila, Mexico, meteorite is an example

of an ordinary hexahedrite (IIA), similar in composition and structure to a number of other hexahedrites. It consists of single crystal kamacite containing profuse small schreibersites generally of

rhabdite morphology, and occasional larger

schreibersites. The larger schreibersites sometimes

border troilite or troilite-daubreelite inclusions,

and in rare instances are associated with cohenite.

Section 3298 is dominated by six troilite-daubreelite inclusions, ranging in size from 5 to 0.3

mm in diameter. Half to three-quarters of the

length of the borders of these sulfides are rimmed

with schreibersite. One small sulfide is bordered

for most of its circumference with schreibersite,

the remaining border being rimmed with cohenite.

Kamacite in the vicinity of these inclusions contains

many subgrain boundaries and is free of schreibersite. Microrhabdites measuring less than a few

microns in maximum length and generally considerably smaller are present in profusion away from

these inclusions. Somewhat larger rhabdites are

present in several areas aligned along Neumann

bands. Several lamellar schreibersites, 5 to 10 /am

wide and extending to lengths that are a major

part of a millimeter or more, are also present. A

summary of the traverses of this section is given

below and specific measurements are listed in

Table 2.

3298, traverse 1: crossed a 150 /xm wide schreibersite bordering

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 extended 350 (im into the surrounding kamacite.

3298, traverse 3: crossed a 100 /u.m wide schreibersite bordering

the 5x 5 mm troilite-daubreelite inclusion and then extended for 350 /xm into surrounding kamacite. This traverse

is reproduced at the upper left in Figure 4.

Section 1641(1) primarily contains regions of

kamacite with evenly distributed rhabdite, the individual rhabdites averaging 200 fim2

 in crosssectional area. Also present in areas of clear kamacite are several large schreibersite inclusions and

schreibersite-cohenite inclusions. The largest

schreibersite inclusion has an area of 0.25 mm2. A

0.1 x 0.6 mm schreibersite is bordered by cohenite, and an adjacent, slightly smaller schreibersite

is partially surrounded by cohenite. One cohenite

area, 0.2 x 0.3 mm, contains six small schreibersites. A 0.4 x 1.4 mm cohenite area contains a

number of small schreibersites and surrounds a

central schreibersite measuring 0.04 x 0.6 mm.

An 80 X 150 fim daubreelite is surrounded by

schreibersite. Two lamellar schreibersites, each

approximately 0.02 X 1.0 mm, are present, and

one of these is partially bordered with cohenite.

1641(1), traverse 4: crossed a large skeletal schreibersite enclosing a kamacite area. The traverse first crossed 450 fjun of

kamacite, then 120 /urn of schreibersite, 70 jxm of included

kamacite, a second 70 (im schreibersite crossing, and terminated after passing through 360 /tm of kamacite. The schreibersite was of uniform composition, and the measurements
14 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

TABLE 2. —Coahuila schreibersite-kamacite interface measurements

Specimen

No.

3298

3298

3298

1641(1)

1641(1)

1641

1641

1641

1641

Traverse

No.

1

2

3

4

5

6

7

8

9

Structure traversed & schreibersites measured

[Ph border of troilite-daubreelite inciusion-a,

1.1 mm]

Schreibersite, 150 pm wide

[Ph border of troilite-daubreelite inclusion-a,

400 ym]

Schreibersite, 40 ym wide

[Ph border of troilite-daubreelite inclusion-a,

450 ym]

Schreibersite, 100 um wide

[a-Ph-a-Ph-a, 1 mm]

Schreibersite 260x500 ym with 60x170 um

included kamacite

120 ym traverse

70 ym traverse

[a-Ph-a, 350 um]

Rhabdite, 25 ym traverse

Enter

Exit

[a-Ph-a, 750 um]

Rhabdite (45x75 um), 45 um traverse

[a-Ph-a, 800 um]

Rhabdite (50x70 um), 70 ym traverse

Exit

[o-Ph-a-Ph-a, 850 ym]

Rhabdite (50x70 ym), 50 um traverse

Rhabdite (25x30 ym), 25 ym traverse

[a-Ph-a, 200 ym]

Rhabdite (20x40 um), 25 ym traverse

Weight

*

NiSch

23.5

25.0

25.0

23.5

23.5

34.5

34.5

27.5

27.0

27.0

29.0

33.0

Percent

3SNi %P

a a

3.4

3.6

3.7

3.5

3.5

4.9

4.9

3.7

4.0

4.1

3.7

4.5

0.07

0.07

0.08

0.08

0.08

0.06

0.06

0.07

0.07

0.07

0.06

0.05

Ni Gradient

Length

(ym) Wt.%Ni

600

250

250

200

250

50

50

175

100

150

200

50

5.5

5.3

5.3

5.3

5.3

5.3

5.3

5.3

5.3

5.3

5.3

5.3

P Gradient

Length

(um) Wt.%P

800

350

300

50

200

Flat

100

100

Flat

Flat

100

Flat

0.16

0.15

0.15

0.10

0.10

0.06

0.07

0.08

0.07

0.07

0.08

0.05

Atomic

*

NiSch

20.1

21.4

21.4

20.1

20.1

29.7

29.7

23.6

23.1

23.1

24.9

28.4

Percent

%N1 %P

a a

3.2

3.4

3.5

3.3

3.3

4.7

4.7

3.5

3.8

3.9

3.5

4.3

0.13

0.13

0.14

0.14

0.14

0.11

0.11

0.13

0.13

0.13

0.11

0.09

in Table 2 are for the initial and final schreibersite-kamacite

interfaces.

1641(1), traverse 5: crossed 100 fim of kamacite, a 25 fim wide

rhabdite and terminated after another 200 fim of kamacite.

This traverse is reproduced at the lower left in Figure 4.

Section 1641 is a neighboring specimen to

1641(1) and is similar in metallography. Evenly

distributed rhabdites averaging about 200 jam2 in

area are the primary feature. Random larger

schreibersites, schreibersite-daubreelite, and

schreibersite-cohenite inclusions similar to those

described above are present, although they are

neither as numerous nor as large as the largest

mentioned above. Two unusually large rhabdites

and two smaller ones were selected for measurement.

1641, traverse 6: crossed 400 fim of kamacite, a 45 fim wide

rhabdite, and 300 fim of kamacite. Values reported in Table

2 are an average of the two similar interface measurements.

This traverse is reproduced upper right in Figure 4.

1641, traverse 7: crossed 230 fim of kamacite, entered the

rhabdite from an area of disturbed kamacite, traversed 70

fim of rhabdite and then 0.5 mm of kamacite. Only the

measurements on leaving the rhabdite were usable.

1641, traverse 8: crossed 370 fim of kamacite, 50 fim of rhabdite,

25 fim of kamacite, 25 fim of rhabdite, and 350 fim of

kamacite. The large rhabdite traversed here is the same one

traversed in number 7 but from a near perpendicular direction. Values reported in Table 2 are averages of two similar

interface measurements.

1641, traverse 9: crossed a smaller rhabdite approximately 1

mm from traverses 6 through 8. 90 fim of kamacite were

crossed, followed by 25 fim of rhabdite and 80 fim of

kamacite. Average interface values are listed in Table 2.

This traverse is reproduced at the lower right in Figure 4.

Ni and P concentration-distance profiles typical

of the Coahuila data outlined above are given in

Figure 4. These diagrams were prepared from

tracings of the computer plotted step-scan data.

Ni concentrations are indicated by the solid lines,

and P concentrations multiplied by 100 by dashed

lines. The Ni profiles are normally readily reproduced from the data charts without ambiguity.

The low level P data contains more scatter; drawing the appropriate line, and particularly selecting
-NUMBER 21 15

5O

45-

40-

o

m

UJ

0 .

£ 3O

O

UJ

S

25

20-

15-

, 0

25

100 (Jm

0.08

3.7

P x 100

Ni

3296

28

0.07

3.7

1641

400

MICRONS

N4.9

300

MICRONS

0.05,,

4.5

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.

A smoothing procedure relying on eyeball examination was used. Ni values for schreibersite and

Ni and P concentrations in kamacite at the kamacite-schreibersite interface are indicated at an appropriate place on the diagrams. The bulk Ni

content of the meteorite is also given and indicated

by a short dashed line.

The four profiles (Figure 4) illustrate the observed range of Ni in Coahuila schreibersite and

the shape and level of the Ni and P gradients in

the surrounding kamacite. They are given from

upper left to lower right in order of decreasing P

concentration at the kamacite-schreibersite interface . Three of the profiles are of large rhabdites

(traverses 6, 5, 9, Table 2), while the one at the

upper left is of a 100 /xm wide schreibersite bordering a large troilite-daubreelite inclusion (traverse

3, Table 2). The high P interface value and the

high level in the surrounding kamacite are of

particular interest. The three rhabdites appear to

increase in Ni, both with decreasing cross-sectional

area and with increasing interface Ni in kamacite

value. The interface P values do not seem to

correlate with either Ni in schreibersite or with

interface Ni in kamacite values. Ni and P gradients

extend for several hundred microns around the

larger schreibersites but appear to be much more

restricted in extent around the smaller ones. The

33% Ni schreibersite has an essentially flat P concentration at 0.05%. For these four cases, the

length and steepness of the P gradient decreases

with decreasing P at the interface.

BELLSBANK

The Bellsbank, South Africa, meteorite is a

phorphorus-rich hexahedrite. Massive and large

skeletal schreibersite crystals, with individual faces

over a centimeter in length and areas larger than

2 cm2, dominate polished surfaces and appear to

be distributed throughout this meteorite (photograph in Henderson, 1965). The kamacite matrix

contains a network of lamellar schreibersite. These

rarely intersecting planar crystals may exceed a

centimeter in length, but are normally only 5 to 10

/u,m thick. Occasional large rhabdites and subgrain

boundary schreibersites are present. Microrhabdites are present in profusion in kamacite areas

away from the other forms of schreibersite. The

Bellsbank meteorite has experienced severe terrestrial weathering. Many of the lamellar schreibersites have been replaced with corrosion products,

and corrosion along kamacite-schreibersite interfaces is common.

Section 2162 contains parts of two massive

schreibersites surrounded by schreibersite-free

kamacite areas. Two lengthy subgrain boundaries

TABLE 3. —Bellsbank schreibersite-kamacite interface measurements

Specimen

No.

2162

2162

2162

2162

2162

2162

Traverse

No.

1

2

3

4

5

6

Structure traversed & schreibersites measured

[a-Ph, 3.6 mm]

Massive schreibersite (7x2 mm), 1 mm traverse

[a-Ph-a, 900 um]

Large rhabdite along sub-grain boundary,

40x50 um, diagonally across body.

[a-Ph-a, 200 um]

Same rhabdite as traverse 2, parallel to 50 pm

direction

Enter

Exit

[a-Ph-a, 300 um]

Rhabdite along sub-grain boundary, 15x120 um

[a-Ph-a, 300 um]

Rhabdite along sub-grain boundary, 15x120 um,

repeat of traverse 2162-4

[a-Ph-a, 130 ym]

Large rhabdite Isolated in micro-rhabdite

area of a, 30x60 pm

Weight

*

NiSch

12.4

22.0

22.0

22.0

22.6

22.8

18.6

Percent

SNi

a

1.6

3.2

3.3

2.7

2.6

2.6

2.8

XP

a

0.09

0.09

0.09

0.05

0.05

0.05

0.09

Ni Gradient

Length

(um)

900

20

40

40

80

90

40

Wt.SSNI

3.9

4.5

4.2

4.2

4.3

4.3

4.2

P Gradient

Length

(um)

1000

50

Flat

60

100

100

30

Wt.JSP

0.20

0.18

0.09

0.12

0.20

0.18

0.20

Atomic

*

N1Sch

10.6

18.8

18.8

18.8

19.3

19.5

15.9

Percent

XN1

a

1.5

3.1

3.1

2.5

2.5

2.5

2.7

%P

a

0.16

0.16

0.16

0.09

0.09

0.09

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,

irregular-shaped, and rhabdite-shaped schreibersites. The kamacite matrix contains profuse microrhabdites. Oxidation products are present at sites

previously occupied by lamellar schreibersite, and

along some Neumann bands and possibly cleavage

planes.

2162, traverse 1: crossed 2.6 mm of kamacite and entered a

large schreibersite and continued for 1 mm. The traverse

was perpendicular to a straight kamacite-schreibersite interface, and the Ni content of the schreibersite was uniform for

the length of the measurement (reproduced at top of Figure

5). Numerical data for this and succeeding Bellsbank traverses are given in Table 3.

2762, traverse 2: crossed 650 fim of kamacite, a rhabdite at a

subgrain boundary and continued into surrounding kamacite

for 200 /urn. Average values for the two interface values are

given in Table 3.

2162, traverse 3: crossed 70 fim of kamacite, the same rhabdite

measured above, and passed into 70 fim of kamacite.

Traverse 2 crossed diagonally, while this one passed through

the middle parallel to the 50 ^im edge. Two different

interface values for Ni and P in kamacite were obtained,

perhaps influenced by the presence of the subgrain boundary

(Table 3 and lower middle of Figure 5).

2162, traverse 4: crossed 170 fim of kamacite, the narrow

direction of an elongated rhabdite situated along a subgrain

boundary, and 100 fim of kamacite (lower right in Figure 5).

2162, traverse 5: a remeasurement of traverse 4.

2762, traverse 6: crossed a large rhabdite surrounded by a

region of clear kamacite within a microrhabdite area (lower

left in Figure 5).

An attempt was made to determine Ni in other

small schreibersites in Bellsbank. A number of

measurements were made on schreibersites ranging down to a minimum of a few microns in

width. Twelve measurements were obtained where

the P value indicated that the microprobe beam

had been completely within a schreibersite. The

Ni values ranged from 22% to a maximum of

32%. Interface measurements were not attempted

on these small bodies.

The four profiles in Figure 5 represent the

traverses described above and illustrate the shapes

and levels of the Ni and P gradients in the surrounding kamacite. The profile at the top is the

interface portion of traverse 1. The low value for

Ni in the schreibersite combines with an unusually

low interface value for Ni and a high interface

value for P in kamacite. The rhabdite profile at

the lower left has the same P interface value, but

higher Ni in both kamacite and schreibersite. The

P gradient in this case is particularly severe. The

rhabdite at the lower right has similar Ni values to

the one on the left, but the P interface value is

considerably lower. The middle profile has uniform Ni in the schreibersite but has two different

sets of P and Ni kamacite interface values, the

only such observation made in this study.

BALLINGER

The Ballinger, Texas, meteorite is a low-Ni

coarse octahedrite. Figure 6 is a photograph of

the slice studied, USNM 824. The area marked PS

shows where material was taken for section preparation. Section 824 is from the back of this surface

and contains a large schreibersite similar in size

and shape to the one at the lower right in Figure

6. Section 824(1) is the outlined surface after repolishing. The overall structure consists of large

kamacite grains and three areas of heiroglyphic

schreibersite. A Widmanstatten pattern is suggested in some areas of the structure, but it is

certainly not well developed. The large schreibersite on the left is completely surrounded by rims

of partially decomposed cohenite. The center

group of schreibersite crystals is partly bordered

by partially decomposed cohenite, while the schreibersite on the right is completely free of cohenite.

Areas of this surface containing the best Widmanstatten pattern development are well away from

the large schreibersite inclusions. It is interesting

to note that the lower left-hand edge of this slice

contains a rim of kamacite heat altered to a2, a

remnant from ablation heating. This structure has

not been previously observed in the Ballinger

meteorite.

Section 824 contains a large schreibersite 0.8

mm long and averaging 1 to 2 mm in width,

surrounded by a narrow area of schreibersite-free

kamacite. The kamacite matrix contains a profusion of both Neumann bands and rhabdites. Neumann band bending, particularly at the interfaces

with the large schreibersite, suggests mild mechanical distortion in Ballinger. The kamacite contains

numerous grain boundaries that are the sites of

grain boundary schreibersites. Terrestrial oxidation has penetrated areas of this section.

824, traverse 1: crossed three areas of the large schreibersite.

500 fim of kamacite were crossed, followed by 350 fim of

schreibersite, 800 fim of kamacite, 400 fim of schreibersite,

450 fim of kamacite, 670 fim of schreibersite, and finally 500

fim of kamacite. The three schreibersite areas traversed are

essentially free of Ni gradients and they all have approxi-
NUMBER 21 19

PS

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

succeeding Ballinger traverses are given in Table 4.

824, traverse 2: crossed two rhabdites along a grain boundary 2

mm away from the massive schreibersite. 400 fim of kamacite

were crossed, followed by two 40 fim rhabdites separated by

25 fim of kamacite, and finally 300 fim of kamacite. Because

of impingement effects, the interface values in Table 4 are

an average of the two similar external measurements.

824, traverse 3: crossed two areas of a 1 x 0.4 mm skeletal

schreibersite 3.5 mm from the massive schreibersite. The

values given in Table 4 are averages of the two external

interface values.

Section 824(1) is from a typical Widmanstatten

area of the Ballinger meteorite, the area outlined

in Figure 6. The kamacite matrix contains Neumann bands and rhabdites in a wide range of

sizes. Kamacite lamellae are separated by grain

boundaries that are alternately sites of grain

boundary schreibersites and taenite-plessite bands.

824(1), traverse 4: crossed 230 fim of kamacite and entered a

small schreibersite embedded in a taenite border, passed

through a narrow band of taenite, and then 220 fim of

kamacite.

824(1), traverse 5: crossed 150 fim of kamacite, a schreibersite

embedded in taenite, 20 fim of taenite, 270 fim of kamacite,

a large rhabdite, and finally 120 /AITI of kamacite (see Figure

2 in experimental section).
20 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

TABLE 4. —Ballinger schreibersite-kamacite interface measurements

Specimen

No.

824

824

824

824(1)

824(1)

824(1)

824(1)

824(1)

Traverse

No.

1

2

3

4

5

6

7

8

Structure traversed & schreibersites measured

[a-Ph-a-Ph-a-Ph-a, 3.7 mm]

Massive schreibersite, 350 ym

Enter

Exit

Massive schreibersite, 400 ym

Enter

Exit

Massive schreibersite, 670 ym

Enter

Exit

[a-Ph-o-Ph-a, 850 ym]

Two large rhabdites, 40x40 um each,

separated by 25 um. Values recorded

are averages of similar values.

[a-Ph-a-Ph-o, 700 um]

Skeletal schreibersite 1x0.4 mm. Values

recorded are average of similar values

from the two exterior Interfaces.

[a-Ph-Y-o» 480 um]

Rhabdite embedded in taenite border,

15x20 um

[a-Ph-Y-a-Ph-a, 650 ym]

Rhabdite embedded In taenite border, 5 um

wide

Large rhabdite (35x25 um), 250 ym into a

from above

[ct-Ph-Y-a, 300 um]

Rhabdite embedded in taenite border,

10x15 um

[a-Ph-a-Ph-a-Ph-a-Ph-a, 1.4 mm]

Grain boundary schreibersite in sequence

with taenite, 20x15 ym

Rhabdite 50 ym from above, 40x50 ym

Rhabdite 400 ym from above, 40x40 ym

Rhabdite 600 ym from above, 30x50 ym

[a-Ph-a, 1.0 mm]

Grain boundary schreibersite in sequence

with taenite, 15x140 ym

Weight

*N1S c h

19.5

19.5

19.5

19.3

19.7

19.5

34.5

35.8

49.8

49.5

41.5

51.0

47.0

44.5

40.0

40.0

48.8

Percent

a

2.9

3.0

2.6

2.6

...

2.6

4.2

4.8

6.0

6.0

5.2

6.3

5.7

5.7

5.0

5.0

6.0

%P

a

0.06

0.06

0.06

0.06

0.06

0.06

0.04

0.05

0.03

0.03

0.05

0.03

0.03

0.04

0.05

0.05

0.03

N1 Gradient

Length

(ym)

500

300

200

150

150

500

200

150

50

50

50

50

50

100

100

100

100

Wt.%N1

5.2

4.3

4.3

4.0

4.0

5.0

6.0

6.8

7.2

7.2

7.2

7.2

7.2

7.2

7.2

7.2

7.0

P Gradient

Length

(um)

150

200

200

150

100

150

100

200

50

50

50

50

50

100

200

200

300

Wt.SP

0.10

0.09

0.08

0.08

0.08

0.08

0.09

0.08

0.05

0.05

0.07

0.06

0.06

0.07

0.09

0.09

0.08

Atomic

*

NiSch

16.6

16.6

16.6

16.5

16.8

16.6

29.7

30.8

43.1

42.9

35.8

44.2

40.7

38.4

34.5

34.5

42.3

Percent

XN1

a

2.8

2.9

2.5

2.5

2.5

2.5

4.0

4.6

5.7

5.7

5.0

6.0

5.4

5.4

4.8

4.8

5.7

XP

a

0.11

o.n

o.n

o.n

o.n

0.11

0.07

0.09

0.05

0.05

0.09

0.05

0.05

0.07

0.09

0.09

0.05

824(1), traverse 6: crossed 80 fj.m of kamacite, a taenite border

schreibersite, 15 fim of taenite, and 150 fim of kamacite.

824(1), traverse 7: crossed 140 /u,m of kamacite, a grain boundary

schreibersite in sequence with taenite, 50 fim of kamacite, a

large rhabdite, 300 (im of kamacite, a large rhabdite, 620

jum of kamacite, a large rhabdite, and finally 80 /um of

kamacite.

824(1), traverse 8: crossed 90 jxm of kamacite, a grain boundary

schreibersite in sequence with taenite, and 1 mm of kamacite.

The massive schreibersite in Ballinger contains

higher Ni than similar sized schreibersite in Bellsbank, and the Ni in kamacite interface values are

higher while the P interface values are lower.

Large rhabdites in Ballinger contain more Ni than

similar ones in Coahuila or Bellsbank, and their

interface values are generally higher in Ni and

lower in P. Grain boundary schreibersite tends to

contain more Ni than kamacite matrix rnabdites,

and taenite border schreibersite contains the highest Ni values observed. As the Ni values in the

schreibersite go up, interface values in kamacite

tend to go up for Ni and down for P. Ni and P

gradients extend over relatively large distances

around the massive schreibersite and over much

shorter distances in the higher Ni forms of schreibersite .
NUMBER 21 21

SANTA LUZIA

The Santa Luzia, Brazil, meteorite is a P-rich

coarse octahedrite, a chemical Group IIB meteorite. The structure of a typical section is illustrated

in Figure 7. Large, hieroglyphic schreibersites with

broad areas of swathing kamacite dominate the

structure. Two such schreibersite areas are shown

in the figure, the central one bordering a large

troilite. The second schreibersite area could also

be associated with a lens-shaped troilite nucleus

that lies outside the plane of this section. The

areas of structure between the troilite-schreibersite-swathing kamacite areas contain a coarse Widmanstatten pattern, particularly well developed in

the top of Figure 7. These Widmanstatten areas

are reminiscent of the schreibersite-free areas of

the Ballinger meteroite. Weathering has penetrated deeply into Santa Luzia, particularly along

the major grain boundaries separating the swathing kamacite areas from the areas of Widmanstatten pattern.

Figure 8 is a sketch of the section of the photograph (Figure 7), indicating how this slice was

sampled for detailed metallographic and electron

microprobe analysis. Four metallographic sections

were prepared from the areas indicated by dashed

lines in the sketch. Section 1618 is from an area of

Widmanstatten pattern, 1618P from the schreibersite-swathing kamacite area, and 1618PS and

1618PS(1) from the sulfide-schreibersite-swathing

kamacite area. The numbered arrows in the sketch

indicate the location of the traverses described

below. Lengthy traverses were made in each of

the two hieroglyphic phosphide areas, with short

ones being made in the Widmanstatten pattern

area.

Section 1618P contains massive schreibersite and

swathing kamacite (Figure 8). Clear kamacite surrounds the large schreibersite, grading into kamacite containing a profusion of microrhabdites.

Neumann bands are most obvious in the transition

areas between clear kamacite and microrhabdite

areas. Occasional subgrain boundaries are present

within the kamacite, and small schreibersites of

various morphologies are associated with them.

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

kamacite, 0.2 mm of massive schreibersite, and 2.6 mm of

kamacite. Numerical data for this and succeeding Santa

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

schreibersite, 0.9 mm of kamacite, and 0.2 mm of massive

schreibersite.

1618P, traverse 3: crossed 0.2 mm of massive schreibersite, 1.0

mm of kamacite, and 0.2 mm of massive schreibersite.

Section 1618PS contains troilite, massive schreibersite and areas of swathing kamacite (Figure 8).

Its metallographic characteristics are similar to

those of section 1618P.

1618PS, traverse 4: crossed 0.2 mm of troilite, 2.9 mm of

kamacite, 140 /tm of massive schreibersite, 0.6 mm of kamacite, 2.9 mm of massive schreibersite, and 0.5 mm of kamacite.

1618PS, traverse 5: crossed 0.4 mm of troilite, 0.7 mm of

massive schreibersite, 4.6 mm of kamacite, 2.5 mm of massive

schreibersite, and 1.8 mm of kamacite. A representative

interface profile for this traverse is reproduced in Figure 9.

1618PS, traverse 6: crossed 0.6 mm of massive schreibersite and

1.9 mm of kamacite.

Section 1618PS(1) contains an outer edge of the

massive schreibersite and spans the swathing kamacite zone, containing a segment of its exterior

boundary. A detailed electron microprobe traverse

was not made on this sample, but sufficient measurements were taken to establish the Ni and P

concentration patterns for the swathing zone. In

the direction away from the schreibersite and

toward the exterior grain boundary, the Ni in

kamacite increased from slightly more than 2% at

the schreibersite interface to approximately 5%

within the first 0.5 mm. Over the next 7 mm the

Ni concentration increased smoothly to approximately 7%. It is in the first 0.2 mm of kamacite

surrounding the schreibersite that the Ni concentration increases most rapidly, and it is this zone

that is free of microrhabdites. The P concentration

increases to approximately 0.1% within this same

region of approximately 0.2 mm bordering the

interface. Upon entering the zone of microrhabdite precipitation, P concentration falls off in kamacite to a uniform level of approximately 0.05%

for the remainder of the swathing zone. Latestage microrhabdite precipitation seems to have

effectively removed the P gradient that must have

been produced within the swathing zone during

the massive schreibersite growth, while apparently

only slightly modifying the Ni gradient.

The clear kamacite surrounding the central

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

kamacite containing profusion of microrhabdites,

and it is in this transitional area that Neumann

bands are frequently best developed. Microrhab-
NUMBER 21 23

TABLE 5. — Santa Luzia schreibersite-kamacite interface measurements

Specimen

No.

1618P

1618P

1618P

1618PS

1618PS

1618PS

1618

1618

1618

Traverse

No.

1

2

3

4

5

6

7

8

9

Structure traversed & schreibersites measured

[Ph-a-Ph-a-Ph-a, 7.3 mm]

Massive schreibersite,

Start

Exit

Massive schreibersite,

Enter

Exit

Massive schreibersite,

Enter

Exit

[a-Ph-a-Ph-a-Ph, 5.3 mm]

Massive schreibersite,

Enter

Exit

Massive schreibersite,

Enter

Exit

Massive schreibersite,

Enter

Stop

[Ph-a-Ph, 1.4 mm]

Massive schreibersite,

Start

Exit

Massive schreibersite,

Enter

Stop

[Troilite-a-Ph-a-Ph-a, 7.4

Massive schreibersite,

Enter

Exit

Massive schreibersite,

Enter

Exit

[Troll1te-Ph-a-Ph-a, 1 cm]

Massive schreibersite,

Start

Exit

Massive schreibersite,

Enter

Exit

[Ph-a, 2.5 nin]

Massive schreibersite,

Start

Exit

[a-Ph-a, 350 ym]

Traverses across grain

site, 12x170 ym

Tip

Center

Tip

[a-Y-Ph-a, 400 ym]

1.3 mm traverse

0.7 mm traverse

250 ym traverse

430 ym traverse

800 ym traverse

200 ym traverse

220 ym traverse

200 ym traverse

mm]

140 ym traverse

2.9 mm traverse

0.7 mm traverse

2.5 mm traverse

0.6 mm traverse

boundary schreiberTaenite border schreibersite, 10x20 ym

[a-Y-Ph-a, 350 ym]

Taenite border schreibersite, 15x40 ym

Weight

*

N1Sch

16.0

16.5

16.5

16.5

17.9

17.9

16.0

16.0

16.2

16.2

16.1

16.1

16.0

16.0

16.2

16.2

17.3

17.3

16.6

17.1

16.6

16.6

16.4

16.6

16.6

16.6

45.5

44.7

45.7

48.0

48.5

Percent

a

2.4

2.3

2.4

2.4

2.4

2.3

2.3

2.2

2.4

2.3

2.3

2.3

2.6

2.3

2.4

2.5

2.5

2.5

2.5

2.3

5.8

6.0

6.2

6.4

6.5

%Pa

0.07

0.06

0.07

0.07

0.07

0.07

0.07

0.07

0.07

0.07

0.07

0.07

0.08

0.06

0.07

0.08

0.07

0.07

0.07

0.07

0.03

0.04

0.04

0.03

0.03

Ni Gradient

Length

(ym)

500

600

200

200

1000

900

1000

600

400

400

200

300

600

300

300

500

900

500

600

600

50

100

50

100

70

Wt.«N1

4.9

4.9

3.8

3.8

5.3

4.0

3.7

3.8

4.2

4.2

4.0

4.0

4.8

3.5

3.5

5.0

4.6

4.6

5.2

4.8

7.3

7.4

7.4

7.4

7.5

P Gradient

Length

(ym)

500

400

100

Flat

800

500

300

400

300

200

300

300

800

300

200

250

700

800

800

500

100

100

100

150

150

Wt.*P

0.15

0.15

0.08

0.07

0.16

0.09

0.09

0.09

0.09

0.09

0.11

0.11

0.16

0.08

0.08

0.14

0.14

0.14

0.18

0.13

0.10

0.10

0.10

0.10

0.10

Atomic

%N1Sch

13.6

14.1

14.1

14.1

15.3

15.3

13.6

13.6

13.8

13.8

13.7

13.7

13.6

13.6

13.8

13.8

14.8

14.8

14.4

14.6

14.2

14.2

14.0

14.2

14.2

14.2

39.4

38.6

39.6

41.5

42.0

Percent

a

2.3

2.2

2.3

2.3

2.3

2.2

2.2

2.1

2.3

2.2

2.2

2.2

2.5

2.2

2.3

2.4

2.4

2.4

2.4

2.2

5.5

5.7

5.9

6.1

6.2

vn

#r

a

0.13

0.11

0.13

0.13

0.13

0.13

0.13

0.13

0.13

0.13

0.13

0.13

0.14

0.11

0.13

0.14

0.13

0.13

0.13

0.13

0.05

0.07

0.07

0.05

0.05
24 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

TABLE 5—continued

Specimen

No.

772

772

772

772

Traverse

No.

10

11

12

13

Structure traversed & schreibersites measured

[a-Ph-a-Ph-a, 3.6 Itm]

Massive schreibersite, 650 um traverse

Enter

Exit

Massive schreibersite, 600 um traverse

Enter

Exit

[a-Ph-a, 2.3 mm]

Grain boundary schreibersite, 400x450 um

Enter

Exit

[a-Ph, 750 um]

Grain boundary schreibersite, 70 um traverse

Enter

Stop

[a-Ph-Y-a, 1.4 mm]

Taenite border schreibersite, 15x20 um

a-border

Y-border

Weight

*

N1Sch

19.3

19.3

19.7

19.7

28.0

28.0

36.2

36.2

47.0

47.5

Percent

%N1a

2.8

2.8

2.8

2.8

3.7

3.9

4.6

6.0

%Pa

0.07

0.07

0.07

0.07

0.06

0.06

0.05

0.03

N1 Gradient

Length

(um)

800

250

250

80

1400

500

500

200

Wt.%N1

5.8

4.7

4.7

5.8

6.9

6.3

7.2

7.4

P Gradient

Length

(um)

500

250

250

600

1000

300

500

200

Wt.%P

0.14

0.10

0.10

0.15

0.11

0.12

0.09

0.07

Atomic

*

N1Sch

16.5

16.5

16.8

16.8

24.0

24.0

31.2

31.2

40.7

41.1

Percent

SN1a

2.7

2.7

2.7

2.7

3.5

3.7

4.4

5.7

0.13

0.13

0.13

0.13

0.11

0.11

0.09

0.05

dites appear to increase in number per unit area

for the first few hundred microns away from the

clear kamacite. As distance increases further, there

appears to be a slight increase in both concentration and coarseness of microrhabdites. Coarser

microrhabdites are occasionally associated with

subgrain boundaries within the kamacite. There

are also occasional aligned microrhabdites, as if

they had precipitated along Neumann bands that

are no longer present. Rare lamellar schreibersites

2 fxm or less in width and as long as 200 ju,m are

present. Near the outer edge of the swathing zone

occasional segments of grain boundary type schreibersite protrude into the swathing kamacite or are

observed enclosed within it. Four areas that contain residual taenite as well as associated schreibersite were observed within the two swathing zones

(Figure 8), and close to their exterior borders.

The swathing zone boundary has suffered severe terrestrial weathering and now contains secondary iron oxides that have penetrated into the

adjoining kamacite. Remnant grain boundary

schreibersite is present, but it is encased in oxides

precluding interface measurements. The amount

of remnant schreibersite suggests that originally

as much as 50% of the length of the grain boundary may have been occupied by schreibersite. A

Ni determination on one of these schreibersites

gave a value of 29%.

Section 1618 is from an area of the large section

containing coarse-structured Widmanstatten pattern (Figure 8). Parts of four kamacite grains 4 to

5 mm wide are present. The general metallography of the section is similar to that described for

Ballinger, with the exception that only microrhabdites are present within kamacite areas. Measurements described below were made along a 1.5 mm

grain boundary containing taenite and taeniteplessite areas in sequence with grain boundary

schreibersite.

1618, traverse 7: combines data from three short (350 fim)

traverses perpendicular to the long direction of a single

grain boundary schreibersite. It measured 170 fim in length

and was separated from taenite by 10 to 20 fim at either end.

Measurements were made near the two ends and in the

middle of the schreibersite. The interface profile for the

center traverse is given in Figure 9.

1618, traverse 8: crossed 200 fim of kamacite, 10 fim of schreibersite, 15 fim of taenite, and 170 fim of kamacite. This

taenite border schreibersite was embedded in the taenite

lamella at one end of the schreibersite in traverse 7, approximately 200 fim away. The schreibersite interface profile is

given in Figure 9.

1618, traverse 9: crossed 200 fim of kamacite, 15 fim of schreibersite, 25 fim of taenite, and 120 fim of kamacite. The

location was 200 fim farther along the same taenite measured

in traverse 8.

Section 772 is from a separate small specimen of

the Santa Luzia meteorite. It combines both the

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

microprobe section of 2 cm2. Figure 10 is a scale

drawing indicating the areas of interest. A massive

schreibersite approximately 7 mm in length is

surrounded by clear kamacite that in turn is enclosed in microrhabdite-containing kamacite. A

major grain boundary containing grain boundary

schreibersite is within 3 mm of one end of the

massive schreibersite. Leading off of that grain

boundary is another that contains taenite as well

as grain boundary schreibersite, within 6 mm of

the massive schreibersite. Measurements have

been made in these areas as indicated in Figure

10.

772, traverse 10: crossed 0.9 mm of kamacite, 0.6 mm of massive

schreibersite, 0.6 mm of kamacite, 0.6 mm of massive schreibersite, 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

boundary schreibersite, and 0.5 mm of kamacite. The kamacite-schreibersite interface on leaving the schreibersite is

given in Figure 11.

772, traverse 12: crossed 0.7 mm of kamacite and 50 fim of

schreibersite. The Ni and P profiles are reproduced in

Figure 11.

772, traverse 13: crossed a taenite boundary schreibersite (see

inset in Figure 10). The major features of this traverse are

given in Figure 11.

The data on the Santa Luzia meteorite may be

of particular significance from the standpoint of

classification considerations. The schreibersite relationships observed for the large swathing zones

and their included schreibersite are in many ways

similar to the Bellsbank meteorite. The schreibersite relationships in the Widmanstatten areas of

Santa Luzia are similar to those observed in Ballinger. Could the differences in these two structures

(Figures 6, 7) be due primarily to differences in

initial P concentration?

LEXINGTON COUNTY

The Lexington County, South Carolina, meteorite is a coarse octahedrite of intermediate Ni
26 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

r

1-2

mm

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-

4 5 -

4 0 -

Z

iJ 35-

E

L

r 30-

s>

u

S

25-

20-

15-

10-

5 '

PxlO

772 *I2

3 6

0.05

4.6 N 772 * 13

4 7

1

—

6.0

0.0

-

-

-

-

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,

333 4

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

areas of well-developed Widmanstatten pattern.

The kamacite of these areas contains rhabdites of

various sizes, grain boundary schreibersites, grain

boundary and residual taenite and plessite, and

Neumann bands. Oxidation has severely penetrated the exterior surface of the specimen and

invaded the interior of the meteorite along major

grain boundaries. Occasional skeletal schreibersites in the millimeter size range are present,

normally completely surrounded by cohenite.

These large schreibersites tend to be surrounded

by areas of swathing kamacite that interrupt the

regularity of the Widmanstatten pattern. Isolated

patches of cohenite are also present.

Section 3334 contains a typical Lexington County

Widmanstatten pattern area and a large schreibersite surrounded by cohenite (6 x 0.3 mm). Edges

of the section and major grain boundaries have

been invaded by oxidation, but the areas selected

for microprobe analysis appear fresh. The paths

of the traverses listed below are indicated in Figure

12 and numerical data are given in Table 6.

3334, traverse 1: crossed 1.4 mm of kamacite, a 30 fim schreibersite partially embedded in cohenite at a cohenite-kamacite

interface, 60 fim of cohenite, 90 fim of schreibersite embedded in cohenite, 0.3 mm of cohenite, 0.2 mm of massive

schreibersite, 0.1 mm of cohenite, 0.2 mm of kamacite, 50

fim of cohenite, 0.2 mm of massive schreibersite, 80 fim of

cohenite, and 0.8 mm of kamacite.

3334, traverse 2: crossed 1.0 mm of kamacite, 50 fim of a large

grain boundary schreibersite, and 1.5 mm of kamacite. •

3334, traverse 3: crossed 0.8 mm of kamacite, a 15 fim wide

taenite border schreibersite, 25 ^im of taenite, 0.3 mm of

kamacite, a 15 fim wide schreibersite in sequence with

taenite, 10 fim of kamacite, a 40 fim taenite, and 0.4 mm of

kamacite. The data for an abbreviated section of this traverse

are given in Figure 3.

3334, traverse 4: crossed 0.1 mm of kamacite, a 10 fim wide

taenite border schreibersite, 25 fim of taenite, and 50 fim of

kamacite.

3334, traverse 5: crossed 50 fim of kamacite, a 5 fim wide

taenite-border schreibersite, 20 /xm of taenite, and 60 fim of

kamacite.

Figure 12 is a tracing of a photomosaic of the

studied area in Lexington County, section 3334.

The path of the traverses, the phases crossed, and

the nature of the structure are indicated diagrammatically. This section illustrates a general problem encountered in obtaining kamacite-schreibersite interface data in the more carbon-rich coarsestructured octahedrites. The large, low-Ni schreibersites in these meteorites are frequently completely surrounded by cohenite. Their Ni values

may be determined, but kamacite interfaces are

not present. Traverse 1, however, illustrates an

interesting schreibersite-cohenite relationship that

will be observed later in other meteorites. Cohenite

borders around large schreibersites occasionally

include small schreibersites, and more frequently

have small schreibersites at cohenite-kamacite in-
NUMBER 21 29

TABLE 6. —Lexington County schreibersite-kamacite interface measurements

Specimen

No.

3334

3334

3334

3334

3334

Traverse

No.

1

2

3

4

5

Structure traversed & schreibersites measured

[a-Ph-cohen1te-Ph-coheni te-Ph-coheni te-acohen1te-Ph-cohen1te-a, 3.5 mm]

Schreibersite embedded in cohenite at aInterface, 25x120 ym

Schreibersite embedded in cohenite, 140x280

ym

Enter

Exit

Massive schreibersite surrounded by cohenite (6x0.3 mm), 200 vm traverse

Enter

Exit

Massive schreibersite surrounded by cohenite (6x0.3 mm), 170 ym traverse

Enter

Exit

[a-Ph-a, 2.5 mm]

Grain boundary schreibersite, 500x40 ym

[a-Ph-Y-a-Ph-a-Y-a, 1.6 mm]

Taenite border schreibersite, 15x40 ym

Schreibersite near taenite, 10x20 ym

[a-Ph-Y-a, 200 ym]

Taenite border schreibersite, 5x10 ym

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

Taenite border schreibersite, 5x10 ym

Weight

*

N i Sch

34.8

33.0

32.5

25.0

24.6

24.1

24.1

39.3

50.0

52.5

50.5

51.0

Percent

XN1a

4.4

5.0

6.0

6.2

6.2

6.3

%

0

0

0

0

0

0

.04

.04

.03

.03

.03

.03

Ni Gradient

Length

(ym)

150

300 .

50

50

50

30

Wt.XNi

6.0

7.0

7.2

7.4

7.3

7.4

P Gradient

Length

(ym)

200

200

150

100

50

30

Wt.%P

0.07

0.06

0.06

0.05

0.05

0.05

Atomic

*

N iSch

29.9

28.3

27.9

21.4

21.1

20.6

20.6

33.9

43.3

45.6

43.6

44.2

Percent

%N1a

4.2

4.8

5.7

5.9

5.9

6.0

XP

a

0.07

0.07

0.05

0.05

0.05

0.05

terfaces. The Ni values in these schreibersites

increase away from the large schreibersite. In this

case, the large schreibersite contains approximately 25% Ni, the small one completely embedded in cohenite has an apparent Ni gradient from

32.5% to 33.0% increasing in the direction toward

kamacite, and the kamacite interface schreibersite

contains 35'% Ni. The measurements in Table 6

are otherwise comparable to those reported above

for similar structures.

BAHJOI

The Bahjoi, India, meteorite is an observed fall

of 1934, the only one included in this study. It is a

typical Group I meteorite of above average Ni

content. Bahjoi contains complex silicate-troilitegraphite-chromite nodules surrounded by rims of

schreibersite, and, in turn, cohenite. Grain boundary schreibersites, rhabdites of various sizes, and

taenite border schreibersites are present. Taenite

and plessite are present in greater abundance than

in the previously examined meteorites, and pearlitic and martensitic forms are common. An area

of the carbide haxonite was observed within a

pearlitic plessite area.

Section 1807(Sil) contains part of a complex

silicate-containing inclusion surrounded by schreibersite and cohenite, and small areas of kamacite.

1807(Sil), traverse 1: started within the complex inclusion, crossing first an island of kamacite surrounded by cohenite. The

phases in sequence were: 0.1 mm of cohenite, 0.2 mm of

kamacite, 0.1 mm of cohenite, 1.5 mm of schreibersite, 0.5

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

given in Table 7.

Section 1807 is of a typical Widmanstatten area

of Bahjoi, free of large inclusions. Rhabdites,

grain boundary schreibersites, and taenite border

schreibersites are present in abundance. Many

taenite-plessite areas are present in a variety of

morphologies. One of these martensitic areas contains a 0.1 x 0.1 mm area of haxonite.

1807, traverse 2: crossed 30 /xm of grain boundary schreibersite,

0.7 mm of kamacite, a 15 fim wide taenite border schreibersite, 20 /xm of taenite, and 10 fim of kamacite.

The observations made on the Bahjoi meteorite
30 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

TABLE 7. —Bahjoi schreibersite-kamacite interface measurements

Specimen

No.

1807(Sil)

1807

Traverse

No.

1

2

Structure traversed & schreibersites measured

[Cohenite-a-cohenite-Ph-cohenite-Ph-a, 3.5 mm]

Massive schreibersite, 10x4 mm, surrounded

by cohenite and containing -30% silicate

Enter

Exit

Schreibersite 100 ym wide embedded in

cohenite at a-border

Enter

Exit

[Ph-a-Ph-a, 750 um]

Grain-boundary schreibersite, 35x250 um

Taenite-border schreibersite, 20x60 wm

Weight

*

N 1Sch

18.8

18.9

32.3

33.5

38.8

49.0

Percent

%M

4.2

5.2

6.2

%P

a

0.04

0.04

0.03

Ni Gradient

Length

( m)

600

150

30

Wt.XNI

6.5

7.2

6.2

P Gradient

Length

( m)

200

150

100

Wt.SP

0.06

0.06

0.06

Atomic Percent

%Ni,. . XN1 %P Sch a a

16.0

16.1

27.7

28.8

33.4 5.0 0.07

42.4 5.9 0.05

are mainly similar in character to examples previously described. Figure 13 is a composition profile

based on data from a part of traverse 1. Part of

the massive schreibersite and its cohenite border

are included. Ni values are low in cohenite and

increase slightly upon moving away from the large

schreibersite. The small exterior schreibersite is

enclosed on three sides by cohenite and contains a

measurable Ni gradient. This type of association

is common in high C Group I meteorites.

GOOSE LAKE

The Goose Lake, California, meteorite is a high

Ni member of Group I with a medium octahedrite

structure. Polished and etched surfaces reveal

many large cohenite bordered, skeletal schreibersite inclusions, and occasional complex silicate -

troilite-graphite-schreibersite-cohenite associations. The cohenite has undergone partial decomposition to kamacite and graphite. Grain boundary

schreibersites, rhabdites in a range of sizes, and

taenite border schreibersites are present. Taeniteplessite areas are more abundant than in the

meteorites examined previously and are present

in a variety of forms. Weathering has invaded the

structure along grain boundaries and is present in

cracks within the large schreibersites.

Section 1332 is dominated by two areas of cohenite bordered schreibersite of approximately 5 X 10

mm each (photograph in Doan and Goldstein,

1969:765). One smaller cohenite-schreibersite inclusion is also present. Kamacite areas around

these inclusions are comparatively free of other

structures for distances up to 2 mm. Beyond this,

normal Widmanstatten areas are present.

1332, traverse 1: began at the border of a taenite area, crossed

2.4 mm of kamacite, and entered a 30 /xm wide grain

boundary schreibersite near its contact with a taenite border

of a plessite area. Numerical data for this and succeeding

Goose Lake traverses are given in Table 8.

1332, traverse 2: crossed 430 /xm of kamacite, 50 fim of schreibersite embedded in a cohenite rim of a large schreibersite,

90 fim of cohenite, 0.2 mm of schreibersite, 50 pm of

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

kamacite, entered a 40 /xm wide schreibersite embayed in

cohenite, 150 fim of cohenite, and 0.3 mm of massive

schreibersite.

1332, traverse 4: started near the initial point of traverse 3

within the taenite border, crossed 0.2 mm of kamacite, a 50

fim wide grain boundary schreibersite, and 0.7 mm of

kamacite.

Section 1332(2) is similar to number 1332 in that

it also contains a 5 X 10 mm skeletal schreibersitecohenite area. This section contains an unusually

large scrfreibersite that is only partially surrounded

by cohenite, thus permitting kamacite-schreibersite interface measurements. Its longest dimension

is 1.4 mm and has a maximum thickness of 0.6

mm. Morphology suggests that it is an unusually

large grain boundary schreibersite rather than a

massive schreibersite of the skeletal variety. The

areas away from these inclusions contained normal

Widmanstatten pattern.

1332(2), traverse 5: crossed 0.1 mm of kamacite, 0.1 mm cohenite, 1.8 mm of massive schreibersite, 170 fxm cohenite, 2.1

mm of kamacite, 20 /tin of cohenite, 0.1 mm of large grain

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

cohenite, 1.2 mm of schreibersite containing a small Ni

gradient over part of its distance, and 0.5 mm of kamacite.

This traverse crosses the length of the large schreibersite in

number 5.

1332(2), traverse 7: crossed 40 /*m tip of large schreibersite, 1.2

mm of kamacite, 0.2 mm of second tip of same schreibersite,

and 0.4 mm of kamacite. This is the same large schreibersite

traversed in numbers 5 and 6.

1332(2), traverse 8: crossed 0.5 mm of kamacite, 150 (im of the

third tip of the large schreibersite measured in traverses 5

through 7, and 0.5 mm of kamacite.

1332(2), traverse 9: crossed 0.4 mm of kamacite, 150 /xm of a

small schreibersite near the large schreibersite in traverses 5

through 8, and 0.3 mm of schreibersite.

1332(2), traverse 10: crossed 0.2 mm of kamacite, 30^im of a

grain boundary schreibersite, and 0.2 mm of kamacite.

1332(2), traverse 11: crossed 0.3 mm of massive schreibersite

close to traverse number 5, 0.1 mm of cohenite, a 20 ^im

wide schreibersite embedded in cohenite, 0.1 mm of kamacite, and 50 fim of cohenite.

The associations measured in Goose Lake are

similar in character to those described above for

other meteorites. The schreibersite precipitates in

this case, however, exist in an environment that is

more Ni-rich.

BALFOUR DOWNS

The Balfour Downs, Western Australia, meteorite is one of the highest Ni members of Group

I. The first prepared surface of this specimen was

perpendicular to the one shown in Figure 14,

along its bottom edge. This area of approximately

12 cm2 was free of large inclusions, having the

regular structure of the upper right-hand area in

Figure 14. On the basis of this casual observation,

Balfour Downs was assumed to be a low P medium

octahedrite. Actually, it contains localized massive

schreibersite enclosing significant quantities of silicates. These schreibersites are normally enclosed

in cohenite and surrounded by swathing kamacite

areas that interrupt the regular Widmanstatten
32 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

TABLE 8. —Goose Lake schreibersite-kamacite interface measurements

Specimen

No.

1332

1332

1332

1332

1332(2)

1332(2)

1332(2)

1332(2)

1332(2)

1332(2)

1332(2)

Traverse

No.

1

2

3

4

5

6

7

8

9

10

11

Structure traversed & schreibersftes measured

[y-a-Ph-y, 2.4 mm]

Schreibersite at y-border, 25 ym wide

[a-Ph-cohenite-Ph-cohenite-a, 1.6 mm]

Schreibersite embaying cohenite, 55 ym traverse

Enter

Exit

Massive schreibersite enclosed in cohenite

(3.4x0.2 mm), 260 ym traverse

[Ph-a-Ph-cohenite-Ph, 3.1 mm]

Schreibersite near taenite, 40x70 ym

Schreibersite embaying cohenite, 30x50 ym

Massive schreibersite (6x2 mm), 300 ym

traverse

[y-a-Ph-a, 950 um]

Grain boundary schreibersite (60x250 ym),

60 ym traverse

[a-cohen1te-Ph-cohen i te-a-cohen1te-Ph-a,

5.3 mm]

Cohenite bordered massive schreibersite

(2x6 mm), 1.8 mm traverse

Enter

Middle

Exit

Large schreibersite with partial cohenite

border (1.4x0.6 mm), 100 um traverse

[a-cohenite-Ph-a, 2.2 mm]

Large schreibersite with partial cohenite

border (1.4x0.6 mm), 1.2 mm traverse

Enter

Middle

Exit

[Ph-a-Ph-a, 1.8 mm]

Large schreibersite of traverses 5 and 6

40 um traverse

200 ym traverse

Enter

Exit

[a-Ph-a, 1.2 mm]

Large schreibersite of traverses 5-7,

150 ym traverse

Enter

Exit

[a-Ph-a, 0.9 mm]

Small schreibersite near traverses 5-8

(250x400 ym), 150 ym

Enter

Exit

[a-Ph-a, 0.5 mm]

Grain boundary schreibersite (0.03x1.6 mm)

associated with plessite, 30 ym traverse

[Ph-cohenite-Ph-a-cohenite, 0.7 mm]

Massive schreibersite (same as traverse 5) ,

0.4 mm traverse

Start

Exit

Schreibersite (20x60 ym) embedded in

cohenite-a border

Weight

*

N1Sch

52.0

35.0

34.0

25.0

46.5

36.5

18.5

41.0

18.7

18.2

18.5

31.3

29.3

27.3

27.3

31.0

27.0

27.0

29.1

29.1

27.2

27.2

44.0

18.3

18.3

37.3

Percent

a

6.0

4.1

6.0

4.3

5.1

3.8

3.6

3.9

3.3

3.5

3.7

3.8

3.1

3.6

5.8

4.5

%P

a

0.03

0.05

0.04

0.05

0.05

0.06

0.07

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.05

0.06

N1 Gradient

Length

(ym)

25

100

50

150

100

200

500

400

600

300

500

400

300

250

100

150

Wt.*N1

7.2

6.4

7.0

5.7

7.2

5.7

5.8

6.0

6.2

6.8

6.5

6.5

6.0

5.5

7.4

6.0

P Gradient

Length

(ym)

100

50

100

200

Flat

100

250

200

150

150

150

150

100

100

200

100

Wt.iSP

0.07

0.06

0.07

0.08

0.05

0.08

0.09

0.08

0.08

0.09

0.09

0.08

0.07

0.07

0.06

0.09

Atomic

*

N1Sch

45.1

30.1

29.2

21.4

40.2

31.4

15.8

35.4

15.9

15.5

15.8

26.9

25.1

23.4

23.4

26.6

23.1

23.1

25.0

25.0

23.3

23.3

38.0

15.6

15.6

32.1

Percent

%M

a

5.7

3.9

5.7

4.1

4.9

3.6

3.4

3.7

3.1

3.3

3.5

3.6

3.0

3.4

5.5

4.3

%P

a

0.05

0.09

0.07

0.09

0.09

o.n

0.13

0.11

o.n

0.11

o.n

o.n

0.13

0.13

0.09

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

Specimen

No.

3202

3202

3202

3202

Traverse

No.

1

2

3

4

Structure traversed & schreibersites measured

[Ph-a-Ph-a-Ph-Y, 3.3mm]

Massive schreibersite,

Start

Exit

1550 ym traverse

Residual schreibersite 30x800 ym, associated with taenite, crossed at two

locations

[Ph-a-Ph-a, 3.1 mm]

Massive schreibersite,

Start

Exit

Massive schreibersite,

Enter

Exit

[o-Ph-o-cohen1te-Ph, 2.3 n

Massive schreibersite,

Enter

Exit

Massive schreibersite,

Enter

End

[Ph-cohenite-a-cohenite-PI

Massive schreibersite.

Start

Exit

Massive schreibersite,

Enter

Stop

600 ym traverse

500 ym traverse

400 ym traverse

1350 ym traverse

,2.1 mm]

800 ym traverse

250 ym traverse

Weight

*

N1Sch

20.5

21.0

41.0

41.0

21.3

21.5

21.5

21.5

21.0

21.0

20.0

20.0

19.5

19.5

20.5

21.0

Percent

*N1

a

2.9

5.3

5.5

2.6

2.8

3.0

2.9

2.8

%P

a

0.05

0.04

0.03

0.05

0.05

0.05

0.05

0.05

Ni Gradient

Length

(ym)

1150

100

50

300

250

1500

250

150

Wt.%N1

6.7

00 00

4.8

4.8

7.0

5.3

4.8

P Gradient

Length

(ym)

200

100

300

200

300

200

150

Wt.fcP

0.09

0.09

0.09

0.09

0.11

0.10

0.08

Atomic

*

N1Sch

17.5

17.9

35.4

35.4

18.2

18.4

18.4

18.4

17.9

17.9

17.1

17.1

16.7

16.7

17.5

17.9

Percent

a

2.8

5.1

5.2

2.5

2.7

2.9

2.7

2.7

%P

a

0.09

0.07

0.05

0.09

0.09

0.09

0.09

0.09

pattern (center left in Figure 14). Grain boundary

schreibersite, schreibersite associated with taeniteplessite areas, and rhabdites in a variety of sizes

are present. Taenite-plessite areas are present in a

variety of forms and Neumann bands are common.

Section 3202 was prepared from the outlined

area in Figure 14. It was selected particularly for

the presence of massive schreibersite that was

partially free of cohenite borders. Figure 15 is a

photograph of a photomosaic of a portion of this

section. The paths of the four traverses described

below are indicated in Figure 16, a sketch based

on Figure 15.

3202, traverse 1: crossed 1.5 mm of massive schreibersite, 4.5

mm of kamacite, a 40 fim wide schreibersite surrounding a

taenite area, 0.1 mm of kamacite, recrossed a 30 fim width

of the same schreibersite previously crossed, 50 /urn of

kamacite, and 50 fim of taenite. Numerical data for this and

succeeding Balfour Downs traverses are given in Table 9.

3202, traverse 2: crossed 0.6 mm of massive schreibersite, 0.6

mm of kamacite, 0.5 mm of massive schreibersite, and 1.5

mm of kamacite.

3202, traverse 3: crossed 0.3 mm of kamacite, 0.4 mm of massive

schreibersite, 0.2 mm of kamacite, 80 jxm of cohenite, and

1.3 mm of massive schreibersite. The path of this traverse is

indicated in Figures 15 and 16 and the electron microprobe

profile is shown in Figure 17.

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

0.2 mm of massive schreibersite.

Also indicated in Figure 16 are Ni concentrations

in various areas of the two massive schreibersites.

One is tempted to conclude that a slight Ni gradient exists, with the lowest Ni values being adjacent to cohenite. The differences, however, are

small, and this should undoubtedly be looked at

more systematically in the future. The Ni and P

profile reproduced in Figure 17 illustrates several

interesting features. The Ni concentration in kamacite is low close to these large schreibersites, as

are the Ni interface values. There also appears to

be a Ni gradient in cohenite, with Ni increasing

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 CONSIDERATIONS OF PHASE GROWTH

The total amount of P present in an iron meteorite has a marked effect on the structural development process, and consequently on the final

structure observed in prepared sections. Studies

of Widmanstatten pattern growth (Wood, 1964;

Goldstein and Ogilvie, 1965b; Goldstein and Axon,

1973) have demonstrated that octahedrite meteorites cooled from high temperatures and were in

equilibrium down to 700° C. Schreibersite growth

studies (Goldstein and Ogilvie, 1963; Doan and

Goldstein, 1969) led to similar conclusions concerning equilibrium temperatures for iron meteorites

in general. In a discussion of the effects of P on

the development of the Widmanstatten pattern,

Goldstein and Doan (1972) estimate that equilbrium in iron meteorites was maintained down to

650° C, and probably to 600° C. Recent work by

Randich (1975) suggests the schreibersite-kamacite

equilibrium is maintained down to 500° C for

diffusion distances as great as 1000 fim. Previous

work, therefore, suggests that many of the schreibersite precipitates observed in iron meteorites

nucleated and grew under equilibrium conditions

during a significant part of their cooling history.

It is the purpose of this section to examine schreibersite growth and related structural development

in the selected group of coarse-structured iron

meteorites in terms of the equilibrium Fe-Ni-P

diagram of Doan and Goldstein (1970). How far

can equilibrium considerations take us in explaining observed structures?

Before proceeding, however, brief mention

must be made of the effects of S, C, and Co on

the Fe-Ni-P system. These elements are present in

iron meteorites in quantities that may be comparable to or in excess of P. The role of S has been

discussed recently by Goldstein and Axon (1973)

and Wai (1974). Goldstein and Axon (1973) point

out that S may be important in lowering the
38 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

temperature of metal melts, thereby mobilizing

metal without melting silicates. This effect, however, would only extend down to temperatures in

the range of 1000° C. At lower temperatures,

material of Fe-FeS eutectic composition becomes

enclosed in solidifying metal, and the solubility of

S in the metallic phases becomes negligibly small.

Sulfur is removed from the metal by this process,

becoming isolated in complex sulfide nodules (El

Goresy, 1965).

The situation with respect to carbon is much

more complex. Graphite or graphite-troilite nodules present in many meteorites represent high

temperature segregations of C as well as S. The

petrography of these complex associations has

been described by El Goresy (1965). Comparatively

large amounts of C, however, remain soluble in

the metal to lower temperatures, temperatures

within the schreibersite growth range. Brett (1967)

has discussed the occurrence of cohenite ((Fe,Ni)3C)

in meteorites and described its formation in terms

of phase equilibria using an extrapolated Fe-Ni-C

diagram. He proposed that cohenite formed

within a narrow range of bulk Ni values in the

temperature range of 650° to 610° C. Scott (1971a)

has discussed carbides in iron meteorites in great

detail from a petrographic point of view. He has

also reported two new carbides, haxonite

((Fe,Ni,Co)23C6) (Scott, 1971b), and a phase designated W-carbide ((Fe,Ni,Co)2.5C), both of which

contain considerably more Ni and Co in their

structure than is observed in cohenite. Scott

(1971a) suggests that cohenite may have formed at

somewhat lower temperatures than those suggested by Brett (1967). Both Scott (1971a) and

Buchwald (1976) point out that cohenite is much

more widely distributed in meteorites than previously realized. Cohenite and schreibersite are frequently observed in intimate association (Drake,

1970; Clarke, 1969; Buchwald, 1976), with cohenite

normally encapsulating large schreibersite inclusions in the high C Group I meteorites. These

schreibersite-bordering cohenites normally enclose

occasional small later-formed schreibersites as has

been illustrated above (Figure 13). This morphology suggests the possibility of concurrent schreibersite-cohenite growth after major amounts of

schreibersite have formed. Carbon, therefore, is

seen to be a component of the system of direct

concern here. Experimental data on carbon's effect on the Fe-Ni-P system at low temperatures is

not available, and it has been assumed that it can

be ignored here except in cases where its physical

presence in the form of cohenite or other carbides

is important. This assumption may well require

reevaluation,when appropriate experimental data

becomes available.

Another simplifying assumption made in this

study is to ignore the presence of Co, an element

present in iron meteorites in the range of 0.3% to

0.7% (Moore et al., 1969). Co distributes itself

between the three phases of interest, following Fe

and seemingly having little effect on the equilibria

involved.

In the following sections, individual meteorites

will be discussed in terms of structural development on equilibrium cooling through the low temperature portion (995° to 550° C) of the Fe-Ni-P

system. The Doan and Goldstein (1970) isothermal

sections were recalculated on an atom percent

basis, and four of these sections plotted on a

triangular coordinate system are given in Figure

18.

Equilibrium in the system at hand may be defined as (1) the phases present are homogeneous,

that is to say, free of measurable concentration

gradients, and (2) the compositions of individual

phases are related to the bulk composition by the

phase diagram. In the case at hand, this means

that (a) the bulk composition is the composition of

the single phase present when it lies within the

kamacite (a) or taenite (y) field of the diagram;

(b) the bulk composition is on a tie line joining the

compositions of the two phases present in the

kamacite + schreibersite (a + Ph), the kamacite +

taenite (a + y), and the taenite + schreibersite (y

+ Ph) fields of the diagram; and (c) the bulk

composition is related to the three phases present

by the compositions at the corners of the triangular

kamacite + taenite + schreibersite (a + y + Ph)

field of the diagram. The proportions of the

phases present in the two- and three-phase fields

may be calculated using lever rule principles. Calculations of this type were carried out for each of

the meteorite compositions considered and are

presented in a series of tables to follow (Tables

10-15). Temperatures, the proportion of phases

present, and their Ni contents are given. These

data will be discussed for each meteorite in terms

of observed structures. Selected compositions representing these meteorites are plotted on a series

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 compositions appropriate to the Coahuila meteorite

(percentage of phases present at indicated

temperatures with their Ni contents)

°c

995

975

925

875

850

750

700

650

600

550

°C

995

975

925

875

850

750

700

650

600

550

0.5

%a

60

75

93

99

99.7

1.1

%a

12

50

82

94

98

98

Atomic

*Y

100

100

100

100

100

40

25

7

1

Atomic 5

%y

100

100

100

100

88

50

18

5

i P, 5

SPh

0.3

5 P, 5

SPh

1

2

2

.3 Atomic

a

3.3

4.0

4.4

5.2

5.2

.3 Atomic 3

a

3.5

3.8

4.5

5.0

5.2

5.2

t N1

%N

5

5

5

5

5

6

9

12

14

. N1

%H

5

5

5

5

5

6

9

11

Y

.3

.3

.3

.3

.3

.5

.2

Y

.3

.3

.3

.3

.6

.9

.1

2Ni Ph

14

*

N i Ph

9.0

13

13

the Fe-Ni-P system from 850° to 550° C in Figure

19.

COAHUILA. —The Coahuila hexahedrite is a meteorite of both low Ni (5.6 weight %, 5.3 atomic

%) and low P (0.3 weight %, 0.5 atomic %). These

bulk composition values are well established in the

literature and are generally consistent with the

major features of the observed structure. The

presence of occasional large schreibersites as described above suggests, however, that the P value

may be*somewhat low. For this reason the calculations given in Table 10 were made using two P

values, the literature value (0.5 atomic %) and one

approximately twice as high (1.1 atomic %). The

equilibrium phases present, their amounts in mole

percent, and their Ni contents in atomic percent

are listed in Table 10 for 10 temperatures between

995° to 550° C.

Due to Caohuila's low P (0.5 atomic %), the

predicted structural development sequence using

the Fe-Ni-P diagram is essentially the same as that

derived using the Fe-Ni diagram. At 995° C (Table

10), taenite of the bulk meteorite composition is

the only phase present, and this situation continues until the meteorite cools to below 850° C. As

the temperature drops an additional 200° C, taenite transforms almost completely to massive single

crystal kamacite. The last of the taenite transforms

to kamacite below 600° C, in the temperature

range where schreibersite first nucleates. This sequence accounts for kamacite containing homogeneously nucleated rhabdite, the most prominent

feature of Caohuila. This low temperature of

initial schreibersite nucleation and the small

amount formed, however, make it difficult to

understand the presence of the occasional larger

schreibersites mentioned above.

If a somewhat higher P content is assumed,

significant differences are revealed. With an assumed content of 1.1 atomic % P (Table 10),

transformation of taenite to kamacite starts more

than 50° C higher, extends over a slightly larger

temperature range, and is complete at a higher

temperature than in the previous case. Schreibersite precipitates, while more than 5% taenite remains in the structure, and a total of 2% schreibersite is present at 600° C. In this case, one would

expect schreibersite to nucleate at taenite-kamacite

boundaries, but no direct evidence for this has

been developed. The actual equilibrium P value

may well lie between the two selected values,

perhaps 0.8 atomic %. This amount of P would

produce upon cooling a somewhat smaller quantity

of schreibersite, initially nucleating at a temperature below 650° C, and consuming any remaining

residual taenite. Both sufficient P and adequate

time at higher temperatures would be available to

permit growth of the isolated larger schreibersites

observed, and the bulk of the meteorite would

appear to have formed under conditions of lower

total P.

The 5.3 atomic % Ni and 0.8 atomic % P compositions are plotted on a series of isothermal

sections in Figure 19. At 850° C, Coahuila's composition is just inside the kamacite-taenite field of

the diagram, yielding a structure containing a

small amount of kamacite within a taenite matrix.

The field moves in relation to this composition

with decreasing temperature, until at 650° C only

a small amount of taenite remains in equilibrium

with a kamacite matrix. At 600° C, the composition

is within the kamacite-schreibersite field, perhaps

having passed through the corner of the kamacitetaenite-schreibersite field. By 550° C, the composition lies well within the kamacite-schreibersite field

of the diagram, with approximately 98% kamacite
NUMBER 21 41

850°C

0

0

100% Fe

ABELLSBAN K

• COAHUILA

• SANTA LUZIA

S-BALLINGER

• LEXINGTON COUNTY

- BAHJOI

AGOOSE LAKE

-BALFOUR DOWNS

5.0

5.3

6.2

6.2

7.1

7.8

%P

3.6

0.8

2.7

1.4

1.8

2.7

10 15

ATOMIC %Ni

20

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,

being in equilibrium with 2% schreibersite containing 13 atomic % Ni and 25 atomic % P.

The development sequence described above accounts for the major features of Coahuila structure. Equilibrium cooling of this bulk composition

produced large single crystals of kamacite containing minor inclusions. The earliest-formed schreibersite nucleated and grew on preexisting sulfide

inclusions where available, or perhaps at kamacitetaenite interfaces. The great majority of the individual schreibersite crystals, however, may be assumed to have nucleated homogeneously within

kamacite. If this process had stopped at 550° C,

the meteorite would contain kamacite of 5.1 atomic

% Ni and 0.5 atomic % P, and schreibersite of 13

atomic % Ni. Phase composition measurements

from the literature and those reported above indicate that diffusion controlled growth continued to

lower temperatures, modifying phase compositions significantly, while producing only subtle

changes in the gross structural features. These

changes in phase compositions and subtle differences in structure will be discussed in some detail

later in this paper.

BALLINGER. —The Ballinger meteorite is a low

Ni octahedrite probably containing more P than

the analytical value given in Table 1 (0.4 weight %

P, 0.7 atomic % P). The Ballinger photograph

(Figure 6) shows three areas containing large

schreibersite inclusions, perhaps suggesting that

the amount of P should be doubled. The calculations given in Table 11, therefore, were made

using 6.2 atomic % Ni (6.5 weight %) and both 0.7

atomic % and 1.4 atomic % P.

During equilibrium cooling in the low P case

(Table 11), taenite is the only phase present until

kamacite appears around 800° C. At 600° C, the

equilibrium structure contains 89% kamacite, 10%

taenite, and only 1% schreibersite. Upon cooling

to 500° C, all of the taenite would be expected to

transform, producing an equilibrium structure of

99% kamacite and 1% schreibersite. Both the presence of the massive schreibersites and residual

taenite in the actual structure argue against this

interpretation. The growth of such large schreibersites requires more P, and the presence of

taenite demonstrates that equilibrium cooling was

not maintained down to 550° C.

The 1.4 atomic % P calculation for Ballinger

(Table 11 ) leads to a more reasonable interpretaTABLE 11. —Equilibrium cooling of compositions appropriate to the Ballinger meteorite

(percentage of phases present at indicated

temperatures with their Ni contents)

"C

995

975

925

875

850

750

700

650

600

550

°C

995

975

925

875

850

750

700

650

600

550

0

%a

15

60

78

89

99

1

%a

5

4

30

62

77

88

96

.7 Atomic

*Y

100

100

100

100

100

85

40

22

10

.4 Atomic

%y

100

100

100

95

95

68

36

20

8

% P, 6

%Ph

1

1

% P, 6

%Ph

1

2

2

3

4

4

.2 Atomic

*N1

a

3.7

4.0

4.9

5.2

6.0

.2 Atomic

XN1

a

3.9

4.0

4.0

4.5

5.0

5.2

5.7

* N1

XN1

y

6.2

6.2

6.2

6.2

6.2

6.6

9.1

11

14

* N1

%N1

y

6.2

6.2

6.2

6.3

6.3

7.2

9.1

11

14

*N1

13

15

XN1

4

5

7

9

13

15

Ph

Ph

.4

.3

.1

.0

tion of the structure. Small amounts of kamacite

form in taenite around 900° C, and by 850° C the

taenite matrix contains 4% kamacite and 1%

schreibersite, a situation that might be expected to

lead to the growth of several rather large schreibersites on subsequent cooling rather than a number of small ones. By 750° C, one-third of the

meteorite is kamacite, undoubtedly the swathing

kamacite surrounding the large schreibersites that

are present. As the temperature falls to 650° C,

the meteorite transforms to 77% kamacite, and

the schreibersite doubles its Ni content to 9.0

atomic %, while increasing its amount by one-half.

As the temperature drops to 600° C, kamacite

increases in volume, schreibersite increases in both

amount and in Ni content, and taenite shrinks to

8%. The higher P content seems to be more

consistent with the observed structure and could

actually be a little low. The presence of taenite

and high Ni schreibersite in the actual structure

(see above) demonstrates that equilibrium cooling

did not persist down to 550° C, and that nonequilibrium phase growth continued to lower temperatures. As in the case with Coahuila, however, the

gross features of the structure are accounted for

by equilibrium cooling of this particular composition (Figure 19).
NUMBER 21 43

SANTA LUZIA. —The Santa Luzia meteorite

structure is dominated by schreibersite and schreibersite-troilite inclusions surrounded by broad

areas of swathing kamacite (Figure 7). The literature values for its bulk composition, 6.3 atomic %

Ni and 1.6 atomic % P (6.6 weight % Ni and 0.9

weight % P, Table 1), are very close to those

assumed for the high P Ballinger composition

(Table 11), only a 0.1% increase in Ni and 0.2% in

P. This apparently minor change, however, has

important ramifications for the structural development process in Santa Luzia. In Ballinger the

expected sequence of phases with falling temperature,

y —> a + y —» a + y + Ph,

is followed. In Santa Luzia this sequence is interrupted around 850° C, where over a range of

temperature taenite is in equilibrium with schreibersite, the sequence of phases being

TABLE 12. —Equilibrium cooling of compositions appropriate to the Santa Luzia meteorite

(percentage of phases present at indicated

temperatures with their Ni contents)

Ph Ph.

The growth of significant amounts of schreibersite

within taenite subsequent to the disappearance of

preexisting kamacite produced conditions that resulted in the development of large volumes of

swathing kamacite and the unusual schreibersite

morphology observed in the meteorite.

The actual bulk P value for Santa Luzia may be

somewhat larger than the 1.6 atomic % value given

in Table 1. The calculations summarized in Table

12 indicate, however, that the same sequence of

phases would be expected whether 1.6 atomic %

or 2.7 atomic % P is assumed. In either case,

kamacite and taenite would have been in equilibrium at 925° C. It is reasonable to assume that this

kamacite nucleated within taenite at the preexisting troilite-taenite grain boundaries, with kamacite

lamellae radiating away from the troilite. Upon

cooling to 875° C, the proportion of kamacite was

markedly reduced at both P levels. This small

amount of kamacite presumably remained localized near the troilite nodules. The taenite interfaces with troilite and the kamacite-taenite interfaces acted as nucleation sites for schreibersite

upon further cooling, producing a skeletal structure that sets the pattern for subsequent schreibersite growth. Nearly half in one case, and more

than half in the other, of the schreibersite that

formed upon cooling the meteorite to 500° C

°c

995

975

925

875

850

750

700

650

600

550

°C

995

975

925

875

850

750

700

650

600

550

1

%a

10

3

26

60

76

87

95

2

%a

43

50

35

4

27

60

76

88

91

.6 Atomic

%y

100

100

90

96

98

71

37

21

8

.7 Atomic 3

*Y

57

50

65

90

93

66

32

16

3

t p, 6

%Ph

1

2

3

3

3

5

5

i P, 6

SPh

6

7

7

8

8

9

9

.3 Atomic

%Ni

a

4.5

4.0

4.0

4.5

5.0

5.2

5.8

.2 Atomic 3

*N1

a

5.3

5.4

4.9

4.0

4.0

4.5

5.0

5.2

5.4

i Ni

SN1

Y

6.3

6.3

6.4

6.4

6.3

7.2

9.1

11

14

i Ni

XN1

Y

6.8

7.1

7.0

6.4

6.3

7.2

9.1

11

14

%Ni

Ph

4.4

4.5

5.3

7.1

9.0

13

15

*

N i Ph

4.4

4.5

5.3

7.1

9.0

13

14

formed at the relatively high temperature of 850°

C. Upon further cooling Ni and P continued to

diffuse into the central schreibersite areas, P contributing to additional growth and Ni contributing

to both growth and Ni enrichment. As a consequence of this localized growth of schreibersite, a

large surrounding volume of low Ni metal was

produced, the swathing kamacite zone of the final

structure. This proposed growth sequence explains the morphology of these large sc(hreibersite

structures without the necessity of invoking precipitation from the liquid, a process that would require a bulk composition of more than 4.9 atomic

% P at high temperatures.

The broad swathing zone developed as the meteorite cooled from 850° C down into the 700° to

650° C range (Figure 7). This transformation takes

place easily at these temperatures as only a relatively small amount of Ni movement is required

for both schreibersite growth and enrichment,

and for production of the taenite border at the

outer edge of the zone. This expanding taenite

border acts as a barrier to isolate the large central

schreibersites from the bulk material beyond the

swathing zone. As the temperature drops and

diffusion becomes increasingly restricted, conditions are met for the nucleation of schreibersite at
44 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

this taenite border. By 550° C the taenite border

has been transformed and replaced with a continuous grain boundary, approximately half of which

is occupied by grain boundary schreibersite. This

is amply supported by examination of Santa Luzia

specimens. The swathing zone boundary contains

a large amount of schreibersite and is, with a few

minor exceptions, free of residual taenite. The

swathing zone itself is also free of large schreibersites and residual taenite, with the exception of

several small areas very close to the swathing zone

border. These appear to have been small areas of

residual taenite that became trapped within the

swathing zone at a very late stage and may have

been part of the taenite border system at higher

temperatures. Competition for P from neighboring schreibersites appears to be the reason that

these areas were not converted completely to

schreibersite.

Schreibersite equilibration with taenite in the

850° C temperature range results in a redistribution of P in the meteorite leading to the development of areas of coarse Widmanstatten pattern

(Figure 7). At these high temperatures P diffuses

with ease, lowering its level even in the distant

bulk metal to approximately 1.0 atomic %. Subsequent to this, the swathing zone development

described above took place, resulting in the isolation of that part of the meteorite from the areas

where the coarse Widmanstatten pattern formed.

The areas beyond the swathing zones developed

an effective bulk composition similar to that of the

low P Ballinger case described above. Widmanstatten pattern nucleated in these areas at temperatures below 750° C, and grain boundary schreibersite probably began nucleating at kamacite-taenite

interfaces around 600° C. The observed metallography of these areas is consistent with this explanation.

The bulk composition of Santa Luzia is plotted

on the diagrams in Figure 19. As in the previous

cases, it is obvious from the observations reported

in the results section that bulk equilibrium was not

maintained down to 550° C, and that phase modification persisted to considerably lower temperatures.

LEXINGTON COUNTY AND BAHJOI. —The bulk

compositions of the Lexington County and Bahjoi

meteorites appear to be poorly established (Table

1). They are structurally similar, carbon- and sulfur-rich members of Group I with intermediate

TABLE 13. —Equilibrium cooling of compositions appropriate to the Lexington County

and Bahjoi meteorites (percentages of phases

present at indicated temperatures with their

Ni contents)

°c

995

975

925

875

850

750

700

650

600

550

°C

995

975

925

875

850

750

700

650

600

550

0.

%a

40

70

79

99

1.

%a

5

10

Tr

42

60

78

94

5 Atomic

%y

100

100

100

100

100

100

60

30

20

8 Atomic 3

%y

100

95

90

98

97

96

54

35

16

t P, 7.

%Ph

0.3

1

i p, 7 .

0.3

2

3

4

4

5

5

6

1 Atomic

%M

a

4.1

4.9

5.2

6.9

1 Atomic

%N1

a

5.5

5.3

(4.2)

4.5

5.0

5.2

6.5

S N1

%M

y

7.1

7.1

7.1

7.1

7.1

7.1

9.2

12

14

i Ni

%N1

y

7.1

7.2

7.3

7.1

7.2

7.2

9.1

11

14

SUM

13

16

%M

5.

5.

5.

5.

7.

9.

13

15

Ph

Ph

2

0

3

3

1

0

bulk Ni. The P values of Lexington County (0.5

atomic % P, 0.3 weight % P) are too low to be

reconciled with the amount of schreibersite observed in prepared sections. The Lexington

County bulk Ni value is also probably somewhat

higher than 7.0 weight % Ni when the contribution

of large schreibersites is taken into consideration.

Much of the kamacite in this meteorite contains

7.0 weight % Ni, and some of it contains slightly

more Ni. Bahjoi also contains isolated large schreibersites, requiring its effective bulk P to be greater

than 0.3 weight % P. For these reasons, the equilibrium phase calculations in Table 13 were made at

the intermediate Ni value of 7.1 atomic % (7.5

weight % Ni), and at 0.5 atomic % P and 1.8

atomic % P (1.0 weight % P). These calculations

suggest that the actual P value probably lies between the two, probably greater than 1 atomic % P.

Structure development in the cooling meteorite

does not start until below 750° C in the low P case

(Table 13), taenite being the only phase present at

higher temperatures. Schreibersite precipitation

does not begin until below 650° C, a temperature

at which more than 70% of the structure has

transformed to kamacite. At 600° C, only 0.3%
NUMBER 21 45

schreibersite is present. Under these conditions

schreibersite would have precipitated at kamacitetaenite interfaces throughout a well-established

Widmanstatten pattern. P could not move through

the Widmanstatten pattern to precipitate as schreibersite borders at isolated sulfide interfaces.

Schreibersite in these meteorites must have precipitated initially prior to extensive Widmanstatten

pattern development, requiring a greater bulk P

content.

The 1.8 atomic % P value used in Table 13 is

somewhat higher than necessary to account for

the observed structure. With this amount of P in

the system, kamacite would precipitate by 975° C

and schreibersite would be present by 925° C. If

the P content were reduced to 1.4 atomic %,

neither kamacite nor schreibersite would be present in the system at 925° C or above. At 875° C,

however, schreibersite would be present growing

within taenite. Kamacite would not nucleate until

below 750° C. The important consideration here is

that within a reasonable range of P contents,

schreibersite nucleates within taenite, undoubtedly

at preexisting taenite interfaces with troilite or

troilite-silicate-graphite inclusions if they are available. Major quantities of schreibersite grow under

these conditions prior to kamacite nucleation and

the onset of Widmanstatten pattern development.

The 1.8 atomic % P value is used in Figure 19,

and it can be seen there that reasonable reduction

in the amount of P changes only the amounts of

the phases present. Nucleation temperatures for

both schreibersite and kamacite would be lowered

somewhat, and smaller amounts of P would produce smaller quantities of schreibersite. Neither

of these meteorites would appear to contain as

much as 6% schreibersite, but certainly they both

contain more than 1 %.

The structural development sequence in Lexington County and Bahjoi is similar to that of Santa

Luzia. With falling temperature a few massive

schreibersites grow within taenite, followed by

nucleation and growth of a swathing kamacite

zone. The Ni values in these massive schreibersites

(16 atomic % Ni for Bahjoi, 21 atomic % Ni for

Lexington County) require that Ni enrichment

continued to below 550° C. At some temperature

below 650° C, however, massive schreibersite

growth is stopped by precipitation of cohenite.

Cohenite contains no detectable P and, therefore,

isolates the massive schreibersite from a source of

P for continued growth. Ni gradients within the

cohenite borders (Figure 13) suggest that Ni may

continue to be supplied to the schreibersite

through the cohenite. Under these circumstances,

a distinct swathing zone morphology is not developed, but the surrounding kamacite does retain a

low Ni level and is comparatively free of laterstage schreibersite precipitation.

Within the bulk metal surrounding these eventual cohenite-schreibersite inclusions, initial Widmanstatten pattern development started above

700° C, following its normal sequence. Bulk Ni is

low enough that a coarse-structured octahedrite

pattern formed. Both of these meteorites have

schreibersite morphologies and distributions similar to a meteorite like Ballinger, but they contain

more grain boundary taenite and residual taeniteplessite areas, particularly Bahjoi.

GOOSE LAKE AND BALFOUR DOWNS. —The Goose

Lake and Balfour Downs meteorites are two of

the highest Ni members of Group I (Table 1), and

they are meteorites that contain significant

amounts of C and S. The analytical P values from

the literature again appear to be too low to explain

the observed metallography. A planometric estimate of 0.9 weight % P (1.6 atomic % P) for Goose

Lake was given by Doan and Goldstein (1969), and

this figure appears to be consistent with the observed structure. Phase calculations are given in

Table 14 using a pair of compositions to represent

the two meteorites, 7.9 atomic % Ni (8.3 weight %

Ni), and 1.4 and 2.7 atomic % P (0.75 and 1.5

weight % P), bracketing what was undoubtedly

their effective bulk compositions.

When 1.4 atomic % P is assumed (Table 14),

schreibersite nucleates within taenite at a temperature above 850° C. If 2.7 atomic % P is assumed,

schreibersite nucleates above 975° C in the presence of 20% kamacite. No structural evidence for

initial precipitation at kamacite-taenite interfaces

has been observed, supporting the suggestion that

the actual P level was in the neighborhood of 1.6

atomic % P. This means that schreibersite nucleated around 900° C, undoubtedly at taenitetroilite boundaries, were they available. The result

was growth of a large amount of schreibersite in

taenite, with nucleation of a kamacite swathing

zone following at a temperature somewhat above

700° C. As the temperature fell, a large supply of

Ni was required to keep these massive schreibersites in equilibrium, a process that seems to have
46 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

TABLE 14. —Equilibrium cooling of compositions appropriate to the Goose Lake and Balfour Downs meteorites (percentage of phases

present at indicated temperatures with their

Ni contents)

°c

995

975

925

875

850

750

700

650

600

550

°C

995

975

925

875

850

750

700

650

600

550

1

%a

25

47

70

92

2

%a

20

23

25

47

70

91

.4 Atomic

%y

100

100

100

100

99

98

72

50

26

4

.7 Atomic

*Y

80

75

96

95

93

92

67

44

21

% P, 7

XPh

1

2

3

3

4

4

% P, 7

%Ph

2

4

5

7

8

8

9

9

9

.9 Atomic

XN1

a

4.5

5.0

5.2

7.1

.8 Atomic

%N1

a

6.2

6.3

4.5

5.0

5.2

7.0

% N1

XN1

Y

7.9

7.9

7.9

7.9

7.9

8.0

9.1

11

14

18

% N1

%N1

Y

8.2

8.3

7.8

7.9

7.9

7.9

9.1

11

14

*

N iPh

6.6

7.0

7.1

9.0

13

16

%N1

Ph

6.0

5.7

6.0

6.2

6.6

7.1

9.0

13

16

continued down to below 550° C (17 to 24 atomic

% Ni in cohenite-enclosed massive schreibersite).

The size of the massive schreibersites requires that

the cohenite borders precipitated at temperatures

below 750° C. Within this same 700° to 550° C

temperature range, the bulk of the metal away

from the large schreibersites transformed to normal Widmanstatten pattern. Grain boundary

schreibersite, rhabdites, and taenite border schreibersites were formed during this part of the sequence.

In Figure 19 the 2.7 atomic % P value is used. It

is obvious here that the same sequence of phases

is obtained if the P value is somewhat reduced.

The higher Ni is the significant structural development factor in this case.

BELLSBANK. —The Bellsbank meteorite is anomalous, unusually low in Ni and high in P. The

gross segregation of schreibersite in its structure

makes it unusually difficult to arrive at a satisfactory bulk P content. For this reason the literature

values of 5.3 weight % Ni and 2 weight % P were

bracketed in Table 15 by assumed compositions of

5.3 weight % Ni and both 1 and 3 weight % P (5.0

atomic % Ni and 1.8 and 3.6 atomic % P, and 4.9

atomic % Ni and 5.3 atomic % P). The intermediate P level proved to be the most reasonable in

terms of explaining the observed structure and

will be considered first.

For the composition 5.0 atomic % Ni and 3.6

atomic % P (Table 15), kamacite is the only equilibrium phase present from 1100° C to below 975° C.

At a temperature somewhat below 975° C taenite

nucleates, and a small amount of it is in equilibrium with kamacite at 925° C. By 875° C, taenite

has grown to be nearly as abundant as kamacite

and the structure contains 6% schreibersite, undoubtedly having nucleated at preexisting kamacite-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

in equilibrium with 10% schreibersite. The schreibersite continues to increase in Ni as it cools,

reaching a value of 11 atomic % Ni at 600° C and

below, with a Ni content in the kamacite of 4.2

atomic percent. Both of these Ni values are in

general agreement with observed values, perhaps

a little on the high side.

If the 1.8 atomic % P value is assumed, kamacite

and taenite are the phases present in the equilibrium structure from 1100° C down to below 850°

C. Schreibersite first appears somewhat above 750°

C, and is present in the amount of 2% when that

temperature is reached. Nucleation under these

circumstances would be expected to distribute

schreibersite along kamacite-taenite borders, these

phases being present in approximately equal

amounts. Taenite disappears from the structure

below 700° C, and by 600° C, 3% schreibersite is in

equilibrium with kamacite. The final Ni contents

of both kamacite and schreibersite are somewhat

higher than in the 3.6 atomic % P case, the higher

P situation agreeing better with actual measurements on the meteorite.

When 5.3 atomic % P is assumed, the bulk

composition lies within the kamacite + liquid field

of the phase diagram from 1100° C to somewhat

above 1010° C. At 995° C, 5% schreibersite is in

equilibrium with kamacite, and this schreibersite

remains constant in amount down to 925° C. During the next 50° drop in temperature one-third of

the structure transforms to taenite and the amount

of schreibersite more than doubles to 13%. The

taenite decreases in volume with further cooling

and disappears below 750° C. By 700° C the equilibrium phases are again kamacite and schreibersite,
NUMBER 21 47

TABLE 15. —Equilibrium cooling of compositions appropriate to the Bellsbank meteorite

(percentage of phases present at indicated

temperatures with their Ni contents)

°c

995

975

925

875

850

750

700

650

600

555

°C

995

975

925

875

850

750

700

650

600

550

°C

995

975

925

875

850

750

700

650

600

550

1

%a

10

30

30

35

55

68

88

97

96

95

3

%a

100

100

85

53

50

63

85

90

88

88

5

U

95

95

95

52

49

62

83

83

81

80

.8 Atomic

%y

90

70

70

65

45

30

9

.6 Atomic

%Y

15

41

42

28

5

.3 Atomic

%y

35

36

22

X P, 5

%Ph

2

3

3

4

5

% P, 5

%Ph

6

8

9

10

10

12

12

X P, 4

XPh

5

5

5

13

15

16

17

17

19

20

.0 Atomic

%N1

a

4.0

4.2

3.8

3.5

4.0

4.0

4.5

4.9

4.7

4.6

.0 Atomic

SN1

a

5.0

5.0

4.7

4.0

4.0

4.0

4.5

4.6

4.2

4.2

.9 Atomic

%Ni

a

4.8

4.8

4.8

4.0

4.0

4.0

4.4

4.4

3.8

3.8

% Ni

%N1

Y

5.1

5.3

5.5

5.8

6.3

7.2

9.1

% N1

*N1

Y

6.6

6.4

6.3

7.2

9.1

% Ni

SN1

Y

6.4

6.3

7.2

«H1p h

5.3

7.1

8.9

12

13

*

N i Ph

4.4

4.4

5.3

7.1

8.0

11

11

%Nip h

6.4

6.0

5.2

4.4

4.4

5.3

7.0

7.5

10

10

schreibersite being present in the amount of 17%.

With further cooling, schreibersite increases in

quantity and in Ni content, with a rather marked

decrease in kamacite Ni content. The final Ni

content of the schreibersite is possibly slightly

lower than the measured value, the quantity of

schreibersite seems too high, and the kamacite Ni

values too low. The 3.6 atomic % P figure, the

one plotted in Figure 19, seems to be the best

choice of the three. A somewhat higher P value,

perhaps 4 atomic %, would probably be even

better, however. This would mean that schreibersite would initially nucleate in the presence of less

taenite, and that final massive schreibersite and

kamacite would have slightly smaller Ni values.

The observed low temperature schreibersite morphology suggests that an extensive Widmanstatten

pattern was not present when schreibersite nucleated .

The phase growth calculations that have been

discussed above account for the major structural

features of the selected group of meteorites. Appropriate compositions cooled under equilibrium

conditions to approximately 600° C would produce

the observed proportion of phases and account

for the general character of the structures. Coahuila is single crystal kamacite containing a few

isolated moderate-size schreibersite inclusions.

Ballinger has a coarse Widmanstatten pattern interrupted by large schreibersites that may be surrounded by cohenite. Santa Luzia contains massive

schreibersite areas surrounded by giant swathing

kamacite, separating areas of coarse Widmanstatten pattern. The higher Ni members of the group

have a finer Widmanstatten pattern with massive

schreibersite morphology explainable in terms of

the amounts of P and C present. Bellsbank contains massive schreibersite and low Ni kamacite

and has similarities to the giant swathing kamacite

areas in Santa Luzia.

It is interesting at this point to summarize equilibrium data given above and compare it with

what would be expected in terms of Widmanstatten pattern growth from the binary Fe-Ni system.

Here Widmanstatten pattern growth is construed

in its conventional usage of simply taenite transforming to kamacite, ignoring the presence of P

either in solution or in schreibersite. The comparison is made in Table 16, where the first two

columns give the compositions of the meteorites

as plotted in Figure 19, the compositions that best

fit the structural arguments given above. These

columns are followed by sets of data for the

ternary system and for the binary system (Figure

1). In each set the first column gives equilibrium

temperatures at which Widmanstatten pattern development starts (y —>a + y) if undercooling

effects are ignored, probably a reasonable assumption when P is present. The second column gives

the expected temperatures at which taenite would

be completely transformed to kamacite (a + y —»

a) if equilibrium were maintained down to these

low temperatures. The third column gives the

expected temperature range for Widmanstatten

pattern growth (ATWPG). The last two columns

show the effect of P on increasing the temperatures of kamacite nucleation (TaNuc) and taenite

disappearance (TyDis) over what would be expected 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

Meteorite

Bellsbank

Lexington County

& Bahjoi

Goose Lake

& Balfour Downs • •

Atomic

Ni

5.0

5.3

6.2

6.2

7.1

7.8

%

P

3.6

0.8

1.4

2.7

1.8

2.7

Fe-NI-P System

(-930°)"

860°

880°

840°

750°

730°

c+Y-a

* 690°

610°

580°

580°

565°

555°

A T W G P

240°

250°

300°

260°

185°

175°

Fe-N1

Y-a+Y <»

775°

765°

755°

755°

740°

725°

System

+Y+*

610°

580°

570°

570°

480°

A T WGP

165°

185°

185°

185°

260°

AT due

Sue

(155°)**

95°

125°

85°

10°

5°

to P

T Y D 1 S

80°

30°

10°

10°

85°

* For this composition WPG starts with the reaction

** Nucleation of Y within a.

The presence of P in the coarsest structured

meteorites, Bellsbank through Santa Luzia, significantly increases the temperature at which Widmanstatten pattern growth starts, increases the

temperature at which transformation of taenite

would be expected to be complete, and also significantly increases the temperature range over which

growth takes place. These three factors all work

in the direction of producing larger kamacite crystals than would be expected from the binary diagram for the same cooling rate. In the coarsestructured meteorites, Lexington County through

Belfour Downs, those with a well-developed Widmanstatten pattern, the amount of P has a less

dramatic effect. The important point here is that

kamacite nucleation takes place at significantly

lower temperatures than is the case for meteorites

with less Ni. The nucleation temperature is also

very close to that which would be expected from

the binary diagram. This leads to much more

restricted conditions for kamacite growth and the

retention of larger amounts of taenite and plessite

in the structure, thereby producing the finer and

more regular Widmanstatten pattern that is observed in these meteorites.

Details of these various structures as given in

the results section, however, are not explained

satisfactorily by this discussion. Taenite and plessite are retained in significant amounts in most of

these meteorites, demonstrating that equilibrium

cooling did not extend to 550° C. Diffusion controlled growth, modifying the equilibrium structure in significant ways continued to lower temperatures. It is this aspect of meteorite structure that

will be discussed below.

Low TEMPERATURE PHASE GROWTH AND THE

EQUILIBRIUM DIAGRAM

The preceding section has shown that the major

structural features of the selected meteorites may

be explained on the basis of nucleation and growth

of phases in a system responding to changing

equilibrium conditions with decreasing temperature. Bulk equilibrium may have pertained down

to 550° C or below for Bellsbank and Coahuila,

but the other six meteorites were not in complete

equilibrium at that temperature (Figure 19). Had

they been, their structures would consist only of

kamacite and schreibersite, and the taenite that

they contain would have transformed. All eight of

the meteorites also contain Ni and P concentration

gradients in kamacite adjacent to schreibersite (Tables 2-9), although the recent work of Randich

(1975) suggests that these gradients formed at

much lower temperatures. Some of the massive

and all of the smaller schreibersites have Ni concentrations that are greatly in excess of those

permitted for schreibersite in equilibrium with

kamacite at 550° C (Figures 18, 25). These observations are explained by the fact that as temperature drops the atomic mobility necessary to maintain bulk equilibrium is lost. Elemental diffusion

becomes slower and compositional changes take

place over much shorter distances, resulting in

composition gradients. Kinetic factors must now

be considered as well as equilibrium states. Upon

cooling under these restrictive.conditions, schreibersite continues to grow in volume, may increase

in Ni content, and late-formed schreibersites nucleate and grow. Taenite shrinks in volume and

increases in Ni, while kamacite decreases in Ni.
NUMBER 21 49

Phase interfaces are still in equilibrium in the

sense that interface compositions represent tie

lines within the Fe-Ni-P system (see below). Schreibersite-kamacite interface measurements on meteorites, therefore, give equilibrium information relevant to the Fe-Ni-P systems at temperatures well

below those that are available experimentally.

Combining this low temperature equilibrium data

with both an analysis of the kinetic record present

in the form of diffusion gradients and a detailed

knowledge of specimen petrography may be expected to provide new possibilities for the elucidation of the structures at hand.

With these ideas in mind, it will be helpful first

to examine the Fe-Ni-P diagram asking what

changes would be expected with decreasing temperature. Meteorite data (Reed 1965a, 1967, 1969,

and Tables 2-9 above) indicate major changes in

the diagram on cooling to low temperatures. The

top part of Figure 20 may be used as a starting

point for a discussion of this transition. It is a

schematic isothermal section of the ternary system

at a temperature slightly below 550° C. The scale

on the Fe-P axis has been interrupted so that the

lower part of the diagram could be enlarged for

clarity. The low P concentrations that are observed

in kamacite and taenite make it impossible to show

the three fields along the Fe-Ni axis and the three

schreibersite-containing fields using the same

scale.

Compositional data on schreibersite-kamacite

and schreibersite-taenite associations suggest the

following trends in the ternary diagram with decreasing temperature. The constant P content of

schreibersite requires that the schreibersite composition corner of the three-phase field (point D

in Figure 20) lie on the 25 atomic % P line regardless of temperature. Ni values for schreibersite

embedded in taenite borders, an association that

approximates the schreibersite corner of the threephase field, range from 41 to 45 atomic %. Point

D, therefore, must move at least that far to the

right with decreasing temperature. Higher Ni contents in schreibersite have been measured (Reed,

1972), but these represent compositions in the

taenite-schreibersite field, while lower Ni contents

represent compositions in the kamacite-schreibersite field. Because of the steepness of the Ni

concentration gradients, no serious attempt has

been made to obtain taenite-schreibersite interface

compositions, but incidental measurements (Figures 9, 11) may be used to bracket the position of

point C. The Ni value in taenite associated with

schreibersite is less than the schreibersite Ni value,

perhaps by about 5 atomic %. This suggestion is

supported also by Reed's (1972) measurements on

the Octibbeha County meteorite and by similar

measurements by the authors on this meteorite

(60 atomic % Ni in schreibersite in contact with 55

atomic % Ni in taenite). Point C moves to the

right, but trails behind point D. The P values in

taenite at these interfaces are also not accurately

known, but sufficient data is available to say that

they are a fraction of the P value in kamacite

associate with the same schreibersite (see discussion below and Figure 22). The Ni and P contents

for point B may be estimated from the kamaciteschreibersite interface values of these taeniteembedded schreibersites. They range from 5.7 to

6.2 atomic % Ni, with constant P values of 0.05

atomic %. In summary the diagram changes as

follows with decreasing temperature: point D

moves to the right to approximately 45 atomic %

Ni with point C moving to approximately 40 atomic

% Ni and to a P value that is much smaller than

0.05 atomic %. As a consequence the a + Ph and

a + y + Ph fields greatly expand in area mainly

at the expense of the y + Ph field. The three

small fields along the Ni axis also lose a large

proportion of their areas, but they still retain

considerable importance for the interpretation of

meteorite structures.

The preceding illustrates the direction and extent of change in the ternary system with decreasing temperature, but it says nothing about what

that temperature range might be. Do meteorite

structures continue to develop to very low temperatures or is growth effectively concluded at temperatures not far below 550° C? The bottom section

of Figure 20 suggests an approach that may be

useful here. We know that D' lies on the 25 atomic

% P line and can assume that it moves continuously

to the right with decreasing temperature. The

points on the Fe-Ni axis can be obtained from the

estimated portion of the Goldstein and Ogilvie

(1965) binary Fe-Ni diagram down to about 350°

C. If A', B', and C could be estimated as a

function of temperature, we would have much of

the information needed for extrapolated isothermal sections.
50 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

0

100 %Fe

20 30

ATOMIC %Ni

40

C'

20

ATOMIC % Ni

30 40

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

kamacite and taenite in equilibrium with schreibersite

TABLE 18. —Temperature versus P saturation concentration in kamacite and taenite taken from Figure 22

Fe-P System* Fe-Ni-P System**

T°C

1010

995

975

925

875

850

750

700

650

600

550

1283

1268

1248

1198

1148

1123

1023

973

923

873

823

1/T ("KxlO"4)

7.79

7.89

8.01

8.35

8.71

8.90

9.78

10.3

10.8

11.5

12.1

Atonic % Pa

4.8

4.1

2.9

1.7

1.3

0.99

0.59

0.45

Atomic % Pa

4.1

3.7

3.0

2.8

1.8

1.2

0.9

0.57

0.47

Atomic * PY

1.9

1.6

1.3

1.2

0.68

0.49

0.27

0.22

0.15

* Doan and Goldstein (1970), table IV.

** Doan and Goldstein (1970), table III .

A', B', and C represent corners of composition

fields, and these particular compositions are very

dilute solutions of P in Fe or in Fe-Ni. Data for

these points at a number of temperatures within

the experimental range are given by Doan and

Goldstein (1970), and their values converted to

atomic % P are given in Table 17. This type of

data is frequently used to extrapolate concentration-temperature relationships to lower temperatures and is plotted accordingly in Figures 21 and

22. Atomic % P on a logarithmic scale is plotted

against 1/T. Figure 21 uses P saturated kamacite

values from the Fe-P system, and Figure 22 uses

both P saturated kamacite and taenite values from

the Fe-Ni-P system. Straight lines were drawn with

ease through the three sets of data, suggesting

that the extrapolations may give useful low temperature composition estimates. The thermodynamic rationale for this procedure is given in

"Appendix."

A particularly interesting observation based on

Figures 21 and 22 is that the kamacite line in the

plot of the Fe-Ni-P system data is identical to the

kamacite line in the plot of Fe-P system. Therefore , both sets of data yield the same extrapolated

P-temperature relationship for kamacite. This

means that line A'B' is parallel to the Fe-Ni axis,

and that the saturation value of P in kamacite is

independent of Ni concentration. Should this actually prove to be the case, it would have interesting consequences for the interpretation of meteorite structures. Kamacite-schreibersite interface

values would not only represent tie lines in an a +

Ph field of the ternary diagram, but the P value in

kamacite at a specific interface would represent

the temperature of the isothermal section to which

T°C

540

527

508

488

473

465

460

454

449

444

436

431

424

417

410

403

394

383

372

362

348

333

315

298

252

Atomic % Pa

0.40

0.35

0.30

0.25

0.22

0.20

0.19

0.18

0.17

0.16

0.15

0.14

0.13

0.12

0.11

0.10

0.090

0.080

0.070

0.060

0.050

0.040

0.030

0.020

0.010

Atomic X PY

0.14

0.12

0.10

0.080

0.070

0.062

0.058

0.054

0.052

0.048

0.046

0.041

0.038

0.034

0.032

0.028

0.025

0.022

0.019

0.015

0.013

0.009

0.007

0.005

0.002

Weight % Pa

Q.ZZ

0.19

0.17

0.14

0.12

0.11

0.11

0.10

0.09

0.09

0.08

0.08

0.07

0.07

0.06

0.06

0.05

0.04

0.04

0.03

0.03

0.02

0.02

0.01

0.006

that tie line belonged. The implication of this

would be that schreibersite growth effectively

stopped over a sequence of temperatures within a

given meteorite. Numerical values for P concentration variation with temperature for both kamacite

and taenite were read from Figure 22 and are

listed in Table 18.

Although a flat line A'B' has several interesting

implications, it is necessary to consider as an alternate possibility that it actually slopes downward

from left to right. The experimental technique

employed by Doan and Goldstein (1970) was

stretched to its limit in obtaining the lower temperature data, resulting in the possibility of significant

error for the problem at hand. There is also

ambiguity in their paper concerning one critical

value. The weight % P in kamacite value for the

Fe-P system at 550° C taken from their table IV is

0.25 ±0.03. The value used on the 550° C isothermal section (their figure 8) is 0.4 weight % P. This

difference probably represents confusion as to

which value is best rather than a simple misprint

in the diagram, a difficulty that can best be resolved by more careful experimental work. If this

0.4 weight % P value had been used in the extrapolation, the line in Figure 21 would have lain to

the right of the kamacite line in Figure 22. This

would mean a sloping line A'B' with P decreasing

as Ni increases. This situation could imply that all

schreibersite within a given meteorite grew down

to the same final temperature. Before attempting
52 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

.001

7 8 9 10 II

FIGURE 21. —Extrapolation of P saturation values in kamacite in Fe-P system to lower temperatures.

12 13 14 15 16 17 18 19 20 21

I/T (°KxlO~ 4)
NUMBER 21 53

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

001

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

meteorite measurements, however, it will be helpful if we first look at the diffusion process as it

applies to schreibersite growth.

DIFFUSION-CONTROLLED SCHREIBERSITE GROWTH

The data presented above on Ni and P gradients

demonstrates that schreibersite grew by drawing

these elements from increasingly restricted volumes of kamacite with decreasing temperature.

Petrographic considerations combined with an understanding of the Fe-Ni-P system suggest the

broad outlines of the nucleation and growth process. With this background in mind, a set of constraints and assumptions can be developed that

allow the qualitative application of the nonisothermal diffusion-controlled growth theory for ternary

systems recently developed by Randich and Goldstein (1975) to be applied to schreibersite growth

in these meteorites (see also Randich, 1975).

Figure 23 may be used to relate schreibersite

growth both to the Fe-Ni-P diagram and to growth

equations. The top part of the diagram is a schematic isothermal section at some temperature below 550° C. The scale for P is broken so that very

low levels of P can be indicated. The tie line

represents schreibersite growing within kamacite.

Its ends give interface P and Ni compositions for

both kamacite (CP, CNi) and schreibersite (Cp,

C^i). The theory of Randich and Goldstein (1975)

requires that equilibrium be maintained to the

extent that Ni and P concentrations in the two

phases are related by a tie line in the ternary

system at the temperature that pertains. For diffusion-controlled conditions, this tie line need not

be the equilibrium tie line (ETL), the one that

passes through the bulk composition of the system.

The lower part of Figure 23 indicates kinetic

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

and Ni values in schreibersite at the interface are

CP and CNi, and the equivalent values in kamacite

are CP and CNi . The P and Ni fluxes from kamacite

into the growing schreibersite are J£ f

+

 and J$U(

+

 >

and the Ni flux from the interface into the schreibersite is Sm.f • The concentration gradients in P

and Ni are dCP/dX and aCNi/aX.

The diffusion controlled growth process may be

described by the mathematical treatment of Randich and Goldstein (1975). Diffusion in the Fe-NiP system may be described using an extension of

Fick's first law. The P and Ni fluxes in terms of

concentration gradients in kamacite and schreibersite are as follows.

T

a

J

 = — P

T

a

 — _

pp

dX

PNi

NiNi

dX

dX

ax

T

Ph

= _r>Fe,Ph

NiNi

ax

,ph aCp

ax

Jp"h = o

(la)

(lb)

(lc)

(Id)

Separate sets of equations are needed for each

phase. The P flux in schreibersite is zero due to

the constant atomic proportion of this element in

schreibersite. The symbols D£pa and D£fN? are

diffusion coefficients, measures of the influence

of the concentration gradients of these two elements on their own flux, where as D£Ni

a

 and D^iP

a

reflect cross effects usually referred to as the

ternary diffusional interaction. The latter coefficients have been found to be so small in this

system that the terms containing them may be

dropped from the above equations (Heyward and

Goldstein, 1973). These equations may then be

simplified by neglecting the diffusional interaction

terms and the P flux in schreibersite.

J

a —P -

Hx

ax

(2a)

(2b)

(2c)

Fick's second law is applied in order to consider

the time dependence of the fluxes. It is assumed

that diffusion coefficients are independent of composition.

at ax

pp

ax2

(3a)
NUMBER 21 55

ATOMIC %Ni

30

20

o

o

10

Ph

Ni

J P 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

dt

at

ax

JNl

ax

NiNi

NiNi

ax2

(3b)

(3c)

The resulting mass balance equations for P and

Ni at a schreibersite-kamacite interface are as follows.

(4a)

dt

(4b)

The two mass balances are related to each other

as the rate of movement of the interface, d£/dt,

must be the same for both elements.

(5)

dt dt dt

A schreibersite growth rate may then be expressed

in terms of interface Ni and P concentrations, the

flux of P in kamacite, and the fluxes of Ni in both

kamacite and schreibersite.

1

dt Cp — Cp

7(-JPV)

dt

(6a)

(6b)

It is beyond the scope of this study to attempt

the difficult numerical analysis required to solve

these growth equations. Randich (1975) has developed a model for this purpose and successfully

applied it to the growth of schreibersite in hexahedrites. He compiled a computer program that

treats diffusion controlled phase growth in ternary

systems when the requisite portions of the phase

diagram are known. The model accomodates ternary interactions, nonisothermal transformations

(variable cooling rates), and impingement (overlapping diffusion fields). It is based on a onedimensional space grid and is rigorous only for

lamellar schreibersite growing within kamacite.

The concepts that this model employs, however,

are valid for meteorites more complex than the

hexahedrites, and they will be used here in a

qualitative interpretation of the data.

Equations 6a and 6b relate P and Ni interface

concentrations to their gradients in kamacite and

schreibersite for the final stages of growth. Cooling is assumed to be uninterrupted, with schreibersite growth continuing down to very low temperatures. Measurements with the electron microprobe, however, do not reveal changes that take

place within less than 1 /urn of an interface. Therefore, as a consequence of this limitation of the

measurement technique, growth apparently stops

when subsequent changes become unresolvable.

Randich's (1975) calculations indicate a lower limit

of 250° C for detectable change.

Equation 6a is a key to the understanding of

schreibersite growth under the conditions that

pertained in the meteorites under study. It demonstrates that the P flux controlled the schreibersite growth rate. The quantity Cp in the equation

is the P content of schreibersite at the interface

(Figure 23), a constant at 25 atomic %. The quantity Cp is the interface P content in kamacite. It is

a small number, 0.2 to 0.05 atomic %, in the

temperature range of formation of the observed

interface relationships. The quantity (CP — CP),

therefore, may be considered to be independent

of temperature even though Cp is not. With this

simplifying assumption, the rate of growth of a

given interface, d£/dt, is seen to be dependent

only on the P flux, J£f

+

. The P flux, however,

will be dependent upon CP at any given temperature. The value of Cp combines with the P level in

the surrounding kamacite to establish the P gradient, aCp/aX, the variable that determines the

flux (equation 2a).

The system responds to the constraint of the P

flux determining schreibersite growth rates by

adjusting individual interface Ni concentrations so

that the growth rate established by P (equation 6a)

is matched by the growth rate due to Ni (equation

6b). This permits individual schreibersites containing markedly different Ni levels to continue to

grow simultaneously within the cooling meteorite.

Kamacite and schreibersite interface Ni and P

concentrations must continue to be related by a tie

line relationship, a requirement that is met by tie

lines shifting from the equilibrium tie line. The

nature of these tie line shifts and their effects on

growth rates of individual schreibersites is illustrated in Figure 24.

Tie line shifts and their effects are illustrated

using hypothetical ternary diagrams in Figure 24.

The symbols for Ni and P are retained as a matter

of convenience, although scale problems preclude
NUMBER 21 57

1

1

1

^ 1

LJ 1

1

1

1

\ V

li H

OJ

u

3

<U

a. EV

ese

a

a

o

d -10 IN % DIW01V !N % OIW01V

2 "° C c a

- "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

higher temperature Ti, ETL(TX), is assumed to

represent bulk equilibrium in the a + Ph region

of the diagram. It passes through the bulk composition indicated by the small circle (5.5 atomic %

Ni and 6.0 atomic % P). Upon cooling to temperature T2 the a + Ph field expands along the a +

Ph/Ph boundary, and the a/a + Ph boundary

moves closer to the Fe-Ni axis. It is assumed that

in this lower temperature range mass transfer is

insufficient to achieve bulk equilibrium. Growth

becomes diffusion controlled and gradients develop within the phases. Tie lines may therefore

shift from the equilibrium tie line so that the Ni

growth rate can match the P growth rate. Three

possible tie lines are indicated at T2, the equilibrium tie line for the same composition used above,

ETL(T2), and tie lines shifted to the right and to

the left of ETL(T2),

The interface relationships for these four tie

lines are illustrated at the right in Figure 24. The

upper diagram shows interface Ni and P compositions for ETL(Tj). The bulk Ni value for the

system is indicated by Co1- The bulk P value is 0.5

atomic % higher and was omitted for clarity. Bulk

equilibrium has been achieved at temperature Tj

as is indicated by the absence of compositional

gradients in both kamacite and schreibersite.

Interface relationships for diffusion controlled

growth at temperature T2 are illustrated in the

lower right side of Figure 24. The P interface

values are assumed to be identical in the three

interface situations shown. The consequence is

similar P fluxes feeding schreibersite growth,

]P,(+, and the establishment of similar growth

rates, d£/dt, for the interfaces through equation

6a. Tie line shifts from ETL(T2) induced by conditions of limited mass transfer produce a continuously varying range of growth rates that permits

Ni flow to match that required by P.

The equilibrium tie line at the lower temperature, ETL(T2), passes through the bulk composition. Its interface Ni and P values are those that

would be observed if the system were at complete

equilibrium. The presence of compositional gradients in both kamacite and schreibersite, however, show that complete equilibrium has not been

reached.

Shifting the tie line to the left results in an

increase in the growth rate due to Ni (equation

6b). The quantity 1/(CNI — CNI) and the Ni flux in

kamacite, ]xU(

+

, become larger and the Ni flux in

schreibersite, J^,f~. becomes smaller. All three of

these changes increase the growth rate over that

of ETL(T2). Ni is supplied to the growing schreibersite in a way that size is increased at the expense

of Ni enrichment.

Moving the tie line to the right of ETL(T2) has

the effect of slowing the growth rate due to Ni.

The quantity 1/(CNI ~ C^i) and the Ni flux in

kamacite, ]yiti

+

, become smaller, and the Ni flux

in schreibersite, J^.f » becomes larger. All three

of these changes decrease the growth rate over

that for ETL(T2). Ni is supplied to the growing

schreibersite in a way that it becomes enriched in

Ni rather than growing in size. A theoretically

infinite range of growth rates may be obtained by

these tie line shifts. One of these possible tie lines

will give a unique growth rate that satisfies both

equations 6a and 6b.

COOLING RATE VARIATIONS

Cooling rates of meteorite parent bodies are

known to vary over several orders of magnitude

(Goldstein and Short, 1967b). These variations will

affect the growth of schreibersite, particularly in

the lower temperature ranges. Compositions and

associations we observe in meteorite specimens

will depend in part on specific cooling histories.

Measurements of schreibersite, therefore, would

be expected to be interpretable in terms of cooling

rates. Randich (1975) has recently demonstrated

the validity of this assumption by calculating cooling rates for hexahedrites. In the course of his

work important observations were made that will

help in the interpretation of more complex meteorite structures. Several aspects of this work will

be reviewed here.

Calculations using the Randich and Goldstein

(1975) schreibersite growth model were made using a bulk composition of 2.1 weight % P, 4.1

weight % Ni, and 93.8 weight % Fe. Cooling rates

of 5 X 10~3, 5 x 10~4, and 5 X 10~50 C per second

were employed over a cooling interval of 900° C to

685° C. Decreasing the cooling rate produced

schreibersites of predicted greater widths and

higher Ni contents. P gradients were predicted in

kamacite and Ni gradients were predicted for both

kamacite and schreibersite. The calculated Ni interface values were significantly lower than those

for the equilibrium tie line for this composition.
NUMBER 21 59

Tie lines had shifted to the left to increase the

growth rate for Ni. An experimental validation of

these calculations was carried out using this same

composition and the 5 x 10~4° C per second (43.2°

C per day) cooling rate. Calculated and observed

compositions agreed within the limits of electron

microprobe measurements.

Additional cooling rate calculations were made

by Randich (1975) in a study of schreibersite nucleation temperatures. Cooling rates in the meteorite range of 1°, 10°, and 100° C per million years

were used with the Coahuila bulk composition of

5.49 weight % Ni and 0.49 weight % P. The

results were similar to those given above, the

slower cooling rates producing slightly larger

schreibersite containing slightly higher Ni values.

The calculated Ni values were within the range of

observed Ni values in hexahedrites, establishing

the validity of the model at cooling rates within

the meteoritic range. Ni gradients were predicted

for the schreibersites, contrary to observations on

meteorites. This is probably explained by the fact

that the ternary diffusion constant for Ni in schreibersite is not known in this temperature range.

These calculations led to a conclusion that is

significant for our purpose. It was shown that if

nucleation occurs above 500° C the amount of

undercooling prior to nucleation in unimportant.

The kamacite-schreibersite assemblage at 500° C

will be in equilibrium, assuming a diffusion field

of 1000 [im or less. Randich (1975) also points out

that this situation leads to the conclusion that

Reed's (1965b) theory that lamellar schreibersite

and rhabdite nucleated simultaneously must be

correct. Had the plate schreibersite grown significantly prior to nucleation of rhabdite, the rhabdite

would have been prevented from nucleating. Both

types are observed together over very short distances, requiring that nucleation temperatures

were very close. Under these circumstances, impingment of P and cooling rate are the two factors

that control schreibersite growth for a given bulk

composition.

INTERFACE DATA AND SCHREIBERSITE

DISTRIBUTION

In a previous section we have seen that the

main petrographic features of coarse-structured

iron meteorites can be understood in terms of

equilibrium cooling of specific compositions in the

Fe-Ni-P system. This approach can be extended

to much lower temperatures with the aid of concepts developed by Randich (1975) in his analysis

of schreibersite growth in hexahedrites. The application of diffusion theory permits the interpretation of schreibersite-kamacite interface data in a

way that yields a more detailed understanding of

schreibersite distribution and composition than

has been possible previously.

Phases and compositions resulting from equilibrium cooling of the meteorite compositions of

interest have been summarized in Tables 10

through 15, and these same compositions have

been plotted on the iron-rich corner of isothermal

sections in Figure 19. At the top of Figure 25

these compositions have also been plotted on the

550°C isothermal section with their respective tie

lines. This is the lowest temperature for which an

experimentally based ternary section is available.

Table 19 lists the equilibrium percentages of kamacite and schreibersite with their Ni contents as

taken from Figure 25. This plot shows that had

complete equilibrium been reached at this temperature for these compositions, taenite would have

disappeared and the observed structures would

consist of kamacite containing inclusions of schreibersite.

The bottom section of Figure 25 contains an

estimated isothermal section at approximately 300°

C. The schreibersite corner of the three phase

field is plotted at 45 atomic % Ni, a value that is

probably close to the real value. The kamacite

corner of the three phase field is plotted at 5.5

atomic % Ni, and this value is considerably less

certain. The compositions used above are again

plotted with their tie lines. Although the diagram

may not be accurate, it does tell us something of

what would be expected if equilibrium cooling

extended to this very low temperature. Proportions of phases and the expected compositions

taken from this diagram are also listed in Table

19.

The bulk compositions plotted in the lower

section of Figure 25 would also consist of kamacite

and schreibersite only, and these two phases would

be uniform in composition. The Bellsbank schreibersite would contain much more Ni than was

observed for its massive schreibersite, and its kamacite would be considerably poorer in Ni than was

observed. The massive schreibersite in Santa Luzia

would be greatly enriched in Ni, its small schrei-
60 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

550° C

y +• P h

too %FE

20 30

ATOMIC % Ni

40

ABELLSBANK

• COAHUILA

• SANTA LUZIA

• BALLINGER

• LEXINGTON COUNTY

- BAHJOI

A GOOSE LAKE

- BALFOUR DOWNS

50

10 20 30

ATOMIC % Ni

40 50

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

would be poorer in Ni. The Coahulia, Ballinger,

and Goose Lake-Balfour Downs compositions all

lie on the same tie line, but would consist of

different proportions of kamacite and schreibersite. The Lexington County-Bahjoi composition

would produce the most Ni rich schreibersite. The

structures suggested by this diagram, therefore,

are markedly different than those actually observed in the meteorites under study. At what
NUMBER 21 61

TABLE 19. —Equilibrium compositions at 550° C

and -300° C taken from Figure 25

Meteorite

Beilsbank

Coahuila

Santa Luzia

Lexington County &

Bahjoi

Goose Lake &

Balfour Downs

Meteorite

Bellsbank

Coahuila

Santa Luzia

Lexington County &

Bahjoi

Goose Lake &

Balfour Downs • • • •

550°C

% a % Ph % Nia % Nip h

87.0 13.0 4.1 11.9

99.3 0.7 5.2 13.3

91.3 8.7 5.4 14.0

96.3 3.7 5.8 15.0

94.8 5.2 6.6 16.2

91.2 8.8 6.9 17.0

-300°C

% a. % Ph % Nia % Nip h

86.4 13.6 2.5 20.8

97.2 2.8 4.3 38.0

90.2 9.8 3.6 30.5

94.5 5.5 4.3 38.0

93.3 6.7 4.7 41.2

90.0 10.0 4.3 38.0

temperatures do these observed differences become introduced into the structures?

Taenite is retained in the form of the Widmanstatten pattern in the six octahedrites studied.

This shows that mass transfer required by the

taenite-kamacite transformation is insufficient at

somewhat above 550° C to achieve bulk equilibrium

in the cooling rate range pertaining. The presence

of the Widmanstatten pattern may also affect subsequent P movement through the structure. Randich's (1975) calculations show that mass transfer

through 1000 /xm diffusion fields in kamacite is

sufficient for kamacite-schreibersite equilibrium to

be maintained down to 500° C in the same range

of cooling rates. Kamacite-taenite transformations

become restricted at higher temperatures than

kamacite-schreibersite transformations. Therefore, it is reasonable to assume that as temperatures drop below 600° C, localized compositional

changes become increasingly more important and

bulk compositional effects become increasingly less

important. As mass flow becomes more restricted,

individual structural units maintain interface equilibrium and respond to compositional gradients in

increasingly small volumes of material. These

processes are responsible for the interface and

gradient data that have been reported above.

Tie lines representing interface data from the

Coahuila, Santa Luzia, and Goose Lake meteorites

(Tables 2, 5, and 8) are plotted on isothermal

sections in Figures 26. Average values were used

where more than one set of interface measurements gave similar values. Any given tie line,

therefore, may represent more than one set of

measurements. The scale used made it impossible

to indicate the differing low P values in kamacite,

so all of these compositions are indicated by points

resting on the Fe-Ni axis. This treatment of the

data may well be an over simplification as various

tie lines within a given meteorite may actually

belong on different isothermal sections. A range

of temperatures may be involved, and this point

will be returned to in a later section. The three

meteorites selected are representative of the group

of eight meteorites studied.

Typical tie lines for the Coahuila meteorite are

given in the top section of Figure 26. They fan

out through a narrow composition range of 3.2 to

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

significant point that will be dealt with later. The

structural associations related to compositions have

been discussed above, but it should be noted here

that the tie line on the left represents early-formed

structures (schreibersite bordering troilite-daubreelite inclusions and a large schreibersite), and

the three on the right are later-formed structures

(large rhabdites). The estimated bulk composition

of Coahuila is indicated on the diagram as a large

dot. The point apparently falls on the right-hand

tie line. All the measured schreibersite compositions in Coahuila then appear to be to the left of

the ETL or on it, as was to be expected on the

basis of Randich's (1975) analysis. Reed (1965a)

has reported Coahuila schreibersite with 34 atomic

% Ni. It is not clear how this value would plot on

Figure 26 as the necessary Ni value in kamacite is

not available.

The relationship of the equilibrium tie line to

observed meteorite tie lines is actually somewhat

more complex then Randich (1975) has implied.

This can be concluded from consideration of Coahuila measurements, and it will become clearer

when measurements on the octahedrites are considered. At temperatures above the final growth

stages represented by the Coahuila tie lines in

Figure 26, mass transfer had become inhibited

except for very short-range interactions. Schreibersite 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

1 S T

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 impingment reduces the P flux to zero, or, for our

purpose, when we can no longer measure interface

relationship changes. Although a theoretical possibility, there is no evidence that any of the schreibersites studied were in the process of dissolving.

As the P flux decreases with decreasing temperature, a given interface tie line would be expected

to shift to the right to conform to the change in

shape of the a + Ph field of the phase diagram. It

would also be expected to move farther to the

right in order to slow the Ni growth rate as the P

supply becomes more limited. The largest schreibersites undoubtedly nucleated first and have

grown at comparatively high rates, drawing P

from large volumes of metal. They have the lowest

Ni concentrations. The smaller schreibersites that

grew at slower rates due to a more limited P

supply have higher Ni levels. The smallest schreibersites would be expected to approach a Ni content given by the tie line whose Ni value in kamacite equals the Ni value of the kamacite in which

they are growing. Kamacite in the Coahuila meteorite has a higher Ni value than it would have had

if complete equilibrium had been reached. It is

therefore possible that microrhabdites in Coahuila

contain more Ni than would be expected from the

low temperature equilibrium tie line. Unfortunately, the necessary measurements needed to

confirm this were beyond the capability of the

techniques available.

Tie lines for the Santa Luzia meteorite are

shown on a single temperature plot in the center

section of Figure 26. Kamacite Ni values range

from 2.2 to 6.2 atomic % with the corresponding

Ni values in schreibersite of 14 to 42 atomic %

covering a much broader range when compared

to Coahuila. Kamacite P values at the interfaces

range from 0.13 to 0.05 atomic %. The three tie

lines at the left represent the early formed massive

schreibersites, the three to their right are from

grain boundary schreibersite, and the one on the

extreme right is from a late-stage structure, a

small schreibersite partially embedded in taenite.

The Santa Luzia bulk composition lies in about

the center of the spread of tie lines, demonstrating

that interface tie lines depend upon local structure

and may fall on either side of the ETL.

Had complete equilibrium been achieved for

Santa Luzia at 550° C, kamacite would have contained 5.4 atomic % Ni and schreibersite 14 atomic

% Ni. Equilibrium extending to the 300° C temperature range would have resulted in kamacite of

3.6 atomic % Ni and schreibersite of 31 atomic %

Ni (Table 19, Figure 25). A very different situation

is indicated by the tie lines in Figure 26. These

represent interfaces from a range of schreibersite

morphologies from massive schreibersite surrounded by large volumes of swathing kamacite to

taenite border schreibersite.

The explanation of these differences lies in the

fact that under conditions of diffusion-controlled

growth, P can be drawn from much greater distances than Ni within a given time period. Complete equilibrium was not reached in the 600° to

500° C temperature range. Massive schreibersites

had nucleated at much higher temperatures and

grown to near their final size. This process had

reduced the level of P in the complete volume of

metal to near the equilibrium level. Even the

volumes of metal most remote from the large

schreibersites probably only contained about 0.6

atomic % P at 550° C (the equilibrium value is 0.4

atomic % P). Massive schreibersite growth may be

viewed as a process that equalizes P levels in

meteoritic metal in the 500° to 600° C temperature

range regardless of initial P concentrations. Differences in initial P level are reflected in the amount

of schreibersite produced.

While P diffusion in this temperature range

resulted in a meteorite with a modest P gradient,

the slower Ni diffusion resulted in an important

segregation of this element. Ni could not be moved

from a sufficiently large distance to maintain bulk

equilibrium in the times available. The massive

schreibersites had grown to near their final size,

but their Ni contents had to have been significantly

below the equilibrium value. Their iriterface tie

lines, therefore, must have been to the left of the

equilibrium tie line. The Ni gradient that probably

already existed within the swathing kamacite zone

became more pronounced. Beyond the swathing

zone, however, sufficient Ni was maintained to

yield the observed Wildmanstatten pattern. These

areas of the meteorite have a higher average Ni

content than the initial bulk Ni content of the

meteorite. As a matter of fact, some areas of

kamacite within the Widmanstatten pattern contain more Ni than the accepted bulk Ni value.

The consequence of massive schreibersite

growth under these circumstances of composition

and cooling is to establish two distinctly different
64 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

structural areas within the meteorite, swathing

kamacite areas containing massive schreibersites

and areas of Widmanstatten pattern (Figure 7).

Both types of areas continue to change with decreasing temperature, but distances involved become increasingly more restricted. The massive

schreibersites continue to grow, their growth rates

being determined by the flux of P from the swathing kamacite. Interface Ni values shift to produce

a tie line with a matching growth rate. The shift is

undoubtedly to the right, resulting in a slower

growth rate due to Ni and Ni enrichment of the

schreibersite. The final Ni values of these schreibersites approximates the equilibrium value at 550°

C.

The outer edge of the swathing zone develops

as a kamacite-taenite boundary that recedes from

the massive schreibersite with decreasing temperature. The final stage in this progression is the

replacement of the boundary in part by grain

boundary schreibersite. P is supplied to the boundary initially by the swathing kamacite, and the

taenite boundary supplies the initial Ni. Nucleation of this type results in concentrations that are

required by the three-phase field of the ternary

diagram. The initial schreibersite-kamacite tie line

will be the a + Ph/a + y + Ph boundary for the

given temperature. With decreasing temperature

the taenite border is replaced with a grain boundary that contains grain boundary schreibersite over

about half of its distance. Nucleation may take

place over a range of temperatures as the taenite

recedes along the forming grain boundary, resulting in different initial Ni contents in the schreibersite and different initial growth rates. As growth

continues at the individual interfaces, Ni concentrations adjust to match the P controlled growth

rate. The general tendency will be for tie lines to

move to the right as the P supply dwindles.

Simultaneously with continued growth of the

massive schreibersites and development of the

exterior swathing zone grain boundary, Widmanstatten pattern development is continuing in the

remaining areas of the meteorite. The Ni level is

high enough in these areas so that a coarse Widmanstatten pattern forms, containing large kamacite lamellae and occasional plessite areas and

grain boundary taenite. Grain boundaries contain

occasional grain boundary schreibersite, and the

kamacite lamellae contain rhabdites and microrhabdites. The rhabdites and grain boundary

schreibersites develop analogously to the similar

morphologies described above for Coahuila, reacting to specific local conditions to achieve appropriate interface values. Nucleation of grain boundary

schreibersites under these circumstances may take

place at lower temperatures, undoubtedly leading

to smaller schreibersites with higher initial Ni

content that continues to increase upon further

cooling. Small rhabdites would also be expected to

reach high Ni values because they are growing in

a relatively Ni rich kamacite.

A final type of schreibersite morphology observed in Santa Luzia is represented by the tie line

at the far right in Figure 26. It represents the

taenite border schreibersite illustrated in Figures

10 and 11. Schreibersite of this type appears to

nucleate within taenite at a taenite-kamacite interface at a very late stage in the structural development process. P is undoubtedly supplied mainly

from the kamacite, with most of the Ni coming

from the taenite. Associations of this type yield

the highest Ni schreibersites to be observed in

coarse octahedrites. The schreibersites may also

contain a slight Ni gradient, undoubtedly due to

the fact that they are associated with kamacite on

one side and taenite on the other. If this type of

schreibersite nucleates earlier or grows more rapidly due to a particularly high P gradient, it isolates

itself from the enclosing taenite by converting it to

kamacite.

Goose Lake meteorite tie lines are shown in the

bottom isothermal section in Figure 26. Differences from the Santa Luzia pattern result from

higher bulk concentrations of both carbon and

Ni. Only the upper portions of the two tie lines on

the left can be indicated. At some point in the

development of these schreibersites, cohenite precipitated in the adjacent metal, transforming the

interface to a cohenite-schreibersite interface and

preventing further schreibersite growth. The partial range of Ni values in kamacite, therefore, is

3.2 to 5.7 atomic %, with Ni values in schreibersite

ranging from 16 to 45 atomic %. The value of 45

atomic % Ni is the highest observed for a schreibersite in contact with kamacite. Kamacite P values

at the interfaces cover the same range as observed

in Santa Luzia, 0.13 to 0.05 atomic %. This range

in P values may indicate that these tie lines do not

all belong on the same isothermal section, a point
NUMBER 21 65

that will be discussed in a later section. The bulk

composition again lies in the center of the spread

of tie lines.

The general outline of the schreibersite growth

process may be summarized on the basis of this

discussion and that given in previous sections. In

the meteorites under study schreibersite petrography has been strongly affected by sequential nucleation. Initial schreibersite nucleation is a function of temperature and bulk composition. It took

place at greater than 850° C for all of the meteorites containing massive schreibersite. The low P

Coahuila meteorite is the only exception. These

massive schreibersites grew rapidly under equilibrium or near equilibrium conditions, at least as

far as P is concerned, for most of their growth

period. Randich (1975) has shown that meteorite

cooling rates are sufficiently slow to allow this. It

is important to note that massive schreibersite has

achieved the major portion of its growth by the

time it has cooled to 600° C. On subsequent cooling, Ni is enriched but size does not increase

appreciably. The massive schreibersite growth

process, therefore, has the effect of lowering the

P level in the bulk metal and isolating it, as

schreibersite enclosed in zones of swathing kamacite, from areas of developing Widmanstatten pattern.

By 600° C the Widmanstatten pattern is well

established in these meteorites (Table 16). Kamacite lamellae have grown to near their final size,

and taenite bands have shrunk to near theirs.

Both taenite and kamacite will continue to increase

in Ni with cooling, but compositional changes will

take place over increasingly short distances. It is

in this environment, one that contains P in kamacite at the 0.5 to 0.8 atomic % level, that rhabdite

nucleation must take place. They nucleate homogeneously in kamacite within a narrow range of

temperatures and grow under conditions of equilibrium combined with severe P impingment. It is

the limited amount of P and the competition for it

from closely spaced schreibersites that controls

their growth rates and consequently their eventual

size. The larger the volume of metal from which a

schreibersite draws P, the faster it grows and the

larger it becomes. Tie line shifts permit Ni levels

to vary depending upon growth rates. The larger

faster-grown rhabdites achieve lower Ni levels than

the smaller slower-grown schreibersites.

A third major type of schreibersite is grain

boundary schreibersite. Its petrography suggests

that it replaces taenite at late stages in structural

development. Heterogeneous nucleation probably

takes place at kamacite-taenite interfaces, resulting

in initial concentrations being controlled by the

three-phase a + y + Ph field of the ternary

diagram. P is supplied to the nucleation site mainly

from the bordering kamacite and Ni comes mainly

from the taenite. Equation 6 does not describe

initial growth rates but may well apply to kamaciteschreibersite interfaces after they become isolated

from the influence of taenite. Nucleation temperatures probably depend upon specific local conditions and may extend to considerably lower temperatures than those appropriate for rhabdite formation.

There are three types of associations that may

be included under the term grain boundary schreibersite. They appear to represent a sequence of

stages in structural development. The first type is

the isolated grain boundary schreibersite. These

are elongated schreibersites that lie along kamacite-kamacite grain boundaries. They frequently

occupy grain boundary junctions and branch accordingly. Their morphology suggests that they

replaced residual taenite as it receded along the

grain boundary. Their Ni values decrease with

distance away from the nearest taenite ribbon or

taenite-plessite area, suggesting that the lowest Ni

grain boundary schreibersite nucleated first. An

alternate possibility is essentially simultaneous nucleation with faster growth accounting for lower

Ni. These schreibersites are frequently somewhat

larger than the higher Ni ones closer to taenite.

There is no obvious reason, however, why P impingment should be more critical at one site than

at another.

The second type of grain boundary schreibersite

is found in close association with taenite and taenite-plessite areas. It appears to have formed similarly to the isolated grain boundary schreibersites,

but it either nucleated later or grew more slowly.

It separated itself from taenite but was not able to

completely absorb it. The final stage in this sequence is the formation of taenite boundary

schreibersites. These are the small, highest Ni

schreibersites that appear to be embedded in taenite at taenite-kamacite borders. These would

seem to be the final schreibersites to form.
66 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

INTERFACE DATA AND THE a/a + PH BOUNDARY

In the earlier discussion of the extrapolation of

experimental data to low temperatures the slope

of the a/a + Ph boundary was mentioned. The

extrapolation shows that the boundary moves toward the Fe-Ni axis with decreasing temperature

and suggests that it is parallel to it (Figures 21,

22). This led to the simplifying assumption that P

saturation values in kamacite are independent of

Ni and Fe concentrations and may be equated to

specific temperatures (Table 18). This in turn

leads to a possible interpretation of the measured

P in kamacite values at kamacite-schreibersite interfaces as being equivalent to a final growth

temperature, a temperature below which subsequent interface changes would be undetectable by

the technique employed.

Observed P interface values cover a range of

concentrations from 0.16 to 0.05 atomic %. A

factor of three is involved, indicating that small

differences in measured concentrations may have

more than trivial significance. The higher P values

are associated with massive schreibersite at interfaces with low Ni kamacite, and the lower P values

are associated with taenite border schreibersite at

interfaces with high Ni kamacite. Assuming that

these values represent solubility limits, we can

equate them to a temperature range of 445° to 350°

C (Table 18). Measurements on the Santa Luzia

meteorite alone covered most of this range, 0.14

to 0.05 atomic % P, equivalent to a temperature

difference of 430° to 350° C. Is it reasonable to

assume that large schreibersites stopped detectable

growth before the later formed schreibersites, or

does the a/a + Ph boundary really have a downward slope with increasing Ni? Are there other

factors that may make a clear choice between these

two alternatives impossible on the basis of the

available data?

The P and Ni kamacite interface data is summarized in Figure 27. The points represent individual

interface Ni and P concentrations in kamacite and

are plotted using the axes at the left and at the

bottom of the diagram. The symbols indicate the

meteorite from which the data were taken. There

is a general trend of decreasing P concentration in

kamacite with increasing Ni concentration. The

relationship of these values to the estimated a/a

+ y boundary of the Fe-Ni binary diagram of

Goldstein and Olgivie (1965) is also indicated. The

scale to the right of the diagram is the temperature

equivalent to the P concentration at the left, using

the data from Table 18. The dashed line is the a/

a + y boundary plotted in relation to temperature

and Ni concentration. It is obvious that interface

values for individual meteorites, as is the case for

the group as a whole, cover a range of both P and

Ni levels. A meteorite has interface values at several P levels, and a range of Ni values may be

covered at any given P level. All of the interface

values are to the left of the a/a + y boundary, the

higher Ni values being closest to it.

There are at least three factors that must be

considered as possible contributors to the observed

distribution in Figure 27. These are (1) cooling

rates, (2) the possibility of a slopping a/a + Ph

boundary, and (3) the possibility of low temperature nucleation at a/y boundaries and subsequent

grain boundary diffusion playing a role in the

growth of grain boundary and taenite border

schreibersite. The available data is insufficient to

sort out these influences in a quantitative way, but

their expected effects can be outlined and suggestive observations mentioned.

Cooling rates have only been reported for four

of the meteorites under consideration here. Values

of 2° C per million years for Lexington County

and Goose Lake and 3.5° C per million years for

Bahjoi were given by Goldstein and Short (1967b).

The recent measurements of Randich (1975) gave

a somewhat higher value of 5° C per million years

for Coahuila, and there is no reported value for

Bellsbank. Kamacite band widths for the other

three meteorites were taken from Buchwald

(1976), and cooling rates were estimated using the

Goldstein and Short (1967b) cooling rate curves.

Ballinger and Balfour Downs gave cooling rates

of 2° C per million years and Santa Luzia gave an

estimated 0.8° C per million years. These numbers

indicate that cooling rate values for this group of

meteorites cluster in a rather narrow range centering around 2° C per million years.

The effect of cooling rate variations on schreibersite-kamacite interface values has been discussed above. It is to be expected that slower

cooling rates would produce lower detectable interface values of P in kamacite and lower levels of

P in kamacite generally. This would be equivalent

to detectable growth continuing to lower temperatures. This type of variation may be present on a

minor scale in the data considered here. Coahuila
NUMBER 21 67

0.30

O25

0.22

0.20

0.19

0.18

0.17

0.16

0.15

0.14

a* o.i2

SJO.II

| 0.10

< 0.09

0.08

0.07

0.06

0.05-

0.04

0.03

0.02

A

•

•

•

O

A

D

a,BELLSBANK

b, COAHUILA

C, SANTA LUZIA

d, BALLIN6ER

e, LEXINGTON CO.

f, BAHJOI

Q, 600SE LAKE

h, BALFOUR DOWNS

a + y

AA

(A CCE A A|» ^ Jft.

*co

500

490

480

470

460

450

440

430

420

410

400

390

380

370

360

350

340

330

320

310

300

29O

280

I 2345678 9

ATOMIC %Ni a

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°

C per million years, and its interface P values and

P levels in kamacite do seem to be somewhat

higher than for some of the other meteorites

studied. Bellsbank also seems to have unusually

high levels of P in kamacite, and this could be the

consequence of a comparatively high cooling rate.

The points representing both Coahuila and Bellsbank interfaces are in the upper part of Figure

27, perhaps meaning that detectable change ceased

at higher temperatures for these two meteorites

due to higher cooling rates.

An a/a + Ph boundary (AB in Figure 20) that

slopes downward from the Fe-P axis toward the

Fe-Ni axis would contribute to the type of data

distribution observed in Figure 27. At any given

temperature lower Ni schreibersites would have a

higher P interface value in kamacite than higher

Ni schreibersites. The range of the data, however,

seems to require much too steep a slope to be

reasonable in terms of the extrapolation given

above. An Fe-P axis intercept value of approximately 0.2 atomic % would be indicated. This

would mean either an unreasonably high P saturation temperature in the neighborhood of 465° C

(Table 18), or that the actual curve bends sharply

to the right of the indicated extrapolation (Figure

21). It also seems unlikely that error in locating

the data points is a major factor. It was pointed

out above that reproducibility of P determinations

was ±0.005 weight % and for Ni was ±0.1 weight

%. This means that the P levels shown are distinguishable from their nearest neighbors with reasonable certainty and that the Ni variations are

meaningful. Ni variations at a given P level range

from 0.8 to 3.0 atomic %. Variations for a specific

meteorite at a given P level are not so wide, but

they do range up to 1.5 atomic %. This range in

Ni values for a single meteorite at a given P level

suggests that factors other than simply a sloping

a/a + Ph boundary are involved. The boundary

may well have a gentle slope, but other factors

must also contribute to the observed distribution.

Tie lines representing three of the P levels in

Figure 27 are plotted on isothermal sections in

Figure 28. The symbols used in Figure 27 are

used at the schreibersite ends of the tie lines to

identify the meteorite represented. Tie lines actually belonging to a given isothermal section will

fan out across the field without overlap. When

overlap occurs it implies either error in the data

or plotting on the wrong isothermal section. We

saw in Figure 26 that all of the measured tie lines

for three of the meteorites can be plotted without

obvious cases of overlap. This could mean they all

belong to the same isothermal section, but it could

also be fortuitous. If interface data for all of the

meteorites could be plotted as tie lines on a single

intelligible drawing, several cases of overlap would

be observed. When tie lines are separated on the

basis of P level as in Figure 28, no serious cases of

overlap are encountered. The two tie lines at the

left on the 0.09 atomic % P section do overlap

slightly, but a small shift in Ni values could do

away with it. The surprising point is how little

overlap is seen when the data are taken directly

from the tables given earlier and plotted in this

way.

Had all of the tie lines in Figure 28 been plotted

on a single isothermal section, there would have

been several cases of overlap. The most serious

offender would be a Bellsbank tie line, the second

one from the left on the 0.09 atomic % P isothermal section. If it were plotted on the top section

of Figure 28, there would have been obvious

overlap with another Bellsbank tie line. All three

of the tie lines in this top isothermal section are

from the Bellsbank meteorite, and there is additional evidence to be discussed below that suggests

that two temperatures are involved.

Plotting tie lines as has been done in Figure 28

supports the idea that a temperature effect is real.

That is to say that late formed schreibersites continued significant growth to temperatures below

those of early formed schreibersites. Unfortunately, these meteorites do not supply us with a

full range of tie lines at any one temperature.

More work will be required before the slope of

the a/a + Ph boundary within this low temperature range can be established with certainty.

A third possibly complicating process that could

be of importance here has been discussed by

Comerford (1969). He pointed out that grain

boundary diffusion might play an important role

in the development of grain boundary schreibersites. He suggested that grain boundary diffusion

would supply Ni to schreibersites growing at low

temperatures more readily than lattice diffusion.

The discussion above has pointed out that P is the

controlling element rather than Ni, but his suggestion would be equally valid for P.

The measurements on grain boundary schrei-
NUMBER 21 69

0.09 ATOMIC %P

395° C

4.4

0

100 % Fe

10 20 30

ATOMIC %Ni

40

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 representative of many such measurements reported above

(Figures 10, 11). Petrographic relationships suggest that this type of schreibersite nucleated at

kamacite-taenite interfaces absorbing residual taenite. Nucleation temperatures were probably low

compared to those for homogeneously nucleated

rhabdite, but the data does not allow a closer

temperature estimate. It would also appear that

nucleation occurred over a range of temperature,

nucleation of the lowest Ni schreibersite farthest

along the grain boundary away from taenite having taken place first (Figure 11, traverse 11). The

Ni concentration of schreibersite nucleated at a

kamacite-taenite border is that of the schreibersite

corner of the three-phase field at the nucleation

temperature. The presence of the phase boundary

probably serves both as a site for nucleation and

as a source of P through the process of grain

boundary diffusion. As the schreibersite becomes

larger, however, the contribution of grain boundary diffusion becomes increasingly less important.

The schreibersite grows rapidly, with its Ni level

increasing only slightly. Its interface with kamacite

represents a tie line that probably tends to shift to

the left with decreasing temperature, as P is drawn

from a relatively large diffusion field.

Along the grain boundary at a location closer to

the final residual taenite a second grain boundary

schreibersite nucleates at a lower temperature (Figure 11, traverse 12). Its initial Ni is higher, and

the P diffusion rate is lower. Grain boundary

diffusion again makes an important contribution

during initial growth, but it probably becomes

increasingly less important as the schreibersite

increases in size.

The observed taenite border schreibersite undoubtedly nucleated at a still lower temperature

(Figure 11, traverse 13). It formed at the taenitekamacite border and had a high initial Ni concentration. Growth was insufficient to separate the

schreibersite completely from taenite. A small

schreibersite that has a Ni gradient due to the

fact that it is in contact with kamacite at one end

and taenite at the other resulted. This type of

schreibersite has both the highest observed Ni

concentrations and the lowest observed P interface

values in kamacite. A reasonable explanation of

these observations is that grain boundary diffusion

plays an increasingly important role both with

small schreibersite size and lower temperature of

growth.

Grain boundary diffusion may also help explain

the Bellsbank interface measurements summarized in Figure 5. Four of the seven interfaces

shown have a P level in kamacite of 0.09 weight %

P (0.16 atomic %), and the other three are at 0.05

weight % P (0.09 atomic %). The schreibersite

interface illustrated at the top of the diagram is a

typical one for massive schreibersite. The schreibersite illustrated at the lower left is a large rhabdite (Table 3, traverse 6) surrounded by clear

kamacite within an area of microrhabdites. There

were no grain boundaries or subgrain boundaries

observed. The schreibersite illustrated at the lower

right is another large elongated rhabdite (Table 3,

traverses 4 and 5) that was observed to be along a

definite subgrain boundary. Both of these rhabdites have extremely large P gradients, and their

P interface values differ by 0.04 weight % P (0.07

atomic %). The center schreibersite in the lower

section of the diagram is a large rhabdite (Table

3, traverses 2 and 3) along a subgrain boundary

with different P and Ni interface values in kamacite on its opposite sides. Tie lines representing

these interfaces are given in Figure 28. The three

high P tie lines are in the top isothermal section

and the fourth, representing the low P interfaces,

is at the left in the middle section. A possible

rationale for these observations is that Bellsbank

cooled more rapidly than the other meteorites

studied. As a result, growth apparently stopped at

higher temperatures than has been the case for

other rhabdite-containing meteorites. The presence of grain boundaries, however, permitted P

flow to continue to significantly lower temperatures for the two schreibersites with the lower

interface levels. A faster type of diffusion permitted detectable interface changes to extend to lower

temperatures. It would be interesting to find similar situations in other meteorites.

SCHREIBERSITE WITH COHENITE BORDERS

A prominent feature of the meteorites selected

for study is the presence of 100-to-200-/Lim-wide

cohenite borders separating massive schreibersite

from kamacite. It is a feature that can be recognized with the unaided eye on many well-prepared

exhibit sections of Group I meteorites, but it has
NUMBER 21 71

been little noted in the literature until relatively

recently. Cohenite borders may partially or completely enclose schreibersite bordering troilite inclusions or massive schreibersite isolated within

kamacite. This type of association has been reported above for the Ballinger (Figure 6), Lexington County (Figure 12), Bahjoi (Figure 13), Goose

Lake, and Balfour Downs (Figures 15, 16, 17)

meteorites. The Ni concentrations in these cohenite enclosed schreibersites range from 16 to 21

atomic %, comparable to those observed for similar unenclosed schreibersites within the same meteorite or within similar meteorites. The cohenite

borders contain small amounts of Ni and no detected P. Their Ni profiles show a gradient varying

from as much as 1.7 weight % Ni at the cohenitekamacite interface to as low as 0.8 weight % Ni at

the cohenite-schreibersite interface (Figures 13,

17).

Similar Ni gradients in cohenite borders have

been reported previously by Drake (1970) and

Scott (1971a). Drake (1970) noted that Ni diffuses

through cohenite, increasing the Ni content of

enclosed kamacite and taenite grains with decreasing temperature, but he felt that cohenite shells

around schreibersite prevented equilibration of

schreibersite with the enclosing metal. Scott

(1971a) interpreted the presence of Ni gradients

in cohenite borders as implying equilibration of

Ni to low temperatures. The observed Ni concentrations in cohenite-enclosed schreibersite supports Scott's interpretation.

Massive schreibersite growth is apparently

blocked by enclosure in cohenite. The absence of

detectable P in, these cohenites indicates that P

solubility is very low. It seems reasonable to assume, therefore, that cohenite borders isolate

schreibersite from a source of additional P. The

situation with Ni, however, is different. The cohenite borders contain appreciable Ni, and the

measured Ni gradients suggest a flow of Ni to the

cohenite-schreibersite interfaces. Under these circumstances the size of the schreibersite indicates

the maximum temperature at which cohenite

could have nucleated. The temperature must have

been low enough to permit schreibersite to reach

its full size. At this maximum temperature, or at

some lower temperature, cohenite encases the

schreibersite. Growth is stopped but Ni enrichment continues with decreasing temperature. The

minimum temperature at which cohenite could

have formed is at or below the temperature at

which the schreibersite achieved its final Ni concentration. According to the Fe-Ni-P diagram this

would be at 550° C or below. In this low temperature range schreibersite would be surrounded by

large volumes of kamacite. Is kamacite a reasonable source for the large amount of carbon required

for cohenite growth? The work of Brett (1967)

and A. D. Romig, Jr. (1977, pers. comm.) on the

Fe-Ni-C system seems to require an alternate explanation.

Consideration of the Goose Lake meteorite

structure in terms of both its cooling history within

the Fe-Ni-P system (Table 14) and Romig's Fe-NiC diagram may be instructive at this point. Goose

Lake is a meteorite that has large amounts of

massive schreibersite. Surface areas totaling more

than 1000 cm2 were examined and found to contain more than 20 areas of clustered massive schreibersites. All of these schreibersites appeared to be

enclosed in cohenite, although poor condition of

surfaces left some ambiguity in one or two cases.

In any event, the ubiquitous nature of cohenite

bordering massive schreibersite in Goose Lake was

established. This suggested that Goose Lake might

be expected to yield a more straight-forward interpretation of cohenite border growth than would

be possible using meteorites where the association

was more sporadic.

Equilibrium compositions of phases derived

from the Fe-Ni-P system for a range of decreasing

temperatures are given in the lower section of

Table 14 for the assumed Goose Lake meteorite

bulk composition. At 925° C schreibersite is in

equilibrium with 96% taenite. On cooling to 750°

C the schreibersite doubles in amount to 8% and

increases in Ni from 5.7 to 6.6 atomic %. Taenite

is slightly reduced in volume during this process,

but its Ni concentration remains essentially constant at 7.9 atomic %. Cooling through this 175° C

range has resulted primarily in the localization of

P. Massive schreibersites have grown within taenite

and have achieved most of their eventual size.

In the 750° to 650° C temperature range major

structural changes take place. Taenite increases in

Ni from 7.9 to 11 atomic % and decreases in

volume by one-half. Schreibersite increases in Ni

from 6.6 to 9.0 atomic % and increases in volume

by approximately 10%. The shrinking taenite is
72 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

converted to kamacite of 5.0 atomic % Ni, which

makes up nearly half the structure at 650° C. With

further decreasing temperature, kamacite expands in volume, taenite shrinks, and schreibersite

holds its own. All three phases continue to increase

in Ni.

What happens if carbon is also present in the

system? Romig's Fe-Ni-C phase diagram gives carbon saturation values for taenite in equilibrium

with cohenite. These are given in Table 20 for

selected taenite compositions for the range of

temperatures covered. The taenite Ni values given

are those of taenite in equilibrium with schreibersite at the indicated temperature and the Goose

Lake bulk composition using the Fe-Ni-P system

(Table 14). Carbon has appreciable solubility in

taenite throughout this temperature range, but its

solubility in kamacite is negligible. This solubility

of carbon in taenite means that a meteorite such

as Goose Lake with a structure consisting of 92%

taenite at 750° C might well contain enough dissolved carbon to produce significant amounts of

cohenite on subsequent cooling. The important

factor is that the amount of taenite present is

drastically reduced with decreasing temperature.

This results in carbon concentration increasing in

the ever decreasing amount of taenite. Supersaturation must result at some temperature if sufficient

carbon were present to start with.

Brett (1967) assumed that P could be neglected

in his discussion of cohenite formation in meteorites. That may be true for some types of cohenite

associations, but it may well not hold for cohenite

bordering massive schreibersite. It is difficult to

understand these associations using the Fe-Ni-C

system alone. Brett (1967) shows that in this system

cohenite grows by the following reaction:

y = cohenite + a.

He also points out that when the Ni content is

high, kamacite and graphite form with falling

temperature. Cohenite is what we see, however,

not significant amounts of graphite in kamacite.

If we assume that carbon is present in taenite,

then we are actually dealing with the Fe-Ni-P-C

system. The more complex system permits the

postulation of the following sequence of events to

account for massive schreibersite with cohenite

borders. Massive schreibersite nucleates at high

temperature and grows upon cooling to near its

TABLE 20. —Carbon saturation values for taenite in

equilibrium with cohenite from the Fe-Ni-C diagram of A. D. Romig, Jr. (taenite Ni values are

equilibrium values using the Goose Lake bulk composition, Table 14).

°c

730

650

600

500

Weight % C

0.75

0.45

0.36

0.18

Atomic % Ni

n

14

•v25

final size, perhaps in the 650° to 700° C temperature range, assuming the Goose Lake bulk composition. These schreibersites are in contact with

taenite that has become saturated in carbon. With

further cooling both taenite and schreibersite require more Ni, and taenite requires a repository

for its carbon. Under these circumstances it is

reasonable to assume the following nucleation reaction taking place at taenite-schreibersite interfaces:

OKQat) + Ph = y + cohenite + Ph.

A cohenite border at the preexisting taenite-schreibersite interface results. Cohenite becomes a repository for carbon and the Ni that previously

resided there is absorbed by taenite and/or schreibersite. As the structure develops, a dynamic equilibrium involving two interfaces is established; carbon saturated taenite-cohenite and coheniteschreibersite. Cohenite grows as long as carbon

saturated taenite is present. Taenite shrinks in

volume, cohenite fills in behind the receding taenite, and schreibersite becomes enriched in Ni. At

some point kamacite precipitates at the cohenitetaenite interface and then kamacite becomes the

source of Ni for transport to schreibersite. The

cohenite border is established and subsequent

changes involve diffusion of Ni and Fe only.

The postulated reaction sequence would permit

cohenite to grow over a broader temperature

range than was suggested by Brett (1967). The

maximum temperature can be estimated from the

size of the cohenite-enclosed schreibersite. What

is the highest temperature that could have produced schreibersite of the observed size starting

with a given bulk composition? In a meteorite like

Goose Lake the temperature might be as high as

750° C. The minimum temperature of nucleation
NUMBER 21 73

would be established by the disappearance of sufficient taenite to supply the required carbon supersaturation. It seems unlikely for the meteorites

that have been studied here that this would be

below 600° C. This postulated growth sequence

could extend Brett's (1967) temperature range of

610° to 650° C upward, but it is unlikely that it

would be lowered appreciably. If Brett's (1967)

estimated rates of cohenite decomposition with

temperature are correct, nucleation within the

lower part of the possible temperature range

would favor preservation of cohenite. Confirmation of this proposed sequence of events will have

to await the results of detailed experimentation.

Summary and Conclusions

The role that schreibersite growth played in the

structural development process in coarse-structured iron meteorites has been examined. The

availability to the writer 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. All of these meteorites contain schreibersite, ranging in amount from easily detectable to

abundant, and they contain additional P in solution in kamacite and taenite. Their detailed metallography indicates that they are comparatively free

of secondary effects such as shock and reheating

that might mask or distort relationships of interest.

The range of bulk compositions covered is one

where small changes in P-Ni concentration relationships may be more important to the structural

development process than would be the case at

higher Ni levels.

Lengthy electron microprobe traverses were

made across structures representative of the observed range of schreibersite associations, with the

exception of microrhabdites. Particular emphasis

was placed on schreibersite-kamacite interface

compositions. Ni and P interface concentrations

in kamacite and Ni interface values in schreibersite

were determined, along with associated Ni and P

concentration gradients in kamacite and Ni gradients in schreibersite. An analysis of these data

has led to a more comprehensive description of

the structural development process in these individual meteorites than has been available previously.

Massive schreibersite, one of the four major

types of schreibersite encountered, may be accounted for by equilibrium considerations. The

Fe-Ni-P equilibrium diagram and estimated bulk

compositions for individual meteorites were used

to explain the basic structures present. 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. No need

was found to invoke eutectic liquids to account for

any of the observed massive schreibersite morphologies. After nucleation, schreibersite and

kamacite grew in volume and increased in Ni

content while taenite receded and increased in Ni

content under equilibrium or near equilibrium

conditions into the 600° C temperature range. 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. The growth of massive schreibersite from

high temperatures into the 600° C temperature

range served both to localize P by reducing its

level throughout the bulk metal and to produce

the large swathing kamacite zones that are observed.

The 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. Growth of these

schreibersites was also under equilibrium conditions to below 500° C. They drew their source of P

from limited diffusion fields when compared to

the massive schreibersites.

The third type of schreibersite is grain boundary

and taenite border schreibersite. It formed at

kamacite-taenite interfaces, absorbing residual taenite. Nucleation took place successively along grain

boundaries over a range of temperatures starting

as high as 500° C or perhaps slightly higher. With

decreasing temperature taenite receded and became richer in Ni and the schreibersite that nucleated at these interfaces had higher Ni. The
74 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

highest Ni schreibersites are the small ones that

nucleated at low temperatures and are observed

still in contact with taenite. 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 supersaturated 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 accounts for the observed range of Ni concentrations in schreibersite. Interface measurements

on meteoritic schreibersites, therefore, yield a

range of tie lines in the low temperature Fe-Ni-P

system. 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.

Schreibersite with cohenite borders is a common

occurrence in many meteorites. It is suggested

that it formed as a reaction in the Fe-Ni-P-C

system in the 750° to 600° C temperature range.

Carbon saturated taenite reacted with schreibersite

to form cohenite. Cohenite then grew in equilibrium with both taenite and schreibersite.

Berzelius' prescient comment of 1832 on the

role of P in the development of the Widmanstatten

pattern has been confirmed and expanded upon.
Appendix

In the discussion of the low-temperature ternary

diagram, experimental values for P saturation concentrations in kamacite and taenite were extrapolated to lower temperatures (Table 17, Figures 21,

22). This is a commonly used procedure when

dealing with dilute solutions and is a variation on

a method used for the determination of partial

molal properties of substances. It is applicable to

solid state transformations in dilute metallic systems and may be justified by the following thermodynamic reasoning adapted from Swalin

(1972:168-171).

At a given temperature (Tx) the end points of a

kamacite-schreibersite tie line in the kamaciteschreibersite field of the equilibrium diagram represent equilibrium concentrations of the two

phases. The relationship between P-saturated

kamacite (ap(Sat)) and schreibersite (Ph) may be

expressed as follows:

ttp(sat) ^ Ph + a AG = 0.

Since the reaction represents equilibrium the

change in the Gibbs free energy (AG) is zero.

Therefore, the chemical potentials (fi) of P in the

two phases are equal,

f^P M'P(sat)

as are their activities (a),

•Ph _ „<*

a P — a P(sa t).

Since schreibersite is a pure substance with respect

toP

requiring that

-•Ph _

aP —

a

P(sat) —— 1 •

If the saturation concentration of P in kamacite is

sufficiently small, P may be assumed to follow

Henry's law and the activity coefficient of P in

kamacite, y?, may be evaluated.

aP(sat) — 7P AP(sat)

1

In order to derive a relationship for the solvus

line (kamacite saturated with P) as a function of

temperature (T), we must consider a situation

where AG for the reaction is not zero. This may

be done by assuming a P concentration in kamacite, Xg, such that Xp" is within the kamacite field

of the equilibrium diagram

X a

P -P(sat)-

Kamacite of this concentration would react with

schreibersite to form P-saturated kamacite,

aP + Ph «* «P(Sat) AGX.

The following equation may be written relating

the Gibbs free energy ( AG2) for the above reaction,

the chemical potentials of the reacting species,

and the activities of these species,

AGX = n% - At°P

ph

 = RT In - ^ •

for a pure substance,

aPh = 1

for a dilute solution following Henry's law

a? = yAX®

and from equation given above

AP

•(sat-m) ) = ^-PCsat)

Substituting these values of aPh and aP in the

expression for AGX given above we obtain

fi$ - n°P

ph

 = RT In Xg - RT In Xp\sat) - RT In (1).

We know that the enthalpy of a component i, H,,

is related to the temperature and chemical potential by

= Hi.

75
76 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

Substituting, we obtain

- Hp h

d(RT In Xg/T) a(RT In

3(1/1)

Xp and (1) are constants, dropping out these two

terms,

HPa - HPh = - Rd(lnXHsat))

a(VT)

3(1/'T)

R

= - 3(ln

In Xp(Sat) =

-AH F

R

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

are observed in our case, and it is reasonable to

assume on the basis of experience in other systems

that AHPa is independent of temperature over a

reasonable temperature range. It would seem then

that an extrapolation over a reasonable temperature range is justified. A similar treatment would

apply to P-saturated taenite data.
Literature Cited

Adler, I., and E. J. Dwornik

1961. Electronprobe Analysis of Schreibersite (Rhabdite) in

the Canyon Diablo Meteorite. U.S. Geological Survey

Professional Papers, 424B(112):B263-B265.

Agrell, S. O., J.V.P. Long, and R. E. Ogilvie

1963. Nickel Content of Kamacite Near the Interface with

Taenite in Iron Meteorites. Nature, 198:749-750.

Anders, Edward

1962. Meteorite Ages. Reviews of Modern Physics, 34:287-

325.

1971. Meteorites and the Early Solar System. Annual Review

of Astronomy and Astrophysics, 9:1-34.

Axon, H. J., andj. Boustead

1967. Kamacite-Taenite Relationships in Iron Meteorites.

Nature, 213:166-167.

Axon, H. J., and D. Faulkner

1970. A Metallographic and Microprobe Study of the Barranca Blanca Meteorite. Mineralogical Magazine,

37:898-904.

Axon, H. J., andj. I. Goldstein

1972. Temperature-Time Relationships from Lunar Two

Phase Metallic Particles (14310, 14163, 14003). Earth

and Planetary Science Letters, 16:439-447.

1973. Metallic Particles of High Cobalt Content in Apollo

15 Soil Samples. Earth and Planetary Science Letters,

18:173-190.

Axon, H. J., and P. L. Smith

1972. A Metallographic Study of Some Iron Meteorites of

High Nickel Content. Mineralogical Magazine, 38:736-

755.

Axon, H.J.,andC. V. Waine

1971. A Metallographic Study of the Angra dos Reis (Iron)

Meteorite. Mineralogical Magazine, 38:94-101.

1972. A Metallographic Study of Some Hexahedrites. Mineralogical Magazine, 38:725-735.

Belaiew, N. T.

1924. On the Genesis of Widmanstatten Structure in Meteorites and in Iron-Nickel and Iron-Carbon Alloys.

Mineralogical Magazine, 20:173-185.

Benedicks, Carl

1910. Synthese du fer meteorique. Nova Ada Reglae Societatis Scientiarum Upsaliensis, series IV, 2(10): 1-26.

Berwerth, Frederick

1907. Steel and Meteoritic Iron. Journal of the Iron and Steel

Institute, 75(3):37-51.

Berzelius, J. J.

1832a. Undersokning af en vid Bohumiliz i Bohmen funnen

jernmassa. Kongelige Svenska Vetenskaps-Academiens

Handlingar, pages 106-119.

1832b. Untersuchung der Bohumulitzer Eisenmasse. Zeitschriftfur Physik und Mathematik, Wein, 1:289-297.

1833. Untersuchung einer bie Bohumiliz in Bohmen

gefundenen Masse. Annalen der Physik und Chemie,

27:118-132.

1834a. Om Meteorstenar. Kongelige Svenska Vetenskaps-Academiens Handlingar, pages 115-183.

1834b. Ueber Meteorsteinen, V: Pallas-Eisen und Pallas-Olivin, VI: Meteoreisen von Elbogen. Annalen der Physik

und Chemie, 33:123-137.

Bild, Richard W.

1974. New Occurrences of Phosphates in Iron Meteorites.

Contributions to Mineralogy and Petrology, 45:91-98.

Brett, Robin

1967. Cohenite: Its Occurrence and a Proposed Origin.

Geochimica et Cosmochimica Ada, 31:143-159.

Brezina, Aristides

1906. Zur Frage der Bildungsweise Eutropischer Gemenge.

Denkschriften der Kaiserlichen Akademie der Wissenschaften, Wein, 78:635-643.

Brown, G. M., A. Peckett, R. Phillips, and C. H. Emeleus

1973. Mineral-Chemical Variations in the Apollo 16 Magnesio-Feldspathic Highland Rocks. In Proceedings of

the Fourth Lunar Science Conference. Geochimica et

Cosmochimica Ada, supplement 4, 1:505-518.

Buchwald, V. F.

1966. The Iron-Nickel-Phosphorous System and the Structure of Iron Meteorites. Ada Polytechnica Scandinavica,

51: 45 pages.

1976. Handbook of Iron Meteorites. Volumes I—III, 1418

pages. Tempe: Center for Meteorite Studies, Arizona

State University; Berkeley: University of California

Press.

Buchwald, V. F., and S. Munck

1965. Catalogue of Meteorite in the Mineralogical Museum

of the University, Copenhagen. Analecta Geologica, 1:

81 pages.

Buseck, Peter R.

1969. Phosphide from Meteorites: Barringerite, a New

Iron-Nickel Mineral. Science, 165:169-171.

Carter, J. L., and E. Padovani

1973. Genetic Implications of Some Unusual Particles in

Apollo 16 Less Than 1 mm Fines 68841,11 and

69941,13. In Proceedings of the Fourth Lunar Science

Conference. Geochimica et Cosmochimica Acta, supplement 4, 1:323-333.

Clarke, Roy S., Jr.

1969. Comments on Cohenite and Schreibersite in Iron

Meteorites (Abstract). Meteoritics, 4:161.

Cohen E.

1894. Meteoreisen-Studien III: Rhabdit. Annalen des K.K.

Naturhistorischen Hofmuseums, IX:97-118.

Comerford, M. F.

1969. Phosphide and Carbide Inclusions in Iron Meteorites.

Pages 780-795 in P. M. Millman, editor, Meteorite

Research. 940 pages. Dordrecht: D. Reidel Publishing

Company.

77
78 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

Daubree, M. A.

1868. Experiences synthetiques relatives aux meteorites. 65 pages.

Paris: Dunod.

De Laeter, J. R., G.J.H. McCall, and S.J.B. Reed

1973. The Redfields Meteorite —A Unique Iron from Western Australia. Mineralogical Magazine, 39:30-35.

Derge, G., and A. R. Kommel

1937. The Structure of Meteoric Irons. American Journal of

Science, 34:203-214.

Doan, A. S., Jr., and J. I. Goldstein

1969. The Formation of Phosphides in Iron Meteorites.

Pages 763-779 in P. M. Millman, editor, Meteorite

Research. 940 pages. Dordrecht: D. Reidel Publishing

Company.

1970. The Ternary Phase Diagram, Fe-Ni-P. Metallurgical

Transactions ,1:17 59-1767.

Drake, John C.

1970. Mineralogical Relationships in Seymour, a Course

Octahedrite. Meteoritics, 5:19-31.

El Goresy, Ahmed

1965. Mineralbestand und Strukturen der Graphit- und

Sulfideinschliisse in Eisenmeteoriten. Geochimica et

Cosmochimica A eta, 29:1131-1151.

El Gorsey, A., P. Ramdohr, and O. Medenback

1973. Lunar Samples from Descartes Site: Opaque Mineralogy and Geochemistry. In Proceedings of the Fourth

Lunar Science Conference. Geochimica et Cosmochimica

Ada, supplement 4, 1:733-750.

Farrington, Oliver C.

1915. Meteorites, Their Structure, Composition, and Terrestrial

Relations. 225 pages. Chicago: Published by the author.

Fricker, P. E., J. I. Goldstein, and A. L. Summers

1970. Cooling Rates and Thermal Histories of Iron and

Stony-iron Meteorites. Geochimica et Cosmochimica Acta,

34:475-491.

Fuchs, Louis H.

1969. The Phosphate Mineralogy of Meteorites. Pages 681-

695 in P. M. Millman, editor, Meteorite Research. 940

pages. Dordrecht: D. Reidel Publishing Company.

Goldberg, E., A. Uchiyama, and H. Brown

1951. The Distribution of Nickel, Cobalt, Gallium, Palladium, and Gold in Iron Meteorites. Geochimica et

Cosmochimica Acta, 2:1-25.

Goldstein, J. I.

1965. The Formation of the Kamacite Phase in Metallic

Meteorites. Journal of Geophysical Research, 70:6223-

6232.

1969. The Classification of Iron Meteorites. Pages 721-737

in P. M. Millman, editor, Meteorite Research. 940

pages. Dordrecht: D. Reidel Publishing Company.

Goldstein, J. I., and H. J. Axon

1973. The Widmanstatten Figure in Iron Meteorites.

Naturwissenschaften, 60:313-321.

Goldstein, J. I., H. J. Axon, and C. F. Yen

1972. Metallic Particles in the Apollo 14 Lunar Soil. In

Proceedings of the Third Lunar Science Conference.

Geochimica et Cosmochimica Acta, supplement 3,1:1037-

1064.

Goldstein, J. I., and A. S. Doan, Jr.

1972. The Effects of Phosphorus on the Formation of the

Widmanstatten Pattern of Iron Meteorites. Geochimica

et Cosmochimica Acta, 36:51-69.

Goldstein, J. I., E. P., Henderson, and H. Yakowitz

1970. Investigation of Lunar Metal Particles. In Proceedings

of the Apollo 11 Lunar Science Conference. Geochimica et Cosmichimica Acta, supplement 1, 1:499-512.

Goldstein, J. I., and R. E. Ogilvie

1963. Electron Microanalysis of Metallic Meteorites, Part 1:

Phosphides and Sulfides. Geochimica et Cosmochimica

Acta, 27:623-637.

1965a. A Re-evaluation of the Iron-Rich Portion of the FeNi System. Transactions of the Metallurgical Society of

AIME, 233:2083-2087.

1965b. The Growth of the Widmanstatten Pattern in Metallic

Meteorites. Geochimica et Cosmochimica Acta, 29:893-

920.

Goldstein, J. I., and J. M. Short

1967a. Cooling Rates of 27 Iron and Stony-iron Meteorites.

Geochimica et Cosmochimica Acta, 31:1001-1023.

1967b. The Iron Meteorites, Their Thermal History and

Parent Bodies. Geochimica et Cosmochimica Acta,

31:1733-1770.

Goldstein, J. I., and H. Yakowitz

1971. Metallic Inclusions and Metal Particles in the Apollo

12 Lunar Soil. In Proceedings of the Second Lunar

Science Conference. Geochimica et Cosmochimica Acta,

supplement 2, 1:177-191.

Goode, George Brown

1877. The Smithsonian Institution 1846-1896. 856 pages.

Washington: The Smithsonian Institution.

Gooley, R. C, R. Brett, and J. L. Warner

1973. Crystallization History of Metal Particles in Apollo 16

Rake Samples. In Proceedings of the Fourth Lunar

Science Conference. Geochimica et Cosmochimica Acta,

supplement 4, 1:799-810.

Grossman, L., and J. W. Larimer

1974. Early Chemical History of the Solar System. Reviews

of Geophysics and Space Physics, 12:71-101.

Haidinger, Wilhelm

1847. Versammlung, am 16, Juli. Berichte uber die Mitteilungen von Freunden der Naturwissenschaften in Wien,

3:69-71.

Hey ward, T. R., and J. I. Goldstein

1973. Ternary Diffusion in the a and y Phases of the FeNi-P System. Metallurgical Transactions, 4:2335-2342.

Henderson, Edward P.

1965. Hexahedrites. Smithsonian Miscellaneous Collections,

148(5): 41 pages.

Henderson, E. P., and S. H. Perry

1958. Studies of Seven Siderites. Proceedings of the United

States National Museum, 107:339-403.

Hornbogen, E., and H. Kreye

1970. The Microstructure of Two Iron Meteorites. Zeitschriftfur Metallkunde ,61:914-923.

Lovering, J. F., W. Nichiporuk, A. Chodos, and H. Brown

1957. The Distribution of Gallium, Germanium, Cobalt,

Chromium, and Copper in Iron and Stony-Iron Meteorites in Relation to Nickel Content and Structure.

Geochimica et Cosmochimica Acta, 11:263-278.

Malissa, H. jun

1974. Electron Microprobe Analysis of Some Minor- and
NUMBER 21 79

Accessory Components in Mesosiderites. TMPM

Tschermaks Mineralogische und Petrographische Mitteilungen, 21:233-245.

Massalski, T. B., and F. R. Park

1962. A Quantitative Metallographic Study of Five Octahedrite Meteorites. Journal of Geophysical Research,

67:2925-2934.

McKay, G. A., S. J. Kridelbaugh, and D. F. Weill

1973. The Occurrence and Origin of Schreibersite-Kamacite Intergrowths in Microbreccia 66055. In Proceedings of the Fourth Lunar Science Conference. Geochimica et Cosmochimica Ada, supplement 4, 1:811-818.

Mehl, R. F., and G. Derge

1937. Studies Upon the Widmanstatten Structure, VIII:

The Gamma-Alpha Transformation in Iron-Nickel

Alloys. Transactions of the American Institute of Mining

and Metallurgical Engineers, Iron and Steel Division,

125:482-500.

Meunier, Stanislas

1880. Imitation synthetique des fers nickeles meteoritiques.

Bulletin de la Societe Mineralogique de France, 3:153-

155.

Moore, C. B., C. F. Lewis, and D. Nava

1969. Superior Analyses of Iron Meteorites. Pages 738-748

in P. M. Millman, editor, Meteorite Research. 940

pages. Dordrecht: D. Reidel Publishing Company.

Olsen, E., and K. Fredriksson

1966. Phosphates in Iron and Pallasite Meteorites. Geochimica et Cosmochimica Ada, 30:459-470.

Olsen, E., and L. H. Fuchs

1967. The State of Oxidation of Some Iron Meteorites.

Icarus, 6:242-253.

Osmond, F., and G. Cartaud

1904. Sur les fers meteoriques. Revue de Metallurgie (Paris),

1:69-79.

Owen, E. A.

1938. Formation of Widmanstatten Figures in Meteorites.

Nature, 142:999-1000.

Owen, E. A., and Y. H. Liu

1949. Further X-Ray Study of the Equilibrium Diagram of

the Iron-Nickel System. Journal of the Iron and Steel

Institute, 163:132-137.

Owen, E. A., and A. H. Sully

1939. The Equilibrium Diagram of Iron-Nickel Alloys. Philosophical Magazine, 7th series, 27:614-636.

Palache, C, H. Berman, and C. Frondel

1944. The System of Mineralogy. 7th edition, volume I, 834

pages. New York: John Wiley & Sons, Inc.

Paneth, F. A.

1960. The Discovery and Earliest Reproduction of the Widmanstatten Figures. Geochimica et Cosmochimica Ada,

18:176-182.

Pauly, Hans

1969. White Cast Iron with Cohenite, Schreibersite, and

Sulfides from Tertiary Basalts on Disko, Greenland.

Bulletin of the Geological Society of Denmark, 19:8-26.

Perry, Stuart H.

1944. The Metallography of Meteoric Iron. United States

National Museum Bulletin, 184: 206 pages.

Phillips, J. P.

1965. J. Lawrence Smith, Pioneer of American Analytical

Chemistry. Journal of Chemical Education, 42:390-391.

Powell, Benjamin N.

1971. Petrology and Chemistry of Mesosiderites, II: Silicate

Textures and Compositions and Metal-Silicate Relationships. Geochimica et Cosmochimica Ada, 35:5-34.

Ramdohr, Paul

1973. The Opaque Minerals in Stony Meteorites. 245 pages.

New York: Elsevier Publishing Company.

Randich, E.

1975. Non-Isothermal Diffusion Controlled Phase Growth in Ternary Systems and Its Application to Iron Meteorites. 125

pages. Ph. D. dissentation, Lehigh University.

Randich, E., and J. I. Goldstein

1975. Non-Isothermal Finite Diffusion-Controlled Growth

in Ternary Systems. Metallurgical Transactions,

6A: 1553-1560.

Reed.S.J.B.

1965a. Electron-Probe Microanalysis of Schreibersite and

Rhabdite in Iron Meteorites. Geochimica et Cosmochimica Ada, 29:513-534.

1965b. Electron-Probe Microanalysis of the Metallic Phases

in Iron Meteorites. Geochimica et Cosmochimica Ada,

29:535-549.

1967. The Distribution of Phosphorus in the Mount Edith

Octahedrite. Geochimica et Cosmochimica Acta, 31:1969-

1974.

1969. Phosphorus in Meteoritic Nickel-Iron. Pages 749-762

in P. M. Millman, editor, Meteorite Research. 940

pages. Dordrecht: D. Reidel Publishing Company.

1972. The Oktibbeha County Iron Meteorite. Mineralogical

Magazine, 38:623-626.

Rose, Gustav

1865. Systematische Eintheilung der Meteoriten. Annalen

der Physik und Chemie ,124:193-213.

Schreibers, C. Von

1820. Beytrdge zur Geschichte und Kenntniss meteorischer Stein

und Metallmassen, und der Erscheinungen, welche der en

Neiderfall zu begleiten pflegen (als Nachtrag zu Herrn

Chladni's neuestem Werke iiber Feuer-Meteore). 97 pages.

Vienna: J. G. Heubner.

Scott, Edward R. D.

1971a. Studies of the Structure and Composition of Iron Meteorites.

179 pages. Ph. D. dissertation, University of Cambridge, Cambridge, England.

1971b. New Carbide, (Fe,Ni)23C6, Found in Iron Meteorites.

Nature Physical Science, 229:61-62.

Scott, E.R.D., and J. T. Wasson

1975. Classification and Properties of Iron Meteorites. Reviews of Geophysics and Space Physics, 13:527-546.

1976. Chemical Classification of Iron Meteorites, VIII:

Groups IC, HE, IIIF, and 97 Other Irons. Geochimica

et Cosmochimica Acta, 40:103-115.

Shepard, Charles Upham

1846. Report on Meteorites. American Journal of Science,

series 2, 2:377-392.

1853. Notice of the Meteoric Iron Found Near Seneca

River, Cayuga County, N.Y. American Journal of Science, series 2, 15:363-366.

Short, J. M., and C. A. Anderson

1965. Electron Microprobe Analyses of the Widmanstatten

Structure of Nine Iron Meteorites.yowrraa/ of Geophys-
80 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

icalResearch, 70:3745-3759.

Short, J. M., and J. I. Goldstein

1967. Rapid Method for Determining Cooling Rates of

Iron and Stony-iron Meteorites. Science, 156:59-61.

Silliman, Benjamin

1886. Memoir of John Lawrence Smith 1818-1883. National

Academy of Science Biographical Memoirs, 2:217-248.

Smith, Cyril Stanley

1960. A History of Metallography. 293 pages. Chicago: University of Chicago Press.

1962. Note on the History of the Widmanstatten Structure.

Geochimica et Cosmochimica Ada, 26:971-972.

Smith, J. Lawrence

1855. Meteoritic Iron from Tazewell County, East Tennessee. American Journal of Science, series 2, 19:153-159.

Smith, S.W.J., and J. Young

1938. Formation of Widmanstatten Figures in Meteorites.

Nature, 142:1162.

1939. The Widmanstatten Structure of Octahedral Meteoric Iron. Nature, 143:384-385.

Sorby, H. C.

1877. On the Structure and Origin of Meteorites. Nature,

15:495-498.

1887. On the Microscopical Structure of Iron and Steel.

Journal of the Iron and Steel Institute, 1:255-288.

Spencer, L.J.

1916. Crystals of Iron Phosphide (Rhabdite) from a Blastfurnace. Mineralogical Magazine, 17:340-343.

Stodart, J., and M. Faraday

1820. Experiments on the Alloys of Steel, Made with a

View to Its Improvement. Quarterly Journal of Science,

9(18):319-330.

Swalin, R. A.

1972. Thermodynamics of Solids. 2nd edition, 387 pages. New

York: John Wiley and Sons.

Thomson, G.

1804. On the Malleable Iron, etc: Essai sur le fer malleable

trouve en Siberie par le Prof. Pallas. Bibliotheque

Britannique, 27(2, October 1804): 135-154, plate opposite page 184; (3, November 1804): 209-229.

1808. Sur ferro malleabile trovato da Pallas in Siberie. Atti

Accademia delle Scienze di Siena, 9:37-57.

Vogel, Rudolf

1927. Uber die Strukturformen des Meteoreisens und Ihre

Spezielle Beeinflussung durch Umwendlung und Beigemengten Phosphor. Abhandlunger der Gesellschaft

der Wissenschaften zu Gottingen, Mathematisch-physikalischeKlasse, 12(2): 51 pages.

1928. Ueber die Strukturformen des Meteoreisen. Archiv

fur das Eisenhuttenwesen, 1:605-611.

1932. Eine umfassendere Deutung der Gefugeerscheinungen des Meteoreisens durch das Zustandsdiagramm

des ternaren Systems Eisen-Nickel-Phosphor. Abhandlungen der Gesellschaft der Wissenschaften zu Gottingen, Mathematisch-physikalische Klasse, 3(6): 31 pages.

1951. Die Gefiigeformen des Meteoreisen und ihre Erklarung auf Grund des Zustandsdiagrammes des

Systems Eisen-Nickel-Phosphor. Neues Jahrbuch fur

Mineralogie, Abhandlungun, 83:23-52.

1952. Uber Rhabdit und Schreibersite. Neues Jahrbuch fur

Mineralogie, Abhandlungen, 84:327-349.

1957. Uber Metabolite und ihre Kennzeichnung durch das

Gefuge. Chemie derErde, 19:147-169.

1964. Uber die Entstehung des Wickelkamazit. Neues Jahrbuch fur Mineralogie, Monatshefte, 2:57-63.

Vogel, R., and H. Baur

1931. Ueber das ternare System Eisen-Nickel-Phosphor.

Archiv fur das Eisenhuttenwesen, 5:269-278.

von Reichenbach, C.

1861. Ueber die nahern Bestandtheile des Meteoreisens.

Annalen der Physik und Chemie, 114:477-491.

Wai, C. M.

1974. Geochemical Affinities of Cobalt and Germanium

toward Metal, Silicate, and Sulfide Phases at High

Temperature. Geochimica et Cosmochimica Ada,

38:1821-1825.

Wasson, J. T.

1974. Meteorites. 316 pages. New York: Springer-Verlag.

Waterston, Charles D.

1965. William Thomson (1761-1806) a Forgotten Benefactor. University of Edinburgh Journal (autumn 1965),

pages 122-134.

Wood, John A.

1964. The Cooling Rates and Parent Planets of Several

Iron Meteorites.Icarus, 3:429-459.

Young, J.

1926. The Crystal Structure of Meteoric Iron as Determined by X-Ray Analysis. Proceedings of the Royal

Society (London), Al 12:630-641.

Ziebold, Thomas O.

1967. Precision and Sensitivity in Electron Microprobe

Analysis. Analytical Chemistry, 39:858-861.