The Disintegration of the

Wolf Creek Meteorite and the

Formation of Pecoraite,

the Nickel Analog of

Clinochrysotile

GEOLOGICAL SURVEY PROFESSIONAL PAPER 384-C
1 mm

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Pecoraite, Wolf Creek meteorite of Australia. Upper, Pecoraite grains separated by hand picking from crack fillings in the meteorite 

(X 25). Lower, Pecoraite grains lining the walls of some cracks in the meteorite. The extent of disintegration of the original metallic phases into new phases, chiefly goethite and maghemite, is apparent (X 2.3).
THE DISINTEGRATION OF THE

WOLF CREEK METEORITE AND THE

FORMATION OF PECORAITE,

THE NICKEL ANALOG OF

CLINOCHRYSOTILE
The Disintegration of the 

Wolf Creek Meteorite and the 

Formation of Pecoraite, 

the Nickel Analog of 

Clinochrysotile

By GEORGE T. FAUST, JOSEPH J. FAHEY, BRIAN H. MASON and 

EDWARD J. DWORNIK

STUDIES OF THE NATURAL PHASES IN THE 

SYSTEM MgO-SiO2 -H2O AND THE SYSTEMS 

CONTAINING THE CONGENERS OF MAGNESIUM

GEOLOGICAL SURVEY PROFESSIONAL PAPER 384-C

Origin of pecoraite elucidated through its 

properties and in terms of the geochemical 

balance in its desert environment

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1973
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress catalog-card No. 73-600160

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402 - Price $1.30

Stock Number 2401-02392
CONTENTS

Page

Abstract- - - - 107

Introduction 107

Acknowledgments 107

Pecoraite 107

Preparation of sample 107

Physical and optical properties 108

Crystallographic studies 108

Electron-microscopic examination 108

X-ray powder diffraction studies 108

Geometrical relations of the curved sheets of pecoraite to the cylindrical structure of clinochry sotile 112

Calculation of tube radii 112

Interpretation of diffraction effects 112

Interpretation of tube morphology 116

Chemical properties - i ig

Pyrognostics i ig

Chemical analysis and composition 117

Microprobe analysis us

Studies on pecoraite and related synthetic phases in the

system H2O-NiO-SiO2 - -- -- 118

Page

Thermal-analysis studies 1 19

Static (dehydration) method 119

Dynamic (differential-thermal-analysis) method - 119 

Disintegration of the Wolf Creek meteorite and the formation of pecoraite 119

Time, temperature, and pressure relations of the natural system - 1 19

Behavior of a model meteorite body in a desert region- 121 

Distribution of temperature and thermal stress- 121 

Stress corrosion (corrosion fatigue) 123

Natural process of disintegration - - - 125

Composition and geochemical balance 125

Stress relations resulting from impact 125

Physical and chemical characteristics and relaSuggested path of the oxidation process 129

Interpretation of the disintegration 131

Nomenclature 132

Literature cited ~ - - 1 33

ILLUSTRATIONS

FRONTISPIECE. Pecoraite from the Wolf Creek meteorite of Australia.

Page

FIGURES 47, 48. Electron micrograph of pecoraite, showing morphology 109, 110

49, 50. Electron micrographs of the synthetic clinochrysotile of Bowen and Tuttle (1949)- 114, 115

51. Thermal-analysis curve of pecoraite obtained by the static method 119

52. Differential-thermal-analysis curves of a partly purified sample of pecoraite from the Wolf Creek meteoritp of Australia 120

53. Graph showing daily and nightly temperature extremes of the air above the soil and of the soil in

54. Photograph showing crack development in the Wolf Creek meteorite 127

TABLES

TABLE X-ray powder diffraction data for pecoraite obtained from powder photographs 111

Calculated values of the thickness of the curved plates of pecoraite, based on measurements derived from powder

Dfittpirm_______________________________________ -_ - * 11 j.

Calculated values of the thickness of the curved plates of pecoraite, based on half width measurements of line

breadth derived from X-ray powder diffractometer patterns 111

X-ray powder diffraction data for synthetic nickel serpentine group minerals - - 112

Half-width measurements of line breadths for clinochrysotile synthesized by Bowen and Tuttle (1949) and for

clinochrysotile from the Currant Creek magnesite deposit, White Pine County, Nev. - 113

Fiber dimensions and wall thickness of chrysotile, synthetic chrysotile, and synthetic pecoraite derived from

X-ray studies and electron microscopy ~ - - - - 117

Chemical analysis of pecoraite from the Wolf Creek meteorite, Wolf Creek, Australia 118

V
VI CONTENTS

Page

TABLE 8. Chemical analysis of some synthetic products related to or containing pecoraite » » - «- » - -- 118

9. Thermal-analysis data obtained by the static method on pecoraite from the Wolf Creek meteorite, Western Australia__________________________________________________________________ 120

10. Calculated temperature and thermal stress in model meteorite bodies in the shape of a sphere, a cylinder, and

a slab___________________________________________________________ 124

11. Distribution of the elements in the disintegration products of the Wolf Creek meteorite and in the source materials a study of geochemical balance - .. . 126
STUDIES OF THE NATURAL PHASES IN THE SYSTEM MgO-SiO 2 -H2O AND 

THE SYSTEMS CONTAINING THE CONGENERS OF MAGNESIUM

THE DISINTEGRATION OF THE WOLF CREEK METEORITE

AND THE FORMATION OF PECORAITE, THE NICKEL

ANALOG OF CLINOCHRYSOTILE

By GEORGE T. FAUST, JOSEPH J. FAHEY, BRIAN H. MASON, 1 and EDWARD J. DWORNIK

ABSTRACT

The Wolf Creek meteorite fell in Western Australia (lat 19°11' S., 

long 127°48' E.) during Pliocene or, at the latest, Pliocene-Pleistocene time and produced an impact crater. Original metallic compounds in the meteor became mechanically unstable in the desert 

environment because of the constant alternation in thermal conditions over a long period of time. The meteorite was heated by the 

sun to over 100°C (212°F) in the daytime, and its temperature 

dropped to about 15°C (59.0°F) at night. This cycle ultimately 

reduced the mechanical strength of parts of the meteorite and 

caused it to rupture and form thin cracks. During the rainy season, 

moisture entered the cracks, and stress corrosion became 

operative, which further reduced the mechanical strength of the 

meteorite and brought about chemical corrosion. The moisture 

also widened the cracks. Sand particles blown by the wind entered 

the cracks, where they were crushed during the cooling cycle. 

Repeated dropping and crushing of the sand further widened the 

cracks. During the heating cycle, these cracks, which contained 

finely crushed silica, nickel ions in solution, and other inactive 

solid phases, behaved like tiny hydrothermal bombs and introduced i their load of nickel ions, by overflow, into the 

hydrothermal solutions in larger cracks, where pecoraite, 

Ni 6Si4O10 (OH) 8 , was formed in very small amounts over long 

periods of time.

The phase pecoraite was described by Faust, Fahey, Mason, and 

Dwornik in 1969, and a detailed description of it is presented here 

to document the species and to provide background for the investigation of the process of disintegration of the meteorite.

INTRODUCTION

White, Henderson, and Mason (1967) studied the 

secondary minerals produced in the disintegration of 

the Wolf Creek meteorite in the Wolf Creek crater in 

Western Australia (lat 19° 11' S., long 127°48' E.) and 

found a green nickel mineral, tentatively identified 

as belonging to the serpentine group, occurring as a 

filling in cracks transecting the meteorite. This green 

phase, the nickel analog of clinochrysotile, was 

briefly described by Faust, Fahey, Mason, and 

Dwornik (1969) and was named "pecoraite"2 (PE 

KAWR'E AIT).

1U.S. National Museum of Natural History, Smithsonian Institution, Washington, 

B.C. 20560.

It is the purpose of this paper first to document, in 

detail, the mineral pecoraite and to add some further 

data on the Wolf Creek meteorite; secondly, to contribute to the knowledge of the natural phases in the 

system H2O-MgO-NiO-SiQ2 ; and thirdly, to present 

a coherent theory for the probable course of the disintegration of the Wolf Creek meteorite in its crater 

and for the development of the phase assemblage 

now observed.

ACKNOWLEDGMENTS

We thank David B. Stewart, J. Stephen Huebner, 

and Mary E. Woodruff, all of the U.S. Geological 

Survey, for preparing the diffractometer traces, and 

we thank John W. White, of the U.S. National 

Museum, for an X-ray pattern and other aid. Michael 

Fleischer translated parts of Russian scientific 

papers and offered advice.

Discussions with our colleagues Paul B. Barton, 

Jr., John L. Haas, Malcolm Ross, and Motoaki Sato 

are greatly appreciated.

PECORAITE

PREPARATION OF SAMPLE

The mineral pecoraite occurs in an aggregate of 

medium- to fine-grained particles in cracks in the 

Wolf Creek meteorite. Pecoraite is present in this 

aggregate as grains approximately 0.1-0.5 mm 

(millimeter) in diameter. All the phases in the 

aggregate are intergrown or are coated by associated 

phases so that preliminary separations were very difficult. Extensive handpicking was required to prepare a pure sample.

The material in the cracks is composed chiefly of 

intergrown maghemite and goethite with minor 

quantities of adventitious quartz sand and clay-size 

material and a very small amount of the green nickel

2Approved by the International Mineralogical Association Commission on New 

Minerals and Mineral Names. . 
108 STUDIES OF THE NATURAL PHASES IN THE SYSTEM MgO-SiO2-H2O

silicate, pecoraite, About 200 g (grams^ of this 

material was scraped from the cracks and crushed to 

pass through a 20-mesh sieve. The amount of 

pecoraite in this sample was visually estimated to be 

1 percent.

Most of the maghemite and intergrown goethite 

was removed by means of an Alnico magnet. These 

minerals made up more than 95 percent of the 

material taken from the cracks in the meteorite. The 

residue at this point consisted of about 25 percent 

pecoraite and an assemblage of adventitious quartz 

sand, clay-size material, goethite, cassidyite, reevesite, and a very small amount of silicon carbide introduced during slicing of the meteorite. The Franz 

Isodynamic Separator did very little to purify the 

sample further, but a heavy liquid (bromoform and 

acetone) was used which increased to about 50 percent the amount of pecoraite present.

The sample was then repeatedly treated with 

acetone and, after decantation, allowed to remain for 

about 10 days at room temperature in an uncovered 

beaker. From this material a sample that weighed 

160 mg (milligrams) and was better than 99.9 percent 

pure pecoraite was obtained by handpicking under a 

binocular microscope.

PHYSICAL AND OPTICAL PROPERTIES

Pecoraite has a hardness of 2l/z. The compact 

masses of pecoraite are brittle and have a conchoidal 

fracture. The color of pecoraite is "Oriental Green" as 

defined and illustrated on Plate XVIII of "Color Standards and Color Nomenclature," by Robert Ridgway 

(1912). Its luster is vitreous to waxy. Photographs of 

the grains and their occurrence in cracks are shown 

in the frontispiece.

The specific gravity of 3.084, as determined by the 

method of Fahey (1961), is in poor agreement with the 

calculated value 3.514 based on Brindley's (1961) 

values for the unit cell of "Ni serpentine." Our low 

value may be due to the large amount (4.1 percent) of 

adsorbed water and too many tiny liquid and gas inclusions. Our value is in better agreement with the 

value of 3.18 determined by the pycnometric method 

on synthetic percoraite by Noll, Kircher, and Sybertz 

(1958).

The mean index of refraction, n, measured on 

many grains ranged from 1.565 to 1.603. This range is 

probably due to variations in the adsorbed water and, 

in a very much lesser part, to variations in NiO, FeO, 

and MgO among the grains. Grains of pecoraite are 

weakly doubly refracting. On some grains, wavy extinctions were observed under crossed polarizers. 

Other grains showed no extinction when the stage of 

the microscope was rotated, indicating an

anomalous condition probably related to the orientation of the submicroscopic particles that constitute 

the grains and which render them isotropic through 

optical compensation. To determine the effect of the 

adsorbed water on the index of refraction of the 

grains, two experiments were made. In the first, the 

grains of pecoraite were placed in a porcelain dish 

and set in an oven at 148°C for 16 days. They were 

then removed from the oven and allowed to cool in a 

dessicator. These dried grains were placed in 

quinoline (n- 1.6245) and allowed to soak for a day or 

more. Most grains then had a mean index of refraction greater than 1.625. In the second experiment, 

pecoraite was dried over the dessicant, anhydrous 

magnesium perchlorate, for 32 days and then examined in an immersion liquid having an index of refraction of 1.650. Most grains then nearly matched 

the index of refraction of the liquid. This mean value 

of 7i=1.650 ± is in good agreement with the value of 

7i=1.654 measured by Noll, Kircher, and Sybertz 

(1960) for synthetic pecoraite. It also agrees well with 

the value of 7i=1.655 calculated with the GladstoneDale relation using a specific gravity of 3.18 as 

measured on synthetic pecoraite by Noll, Kircher, 

and Sybertz (1958).

CRYSTALLOGRAPHIC STUDIES

/ELECTRON-MICROSCOPIC EXAMINATION

Part of the carefully purified sample prepared for 

the chemical analysis was examined under the electron microscope. The electron micrographs (figs. 

47, 48) disclose the morphology of pecoraite to be 

curved plates which are in various degrees of curling. 

Some are curved to form incomplete coils, whereas 

others have curved to form one or more complete 

turns and have thus generated spirals. When the 

spirals stand on end, as in the aggregates or clusters, 

they appear to be circular. The diameter of the spirals 

is approximately 250 A, and the width of the individual coil is approximately 70 A.

A search of several fields under the electron microscope did not disclose any spirals with enough complete revolutions to form long coils such as those 

observed for lipids by Glauert and Lucy (1969a,b). 

The spirals shown in figures 47 and 48 formed only 

three, four, or five complete revolutions about the 

spiral axis.

X-RAY POWDER DII I-RAC. I ION Ml'DIES

X-ray powder diffraction patterns were made of 

pecoraite using both the camera and film and the diffractometer techniques. More reflections were recorded on the films than on the diffractometer 

patterns. The diffractometer patterns, however, gave 

information on the character of the peaks that was
WOLF CREEK METEORITE AND THE FORMATION OF PECORAITE 109

FIGURE 47. Electron micrograph of pecoraite, showing morphology of curved plates in various degrees of coiling; A, 

incomplete coils; B, spirals standing on end (supported in cluster groups). About X 180,000.

not discernible from the lines recorded on the films. 

The films yielded a sufficient number of reflections to 

identify the phase.

Specimens of the ground pecoraite were tamped 

into specially drawn capillary tubes of 0.2 mm 

diameter. X-ray powder photographs and diffractometer traces were obtained for both the natural dry 

sample and the ethylene glycol permeated pecoraite. 

The X-ray powder diffraction pattern of pecoraite 

consists of a series of rather broad lines, probably 

resulting from the fine grain size of the material 

(Scherrer, 1918). The broadening of the lines in-
110 STUDIES OF THE NATURAL PHASES IN THE SYSTEM MgO-SiO2-H2O

FIGURE 48. Electron micrograph of pecoraite, showing, in detail, the incompletely curved plates (A). About X 960,000.

troduces uncertainty into the measurement of the 

position of the lines and, consequently, of the dspacings.

Ross (1968) has also shown that for thin plates, 

generally, the first-order OOZ reflection shifts to

smaller values of 26 and the apparent d 001 increases. 

Higher order reflections may shift to either smaller 

or larger values of 26 depending on the nature of the 

Fourier transform of the single layer. Line 

broadening also results from order-disorder 

phenomena.

Three X-ray powder diffraction patterns of 

pecoraite were measured, and the results are given in 

table 1. From these data of the line-breadth, 

calculations were made to determine particle size by 

means of the von Laue (1926) and Scherrer (1918) 

relation as modified by Wilson (1949) and Stokes and 

Wilson (1942, 1944).

The von Laue and Scherrer relation is

y2=-

t cos 6

where

&V2 = measured line breadth at half intensity 

(given in radians 26) (on powder photographs the eye estimates a value equal 

to approximately one-half width because 

of its inability to discriminate between 

background and line boundaries); 

A. = wavelength of radiation, in angstroms; 

t = a linear dimension of the particle, in angstroms;

6 - Bragg angle; and

K - the Scherrer constant, commonly about 

0.90 but varying a few percent with particle shape.

Electron-microscope study of part of the sample of 

pecoraite used in the chemical analysis suggested 

that some of the particles were curved plates. Using 

the von Laue and Scherrer relation, the thickness of 

the plates of pecoraite was calculated from the X-ray 

powder diffraction patterns (table 2). The average 

thickness of the curved plates of pecoraite calculated 

from these data is 74.1 A. Assuming, for comparison, 

the thickness of a single layer3 of clinochrysotile to be 

7.3 A and dividing this into 74.1 A gives 10.1 as the 

probable number of layers in the pecoraite plates.

Similar data were obtained from diffractometer 

traces of pecoraite taken with copper radiation and 

using a nickel filter. These patterns exhibited a small 

number of broad rounded peaks having subdued 

peak heights. Such diffraction effects arise from the 

fine particle size, which causes a broadening of the reflections.

Using the von Laue (1926) and Scherrer (1918) 

relation, as before, the data for the particle thickness, 

t, of pecoraite curved plates as determined from the 

half-width line-breadth measurements of the dif3Whittaker (1957), for computation, used the halved value of c (7.3 A) for a single layer.
WOLF CREEK METEORITE AND THE FORMATION OF PECORAITE 111

TABLE 1. X-ray powder diffraction data for pecoraite obtained from powder photographs

[Copper radiation and nickel filter,A= 1.54050 A; films uncorrected for shrinkage; camera diameter, 114.59 mm. The intensity, I, based 

on the V2 scale, was visually estimated and converted to a linear scale. The line breadth is the average of 6 measurements and 

is given as the difference 2G"-26' at approximately '^ width. Db, diffuse band; P, present, but too diffuse to measure; D, diffuse; 

B, broad]

hkV

002 

020 

004 

130 

202 

202 

204 

060 

402

Film 11006- 

U.S. Geological Survey

rf obs 

(angstroms)

7.43 

4.50 

3.66 

2.620

2.447 

P 

1.529 

1.303

I

100 

35 

50 

35 

Db 

25 

D 

50 

Db

Line 

breadth 

(radians)

0.01919 

.02094 

.02269 

.02792

.02443 

B 

.02269 

B

Film 3671 

U.S. National Museum

rf obs 

(angstroms)

7.43 

4.49 

3.63 

2.63

2.447 

2.106 

1.524 

1.300

I

100 

17 

36 

25 

Db 

17 

13 

25 

Db

Line 

breadth 

(radians)

0.01919 

.02094 

.02094 

.01745

.01745 

B 

.01745 

B

Film 10972* 

U.S. Geological Survey

rf obs 

(angstroms)

7.43 

4.49 

3.65 

2.627 

2.554 

2.443 

2.116 

1.527 

1.305

I

100 

35 

50 

50 

Db 

35 

13 

50 

Db

Line 

breadth 

(radians)

0.02094 

.01745 

.02443 

.01745

.02094 

B 

.02269 

B

'The indexing is probable and is based on analogy to the indexing of Whittaker and Zussman (1956). 

2Obtained with a special low-angle slit assembly.

'The accidental recording of a "brass line" on film 10972 produced a sharp, narrow line in contrast to the lines of pecoraite and 

served both to emphasize the difference in the diffracting characteristics of the two substances and as a check on the calibration.

fractometer traces are given in table 3. For peaks 

with asymmetrical bases the geometric method of 

Halla and Mark (1937, fig. 158, p. 318) was used to obtain 1/2. The average value of t from the two 

calibrated measurements is 71 A which, when 

divided by 7.3 A (the thickness of a single layer), 

yields 9.8 multiples of the "clinochrysotile layers." 

This is in good agreement with the data from the Xray powder diffraction pattern in table 2. In these 

calculations no consideration was given to the 

broadening of the lines owing to order-disorder 

phenomena which are believed to be of lesser importance.

TABLE 2. Calculated values of the thickness of the curved plates 

of pecoraite, based on measurements derived from powder 

patterns

[Copper radiation and nickel filter;\=l.54050 A]

Film

11006

11006

3671

3671

10972

10972

hkl

002

004

002

004

002

004

breadth 

(radians)

0.01919

.02269

.01919

.02094

.02094

.02443

26 uncorrected 

(degrees)

11.90

24.30

11.90

24.50

11.90

24.35

6 uncorrected 

(degrees)

5.95

12.15

5.95

12.25

5.95

12.17

Cos6

0.99461

.97759

.99461

.97723

.99461

.97750

Thickness

of curved 

plates 

(angstroms)

80.7

69.5

80.7

75.3

74.0

64.5

The data in table 1 show that d002 for pecoraite is 

7.43 A, and in table 3 the values range up to 7.46 A. 

These values are much larger than c?0o2 for clinochrysotile, which is 7.36 A. The substitution of nickel 

for magnesium would not be expected to cause this 

difference, for the difference between the ionic radii 

of nickel (0.69 A) and magnesium (0.66 A) is not large. 

The explanation must be related to the small size and 

thickness of the spirals of pecoraite. Malcolm Ross 

(written commun., 1969) calculated the shift in

position of the 002 reflection (c? 002=7.16 A) of dickite 

for platelets of various thickness, and he found that 

for a particle 10 platelets thick, the shift to a larger 

spacing amounts to 0.1 A; for a particle five platelets 

thick, the shift is an increase of 0.4 A. The wall 

thickness of pecoraite is approximately 73 A, or about 

10 layers, so the shift from 7.36 A to 7.43-7.46 A is consistent with Ross' calculation.

I ABl.F 3. Calculated values of the thickness of the curved plates 

of pecoraite, based on half-width measurements of line breadth 

derived from X-ray powder diffractometer patterns

[Copper radiation and nickel filter;A= 1.54050 A]

Record hkl

WC-2-1 002

WC-2-2 002

WC-2-3 002

WC-2-4 002

rfgj^A 20"uncorrected 

(degrees)

'7.46 12.22

'7.43 12.40

(2 ) 12.39

( 2 ) 12.18

26'uncorrected 

(degrees)

11.02

11.08

10.98

10.90

(radians)

0.02094

.02304

.02461

.02234

Thickness of 

curved plates 

(angstroms)

74

67

63

69

'Calibrated with quartz.

 Uncalibrated, poor pattern with broad peaks.

After examining the data obtained in this study, 

Malcolm Ross (written commun., 1969) submitted the 

following two observations:

1. Using the data for c?uu2 of record WC-2-1 in table 3 and the 

relationship of Ross (1968),

0.90X

t

and setting

I calculated a thickness of 67 A. This calculation serves as an 

independent check on the results given in this paper.

2. It is of interest to note that the shift of the d u04 spacing for 8- to 

10-layer models predicted on the basis of Ross'(1968) calculation 

for dickite is +0.004 A; thus, the d 004 value of pecoraite will give the 

essentially true spacing if the dickite model is valid for the 

pecoraite structure, as I think it is. The doubled values of the d004

508-391 O - 74 - 2
112 STUDIES OF THE NATURAL PHASES IN THE SYSTEM MgO-SiO2-H2O

spacing from table 1 are 7.32, 7.26, and 7.30, suggesting a true d 002 

of 7.29 A. The average d002 is 7.43 A a difference of 0.14 A. Such a 

shift suggests an eight-layer model. The average data in table 3 

suggest a nine-layer model. The agreement may be fortuitous, but 

there is agreement between two completely unrelated calculations

The X-ray diffraction powder data (table 4) for 

synthetic nickel chrysotile and cobalt chrysotile were 

measured by Feitknecht and Berger (1942) and 

recalculated by Brindley (1961). Roy and Roy (1954) 

also prepared synthetic nickel serpentine and 

recorded the X-ray powder diffraction data (table 4). 

These data are similar to those for natural 

pecoraite (table 1) but represent better crystallized 

phases. Longuet (1947) gave X-ray powder data for a 

nickel serpentine which she called nickel antigorite, 

and these data are in table 4.

To be certain that there was no admixture with a 

phase having a spacing larger than 7.5 A, runs were 

made, using both powder cameras and the diffractometer, with samples treated with ethylene glycol. 

Observations were also made with a camera having 

special slits designed to record low-angle spacings. 

The results of all these tests failed to disclose any additional phases.

TABLE 4. X-ray powder diffraction data for synthetic nickel 

serpentine group minerals

[I, intensity]

Feitknecht and Berger (1942)

hkl1

001

020

._.

002

200

201

202

203

060

061

260, 063

d1 (angstroms)

7.28

4.62

3.65

2.69

2.506

2.163

1.778

1.542

1.500

1.310

I

10

2

-

10

6

10

6

1

10

2

6

Roy and Roy (1954)

rf 1 (angstroms)

7.26

4.2

3.629

2.50

2.14

1.52

I

10

M;

8

7

4

4

Longuet (1947)

d (angstroms)

7.35

4.52

3.58'

2.65

2.51

2.16

1.72

1.52

1.30

'Data recalculated by Brindley (1961) on the basis of orthogonal cells.

GEOMETRICAL RELATIONS OF THE CURVED SHEETS OF

PECORAITE TO THE CYLINDRICAL STRUCTURE OF

CLINOCHRYSOTILE

To interpret the geometrical relations of the curved 

plateg, coils, and spirals of pecoraite to the cylindrical structure of clinochrysotile, it was necessary to 

consider the structural makeup of the tubes of clinochrysotile and of the synthetic pecoraite. This was 

done by (a) calculating the tube radii (and diameters) 

of two clinochrysotiles which gave X-ray powder diffraction patterns almost identical with the pattern of 

pecoraite, (b) interpreting the diffraction effects of 

pecoraite and clinochrysotile, and (c) gathering the 

data in the literature on tube morphology and interpreting all these data in terms of the theoretical 

model of Whittaker (1957).

CALCULATION OF TUBE RADII

The clinochrysotile (sample F-41) from Currant 

Creek, Nev., and the synthetic clinochrysotile (sample F-54), No. T-2-29C of Bowen and Tuttle (1949), 

gave X-ray powder diffraction patterns which resemble most closely the pattern of pecoraite. They both 

show tubular structure in the electron micrographs 

(Edward J. Dwornik and George T. Faust, unpub. 

studies; Bowen and Tuttle, 1949).

The tube radii (and diameters) of these clinochrysotiles were calculated using the modification of the 

von Laue and Scherrer relation by Wilson (1949) in 

which the quantity e is defined as the apparent 

particle size. For long thin cylinders the equation for 

cequals (16/877) r esc <£, in which r is the radius of the 

cylinder, <£ is the angle between the normal to the 

reflecting planes and the axis of the cylinder, and b is 

the measured breadth. Thus the complete relation 

becomes:

b= * .

 r esc (b cos 6

37T

To evaluate the term cosecant (/>, the stacking 

arrangement of the unit cells of clinochrysotile 

around the cylinder must be known. This arrangement was determined by Whittaker (1956) and summarized by Brindley (1961, p. 90) as follows, using the 

convention that the a parameter (a=5.34 A) corresponds to the fiber axis and that the unit cell contains 

two layers: "With these parameters, a is parallel to 

the fiber length, and if its structure is tabular, then 

the b axis follows the circumference of the tube, and c 

is radial but inclined to the tube axis at the angle /?." 

For clinochrysotile, 0=93°16'.

X-ray data for only the QQl reflexions were examined; for these, 4>=90° and csc<£=l, so the above 

equation reduces to

\

16 

STT

r cos 6

Using data obtained from the diffractometer traces 

of samples F-41 and F-54, described above, which 

were made with copper radiation and a nickel filter 

(\=1.54050 A), the tube radii (and diameters) were 

computed. The results are given in table 5.

INTERPRETATION OF DIFFRACTION EFFECTS

Pecoraite and the tubular clinochrysotiles F-41 and 

F-54 have similar X-ray powder diffraction patterns, 

and they also show a similar large broadening of the 

line-breadth profiles on diffractometer traces. The 

dimensions of the coils and spirals of pecoraite have 

been described in the previous section on electron
WOLF CREEK METEORITE AND THE FORMATION OF PECORAITE 113

TABLE 5. Half-width measurements of line breadths for clinochrysotile synthesized by Bowen and Tuttle (1949) and for 

clinochrysotile from the Currant Creek magnesite deposit, White Pine County, Nev. 

[Diffractometer traces made with copper radiation using a nickel filter, A=1.54050 A]

Sample hkl

20"

un corrected 

(degrees)

20'

uncorrected 

(degrees)

/3./,=<29"-2e') 

(radians)

Radius of

cylinder

(angstroms)

Diameter of

cylinder 

(angstroms)

Synthetic clinochrysotile, 

T-2-29C (F-54):

Dn

p.

Clinochrysotile, Currant 

Creek, Nev. (F-41):

Dn

004

002

004

002

004

24.50

1 9 O8

O/1 Q8

1 Q O8

QQ QQ

11.50

oq Q.?

i M o£!

u.uuaiz 

OOQ07

.01012

OOQO7

09478

.01745

109

90

102

07

904

i on

904

74

microscopy. The electron microscopy of the synthetic 

clinochrysotile given by Bowen and Tuttle (1949) was 

also studied in more detail by us. Figure 49 shows 

that the sample consists of tubes of variable diameter ranging from 166 to 1,390 A. Moreover, in 

photographs with magnifications of X 40,000 to X 

50,000, some tubes show a variability in diameter 

that is expressed as an apparent steeply tapered stepwise increase or decrease in diameter along the tube 

length. Bates (1958) observed this same structure and 

differentiated between two types described as "telescoped cylinders" and "cone-in-cone" structures in the 

synthetic preparation of Roy and Roy (1954). In electron micrographs of synthetic clinochrysotile of 

Bowen and Tuttle (1949) with a magnification of X 

90,000, these structures are resolved into spiral 

growth layers winding around the last completed 

inner tubular layer (fig. 50). At this magnification, 

the centers of the tubes appear to be right cylinders, 

and they appear to be empty.

The middle and upper parts of the tube marked A 

(fig. 50) show a structure which may be interpreted as 

a poorly fitting spiral coil wound on a helical path on 

the outer surface of the tube with a flaring out of the 

coil at its top edge. The coiled sheet is assumed to be 

10 layer units (about 73 A) thick, and the very narrow 

ramplike surface formed by the spiraling edge of the 

sheet may be described as a helicoid. A in figure 50 is 

oriented so that the spiral appears to be descending 

along the tube and, as it is wound in a clockwise 

fashion starting at the top of the coil, it is called a 

right-handed helicoid. 4

The few helicoids in this photograph appear to be 

right-handed. The helical angle is about 9°. The

4The helicoid and related geometrical terms were defined by Schumann (1938) as 

follows: "The helicoid is a warped surface generated by a straight line moving so that it 

constantly touches a helix and its axis and makes a constant angle with that axis"(p281). "When the generatrix makes an angle of 90° with the axis the helicoid is right" (p.

282). "When the generatrix makes an angle with the axis other than 90° the helicoid is 

oblique" (p. 281). "A warped surface is one generated by a straight line moving so that no 

two consecutive positions lie in the same plane" (p. 261). [Italics added.]

structure marked B (fig. 50) appears to be a series of 

helical growth layers forming incomplete spirals. 

Their staggering about the tube is probably related to 

periodic defects on the outer tube surface upon which 

they grow. These structures resemble the "cone-incone" structures of Bates (1958), but there is no trace 

of an internal cone. Conical terminations could arise 

as defect structures. The structure marked C (fig. 50) 

shows bands growing on a central tube in telescopic 

fashion. Clinochrysotile tubes are probably limited 

in their diametral growth by the accumulation of 

defects along the edges of the spirals which curl and 

join to form tubes. When the defects become too 

numerous, the tube diameters cease to increase. 

Within short lengths of the tube, growth may still occur as oriented overgrowths, and in this manner the 

bands can form. The tapering at the edge of the bands 

is caused by the spiral growth. These structures give 

rise to abnormal fibril diameters. The radii of the 

tubes calculated from the X-ray data and shown in 

table 5 yield an average radius of 98 A, or a cylinder 

diameter of 196 A. A strict comparison of this value 

with the electron-microscope data is difficult, but it is 

obvious that the calculated values are an average for 

heterogenous size distribution. The synthetic clinochrysotile of Roy and Roy (1954) studied by Bates 

(1958) showed a similar variation, and Bates found 

inner radii of 25-50 A and outer radii of 110-450 A for 

their preparation. The studies reported here show 

that the broadening of the lines on the diffraction 

pattern of the synthetic clinochrysotile arises from 

the abundance of extremely thin tubes.

The calculated tube radii for the clinochrysotile 

from Currant Creek, Nev., based on diffractometer 

traces, yield an average radius of 45 A and a cylinder 

diameter of 90 A. The electron micrograph of this 

sample shows two components. The first type consists of tubes with an average inner diameter of 52 A 

and an average outer diameter of 207 A, which, by
114

r

STUDIES OF THE NATURAL PHASES IN THE SYSTEM MgO-SiO2-H2O

\ vr

FIGURE'49. Electron micrograph of the synthetic clinochrysotile of Bowen and Turtle (1949). Note the tubular structure of the 

clinochrysotile and the much larger percentage of small tubes than of tubes having larger diameters.
WOLF CREEK METEORITE AND THE FORMATION OF PECORAITE 115

B C

FIGURE 50. Electron micrograph of the synthetic clinochrysotile of Bowen and Tuttle (1949). A, a poorly fitting spiral coil wound on 

the middle part of the tube; B, a series of helical growth layers forming incomplete spirals; C a band structure growing on the 

central tube in telescopic fashion. The proportion of the labeled structures is small relative to the amount of very fine tubes.
116 STUDIES OF THE NATURAL PHASES IN THE SYSTEM MgO-SiO2-H2O

difference, yields a wall thickness of 78 A, or approximately 10 multiples of the layer unit. The second 

type consists of wisplike threads of extremely thin 

diameter. The radius calculated for this sample from 

line-broadening data is thus an average value for two 

different size fibers.

The similarity in the three X-ray powder patterns 

of pecoraite, the clinochrysotile from Nevada, and 

the synthetic clinochrysotile results from the 

presence in all of significant amounts of exceedingly 

fine particles. The size distribution in these three 

samples is, however, markedly different. In 

pecoraite, the size interval is small, and the coils and 

spirals are all short. In the synthetic clinochrysotile, 

even though there is a gradual change in tube diameter, the preponderant narrow tubes contribute 

greatly to the broadening of the line width on the Xray powder patterns. In the clinochrysotile from 

Nevada, there is a collection of rather closely sized 

tubes and an abundance of wisplike threads; the 

latter cause most of the broadening of the line widths.

INTERPRETATION OF TUBE MORPHOLOGY

The data in the literature on the dimensions of the 

tubular form of chrysotile are given in table 6. These 

data have been derived from both electron-microscope and X-ray diffraction studies. The earlier work 

on fibril diameters is included to show agreement in 

the order of size with the later measurements and 

with Whittaker's (1957) theoretical calculation.

Whittaker (1957) calculated the number of layers of 

clinochrysotile which would be present in the tube 

wall. He found that the most common values for the 

inner and outer radii were 55 and 130 A, respectively. 

The difference between these values is 75 A, and this 

is 10 times one-half of the c dimension of the unit cell 

(l§-7.32 A). He thus derived the multiple to be 10.

The data in table 6 were used to calculate the 

number of layer multiples (col. 7) of the clinochrysotile in the walls of the tubes. Eleven sets of data in 

dicate the multiples to be six to 13 units, and eight of 

these are 9 to 11 units. Eight sets of data have 

multiples ranging from 17 to 24 units and averaging 

20 units pf clinochrysotile.

The comparison of data on inner and outer radii of 

tubular crystals of clinochrysotile obtained from electron micrographs and from low-angle scattering experiments by X-rays was criticized by Jagodzinski 

(1961, p. 314), who felt that the data obtained from 

electron-microscope techniques was of questionable 

value because of deformation in sample preparation. 

Yada (1967) therefore reexamined a clinochrysotile 

from Canada with a high-resolution electron micro 

scope (table 6) and found little evidence for the dis 

tortion of the tubes in his method of preparation 

Yada (1967, p. 707) further found that:

the increment of the number of unit cells in the circumferential 

direction is five for each additional layer of radial direction. As the 

spacing of the observed radial fringes corresponds to the interplanar spacing of the (020) plane, the corresponding increment of 

radial fringes increases by 10 for each additional layer.

Jagodzinski (1961) found that the clinochrysotile 

from Poschiavo, Switzerland, contained 10 multiples 

of the layer unit of clinochrysotile in the tube walls, 

but using the same technique he also found multiples 

ranging from 17 to 24 for four other clinochrysotiles. 

Multiples of 20 to 23 were found by Kheiker, 

Flantsbaum, and Bubeleva (1967). Toman and Frueh 

(1968a,b) reinvestigated clinochrysotile and confirmed the earlier work of Whittaker (1957), which 

shows the number of multiples to be 10. If 20 

multiples is correct, then it may be explained by polytypism in clinochrysotile, or the larger multiple may 

arise from undetected oriented overgrowths occurring as long bands (fig. 50).

Whittaker's (1957) theoretical model for fibril diameters and for 10 multiples of the layer unit 7.3 A in 

the wall thickness is supported by the literature 

surveyed here. Yada's (1967) study also supports 

Whittaker's (1957) model. The data on wall thickness 

of pecoraite as determined in this study by electron 

microscopy (70 A) and by calculations from X-ray diffraction data (74 and 71 A) are in accord with Whittaker's (1957) model of 10 multiples of the layer unit 

in the wall. The value of 20 multiples of the unit in the 

fibril wall needs further study. After this report had 

been completed, Yada (1971) published a contribution which shows cross sections of chrysotile 

tubes having single spiral, multispiral, and concentric structures. He also measured the frequency distribution of inner and outer diameters for six 

different chrysotile specimens and observed that the 

outer diameters may vary over a wide range in some 

specimens and that this variation in fibril diameter 

may vary from sample to sample.

The discrepancy between the various sets of data 

for the fibril diameters may arise from (1) the 

presence of tubes having variable outer diameters 

(tig. 49), (2) the presence of oriented overgrowths occurring as long bands (fig. 50C), and (3) the phenomena observed by Maser, Rice, and Klug (1960) of 

chrysotile fibers consisting of tubes within tubes.

CHEMICAL PROPERTIES

PYROGNOSTICS

Pecoraite is slightly attacked by 1:1 HC1 at room 

temperature, producing a bleaching of the color and a 

decrease in the mean index of refraction. Upon heating or boiling it decomposes, leaving a siliceous 

skeleton that has the same gross form as the individual mineral particles did before treatment with
WOLF CREEK METEORITE AND THE FORMATION OF PECORAITE 117

TABLE 6. Fiber dimensions and wall thickness of chrysotile, synthetic chrysotile, and synthetic pecoraite derived from X-ray

studies and electron microscopy

[Measurements are given in angstroms, avg., average]

Natural or

synthetic

phase

, r. Inner radius Outer radius Fibril Locality or source oftubeg oftubes diameter Wall 

thickness

Approximate number

of multiples of layer

thickness (=7.3 A)

Method Observers

Chrysotile------ Thetford, Quebec, Canada- - - ------ 195

Chrysotile United States - ----- ---- 250

(harsh). 

Chrysotile ...^........._.....do................. ........ ........ 226

(soft). 

Chrysotile---- Italy----- --- ---- ---- 218

Chrysotile- - Kings, Canada- - - 100

Chrysotile----- ---(?) - - - ----- 180

Chrysotile- -- Natal Province, 90 170

Union of South Africa. 

Chrysotile - Canada 130

Chrysotile- .. 55 avg. 130 avg.

Chrysotile ----- Globe district, 25 65

Arizona. 

Chrysotile---- Synthetic of 25-50 100-450

Roy and Roy (1954). 

Chrysotile---- Synthetic------ ----- 37 120

Nickel Synthetic pecoraite -- 45 109

chrysotile. 

Chrysotile-- - Madison County, 50-88 182-220

Montana. 

Chrysotile - Globe district, \ (

Arizona. I 10-95, 57-425, ) ... 

Chrysotile----- Thetford, / mean 34 mean 125 |

Quebec, Canada. *

Chrysotile Transvaal, ('340 

1 (silky fibers). Union of South Africa. ( 2 80 

Chrysotile Poschiavo, Orisons Canton, 175 ± 5 248 ± 5

(brittle). Switzerland. 

Chrysotile Transvaal, ..... ........

Union of South Africa. 

Chrysotile-- - New York (?)-- --- - ---- 

Chrysotile - - Quebec, Canada (?) - - ........

Chrysotile---- Thetford, ----

Quebec, Canada. 

Chrysotile - Bazhenovo, S. Sverdlovsk 25 170

oblast, Russia (normal).

Chrysotile- Kiembai, U.S.S.R. -- 30 190

Chrysotile ------ Ak-Tovrak, U.S.S.R.------ 25 195

Chrysotile- - Dzhetygara, U.S.S.R.- - 35 192

Chrysotile ----- Jeffrey Mine, 25-40 75-500

Asbestos, Canada. avg. 250

Chrysotile- Average of (15) specimens: 

Quebec, Canada (9) 

British Columbia, Canada (1) 

Australia (3) 

Russia (1) 

Arizona (1)

234 (mean)

75

40

83

64

91

40

70

73

124

132

146

180

145

160

170

157

77

6

10

10

17

18

20

24

20

22

23

21

X-ray, low-angle 

scattering.

 do- 

_..... -do 

........do - 

Electron 

microscopy.

 ..-doX-ray, low-angle 

scattering.

.. do - 

Theoretical modelElectron 

microscopy. 

_...... do 

 do------

........do-- 

 .... do 

. do 

 .... do 

........ doX-ray, low-angle 

scattering. 

........do- -

........do -

 -do 

....... -do 

High-resolution

electron

microscopy.

X-rayFankucken and 

Schneider (1944). 

Do.

Do.

Do. 

O'Daniel and

Kedesdy (1S47). 

Turkevich and

Hillier (1949). 

Jagodzinski and

Kunze (1954a, b, c; 1956).

Do.

Whittaker (1957). 

Bates [1958].

Do.

Noll, Kircher, and 

Sybertz (1958). 

Do

Bates and Comer 

(1959).

Do.

Do.

Maser, Rice, and

Klug (1960). 

Jagodzinski (1961).

Do.

Do. 

Do. 

Do.

Kheiker,

Flantsbaum, and 

Bubeleva (1967). 

Do. 

Do. 

Do. 

Yada (1967).

Toman and 

Frueh (1968a, b).

'Average of outer tubes. 

^Average of inner tubes.

acid. Pecoraite yields water when heated in a closed 

tube.

CHEMICAL ANALYSIS AND COMPOSITION

The chemical analysis (table 7) was made on 0.1044 

g of concentrate. The oxides other than H2 O were 

determined on the residue which remained after the 

dehydration study. The dehydration data, when 

plotted as a thermal-analysis curve (fig. 51), failed to 

show a change in direction which would indicate that 

all the adsorbed water had been driven off at a low 

temperature. The X-ray powder pattern indicated a 

chrysotilelike structure, which was confirmed by the 

NiO-SiO2 ratio of 3:2 in the analysis.Therefore, only 

9.7 percent of the total (13.8 percent) water in the 

analysis is combined water. This adjustment gave a 

NiO-SiO2 -H2 O ratio of 3:2:2 for the nickel analog of 

chrysotile, pecoraite.

Assuming the structural formula for pecoraite to be

Ni6 Si 4010 (C)H)8

and using the equivalent weights for the oxides and 

the method of calculation given by Faust and Fahey 

(1962, p. 27, 32), the structural formula is computed 

from the analyses to be:

2-5.81 

oct

I "4.05 

tet

Theoretically, pecoraite should have 6.00 ions in octahedral coordination and 4.00 ions in tetrahedral coordination, but bearing in mind the small size of the 

sample and the low summation of the analysis, the 

agreement between theory and experiment is very 

good. The indicated discrepancy in the filling of octahedral sites is probably not real.
118 STUDIES OF THE NATURAL PHASES IN THE SYSTEM MgO-SiO2 -H2O

TABLE 7. Chemical analysis of pecoraite from the Wolf Creek 

meteorite, Wolf Creek, Australia

[Analyst, Joseph J. Fahey]

I

SiO

NiO ---------

FeO --------

MirO

A1'O< -

FLO+ (combined)- 

HiO- (adsorbed)- 

CaO - ------

MnO - -

3ercentage

51.5

.7

.5

9.7 

4.1

.0

Molecular 

weight

60.09

74.71

71 ftr^

101.96

18.02

Mole Mole 

number ratio

AflQ1? I

0124 /°-7251 2 '9=3

'.0131'

.5383 2.1=2

Theoretical 

composition

, 58.92

/

1

9.48

99.3 100.00

MICROPROBK ANAI .VSIS

Specimens of pecoraite were analyzed by means of 

a microprobe by Mason, who found SiO2~30 percent, 

NiO ^55 percent, and FeO, MgO, CaO, and A12O3 all 

equal to or less than the background approximately 

less than 0.5 percent. Considering the small chemical 

variability within the sample and the difference in 

the size of the samples used for chemical and microprobe analysis, the results are in good agreement.

The actual variation in the distribution of nickel 

and iron in individual grains of pecoraite was investigated with the electron probe by Dwornik, who 

found that nickel oxide ranged from 50.4 to 52.6 

percent, and the iron, from 0.0 to approximately 0.5 

percent. Phosphorus was not detected in the 

analyzed sample, which indicates the total absence 

of the phosphate minerals lipscombite, cassidyite, 

and apatite.

STUDIES ON PECORAITE AND RELATED

SYNTHETIC PHASES IN THE SYSTEM

H2O-NiO-SiO2

Feitknecht and Berger (1942) heated a suspension 

of carefully purified nickel hydroxide in a glass tube 

bomb at about 200°C and obtained a pale-green 

product that gave an X-ray pattern resembling that 

of a synthetic serpentine-group mineral. The X-ray 

powder data obtained on this material are given in 

table 4. The c?-spacings were computed by Brindley 

(1961) from the original measurement of Feitknecht 

and Berger. To obtain the chemical composition of 

this synthetic mineral, Feitknecht and Berger (1942) 

digested the green product, which contained unreacted nickelous oxide and possibly other materials, 

in hydrochloric acid; the analysis of the product is 

given in table 8, along with the theoretical composition of pecoraite. The chemical analysis and the 

X-ray data of Feitknecht and Berger (1942) show that 

this product is a mixture of a serpentine-group 

mineral and silica. The presence of siliceous plates

and flakes in the products of solution studies on 

phyllosilicates is now well known, and it can account 

for the discord between the chemistry and X-ray 

study of Feitknecht and Berger (1942). Longuet (1947) 

prepared a nickel serpentine, which was called nickel 

antigorite, and gave X-ray powder diffraction data in 

support of the synthesis (table 4).

Hydrous nickel silicates were also synthesized by 

van Voorthuijsen and Franzen (1950, 1951), who 

boiled solutions of nickel nitrate and alkali metasilicates to produce gels. X-ray studies showed these 

gels to yield "rather vague" patterns resembling 

those of phyllosilicates. Further heating of these gels 

at 250°C for 50 hours yielded "well-defined" hydrous 

nickel silicates. Examination of these products by Xray diffraction, electron diffraction, and thermal 

analysis showed the presence of a nickel serpentine 

group mineral and pimelite (synthetic nickel montmorillonite).

Noll and Kircher (1952) synthesized nickel chrysotile from solutions of nickel chloride, water glass, and 

sodium hydroxide held in a hydrothermal bomb for 3 

hours at 300°C at a water-vapor pressure of 87 

kg/cm2 (84 atms). The X-ray powder photographs 

showed the phase to be a chrysotile type, and the electron-microscope photographs showed the 

characteristic tubular structure of chrysotile. Comparison of the studies on this material with similar 

studies on "garnierite" from Noumea, New 

Caledonia, by Noll and Kircher (1952) demonstrated 

the morphologic identity of their product with one of 

the phases in the natural "garnierite."

Using coprecipitated gels, Roy and Roy (1954) 

synthesized phases in the system H2O-NiO-SiO2 by 

hydrothermal methods in the temperature range of 

220°C to 500°C and at pressures ranging from 5,000 

to 10,000 psi (340 to 680 atms) for periods ranging from 24 to 215 hours. They observed in the electron microscope good tubes and very fine plates. On 

the basis of their X-ray powder diffraction data (table 

4) and their electron-microscope photographs, they 

concluded that their products contained a nickel 

serpentine.

TABLE 8. Chemical analyses of some synthetic products related 

to or containing pecoraite

[Analyst, Dr. O. Steinhauser]

Pale-green product Synthetic pecoraite Synthetic pecoraite 

(Feitknecht and of Noll, Kircher, and of Noll, Kircher, and Theoretical 

Berger, 1942) Sybertz (1960) Sybertz recalculated pecoraite

omitting H^O~

SiO 2 NiO ---

HH 2 0+l ,0- J "" ""

47.5

37.3

13.7

28.8

55.3

(10.2 

j 5.2

30.5

58.7

10.8

31.60

58.92

9.48

, --- 98.5 99.5 100.0
WOLF CREEK METEORITE AND THE FORMATION OF PECORAITE 119

Noll, Kircher, and Sybertz (1958) proposed that 

chrysotile, nickel chrysotile, and "halloysite" be 

designated "solensilicates" because of their 

characteristic tubelike habit and that this be a subgroup of the phyllosilicate group of the Strunz classification (1938, 1966). They added data on tube 

diameters, tube lengths, and absorption characteristics and detailed electron-microscope studies. The 

electron-microscope photographs leave no doubt as 

to their conclusion that they synthesized nickel 

chrysotile (^synthetic pecoraite). Noll, Kircher, and 

Sybertz (1960) published further data on the nickel 

chrysotile (^synthetic pecoraite). They gave its color 

as light green, its mean index of refraction for sodium 

light as 1.654, and its chemical analysis (table 8). 

Recalculation of this analysis omitting H2O- gives a 

good agreement with the theoretical formula.

Kozyrin, Bezrukov, Gusel'nikov, and Litvin (1968) 

apparently synthesized both pecoraite and another 

phase, as suggested by their electron micrographs.

The work of Noll and Kircher (1952), Noll, Kircher, 

and Sybertz (1958), and Roy and Roy (1954) shows 

the existence of nickel chrysotile, which is the 

synthetic equivalent of pecoraite.

THERMAL-ANALYSIS STUDIES

STATIC (DEHYDRATION) METHOD

The sample of pecoraite picked for the chemical 

analysis was also used for the dehydration study. It 

was heated in a platinum crucible in a furnace 

thermostated to a given temperature until its weight 

approached constancy. It was removed from the 

furnace after a given period of time, placed in a dessicator, and then weighed. The results of these 

heatings and weighings are given in table 9 and are 

plotted to give the curve in figure 51. This curve is 

similar to the curve for clinochrysotile from Quebec, 

Canada (F-10), in Faust and Fahey (1962).

DYNAMIC (DIFFERENTIAL-THERMAL-ANALYSIS) METHOD

Differential thermal analyses were made on impure samples of pecoraite by the techniques described 

by Faust (1948,1950). The samples were heated in air 

at the rate of 10°C per minute. The records were obtained as photographs. Because of the scarcity of 

purified material, the DTA (differential-thermalanalysis) records were obtained on partly purified 

samples which contained only a small amount of 

pecoraite, significant quantities of adsorbed water, 

and a small amount of organic matter. The loss of 

this adsorbed water produced a broad, low, 

temperature endotherm (fig. 52). The trough 

representing the decomposition of pecoraite is broad 

for a serpentine-group mineral, and its shape is attributed to the fine grain size of the particles. The

200 400 600 800 

TEMPERATURE, IN °C

1000

(FIGURE 51. Thermal-analysis curve of pecoraite from the Wolf Creek meteorite, 

Western Australia, obtained by the static method.

temperature of this trough is 562°C on record C-988 

and 575°C on record C-961, which has a broader endotherm. The exothermic reaction in the temperature 

range of about 250 °C to 475° C is probably due chiefly 

to admixed maghemite and, to a lesser extent, to 

organic matter, some of which may have been introduced in the process of concentrating the 

pecoraite. The absence of goethite in these partly 

purified materials is evidenced by the lack of an endotherm in the range of 350°C to 400°C.

DISINTEGRATION OF THE

WOLF CREEK METEORITE

AND THE FORMATION OF PECORAITE

TIME, TEMPERATURE, AND PRESSURE RELATIONS 

OF THE NATURAL SYSTEM

To determine the probable origin of pecoraite, it is 

first necessary to evaluate the time, temperature, and 

pressure relations of the Wolf Creek meteorite in its 

earthly environment. Shortly after its impact with 

the earth, the fragmented meteorite was exposed to 

the agencies of disintegration the heat of the sun, 

the atmosphere, and the occasional torrential rains.

The meteorite fragments are now in sedimentary 

material, which consists of a superficial layer of sand 

1 to 2 feet thick that overlies a Tertiary laterite that

508-391 O - 74 - 3
120 STUDIES OF THE NATURAL PHASES IN THE SYSTEM MgO-SiO2 -H2O

TABLE 9. Thermal-analysis data obtained by the static method 

on pecoraite from the Wolf Creek meteorite, Western Australia

[Dehydration made on 0.1044-gram sample]

Temperature

25

77

101

110

125

150

174

205

225

250

275

300

325

350

375

400

420

450

470

500

525

550

575

600

625

650

675

725

790

900

1,000

1,080

1,100

Weight loss 

(grams, ± 0.0002)

0.0000

.0018

.0020

.0022

.0023

.0032

.0034

.0035

.0038

.0038

.0041

.0042

.0045

.0051

.0057

.0066

.0070

.0076

.0088

.0097

.0111

.0199

.0123

.0126

0127

.0128

.0128

.0133

.0136

.0139

.0144

.0146

.0144

Cumulative 

weight loss

(percent)

1.72

1.92

2.11

2.20

3.07

3.27

3.35

3.64

3.64

3.93

4.02

4.31

4.89

5.46

6.32

6.70

7.28

8.43

9.29

10.63

11.40

11.78

12.07

12.16

12.26

12.26

12.74

13.03

13.31

13.79

13.98

13.79

Change in 

weight loss

(percent)

1.72

.20

.19

.09

.87

.20

.08

.29

.0

.29

.09

.09

.58

.57

.86

.38

.58

1.15

.86

1.34

.77

.38

.29

.09

.10

.0

.48

.29

.28

.48

.19

-.19

Ar

( C)

52

24

9

15

25

24

31

20

25

25

25

25

25

25

25

20

30

20

30

25

25

25

25

25

25

25

50

65

110

100

80

20

has a typical coarse pisolitic structure. The laterite 

layer is about 10 to 20 feet thick. Most specimens of 

the meteorite that have been collected are from the 

outer surface of the crater mound.

The time span during which the alteration of the 

Wolf Creek meteorite has taken place has been estimated to be at least a million years by McCall (1965, 

p. 991), who stated: "The morphological state of the 

crater probably limits the possible age to Pliocene or 

at the very latest to Plio-Pleistocene."

The temperature of the environment is that of a 

desert in a tropical region. It is hot during the day and 

cool at night. Mason (1968), during his field trip to the 

Wolf Creek crater, observed that the nights, though 

cool, were dew free, and that, consequently, frosts do 

not occur. He noted also that torrential rains occur 

only during the monsoon season.

The Commonwealth Bureau of Meteorology makes 

observations at Halls Creek and at Balgo Mission, 

and some of their data are given in Casey and Wells 

(1964). The average annual rainfall of 12 inches near 

Wolf Creek occurs during the northwest's monsoon 

season, but some rain falls during the winter because 

of the influence of the southern Australian weather. 

At Balgo Mission the mean maximum air 

temperatures range from 25.9°C (78.6°F) in July to 

39.9°C (103.8°F) in December; correspondingly, the 

me?n minimum temperatures range from 11.9°C 

(53.5°F) to 25.7°C (78.2°F). In an area close to the 

Wolf Creek crater, Roberts, Halligan, and Playford 

(1969) reported an average annual rainfall of 16

inches in the south and 20 inches in the north. The 

daily maximum air temperature in January is 38°C 

(100°F), and the minimum is 24°C (75°F); in July the 

values are 27°C (80°F) and 10°C (50°F), respectively. 

Probably because of its lower elevation, the air 

temperatures at the Wolf Creek crater are appreciably higher than those at Balgo Mission.

Cassidy (1954) observed that the "vegetation inside the crater is much more luxuriant than outside, 

reflecting the rain-catching propensity of the great 

bowl."

For this study, we felt that something should be 

known of the temperature variations in the meteorite 

during an average day. To assess these variations 

the diurnal and nocturnal temperature variations, 

both in the atmosphere and in the ground below it, 

must also be known. Such data do not exist for the 

Wolf Creek crater, however, so they were estimated 

from studies made in other deserts.

Observations of the air temperatures and of the 

temperatures in the ground were made by Sinclair 

(1922) at the Desert Laboratory, Carnegie Institution 

of Washington, near Tucson, Ariz. Some of these data 

were replotted by Brinkmann in Kayser (1967), and 

his diagram is given in figure 53. The measurements 

were taken from 175 cm above the ground to 200 cm 

below the surface. The temperature curve demonstrates the character of a single cycle to a depth of 50 

cm, below which the contribution from solar heat becomes minimal. The minimum temperature of the air 

above the surface (25 cm) was 10°C (50°F), and the 

maximum was about 48°C (118°F); at the ground 

surface the minimum and maximum temperatures 

were 17.5°C (64°F) and 72°C (162°F), respectively. At 

50-cm depth the curves form a cusp point, at approximately 28°C (82°F), at which the two curves 

touch but through which they do not pass. The curve

C-988

562

C-961

897

575

INCREASING TEMPERATURE, IN °C

FIGURE 52. Differential-thermal-analysis curves of a partly purified sample of pecoraite from the Wolf Creek meteorite of Australia. Resistance of 600 ohms in 

the galvanometer circuit.
WOLF CREEK METEORITE AND THE FORMATION OF PECORAITE 121

shows the notable temperature change of the air, 

AT=38°C (97°F), and the exceptionally larger 

ground-surface temperature variations, A !T=55°C 

(131°F). Horner (1936,1957), gave similar data for the 

region of Etsin Gol, Gobi Desert, central Asia. For 

June 6, 1932, the minimum air temperature was 

about 18°C (64°F), and the maximum, about 35°C 

(95 °F); 2 mm below the surface the minimum was 

about 8°C (46°F), and the maximum, about 58°C 

(136°F). Fifty centimeters below the surface, 

temperature was almost constant at 18°C (64°F). 

Likewise, Dubief (1959, p. 90) gave for Touggart, 

Sahara Desert, North Africa, data for July 6,1931, as 

follows: Close to the surface the minimum was 24°C 

(75°F), and the maximum, 58°C (136°F); 10 cm below 

the surface the minimum was 34°C (93°F), and the 

maximum, 41°C (106°F); 20 cm below the surface the 

minimum was 34°C (93°F), and the maximum, 36°C 

(97°F). For similar depths these several sets of data 

are comparable. Superimposed upon the daily variation is the annual regional variation (seasonal variation) which for the tropics results in lower temperatures in the rainy season and higher temperatures in 

the dry season; data for these observations are contained in the works cited above. These data suffice to 

illustrate typical temperature conditions in world 

deserts.

To assess the probable conditions in the desert area 

of the Wolf Creek crater, we may compare the contours of the total thermal energy reaching the 

horizontal surface of the earth at Wolf Creek; Tucson, 

Ariz.; Gobi; and the Sahara, as given by Budyko 

(1958). The Wolf Creek area receives a total thermal 

energy of about 180 kilolangleys5 per year; the Tucson area receives 180-200 kilolangleys per year; the 

Sahara, 180-200 kilolangleys per year; and the Gobi, 

about 140-160 kilolangleys per year. We may. 

therefore, use the more complete data of Sinclair 

(1922) from the Tucson area as those representative 

of the probable conditions at Wolf Creek.

The external pressure of the environment in which 

the meteorite lay was 1 atm. The internal pressure of 

the meteorite over the long time span during its alteration was not constant. Although internal pressure 

cannot be measured directly, it can be inferred from 

the stress relations. When the meteorite was largely 

metallic in composition, it was under strain, chiefly 

because of the thermal stresses induced by the cyclic 

process of solar radiation during the day and of cooling at night. As the meteorite became largely altered 

and the metallic compounds were converted to oxides 

and hydroxides, the internal stresses were greatly decreased.

Having now estimated approximate coordinates of

5 1 kilolangley= 1,000 langleys. 1 langley=l g cal (15°C)/cm'!=4.1855 joules/cm2 .

the time, temperature, and pressure variables of the 

Wolf Creek meteorite during its earth history, an 

attempt is made here to evaluate the effect of these 

variables on the state of the meteorite. To do this we 

will calculate their effects on a model meteorite.

BEHAVIOR OF A MODEL METEORITE BODY 

IN A DESERT REGION

DISTRIBUTION OF TEMPERATURE AND THERMAL STRESS

The thermal regime of a meteorite partly embedded 

in the sediments of a desert and exposed to the heat of 

the sun can be assessed by comparing the meteorite 

to a model body of similar size, shape, and physical 

properties and similarly embedded.

FIGURE 53. Daily and nightly temperature extremes of the air above surface 

and of the soil in the desert floor at the Desert Laboratory, Carnegie Institution of Washington, near Tucson, Ariz., as measured by John G. Sinclair 

on June 21, 1955. Plot, reproduced by permission of author and publisher, by 

Roland Brinkmann, in Kayser (1967, p. 44).

Meteorite fragments are of various sizes and 

shapes, and it is only necessary to select two different 

masses and assume the shapes of a sphere, cylinder, 

and slab for purposes of calculation and comparison. 

A large mass weighing approximately 226.8 kg (500 

lb)6 was observed by Mason (1968) at Wolf Creek. To 

calculate the desired geometrical volume for a mass 

of this value, it is necessary to know its density. The 

nickel content of the Wolf Creek meteorite is 8.6 percent, and the average density of 360 iron-nickel 

meteorites having an average nickel content of 8.71 

percent is 7.693 g/cm3 . Using this value of the density 

and the weight of 226.8 kg, the dimensions of the 

sphere are volume 29,481 cm3 and radius (a) 19.164

6This weight represents an altered mass whose original weight would have been less 

than this value. It is merely selected as a convenient approximation for the weight of the 

model body. Larger masses may have been found and removed.
122 STUDIES OF THE NATURAL PHASES IN THE SYSTEM MgO-SiO2-H2O

cm. For other shapes having this same volume, the 

dimensions of a cylinder are radius 19.164 cm, 

altitude 25.55 cm; and for a slab are thickness (2o) 

38.328 cm, base 15.38x50 cm. An arbitrarily chosen 

mass (2,268 kg), 10 times the previous value, has a 

volume of 294,813 cm3 . The dimensions are, for the 

sphere, radius 41.288 cm; for the cylinder, radius 

41.288 cm, altitude 55.049 cm; and for the slab, thickness (2o) 82.576 cm, base 35.7x100 cm.

On the basis of the climatological data, previously 

presented, a temperature span of 0°C to 100°C was 

selected to bound the possible reaction coordinates. A 

search of various compilations of the pertinent 

physical properties shows that the most plentiful 

data are for the element iron, although there are some 

data for iron-nickel alloys. Few data for meteorites 

exist, but the coefficient of thermal expansion (a) of 

the meteorite from La Caille, Grasse, Alpes 

Maritimes, France, was measured by Fizeau (1869) 

and was found to be llxlO"6 per °C. In choosing the 

various constants for use in the calculations, values 

for iron were used, but the data for iron alloys containing nickel were considered in making the selections.

The thermal behavior of a spherical body while it is 

being heated from the outside was investigated by 

Williamson (1919), who derived the various 

equations for calculating the rate of heating of the 

sphere and for calculating the radial and tangential 

(circumferential) stresses developed as a result of 

heating. Assuming for a first approximation a constant rate of heating, Williamson (1919) observed 

that each element of the sphere experiences an expansion due to the temperature rise and a restoration 

of the equilibrium position by the induced internal 

stresses. The equations were extended by Williamson 

and Adams (1919), and their most recent form, 

together with extensive tables for facilitating the calculations, were given by Adams and Waxier (1960). 

For a sphere heated from the outside at a constant 

linear rate,

1 f\ »x

in which

0 ^temperature at a point x/a from the center; 

Oi =initial uniform temperature; 

h =rate at which the surface is heated, in °C/sec; 

t =time, in seconds; 

K =thermal diffusiyity; 

a =radius of the sphere; 

r =point inside the sphere; and 

£ =distance from the center of the sphere expressed 

a in terms of the radius. 

The solution of this equation permits the determin-i

ation of the temperature at any point within the 

sphere.

To determine the radial stress:

FY= 

r I5*(e-f)

(2)

To determine the tangential (or circumferential) stress:

(a2-2*2)

' (3) 1 15*(e-/)

a=thermal linear coefficient of expansion. 

( -)=Young's modulus of elasticity^.

( -£ )=Poisson's ratio=<r.

(e-f)=a parameter for the elastic effects.

The values used in the calculations, and their source, 

are:

a = ll.71x10- 6 value for iron (Holborn and Day 

1901).

E=Young's modulus=21.7xlOn dynes/cm2 

(Zisman, 1933).

o-=Poisson's ratio=0.317 (Zisman, 1933).

K=Bulk modulus=16.7xlOn dynes/cm2 (Zisman, 1933).

G=Rigidity modulus=8.38xlOn dynes/cm2 

(Ide, 1936).

K=thermal diffusivity=0.181 cmVsec (Ingersoll 

and others, 1948).

(e-/)=0.0315xlO~ n cmVdyne based on crand E.

(e-/)=0.0332xlO- 11 cmVdyne based on K and G. 

Similar equations were also derived for the geometric forms of the slab and the cylinder.

in addition to the thermal constants for the model 

meteorite, we need the thermal constants for adobe 

soil. The thermal conductivity (k) of adobe, calculated by the method of Smith (1942) and using data 

for adobe from Chang (1941) and other data from 

Ingersoll, Zobel, and Ingersoll (1954), was found to be 

&=0.00066 cal/cm/sec/°C. From this the value of 

the thermal diffussivity for adobe, K=0.002 cmVsec, 

was obtained. The consistency of the measured temperature data in Sinclair's (1922) table was checked 

by calculating the temperature at a depth of 2 cm 

in the adobe using the standard equations for periodic heat flow as given in Jakob and Hawkins (1942) 

and in Ingersoll, Zobel, and Ingersoll (1954) together 

with the value of K for adobe given above and Sinclair's data for the temperature 0.4 cm below the 

ground surface. The results showed that for the 

measured maximum of 62.1°C, the calculated value is 

65.2°C; and for the measured minimum of 22.0°C,the 

calculated value is 23.2°C.
WOLF CREEK METEORITE AND THE FORMATION OF PECORAITE 123

For the model meteorite body it is assumed (1) that 

a slab of metal lies buried to a depth, a (equal to 

l/2 of its thickness=2a), below the surface of the desert 

floor, (2) that the adobe floor and the model meteorite 

body are heated at a constant rate, and (3) that the 

quantity of solar heat striking both the desert adobe 

floor and the metal is the same.

With these assumptions it is now possible to evaluate the heating rate h for the adobe floor and the 

metal by solving the Williamson and Adams (1919) 

equation for a slab for both the media and then by 

equating the resulting two values for h as follows;

h=-

(a'2-x2)

t6adobe-6/

t2K

tSinclair (1922) measured the temperatures in the 

adobe soil at a depth of 0.4 cm and found a minimum 

value of 0=17°C at 5:30 a.m. and a maximum value 

0=71.5°C at 1:00 p.m. Substituting these data in the 

above equation, a maximum temperature of 115°C 

was calculated for the model meteorite body. In this 

calculation the modifications introduced by the 

boundary conditions of the metal have been 

neglected. The metal is surrounded by and underlain 

by adobe, and as these media have widely different 

conductivities and diffusivities, the adobe acts as an 

insulator, causing a higher overall temperature in 

the model meteorite body because it retains some of 

its heat for a longer time in the daily cycle. The magnitude of this effect was estimated by calculating the 

temperature in a slab of the metal at the contact of the 

metal and the adobe using the standard methods 

(Jakob and Hawkins, 1957). For a slab having a mass 

of 226.8 kg and a thickness of 38.328 cm, the temperature at the ground surface is 114.6°C and at the contact is 113.8°C. Similarly, for a slab having amass of 

2,268 kg and a thickness of 82.576 cm, the temperature at the surface is 114.6°C and at the contact is 

112.9°C.

The temperature distribution in model meteorite 

bodies having the shape of a sphere, cylinder, and 

slab are given in table 10 for metallic masses of 226.8 

kg and 2,268 kg. These data are calculated for the 

upper half of the model. The calculated values are, of 

course, carried to more significant figures than are 

needed for this study, but they have been retained to 

permit checking.

A study of the data in table 10 shows that a tenfold 

increase in mass produces a corresponding 41/2-fold

temperature difference within each geometric shape. 

For the sphere the temperature difference changed 

from 1.2°C to 5.7°C; for the cylinder it changed from 

1.8°C to 8.5°C; and for the slab it changed from 3.8°C 

to 17.0°C. For very small fragments the temperature 

differences should be negligible. The shape of the 

body does significantly alter the temperature distribution within it. The temperature differences within 

a slab are approximately twice as great as those 

within the sphere and cylinder, and the cylinder is 

more divergent than the sphere.

The stresses within the heated bodies are taken 

following Williamson (1919) as Fa , FF , and F^,, which 

are, respectively, parallel to the axis and radially and 

tangentially in the plane at right angles to the axis. X 

represents the displacement parallel to the axis of the 

element which was originally defined by the 

coordinates (x, r), where r is the radius. For a sphere 

the condition of equilibrium is given by:

r 2\dr

which relates the tangential and radial stresses. Considering the second member on the right-hand side of 

the equation, it is apparent that at the center (r=0)ithe 

tangential and the radial stress are equal and-are 

positive in sign, and thus they represent tensional 

forces. The calculations show that the tangential 

stress (a tension) diminishes as r increases, passes 

through the spherical shell of no strain, and becomes 

a compressive force.

For all of the geometric shapes chosen for the 

model, the bottom of the model is embedded in the soil 

well above the level at which no heat is received from 

solar radiation (50-cm depth in the normal soil profile). The sphere lies buried to a depth of its radius, a, 

and the cylinder lies with its altitude parallel to the 

desert floor and buried to a depth of its radius, a. For 

purposes of simplicity, the depression of this temperature level, under the meteorite, has been ignored. 

The position of the level of constant temperature 

would be depressed even further under the slab cited 

above.

The analysis of the thermal and stress behavior of 

a model meteorite body presented here is based on 

data which are reasonable estimates of conditions in 

the Wolf Creek desert. It yields a solution which is 

realistic.

STRESS CORROSION (CORROSION FATIGUE)

'Stress corrosion" is a term used by metallurgists 

to describe the simultaneous action of stress and 

corrosion in bringing about the mechanical failure of
124 STUDIES OF THE NATURAL PHASES IN THE SYSTEM MgO-SiO2-H2O

TABLE 10. Calculated temperature and thermal stress in model meteorite bodies in the shape of a sphere,

a cylinder, and a slab

[6 is the temperature at a distance x from the center of a sphere of radius a, at a distance x from center of a cylinder of radius 

a, and at a distance x from the center of a slab whose thickness is 2a. See text for the dimensions of the various shapes having 

a given volume. A positive number indicates tension; a negative number, compression]

Sphere Cylinder Slab

Tangential

x

a Q <°C)

Radi

stress.

kg, cm2

al

Fr

psi

(circumle

stress.

kg, cm2

rential)

Fl

psi TO

Radial

stress, Fr

kg, cm2 psi

langcntial

(cinum

strc

kg cm 2

Mass 226.8 kg (500 Ib), volume 29,481

0 113.4

.333 113.5

.500 113.7

.667 113.9

.800 114.2

1.000 114.6

15.93

14.16

11.95

8.84

5.73

0

226.58

201.40

169.97

125.73

81 50

0

15.93

12.40

7.96

1.76

-4.46

-15.93

226.58

176.37

113.22

25.03

-63.44

-226.78

112.8

1 1 3.0

113.2

1136

113.9

114.6

14.93 21236

13.27 18874

1120 15930

8.29 117.91

5 38 76.52

0 0

1493

9.96

3.73

-5.00

-13.73

-29.87

Icientull

ss, /,

psi

cmj

21236

141 67

53.05

-71.12

-195.29

-424.85

Axial st

kg cm2

2987

23 24

1493

3 29

-8 36

-29 87

ress, fa

Q

psi (°C)

424.84 110.8

330.55 111.3

21236 111.9

46.79 1126

-11891 113.3

-424.85 1 14.6

tace ol slih

kg cm : psi

39 82 556.38

26.57 37792

9.96 141 67

-13.33 -18960

-36.63 -521.00

-79.64 -1,132.75

Mass 2,268 kg (5,000 Ib), volume 294,813 cm3

0 108.9

.333 109.6

500 110.3

667 111.5

.800 112.6

1000 114.6

75.4

67.1

56.6

41.9

27.2

0

1,072.8

953.8

804.6

595.5

386.2

0

75.4

58.7

34.5

8.3

-21.1

-75.4

1,072.8

834.9

491.1

1IX.2

-300.4

-1,072.8

106.1

107.0

108.2

1 10. 1

111.7

114.6

70.7 1.0057

62.9 894 2

53.0 754 3

39 3 558.3

25.5 362 1

0 0

70.7

47.2

17.7

-23.7

-65.1

-141 4

1,005 7

671.2

251 2

-336 5

-925.2

-2,011.4

138 6

107.9

645

153

-38 8

-1386

1.971 9 97.6

1,534.6 99.5

9026 101.8

217.3 105 1

-552.1 108.1

-1,971 9 114.6

188.6 2,682.1

125.9 1,790.0

47.1 669.9

-63.1 -897.6

-1735 -2,467.4

-377.1 -5,3640

metals and alloys. Birchenall (1959) limited the term 

"stress corrosion" to those systems in which the 

stress is static and applied the term "corrosion 

fatigue" to those systems in which the stress is cyclic. 

In the previous section it was shown that the model 

meteorite body lying in the desert sand had 

developed a state of stress through cyclical processes. The surface layers of the body, during 

heating, contained residual stress and were in a state 

of compression, whereas the core, or the central portions, also containing residual stress, was in a state 

of tension. The cyclical repetition of the stressing 

conditions thus produced fatigue within the model 

meteorite body

The tensile stresses developed in the iron geometric-shape bodies with a mass of 226.8 kg, at the 

highest temperature reached by the bodies, ranged 

from 0 to 39.82 kg/cm2, and the compressive stresses 

ranged from 0 to -79.64 kg/cm2 . Similarly, in the 

mass 10 times heavier (2,268 kg), the tensile stresses 

ranged from 0 to 188.6 kg/cm2, and the compressive 

stresses ranged from 0 to -377.13 kg/cm2 . These tensile and >the compressive stresses are considerably 

below the ultimate tensile and compressive strength 

of 3,500 kg/cm2 for wrought iron and of 4,200 kg/cm2 

for structural steel, determined under static conditions on test pieces that are free of, or at least 

almost free of, existing stress. Thus, the model 

meteorite body never developed a sufficient stress to 

exceed its ultimate strength and rupture. 

Metallurgists have found, however, that metals subjected to the alternate application and removal of a 

stressing force become permanently stressed and 

develop a state of fatigue. Fatigued metals are capable of rupturing at stresses greatly below their 

ultimate strengths. The solids are weakened by such 

phenomena as embrittlement at grain boundaries 

and movements along slip and other structural 

planes. Any process which develops these surface 

defects leads to the development of cracks. Gensamer 

(1941, p. 26) stated "Ultimate failure in fatigue results from the propagation of a crack caused in the 

first place by repeated minute amounts of plastic deformation." The crack, once formed, spreads as the 

deforming cycles are repeated, and ultimately a large 

crack forms rapidly. According to Gensamer (1941, p. 

26), "The crack nearly always follows such a direction as to be everywhere perpendicular to the greatest 

normal stress (o^), both in the stage of slow development and during its final rapid propagation."

Fatigue tests on steels, under atmospheric conditions, show that the curve relating stress to the 

number of cycles to produce failure shows a knee in 

the curve at a value between 6 and 7 million cycles; a 

value of 10 million cycles is usually adopted 

(Woodward, 1948). After the initial crack forms, the 

limit fatigue stress falls by as much as 75 percent for 

high tensile alloy steels (Woodward, 1948). In a 

million years a model meteorite body lying in the 

desert would undergo 365 million cycles, so more 

than enough time would be available for stress 

failure.

After the first crack or cracks are formed, moisture 

can enter into the process, and the disintegration 

proceeds rapidly. Woodward (1948) stated that fresh 

water reduces the endurance limits of mild steels by 

50 percent, and salt water, by 75 percent. The 

solutions cause pitting of the metal, and numerous
WOLF CREEK METEORITE AND THE FORMATION OF PECORAITE 125

cracks can radiate from the pitted surfaces and lead 

to failure.

The interpretation of the mechanisms by which 

stressed bodies disintegrate is based on thermodynamic considerations. Stressed phases are 

generally thermodynamically much less stable than 

their unstressed equivalents and are therefore more 

easily destroyed. The rate of the chemical reactions is 

controlled in part by the phase assemblages, by the 

temperature, and by those structures which give 

access to, or facilitate development of, surface 

area such as cracks, cleavages, slip planes, and 

lamellae of dissimilar phases. These characteristics 

affect the rate of disintegration of both the model 

meteorite body and the real meteorite.

The model meteorite body is assumed to be a 

medium octahedrite, for which Mason (1962, p. 53-68) 

gave the following characterization: (1) The 

mineralogy consists chiefly of the iron-nickel alloys 

kamacite (a-iron, containing about 5.5 percent Ni) 

and taenite (X-iron, containing about 27-65 percent 

Ni), with very small amounts of troilite, FeS, schreibersite, (Fe, Ni)aP, and (rarely) cohenite, Fe3C, and 

daubreelite, FeCr 2 S 4 ; and (2) the texture and structures are dominated by the Widmanstatten lamellae, 

containing bands of kamacite 0.5 to 1.5 mm wide, 

that may conceal previous grain boundaries. 

Neumann lines arising from mechanical twinning 

and slip bands produced by small displacements 

along gliding planes may also be present. The Neumann lines within the bands of kamacite and the slip 

bands may have appeared under conditions of shock 

metamorphism resulting from impact.

Evidence on the manner of chemical attack on 

meteorites was given by Perry (1944), whose photomicrographs clearly show that limonite invades from 

the surface along twinning planes and attacks the 

kamacite. Most of the kamacite is replaced, until only 

the taenite lamellae remain. The taenite lamellae frequently outline the traces of the Widmanstatten 

pattern. If the taenite is replaced, it shows that 

significant amounts of nickel will be released in the 

last stage of the replacement process.

NATURAL PROCESS OF DISINTEGRATION

COMPOSITION AND CEOCHEMICAL BALANCE

The exact phase assemblage of the meteorite from 

Wolf Creek crater, Australia, at the time of its impact 

with earth is not known. Taylor (1965) collected fragments of a medium octahedrite in the desert about 4 

km southwest of the crater. Analysis of this meteorite showed 8.6 percent nickel and 0.4 percent cobalt. 

Well-developed Widmanstatten lamellae about 0.9 

mm wide dominate the fragments. According to

Taylor (1965), these fragments may be part of the 

Wolf Creek meteorite. White, Henderson, and Mason 

(1967) observed pseudomorphic textures in the Wolf 

Creek meteorite which show that the newly formed 

lipscombite occurs as crusts lining the lath-shaped 

cavities of former schreibersite crystals and that 

jarosite occurs as "small crystals in cavities which 

probably held troilite" (p. 1195). The composition of 

the Wolf Creek meteorite at the time of impact can 

also be estimated from the present mineralogy. The 

present mineralogy of the Wolf Creek meteorite, the 

probable mineralogy of a medium octahedrite, the 

mineralogy of the desert sand, and the composition of 

rainwater from Australia (Carroll, 1962), are summarized in table 11 in the form of a geochemical 

balance relating the system before and after disintegration. From this we deduce that the Wolf Creek 

meteorite was composed principally of kamacite and 

taenite and that at least minor amounts of 

schreibersite and troilite were originally present. It 

also seems likely, from other studies, that if 

chromium was present in this meteorite, it would 

have been in chromite rather than daubreelite.

STRESS RELATIONS RESULTING FROM IMPACT

The theoretical behavior of a model meteorite body 

previously given and the descriptive mineralogy of 

the Wolf Creek meteorite presented in table 11 are 

bases for deducing the probable processes leading to 

the disintegration of the meteorite. The disintegration advanced in stages.

The first stage took place on impact. The meteorite 

was intensely shocked and was in large part fragmented by its impact with the earth. Heymann, 

Lipschutz, Nielsen, and Anders (1966, p. 622) described the thermal and pressure history of a typical 

fragment formed by the breakup of a meteoroid on 

impact as follows:

A typical fragment must have passed through the following stages 

during impact. First, it was heated and compressed by the shock 

wave. Its temperature at this point was equal to the sum of an 

"adiabatic" and a "residual" term and depended strictly on the 

shock intensity. A fraction of a millisecond later the rarefaction 

wave decompressed it and cooled it to the residual temperature and 

threw it clear of the explosion area. From then on the fragment 

cooled by radiation and conduction. Its cooling time therefore 

depended on its size and ejection velocity and might be expected to 

range from a few seconds to a few minutes.

These investigators studied a collection of meteorite 

fragments from the rim of the Canyon Diablo crater 

and found that 72 percent of the fragments were 

strongly shocked at pressures ^750 kilobars and that 

28 percent were moderately shocked at pressures of 

130-750 kilobars. Temperatures in the fragments 

were estimated to have been ^ 760°C. Similar con-
126 STUDIES OF THE NATURAL PHASES IN THE SYSTEM MgO-SiO2-H2O

TABLE 111. Distribution of the elements in the disintegration products of the Wolf Creek meteorite and in the source materialsa study of geochemical balance

[Data for Wolf Creek meteorite (except for pecoraite) from White, Henderson, and Mason (1967); data for medium octahedrite from Mason 

(1962, p. 62). M, major; n.d., not detected; vm, very minor; m, minor. Values for disintegration products are in weight percent 

of oxide formed with element listed in col. 1; values for source materials, in weight percent of element]

Element

Principal 

phases

Disintegration products

Wolf Creek meteorite

Minor phases

Source materials

Typical medium octahedrite

Principal 

phases Minor phases Sand

Rainwater

C C q^ ,»_» ',£ Oj CJ ^* K "tn

^ Il| _^w> 1 i to I ~; i Js- a ** i i 11 S r: .« t~ i 773 1 qj I m i o * i»< i en i* < 13 

O S 

o g 

^ 

y> « 

* .Sgo.O.S 

o.2=s=s8 

CD 

E S 

* ct 

S^xs^o.^

ri OCU9>>rt

o 2 « o J^<o^w H^^OQC?OJ

S ----- - - - - ... M ... ... ... ... - M - - M - M

Ca------ - - - 32.3 - - M - - - - - - - - - M

Si------ - - - - - - - M 31.0 - - - - - - M

H,O -------- M - M - - M M M 9.7 - - - - - - - M M

Al - - - ... ... ... ... 14 ... ... - ... ... ... M

Cr ... ... ... ... M

M

'MgCl,. 

4CaCl:.

ditions must have prevailed at the Wolf Creek crater. 

Under these conditions of pressure and temperature, 

all macroscopic and microscopic cracks would have 

been healed.

PHYSICAL AND CHEMICAL CHARACTERISTICS 

AND RELATIONSHIPS

The second stage in the disintegration was 

probably the longest in duration. It consisted chiefly 

of the mechanical cracking of the meteorite. As 

White, Henderson, and Mason (1967) observed, the 

meteorite is remarkably preserved as a 

pseudomorph, even though it contains a system of 

cracks. They noted (p. 1196) only a minimal amount 

of "flaking-off of successive layers of oxide ***" and 

attributed this coherence to the absence of frost in the 

region; this certainly was an important factor. The 

coherence of the phases into a single unit was also 

determined by the manner of crack development. 

Thermal stressing caused cracks to extend from the 

interior of the meteorite to the surface and permitted 

reactions to take place throughout the fragment. A 

purely surface attack would have caused reaction 

rinds, and the spalling of the rind would easily follow 

such a process. The crack development in one slice of 

the meteorite is shown in figure 54. It consists of 

several systems of cracks; one major system of sub-

»(CO j). 

6Carbon.

'CO,. 

8NaCl.

parallel cracks is nearly parallel to the scale in the 

photograph.

McCall (1965, p. 978-991) described the surface 

morphology of the weathered meteorite fragments 

and called the fragments iron shale balls. These fragments have a breadcrusted surface suggestive of volcanic bombs and a tail that McCall suggested has the 

form "of molten drops separated from a larger molten 

mass in flight."

With these observations as a guide and the 

knowledge of the model meteorite theory as 

developed in this report, one can deduce the probable 

manner in which the second stage progressed. The 

fragments of the meteorite were stressed at the time 

of impact (Heymann and others, 1966) and were still 

in a state of stress when alteration began. The 

thermal stress was added during a daily heating and 

cooling cycle. Calculations show that during the 

heating cycle, the fragments were in a state of tension from the center to the thin zone of no strain, but 

were in a state of compression from the thin zone of 

no strain to the outer surface. During the cooling cycle, the mechanical state of the fragments reversed, 

and the outer shells were in a state of tension while 

the part from the thin zone of no strain to the center 

was in a state of compression. The alternating cycle 

of heating and cooling with its thermal stressing al-
WOLF CREEK METEORITE AND THE FORMATION OF PECORAITE 127

FlGl'RE 54. Crack development in the Wolf Creek meteorite. Subparallel 

system parallels the 6-inch white scale.

ternating between tension and compression was repeated every day. The magnitude of these effects 

varied slightly with the seasons and was interrupted 

by occasional torrential rains. Repeated application of these stressing forces ultimately induced a 

state of fatigue in the metal phases which in turn led 

to crack formation. Movement along Widmanstatten band boundaries and Neumann lines 

also produced some cracks. Ultimately the fragments cracked from fatigue alone. When the cracks 

had widened enough to permit the entrance of water, 

mechanical disintegration advanced rapidly, and 

chemical reaction also occurred. The water films 

helped to widen the cracks, because they opposed the 

compressive forces attendant upon cooling. The 

water films were trapped in the cracks of the 

meteorite at night. During the morning heating cycle, the water films behaved as tiny hydrothermal 

bombs until expansion permitted some of the water to 

escape by leaking.

Rainwater entering the cracks set off chemical reactions in this system, and as a result of these reactions, the system underwent physical changes. The 

cracks were produced slowly and eventually extended to the center and then to the bottom. They 

were the principal paths along which the films of 

rainwater moved. The chemical system, so formed, 

consisted chiefly of taenite, kamacite, and water containing dissolved air and behaved as a collection of 

tiny electrochemical cells. During the disintegration 

of the meteorite, some important volume change 

must have occurred, for the molar volumes of the reactants iron, nickel, and water are 7.09, 6.59, and

18.009 cm3, respectively, whereas the molar volumes 

of the principal products, goethite and maghemite, 

are 20.82 and 43.44 cm3. A determination (by Fahey) 

of the water content of selected fragments representative of the meteorite, gave 1.12 percent water 

lost when heated below 110°C and 9.28 percent water 

lost above 110°C. As goethite loses 10.14 percent 

water above 110°C, these data suggest that the Wolf 

Creek meteorite is chiefly goethite. A specific-gravity 

determination on several fragments of the meteorite 

yielded the value of 3.98, and this supports the conclusion that the Wolf Creek meteorite studied by us is 

chiefly goethite. If one assumes that the Wolf Creek 

meteorite had a specific gravity of 7.69 before disintegration and uses the value of 3.98 for the present 

specific gravity of the meteorite, an increase of 

volume of just under 50 percent is indicated. As the 

reactions took place very slowly, over long periods of 

time, the volume-expansion process due to oxidation 

did not rupture or fracture the meteorite into myriads 

of small fragments.

The assemblage of phases, which formed the reactants, was dominantly kamacite and taenite and 

very subordinately troilite (FeS) and schreibersite 

(Fe,Ni)3P. These may, for simplification, be referred 

to the system Fe-Ni-H2O-O2 at temperatures from 

25°C to 100°C. The system was elastically stressed, 

and accordingly the strained phases were even less 

stable thermodynamically than their unstrained 

equivalents. Phase relations for this system at these 

temperatures are incomplete, and Eh-pH diagrams 

containing both nickeloan maghemite and goethite 

are not available. Although the complete Eh-pH diagrams do not exist, sufficient data are available to indicate the probable physicochemical environment 

and the course of the reactions.

To interpret the course of the reactions at this early 

stage in the disintegration of the meteorite, the geochemical balance (table 11) may be examined further. 

The principal solid phases, the reactants, are the two 

alloys of iron and nickel, kamacite (nickel-poor) and 

taenite (nickel-rich). The relative stability of these 

two metallic phases can be observed directly in 

polished surfaces of altered meteorites where the 

relict taenite lamellae are encased by goethite (Perry, 

1944). The oxidation and hydration of the kamacite 

to form goethite proceeds along the planes of Widmanstatten patterns, along grain boundaries, and 

probably along the Neumann lines. The liquid phase 

and its vapor which accomplished this disintegration was rainwater which contained some dissolved 

oxygen absorbed from the air. This dissolved gas was 

active in the reaction forming goethite and maghemite. Coker (1954) gave the solubility of air in 

water as follows: 14.6 ppm (parts per million) at 0°C,
128 STUDIES OF THE NATURAL PHASES IN THE SYSTEM MgO-SiO2-H2O

10.1 ppm at 15°C, and 7.5 ppm at 30°C. Rainwaters 

from various localities, taken under a wide variety of 

conditions, have a pH range of 4 to 6 and a corresponding Eh range of+0.55 to +0.45 volts, and they are 

accordingly oxidizing in character (Baas-Becking 

and others, 1960). Carroll (1962) summarized data for 

rainwaters in Australia, and from these data we assume that the pH of the rainwater was approximately 6.0.

The electrochemical reaction of metals in contact 

with distilled water and with salt water of various 

concentrations has been studied by Whitmore (1933), 

who found that a slab of iron immersed in distilled 

water at 25 °C produced an environment that became 

more alkaline with time. Thus at the end of 25 hours, 

the system iron-distilled water had a pH of 6.7l(practically identical with that of the blank solution), but 

after 400 hours its pH had risen to 7.5. Nickel reacted 

more quicklyjhaving a solution with a pH of'7.1|after 

24 hours and 7.3 after 400 hours. Whitmore (1933) 

showed that steel slabs immersed in salt water with 

free access to oxygen of the air form brown ferric 

hydroxide, whereas steel slabs coated with pyroxylin 

primer to exclude the air form green ferrous hydroxide under the coating. He measured the pH of a saturated solution of ferric hydroxide to be 7.0 and of 

ferrous hydroxide to be 9.5. These experiments suggest that the formation of the hydroxide controls the 

pH. A similar increase in pH to about 7.0 probably 

took place when the rainwater moved down the 

cracks and into the meteorite.

Examination of the Eh-pH diagrams in Garrels 

and Christ (1965) for the iron and nickel systems at 

25°C shows that, at the Eh-pH conditions of rainwater, the metals iron and nickel are completely unstable with respect to hematite, Fe2O3 , Fe(OH)3 , and 

the nickel ion in solution (Ni+2 aq),'and that these 

metals could form such compounds or ions in 

solution. The stability field of magnetite on their diagrams excludes this phase as a possible member of 

the natural assemblage. Stability fields for goethite 

and maghemite are not given. Langmuir (1969) gave 

the free-energy data for maghemite and, on the basis 

of the activity product K=(aFe+3)( a (OH-))3 , suggested that maghemite is less stable than goethite at 

25°C. John L. Haas, Jr. (written commun., 1970, 

stated that nickeloan maghemite is as stable as 

goethite. The constant alternating elastic stress in 

the natural system may tend to form metastable 

phases.

The abundance of goethite in the Wolf Creek meteorite as the chief reaction product suggests that the 

system was entirely oxidizing in character. This reaction forming goethite released nickel, and since the 

alloy kamacite may contain as much as 5.5 percent

nickel, the nickel must be accounted for in the replacement products. The microprobe analyses show 

that the goethite contains only 0.7 to 1.30 percent NiO 

(0.55 to 1.02 percent Ni). The second most abundant 

product of the reaction is the phase nickeloan 

maghemite, which is shown by microprobe analysis 

to contain as much as 8.7 percent NiO (6.84 percent 

Ni).

Probable reactions to produce goethite, or its polymorph, lepidocrocite, and maghemite were given by 

Doyle (1968). In his experimental study he investigated the reaction

4Fe+ 2+O<2 *-6H 2O' +* 4FeOOH+ -8H+4E', 

and he found that under his experimental conditions 

it was almost impossible to prevent the oxidation of 

some of the ferrous iron to ferric iron because maghemite formed with the goethite. This is a close 

parallel to the assemblage in the Wolf Creek meteorite.

Coker (1954) found the solubility of oxygen from 

the atmosphere to be 15 ppm in rainwater. Using this 

yalue and the above equation, 213 liters of rainwater 

are required to furnish the oxygen necessary to convert 1 gram of pure iron to goethite. Obviously other 

sources of oxygen are available, such as the air which 

enters the cracks and oxygen liberated in the 

catalytic decomposition of water in the hydrothermal system. The reaction given above would 

probably occur more rapidly during the heating cycle 

and the early part of the cooling cycle.

It is desirable to evaluate the probable reaction 

time necessary to convert the iron-nickel alloys to 

goethite and nickeloan maghemite, and fortunately 

such experimental data are available so a comparison can be made. The rate at which iron and iron 

alloys corrode has been studied extensively by electrochemists. In thoroughly dry atmospheres, iron 

may not be detectably corroded; in moist atmospheres, higher nickel contents of the alloy cause 

greater stability. This is in accord with the alteration 

of meteorites where the greater stability of taenite 

over kamacite is abundantly demonstrated and 

easily observed as in the photomicrographs of Perry 

(1944). Iron and steel sheets, under no stress and not 

cracked but exposed to the earth's atmosphere, 

corrode at the rate of 6 to 40 milligrams per square 

decimeter per day. Expressed in other units, this rate 

of corrosion is about 0.0028 to 0.0188 centimeters per 

year, or 2.8 to 18.8 centimeters per thousand years 

(McKay and Worthington, 1936, p. 243). In time, even 

the nickel-rich alloys corrode, just as their natural 

equivalent, taenite, does. These time data for the rate 

of corrosion on test slabs are a realistic estimate for 

the probable rate of disintegration of the Wolf Creek 

meteorite.
WOLF CREEK METEORITE AND THE FORMATION OF PECORAITE 129

SUGGESTED PATH OF THE OXIDATION PROCESS

John L. Haas, Jr., U.S. Geological Survey, examined our data and suggested a pattern for the oxidation of kamacite (nickel-poor alloy) and taenite 

(nickel-rich alloy) based on thermodynamic considerations. He kindly allowed us to use his notes and 

calculations (Haas, written commun., June 19,1970), 

and they are summarized herewith.

Steps in the path of oxidation of a meteorite composed of kamacite and taenite 

Kamacite (nickel-poor alloy) Taenite (nickel-rich alloy)

Step 1 

Magnetite

Step 2

Magnetite-trevorite solid solution

Step 3

t

Goethite 

(final state)

Step 4

Goethite + maghemite-trevorite 

solid solution 

(final state)

Thus, the final state of these reactions is goethite 

and goethite+maghemite-trevorite solid solution.

The thermochemical data used in the following 

calculations and given below are taken from Langmuir (1969), except for NiFe2 04 and Ni*~, which are 

taken from Wagman and others (1969).

Species AF (kcal mole)

Trevorite, NiFe2O< -232.6

Magnetite, FeFeO4 , -243.09

Maghemite fcy3Fe2/,)Fe.,O 4 -226.35 (adjusted to agree with defect formula)

Goethite, FeO-OH -116.375

Fe+2 -21.80

Fe+3 -4.02

Ni+2 -10.9

H 2O(1) -56.688

The maghemite in the meteorite contains as much 

as 8.7 percent NiO (6.8 percent Ni). The formula 

weight of trevorite is 234.41 g/mole, and that of maghemite is 212.93 g/mole. A ratio of 1:3 of trevorite to 

maghemite contains 6.8 percent of nickel, and this 

corresponds to the amount of nickel in the nickel-rich 

alloy, taenite, in the meteorite. This ratio, which was 

obtained from both the maghemite-trevorite solid 

solution and the taenite, will be used when the ratios 

of end members are considered in further calculations. The abbreviation ss refers to the solid 

solution between trevorite and maghemite which has 

been observed, and the abbreviation ss 'refers to the 

solid solution between trevorite and magnetite which 

is theoretically possible. The solid solutions are assumed to be ideal that is, the activity of the end 

members a i is equal to the mole fraction NReaction involved in step 1, kamacite^magnetite. Kamacite is irreversibly oxidized 

to magnetite. Because of its nickel content, taenite is 

not reactive relative to kamacite and undergoes no alteration at this point. First, water in 

contact with the kamacite spontaneously attacks the 

kamacite, producing magnetite and hydrogen gas. 

The hydrogen gas diffuses out of the meteorite body. 

No oxygen is needed, though any supplied increases 

the rate of reaction. The rate of the irreversible reaction also increases in the presence of carbonates, 

sulfates, and phosphates which are in the water as 

the result of solubility in rainwater or oxidation 

occurring in the outer parts of the body. Increase in 

the temperature to 100°C plus hastens the deterioration even more.

Reaction involved, in step!2, taenite magnetitetrevorite solid solution (ss'). After the immediately 

available kamacite has been oxidized to magnetite, 

the water and its dissolved salts begin to attack the 

taenite. The product will be a solid solution between 

trevorite and magnetite which is controlled by the 

ratio of nickel to iron in the taenite and by the distribution coefficient of nickel between the water and 

the oxide solid solution. Here it is assumed that there 

is no partitioning of the nickel into the aqueous phase 

and depleting of the solid solution below that which 

would be expected from simple oxidation of taenite 

and that which is actually observed in the trevoritemaghemite (ss). This reaction is irreversible, and all 

the attributes which accompany the oxidation of 

iron-rich kamacite also apply to the oxidation of 

taenite to ss'.

At, this level of oxidation, the Ni+2 and Fe +2-andFe+3 concentrations in solution can be calculated. 

Consider the reaction of solid solution ss' with H2p 

(1) in the proximity of metal and thus in the presence 

of about 1 atmosphere H2 pressure. Ionic-strength 

effects are neglected.

FeFe2 O4 (ssO+eH^H^SFe^MHp 

For this reaction,

log #=35.96=3 log [Fe* 2 ]-log [FeFe 2O4 (ss')] 

+6pH-logP(H2 )

Whitmore (1933) measured the pH of solutions in contact with weathering nickel-iron alloys and obtained 

a mean value of 7.0. Using this value and 0.75 for 

,[FeFe204 (ss')],

log [Fe +2]=-2.052

[Fe+2j=0.0089 molal

Under these conditions there will be no Fe +3 in the 

solution, so [Fe +3]-0 (for all practical purposes).

Next, consider the solution of the trevorite component:
130 STUDIES OF THE NATURAL PHASES IN THE SYSTEM MgO-SiO2-H2O

NiFe2O4 (ss')+6H++H2=Ni+2 +2Fe+2 +4H2 O 

subject to the following conditions: 

H2 -l atm 

pH=7

[NiFe 2O4 (ss')]=0.25 

[Fe +2 ]=0.0089 molal 

For this reaction 

log #=35.67=logLNi +2 J+2 log [Fe +2 ]

-log [NiFe,O4 (ss')]+6 pH-log P(H2) 

The following concentrations can be calculated: 

log [Ni + 2> -12.828, [Ni+2>0.0015 molal 

[Ni+ 2]/([Fe+2 ]+[Fe+3 ])=0.17

At this level of oxidation the assemblage is:

Solids

Magnetite (from kamacite) 

Magnetite-trevorite solid solution

Proximity of unreacted kamacite and taenite

Solution

Fe<- 2=0.0089 molal 

Fe+ a=0 

Ni-> 2=0.0015 molal

=0.17

[Fe+2]+[Fe+3]

Reaction involved in step 3, 

magnetite goethite. Only long after the oxidation front caused by the spontaneous reaction of 

H2 O with metal has passed a given point in the 

meteorite does the site begin to see the effect of atmospheric oxygen. After the spinels form, the dissolved 

oxygen is the only oxidant.which causes further oxidation. Goethite is produced in proportion to the rate 

at which oxygen is provided.

The concentration of Ni+2 and (Fe +2 + Fe+3 ) in 

solution at the magnetite?-goethite boundary can be 

calculated. The term"magnetite?" is used because 

there is some question as to what spinel is stable at 

the boundary. Although it is a solid solution with 

trevorite in the 1:3 ratio, there is a question whether 

the defect structure is applicable at this point. In the 

pureFe-O system at 25°C, there is no appreciable 

defect in the magnetite at the magnetite-hematite or 

magnetite-goethite boundary. It is assumed here that 

there is no defect at the "spmel"-hematite boundary' 

where the spinel is in the system NiFe 2 O4-FeFe2 O4 

(D i/3 Fe'va )Fe.2O4> which is to say that the spinel lies 

on the magnetite-trevorite boundary, and there is no 

significant maghemite molecule in the solid solution. 

This is most probable because a large excess of nearly 

pure magnetite is present which was produced from 

the oxidation of kamacite. If the spinel is assumed to 

be free of defects, the fugacity of oxygen at the spinelgoethite boundary is calculated as follows:

2FeFe2O4 (ss')+3H2O+1/2O2-6 FeOOH 

For this reaction log #=30.8.

To calculate the fugacity (/) of oxygen, we have for 

7V(FeFe2O4 )=0.75 the following:

log#=30.8=-2 log FeFe2O4 -1/2logO2 , 

from which log O2=-61.1, and /o ^lO"61 - 1 atm.

From the

log H2 =

equilibria, 

(-83.1-log

and thus log H2=- 11.0. From these data it is possible 

to calculate as before

Fe +2=0.00000l9 molal

Ni* 2=0.00000032 molal

(Ni +2 /Fe+2 )=0.167

Thus the iron concentration in the solution is still 

much greater than the nickel concentration. The 

Fe* 3 concentration is still insignificant. The magnetite is oxidized to goethite by oxygen carried in the 

water. Here, no change occurs in the magnetitetrevorite (ss') because of the stabilizing influence of 

Ni +2.

Reactions involved in step 4 and the attainment of 

the final state In this reaction, the magnetitetrevorite (ss') is oxidized to the maghemite-trevorite 

(ss) and goethite, at pO2=0.20 atm. The ion concentrations are calculated as follows:

FeOOH+3H+ =Fe +3 +2H2O

log #=0.75

log #=0.75=log Fe* 3 -3 log H +.

For a rainwater with pH=6, log Fe+3 --17.25, or 

Fet3'=10- 17 - 2h molal. The nickel concentration is found 

from

NiFe 2(V8H+ =Ni+2 +2Fe +3 +4H2O 

and 

log #=9.59=log Ni+2 +2 log Fe +3 -8 log H +

-log NiFe 2 O4 (ss)

For pH=6 (that of rainwater), log Fe+3 =-17.25, and 

7V(NiFe 2 O4 ) (ss)=0.25, log Ni +2 =-4.50 and Ni* 2

=0.000031 molal

AtpO2=0.2 atm there is no Fe* 2 in solution, and the 

spinel is the observed trevorite-maghemite solid 

solution. The ratio Ni/Fe=0.000031/10-17- 25 is greater 

than 1.0, and therefore there is no available iron in 

solution to enter into the silicate phase. At any other 

time, the dissolved iron would be sufficient in the 

solution to produce a mixed silicate of iron and nickel 

or an iron silicate.

In summary, at the final state with goethite and 

maghemite-trevorite (ss') at pO?=0 20 atm:

Fe* 3=10' 17 - 25 molal

Fe * 2-zero!

Ni +2 =0.000031 molal

Ni/Fe=»l

Pecoraite, an iron-free silicate, could form only 

when the nickel ion is sufficiently concentrated in the 

solution and the activity of aqueous iron is very low. 

These conditions could be realized if hot water and 

traces of silica at approximately 80°C react with
WOLF CREEK METEORITE AND THE FORMATION OF PECORAITE 131

nickeloan maghemite. The silica could be obtained 

from the quartz which is remobilized by water to 

produce opal. Only in the last state of the alteration of 

the meteorite would the concentration of aqueous 

nickel exceed that of aqueous iron, and at this stage 

pecoraite can form.

INTERPRETATION OF THE DISINTEGRATION

The preceding analysis supplied information on 

the stability of iron and iron-nickel alloys in a corroding environment. The data and the theory permit 

evaluation of the stability, the electrochemical process of alteration, the rate of alteration, and the 

relative concentration of nickel and iron ions in the liquid phase at various stages in the alteration process. The metallographic observations made by 

Perry (1944) on altered meteorites show clearly that 

the kamacite in the meteorite is altered first, before 

the taenite. The alteration process is so selective that 

kamacite is completely replaced, and a pseudomorph 

of goethite and nickeloan maghemite that remains 

contains the taenite lamellae which outline the 

former grains and also preserve the outlines of the octahedral pattern. Only after a long and intensive 

alteration process do the taenite lamellae yield to the 

attacking solutions.

The first stage in the oxidation of the meteorite, accomplished in a succession of steps, involved the replacement of the major phase kamacite and the redistribution of the nickel between the most abundant 

phase goethite, the nickeloan maghemite, and the liquid phase which remained after oxidizing the 

alloys. The amount of nickel ion in solution at this 

late stage in the replacement of kamacite must have 

been very small, as judged by comparison with the 

calculations of John L. Haas for the nickel-rich alloy, 

taenite. Most, if not all, of the nickel may have remained in place in the pseudomorph.

Physically, the partly oxidized meteorite was 

cracked by the alteration, as previously described, 

and this was a major system of large cracks. In addition, a minor system of numerous tiny cracks must 

have formed in the replacement of the kamacite accompanying the increase in volume of the mass.

A network of tiny cracks was required to allow the 

solutions to pass through, along, or around replaced 

plates and grains of kamacite to reach and replace 

the more resistant taenite lamellae and later escape 

with their load of dissolved ions into the major crack 

system. Perry (1944, p. 24) noted, from metallographic studies, that "The hydroxide [goethite] 

tends to penetrate octahedrites along the planes of 

the Widmanstatten pattern and also along grain 

boundaries, and sometimes along the planes of the

Neumann lines." All these paths for the solutions to 

migrate into the altered mass must have been 

available for the late-stage replacement of taenite.

Such an assemblage of tiny cracks can be likened 

to a large collection of tiny misshapen hydrothermal 

bombs. At night, when the whole meteorite cools 

down to about 17°C, the cracks are partly filled with 

trapped liquids and their associated vapor phase. 

The composition of the liquid phase may vary 

slightly throughout the mass. During the day, the 

daily heating cycle causes the temperature of the 

assemblage to rise, and the vapor pressure increases 

correspondingly. As the temperature and vapor pressure increase, a state is reached at which the cracks 

can no longer contain the microsystem, and they begin to leak. The leachate, with its load of dissolved 

constituents, leaks into the low-temperature hydrothermal environment of the major crack system. 

There is a certain similarity between the character of 

the assemblage postulated above and some accidental experimental observations.

Dr. George W. Morey (oral commun., 1960), while at 

the Geophysical Laboratory, Carnegie Institution of 

Washington, accidentally produced nickel talc in 

cracks in steel bombs that had leaked during hydrothermal experiments. His experiments were made at 

much higher temperatures and pressures than those 

in the natural system studied in this report. Other investigators, including Dr. Albert C. Walker, Bell Telephone Laboratories (oral commun., 1960), and Professor O. F. Tuttle, then at the Pennsylvania State 

University (oral commun., 1960), also observed 

hydrous nickel silicates formed by the leaking of 

heated silica-rich solutions into cracks in the nickelsteel bombs. The reactions proceeded rapidly.

Corrosion of the meteorite progressed as thermal 

cycles repeated. The cracks at the surface widened, 

and when they became wide enough, adventitious 

sand grains, composed chiefly of quartz, were blown 

into them. As the cracks expanded during the daily 

heating cycle, the grains dropped deeper. When the 

nightly cooling cycle took place, the grains contracted much more slowly than the other phases. 

This resistance helped widen the cracks, and the 

grains were themselves crushed in the process. The 

finer crushed grains of quartz could thus drop deeper 

in the cracks. Evidence of the crushed grains can be 

detected in association with pecoraite, and this admixture was clearly revealed in the X-ray patterns of 

pecoraite. Grain-size determinations based on 

measurements obtained from diffractometer traces 

give values of 542 A for the diameter of these crushed 

quartz grains in sample WC-2-1, and 371 A for those 

in sample WC-2-2.
132 STUDIES OF THE NATURAL PHASES IN THE SYSTEM MgO-SiO2-H2O

The sand grains were confined to the cracks in the 

meteorite which were wide enough to receive them. 

Here they were crushed until they were fine enough to 

be attacked by the solutions and remobilized as an 

opaline form of silica. The addition of silica to the 

system introduced another component, thus forming, 

in these limited regions of the meteorite, the five-component system Fe-Ni-H2O-O2-SiO2 , No diagram for 

this system exists. The Eh-pH diagrams for the iron 

and the nickel systems, at 25°C, show that regions of 

stability of the (Fe+ 2 aq) ion lie outside the regions of 

probable Eh-pH relations in the meteorite, whereas 

the region of stability of (Ni + 2 aq) ions is quite large 

and includes the entire probable Eh-pH region of the 

system in the meteorite. The concentration of nickel 

ions became important when the taenite lamellae 

were attacked by the heated solutions in the minor 

crack system. The calculations by Haas (p. 129) show 

the relative concentrations of Fe+3 , Fe+2, and Ni+2 in 

the solutions. It is only in the last step in the oxidation process, when the magnetite-trevorite solid 

solution is being oxidized to the maghemite-trevorite 

solid solution, that the concentration of the nickel 

ions in solution grossly preponderates over the concentration of the Fet3 and Fe +2 ions in solution. It is 

very likely for these reasons that pecoraite does form 

and an iron silicate does not form. The only silicate 

mineral in the meteorite is pecoraite, and it contains 

only very minor amounts of iron. In this connection, 

the general nonexistence, in sedimentary and other 

low-temperature environments, of minerals composed of FeSiO3 , or ^FeSiO3 -yH2<), has long been ob- 

'served by mineralogists and interpreted as evidence 

of the reluctance of ferrous iron and silica to react in 

such environments.

The greatest concentrations of pecoraite occur in 

the widest cracks, and so it is evident that pecoraite 

formed late in the paragenesis when the taenite was 

decomposing and the concentration of nickel ions 

was at its highest value in the solution. At this stage 

in the disintegration, the upper part of the meteorite 

was chiefly goethite and nickeloan maghemite with 

only subordinate amounts of taenite. The lower part 

of the meteorite probably contained some undecomposed kamacite and most of its original taenite but 

was chiefly composed of goethite and nickeloan 

maghemite. Determining the temperature relations 

in this heterogeneous assembly is difficult; Values 

for the thermal conductivity of goethite and maghemite do not exist. However, a 226.8-kg sphere of 

magnetite, whose thermal conductivity would 

probably be close to that of maghemite, would have 

attained, under the assumed heating rate, a 

temperature of 100.7°C at its surface. An average 

temperature for the meteorite and for the reacting

system at this stage, when composed of the mixture 

of decomposed and undecomposed phases, would 

probably be at least 85°C-90°C at the center at the 

peak of the heating cycle. Thus the temperature in the 

meteorite would range from 85°C-90°C at the center 

to 100.7°C at the surface.

Pecoraite grew in this stress-controlled environment, and its particle size is very small. The particle 

size may have been controlled by the cyclical interruption of the growth process. Providing an ample 

supply of solution was available, growth probably 

took place during the daily period of 8 hours when the 

temperature was high. The growth process would be 

interrupted as the water evaporated and during the 

long dry periods when sufficient rainwater was not 

available. The jostling of the grains during the expansion and contraction cycles of the cracks would 

have put the grains under stress. Any or all of these 

conditions could have limited the growth of the 

pecoraite crystals.

NOMENCLATURE

The nomenclature of the hydrous nickel-magnesium silicates, the garnierite group, was critically 

evaluated by Faust (1966), who pointed out the vague 

and contradictory usages then extant. On the basis of 

his studies he concluded that a nickel end-member of 

the clinochrysotile group equivalent to nickel clinochrysotile existed and indicated that a name would 

be proposed for it later.

The name "nickel chrysotile" was previously proposed by Gritsaenko and Iskyul (1937) for a filling 

material in a crevice in leached serpentines in the 

nickel deposits of Kempersai, Western Kazakhstan, 

Russia. They stated concerning this material:

On the basis of these data it is difficult to say whether we are dealing with replacement of Mg by Ni in chrysotile asbestos or with a 

pseudomorph of nickel silicate after it. Because the analysis gives 

a rati^ of oxides closer to nickel chrysotile than to garnierite we 

name it nickel-nhrysotile.'

They gave a chemical analysis which shows only 

0.55 percentH2 O, certainly a misprint for 10.55 as 

judged by their ratios. This corrected analysis when 

computed in terms of the end-members, clinochrysotile and pecoraite, gave less than 50 percent for the 

nickel end-member and as such did not justify a new 

name. The data are given in the following tabulation:

Nickel chrysotile of

Gritsaenko and Iskyul (1937)

(weight percent)

50:50 mixture of theoretical

pecoraite and clinochrysotile

(weight percent)

MgO -----------

H'O ----------

Total -- .--

41.20 

20.50 

28.20 

'10.55 

100.45

37.47

21.83

29.46

11.24

100.00

'Given as 0.55, obviously a misprint, by Gritsaenko and Iskyul (1937).

7 As translated by Michael Fleischer, U.S.Geological Survey.
WOLF CREEK METEORITE AND THE FORMATION OF PECORAITE 133

Written as a structural formula, their analysis 

becomes

[Mg31B Ni 2 .34][Si 4 .25]Oio(OH) 8 

1-5.49 2=4.25 

oct tet

which again emphasizes the dominance of Mg ions 

over Ni ions. The excess silica suggests a mixture of 

phases.

The physical properties reported by these authors 

for their nickel chrysotile, the above chemical data, 

and their indecision as to the purity of their sample 

render their name meaningless.

Shubnikova (1938), in her summary of data on new 

minerals, termed the substance described by Gritsaenko and Iskyul (1937) nickel-chrysotile(?), indicating by the question mark her reservation about 

this so-called species. She repeated their incorrect 

statement of their analysis.

Noll and Kircher (1952) compared their synthetic 

products with a sample of "garnierite" from Noumea, 

New Caledonia. The electron micrographs of the 

natural material showed tubes just like those in the 

synthetic products and demonstrated that a nickel- 

'rich serpentine-group mineral is one of the phases in 

the garnierite from New Caledonia.

This uncertainty surrounding the identity of the 

phase (or phases) to which the name "nickel chrysotile" had been applied clearly justified Faust, Fahey, 

Mason, and Dwornik (1969) in laying aside that 

name and proposing the name "pecoraite" for the 

phase which they described as the nickel analog of 

clinochrysotile.

A recently recorded occurrence of pecoraite in 

nickel-bearing rocks in South Africa has been noted 

by Liebenberg (1970).

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