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14th International Symposium on Experimental Methods for Microgravity Materials Science - June 2002

STONY-IRON METEORITES (PALLASITES) – A STUDY OF NATURE’S MICROGRAVITY SPECIMENS

Phyllis Z. Budka, Technical Communications Unlimited, 2135 Morrow Avenue, Niskayuna, New York 12309-2332 e-mail: [email protected]

ABSTRACTThe interpretation of metallographic structures is widely used in materials engineering to gain insight into a material’s history. This paper presents Imilac stony-iron meteorite (pallasite) color micrographs that show interrelated regions at low magnification. Logically, stony-iron meteorites such as Imilac formed in a low gravity environment. Color and shape cues can be used to “reconstruct” the last stages of Imilac microstructural evolution before final solidification. The role of gravity as a variable in pallasite microstructural evolution needs study. Micrographs are presented to stimulate interest and gain new insights into pallasite formation conditions as well as microgravity solidification.

Table of ContentsI. INTRODUCTION…………………………………………2

II. A VISUAL OVERVIEW: FROM PALLASITESTO NICKEL-IRON METEORITES……………………..2

III. IMILAC METALLOGRAPHIC STUDY…….…………..9

Piece A Side 1: Pages 10 - 17

Piece A Side 2: Pages 18 - 25

Piece B Side 1: Pages 26 - 33

Piece B Side 2: Pages 34 – 39

IV. Conclusions…………………………………………….40

V. References………………………………………………40

VI. Acknowledgments……………………………………..41

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I. INTRODUCTION

The interpretation of metallographic structures is a simple, effective approach, long used in materials engineering to gain insight into conditions experienced by a material during its history. The same approach can be applied to stony-iron (pallasite) meteorites, nature’s microgravity solidification specimens, to glean information on conditions in a mushy melt, during the last stages of low gravity solidification.

Micrographs of both typical and anomalous stony-iron (pallasite) and nickel-iron meteorite microstructures are first presented in a visual progression overview (Part II). Next (Part III), a low magnification study of 2 pieces of Imilac pallasite gives insights into microstructural development before the final stages of solidification.

II. A VISUAL OVERVIEW: FROM PALLASITES TO NICKEL-IRON METEORITES

Springwater Pallasite

Initial insights for the concept that stony-iron meteorites are formed by non-equilibrium solidification under microgravity conditions came from the Springwater Pallasite specimen shown in Figure 1 [1, 2, 3, 4]. The yellow-green phase is olivine, a magnesium – iron silicate in the orthorhombic system; it is an isomorphous series with end members Mg2SiO4 (forsterite) and Fe2SiO4 (fayalite). Olivine is set in a matrix of body-centered cubic iron with approximately 7-16 vol% nickel [5]. This combination of low density silicate in a matrix of high density metal does not occur naturally on earth.

Imilac Pallasite

Imilac Pieces A and B, Figure 2, are the subjects of the detailed metallographic study in Part III. Table 1 gives size and mass details for Pieces A and B.

Table 1 Imilac Pieces A and B

Imilac Piece A – ~50-50 Metal/Silicate

Imilac Piece B – ~95-5 Metal Silicate

Mass: 21.20 grams Mass: 8.28 grams

Length: 4.04 cm Length: 3.58 cm

Width: 2.69 cm Width: 2.46 cm

Thickness: 0.45 – 0.46 cm Thickness: 0.45 – 0.50 cm

Brenham Pallasite

This Brenham image, Figure 3a, is often included in meteorite books for its unusual microstructure, a combination of stony-iron meteorite and characteristic nickel-iron meteorite Widmanstatten structure. Since Figure 3a does not have a scale bar, the Brenham Figures 3b and c are included for scale.

Agpalilik and Gibeon Nickel-Irons

Agpalilik and Gibeon show the typical meteoritic Widmanstatten structure (Figure 4a and b). The major microstructural feature is body-centered cubic iron (kamacite) with ~7.5% iron [6].

Albion Nickel-Iron

Albion (Figure 5a-c) contains an unusual void within the Widmanstatten structure, a very rare microstructural feature.

8 mm

Figure 1: Springwater Stony-Iron Meteorite

Figure 2: Imilac Stony-Iron Meteorite – Back lighting highlights translucent regions

Scale Bar 10 mm

Scale Bar 30 mm

Brenham Pallasite with typical Widmanstatten Structure

Figure 3a

Figure 3a courtesy of Carleton Moore, Arizona State University, Center for Meteorite StudiesFigures 3b and c from “Handbook of Iron Meteorites,” Vagn F. Buchwald, University of California Press, 1975.

Figure 3c

Figure 3b

Meteoritic Widmanstatten Structure

Kamacite - Body-centered cubic iron -”Ferrite”Ni: 4 - 7.5% Co: 0.4 - 0.6%

Taenite - Face-centered cubic iron - “Austenite”Ni: 25 - 50% Co: .3 - .8% C: 0.05 - 0.5% P: 0.05 - 0.1%

Kamacite: Brown or Blue Etching Phase

Taenite: White Etching PhaseAgpalilik 2.25 cm

Figure 4aCourtesy of Vagn F. Buchwald

Figure 4b Courtesy of G. Vander VoortGibeon

Figure 4: Typical Widmanstatten Structure

10 mmFrom 22 kg mass

Albion Widmanstatten Structure

Figure 5a

10 mmFigure 5b

Photos Courtesy of Russell W. KemptonNew England Meteoritical Services

10 mmFigure 5c

III. IMILAC METALLOGRAPHIC STUDYThis section presents a study of both sides of Imilac Pieces A and B; a photo of each side is given first, then a visual map of that same image keyed to the higher magnification images (~18X) that follow. It is common practice for specimen preparers to fill voids created during cutting with epoxy. The epoxy appears as bubble artifacts in olivine regions. These specimens are shown as purchased and have not received metallographic preparation.

01 A

Imilac Piece A Side 1

1

65

3

4

2


24 A41


23 A32


28 A53


22 A24


26 A65


21 A16


02 B


02 B WITH OUTLINE1

65

4

3

2


17 B31


18 B42


16 B23


19 B54


15 B15


20 B66


04 D Flip

Imilac Piece B Side 1

04 D Flip

654

312


05 D11


06 D22


07 D33


10 D64


09 D55


08 D46


03 C Flip


03 C Flip

43

21


11 C11


12 C22


14 C43


13 C34


IV. CONCLUSIONS

Using color and shape cues and simple digital image tools applied to low magnification micrographs, it is possible to reconstruct the last stages of Imilac microstructural evolution before final solidification. Several pieces of related olivines and the order of their position in the pre-existing “parent” olivine cluster can be determined and the “parent” olivine cluster reconstructed. In Imilac Piece B, a region of liquid metal invasion into the parent olivine cluster can be identified. As more liquid metal invaded the cluster, several olivine pieces separated and were pushed a few millimeters in a gentle movement before final solidification. This same simple reconstruction methodology is possible with Imilac Piece A and the Springwater pallasite piece in Figure 1. It is, thus, a general and powerful technique to visualize the pallasite mushy melt as it freezes in microgravity.

The role of gravity as a variable in pallasite microstructural evolution needs study. These micrographs are presented to stimulate interest and gain new insights into pallasite formation conditions as well as microgravity solidification.

V. REFERENCES

1) Budka, P.Z., “The Formation of Nickel-Iron and Stony-Iron Meteorites: Evidence for Rapid Solidification Under Microgravity Conditions,” Masters Thesis, Union College, Schenectady, NY, (1982).2) Budka, P.Z., “Meteorites as Specimens for Microgravity Research,” Metallurgical Transactions A, Vol. 19A, August 1988, pp. 1919 – 1923.3) Budka, P.Z., Viertl, J.R.M., Thamboo, S.V., Schiffman, R.A., “Gravity Independent Macro/Micro-Structural Features: Lessons from Nickel-Iron Meteorites,” 7th International Symposium on Experimental Methods for Microgravity Materials Science,” TMS 1995, pp. 27 – 36.4) Budka, P.Z., Viertl, J.R.M., Thamboo, S.V., “Microgravity Solidification Microstructures as Illustrated by Nickel-Iron and Stony-Iron Meteorites,” 8th International Symposium on Experimental Methods for Microgravity Materials Science,” TMS 1996.5) Norton, O. Richard, “The Cambridge Encyclopedia of Meteorites,” Cambridge University Press, p.203, 2002.6) Buchwald, Vagn F., “Handbook of Iron Meteorites,” University of California Press, 1975.7) Marvin, Ursula B., Petaev, M.I., Kempton, R.W., “Preliminary Observations On Drusy Vugs in the Albion Iron Meteorite,” Harvard -Smithsonian Center for Astrophysics, New England Meteoritical Services, Presented at the 27th Lunar and Planetary Science Conference, Lunar and Planetary Institute.

VI. ACKNOWLEDGMENTS

The Author is grateful for the help of Dr. Niko Gjaja, Ms. Tymm Schumaker and Mr. Kevin Shoemaker, The M&P Lab, Schenectady, NY. This work has significantly benefited from the photographic skills and techniques of Ms. Schumaker and Mr. Shoemaker. Mr. Russell W. Kempton, New England Meteoritical Services; Dr. Carleton Moore, Center for Meteorite Studies, Arizona State University, Tempe, AZ; and Mr. George VanderVoort, Buehler Ltd. are thanked for their contribution of images to this paper. Dr. Vagn F. Buchwald, Denmark, is thanked for the Agpalilik specimen. The help of Dr. J.R.M. Viertl and Dr. Mark Markovitz, of Schenectady, NY, is gratefully acknowledged.

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