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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……………………………………..41Page 1
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.
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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.
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8 mm
Figure 1: Springwater Stony-Iron Meteorite
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Figure 2: Imilac Stony-Iron Meteorite – Back lighting highlights translucent regionsPage 5
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
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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 Page 7
10 mmFrom 22 kg mass
Albion Widmanstatten Structure
Figure 5a
10 mmFigure 5b
Photos Courtesy of Russell W. KemptonNew England Meteoritical Services
10 mmFigure 5cPage 8
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.
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01 A
Imilac Piece A Side 1 Page 10
1
65
3
4
2
Imilac Piece A Side 1 Page 11
24 A41
Imilac Piece A Side 1 Page 12
23 A32
Imilac Piece A Side 1 Page 13
28 A53
Imilac Piece A Side 1 Page 14
22 A24
Imilac Piece A Side 1 Page 15
26 A65
Imilac Piece A Side 1 Page 16
21 A16
Imilac Piece A Side 1 Page 17
02 B
Imilac Piece A Side 2 Page 18
02 B WITH OUTLINE1
65
4
3
2
Imilac Piece A Side 2 Page 19
17 B31
Imilac Piece A Side 2 Page 20
18 B42
Imilac Piece A Side 2 Page 21
16 B23
Imilac Piece A Side 2 Page 22
19 B54
Imilac Piece A Side 2 Page 23
15 B15
Imilac Piece A Side 2 Page 24
20 B66
Imilac Piece A Side 2 Page 25
04 D Flip
Imilac Piece B Side 1 Page 26
04 D Flip
654
312
Imilac Piece B Side 1 Page 27
05 D11
Imilac Piece B Side 1 Page 28
06 D22
Imilac Piece B Side 1 Page 29
07 D33
Imilac Piece B Side 1 Page 30
10 D64
Imilac Piece B Side 1 Page 31
09 D55
Imilac Piece B Side 1 Page 32
08 D46
Imilac Piece B Side 1 Page 33
03 C Flip
Imilac Piece B Side 2 Page 34
03 C Flip
43
21
Imilac Piece B Side 2 Page 35
11 C11
Imilac Piece B Side 2 Page 36
12 C22
Imilac Piece B Side 2 Page 37
14 C43
Imilac Piece B Side 2 Page 38
13 C34
Imilac Piece B Side 2 Page 39
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.
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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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