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

Transcription

1 14 th 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 abudka@nycap.rr.com ABSTRACT The 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 Contents I. INTRODUCTION 2 II. III. IV. A VISUAL OVERVIEW: FROM PALLASITES TO NICKEL-IRON METEORITES..2 IMILAC METALLOGRAPHIC STUDY...9 Piece A Side 1: Pages Piece A Side 2: Pages Piece B Side 1: Pages Piece B Side 2: Pages Conclusions.40 V. References 40 VI. Acknowledgments..41 Page 1

2 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 Mg 2 SiO 4 (forsterite) and Fe 2 SiO 4 (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. Page 2

3 Table 1 Imilac Pieces A and B Imilac Piece A ~50-50 Metal/Silicate Mass: grams Imilac Piece B ~95-5 Metal Silicate Mass: 8.28 grams Length: 4.04 cm Length: 3.58 cm Width: 2.69 cm Width: 2.46 cm Thickness: cm Thickness: cm Brenham Pallasite This Brenham image, Figure 3a, is often included in meteorite books for its unusual microstructure, a combination of stonyiron 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. Page 3

4 8 mm Figure 1: Springwater Stony-Iron Meteorite Page 4

5 Figure 2: Imilac Stony-Iron Meteorite Back lighting highlights translucent regions Page 5

6 Scale Bar 10 mm Brenham Pallasite with typical Widmanstatten Structure Figure 3b Figure 3a Figure 3a courtesy of Carleton Moore, Arizona State University, Center for Meteorite Studies Figures 3b and c from Handbook of Iron Meteorites, Vagn F. Buchwald, University of California Press, Figure 3c Scale Bar 30 mm Page 6

7 Meteoritic Widmanstatten Structure Kamacite - Body-centered cubic iron - Ferrite Ni: 4-7.5% Co: % Taenite - Face-centered cubic iron - Austenite Ni: 25-50% Co:.3 -.8% C: % P: % Agpalilik 2.25 cm Figure 4a Courtesy of Vagn F. Buchwald Kamacite: Brown or Blue Etching Phase Taenite: White Etching Phase Figure 4b Gibeon Courtesy of G. Vander Voort Figure 4: Typical Widmanstatten Structure Page 7

8 Albion Widmanstatten Structure From 22 kg mass 10 mm Figure 5a Figure 5b 10 mm Photos Courtesy of Russell W. Kempton New England Meteoritical Services Figure 5c Page 8 10 mm

9 III. IMILAC METALLOGRAPHIC STUDY This 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. Page 9

10 01 A Imilac Piece A Side 1 Page 10

11 Imilac Piece A Side 1 Page 11

12 1 24 A4 Imilac Piece A Side 1 Page 12

13 2 23 A3 Imilac Piece A Side 1 Page 13

14 3 28 A5 Imilac Piece A Side 1 Page 14

15 4 22 A2 Imilac Piece A Side 1 Page 15

16 5 26 A6 Imilac Piece A Side 1 Page 16

17 6 21 A1 Imilac Piece A Side 1 Page 17

18 02 B Imilac Piece A Side 2 Page 18

19 1 02 B WITH OUTLINE Imilac Piece A Side 2 Page 19

20 1 17 B3 Imilac Piece A Side 2 Page 20

21 2 18 B4 Imilac Piece A Side 2 Page 21

22 3 16 B2 Imilac Piece A Side 2 Page 22

23 4 19 B5 Imilac Piece A Side 2 Page 23

24 5 15 B1 Imilac Piece A Side 2 Page 24

25 6 20 B6 Imilac Piece A Side 2 Page 25

26 04 D Flip Imilac Piece B Side 1 Page 26

27 1 04 D Flip Imilac Piece B Side 1 Page 27

28 1 05 D1 Imilac Piece B Side 1 Page 28

29 2 06 D2 Imilac Piece B Side 1 Page 29

30 3 07 D3 Imilac Piece B Side 1 Page 30

31 4 10 D6 Imilac Piece B Side 1 Page 31

32 5 09 D5 Imilac Piece B Side 1 Page 32

33 6 08 D4 Imilac Piece B Side 1 Page 33

34 03 C Flip Imilac Piece B Side 2 Page 34

35 03 C Flip Imilac Piece B Side 2 Page 35

36 1 11 C1 Imilac Piece B Side 2 Page 36

37 2 12 C2 Imilac Piece B Side 2 Page 37

38 3 14 C4 Imilac Piece B Side 2 Page 38

39 4 13 C3 Imilac Piece B Side 2 Page 39

40 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 ) Budka, P.Z., Viertl, J.R.M., Thamboo, S.V., Schiffman, R.A., Gravity Independent Macro/Micro-Structural Features: Lessons from Nickel-Iron Meteorites, 7 th International Symposium on Experimental Methods for Microgravity Materials Science, TMS 1995, pp ) Budka, P.Z., Viertl, J.R.M., Thamboo, S.V., Microgravity Solidification Microstructures as Illustrated by Nickel-Iron and Stony-Iron Meteorites, 8 th International Symposium on Experimental Methods for Microgravity Materials Science, TMS ) Norton, O. Richard, The Cambridge Encyclopedia of Meteorites, Cambridge University Press, p.203, ) Buchwald, Vagn F., Handbook of Iron Meteorites, University of California Press, ) 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. Page 40

41 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. Page 41