Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.332b&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 332
Other
Cooling Rates, Iron Nickel, Microgravity, Mundrabilla, Octahedrite, Solidification Time
Scientific paper
The name "Mundrabilla" is applied to two nickel-iron meteorite masses (combined mass over 22,700 kg), which apparently were a single mass before atmospheric entry [1]. A medium octahedrite, Mundrabilla exhibits the microstructural features common to other nickel-iron meteorites such as Widmanstatten structure and troilite; however, its macrostructure is anything but common. Described by Buchwald as "anomalous" [1], Mundrabilla's macrostructural morphology is characterized by strikingly prominent, rounded Widmanstatten areas separated by regions of sulfur segregation (Fig. 1). While microstructural development of a metal can reflect both solidification and solid state reactions, macrostructural features are determined during solidification. Thus, a typical metallurgist, unfamiliar with microgravity solidification, might describe Mundrabilla's macrostructure as an "anomalous" casting. Those familiar with microgravity solidification might characterize Mundrabilla's macrostructural features as due to solidification of two immiscible liquids [2]--one rich in nickel-iron, the other rich in sulfur. Combining these observations, Mundrabilla's macrostructural features are consistent with that of a liquid mass solidified under microgravity conditions [3,4]. Since nickel-iron meteorite cooling rates often serve as the foundation for assumptions about the formation of solar system bodies, information on the solidification time for the Mundrabilla mass may give additional insights. How long did it take for Mundrabilla, with a minimum "as received" mass of approximately 22,700 kg to solidify? Because Mundrabilla's mass before atmospheric entry is unknown, we take as an upper boundary a mass of 4.1 x 10^15kg. These masses, assumed spherical, range in diameter between 1.8 meters and 10 kilometers, respectively. Mundrabilla can be idealized as a pure iron liquid mass cooling from the melting point of pure iron (1535C) by radiation into space at absolute zero. The latent heat of transformation for iron is used to calculate "excess temperature," i.e., the amount the mass temperature can be raised due to recalescence. Solidification is considered complete when the center of the mass is solid. Fig. 2, is a plot of the solidification times for an iron mass in the range 1.8 meters to 10 kilometers in diameter. At the lower bound, solidification time is about 1.6 hours; at the upper bound, solidification time is on the order of 3,400 years. References: [1] Buchwald V. F. (1975) Handbook of Iron Meteorites, University of California, Berkeley. [2] Carlberg T. and Fredriksson H. (1980) Metallurgical Transactions A, 11A, 1665-1676. [3] Budka P. Z. (1988) Metallurgical Transactions A, 19A, 1919-1923. [4] Budka P. Z. (1988) J. Metals, 40, 9, 6-9. Fig. 1, which appears here in the hard copy, shows Mundrabilla--a scale in inches. Figure 2, which appears here in the hard copy, shows solidification time vs. diameter.
Budka P. Z.
Viertl R. M. J.
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