The spectral and physical properties of metal in meteorite assemblages: Implications for asteroid surface materials

doi:10.1016/0019-1035(86)90086-2

Icarus

Volume 66, Issue 3, June 1986, Pages 468-486

https://doi.org/10.1016/0019-1035(86)90086-2 Get rights and content

Abstract

The metal grains in chondritic meteorites from terrestrial collections are coated with an optically thick surface layer, probably composed of iron oxide and/or iron sulfide. This coat on the metal grains suppresses the spectral contribution of NiFe metal in the reflectance curves of these meteorites. Only if this surface layer is disrupted will the strongly reddened signature of metallic NiFe be seen in chrondritic spectra. While origin of this surface layer is not yet established, it is probable that it is either pre-terrestrial or formed by the weathering of an unstable mineral species, such as lawrencite (FeCl₂), which was present as a thin, pre-terrestrial veneer on the chondritic metal grains. In either case, the surfaces of intact metal grains in asteroidal chondritic assemblages most probably will not resemble NiFe metal. Low-nickel metal grains, such as those in H-type chondrites, will be brittle at asteroid surface temperatures. High-nickel metal grains, such as those in LL-type chondrites, remain ductile down to at least 50°K, below even asteroid night side temperatures. The metal phase, even when brittle, will be at least as strong as the silicate phase in asteroid regoliths. Therefore, preferential fragmentation of brittle metal is not a viable mechanism to increase the spectral contribution of the NiFe phase in an asteroid regolith. Under plausible proposed regolith processes, only the metal-rich H-type subset of the ordinary chondrites can be expected to produce an S-type asteroid spectrum from an undifferentiated assemblage, and then only if optically thick metal grain coats are absent. Known regolith processes cannot reasonably produce an S-type spectrum from metal-poor L-, LL-, or C3-type assemblages. The strong NiFe signatures and the mafic silicate features in the reflectance spectra of the S-type asteroids appear to require that the most of them represent metal-rich, differentiated assemblages. The spectral properties of M-type asteroids do not require metal-rich or differentiated surface materials, although it is plausible that this is the case.

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Metal optical constants in the literature: In order to model reflectance spectra for intimate mixtures containing opaque metal, the optical constants of the metal must be supplied. Samples of iron meteorites or terrestrial iron may be altered by even brief exposure to the humid terrestrial atmosphere (Gaffey, 1986), producing coatings that can interfere with determination of the spectral properties of the unaltered metal as it would exist in space. Hence, values for n and k of iron have in the past been selected from the applied physics and materials science literature (Johnson and Christy, 1974; Paquin, 1995).

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