Vermillion meteorite, Marshall County, Kansas, USAi

Regional Level Types
Vermillion meteorite	Meteorite Fall Location
Marshall County	County
Kansas	State
USA	Country

Pallasite, ungrouped
Find, 1991; 34.36 kg

An iron-rich mass was found by M. & G. Farrell while planting in a grain field. While the Fe-Ni metal content (~86 vol%) is the predominant constituent of the Vermillion meteorite, the silicate constituents (mostly rounded Mg-rich olivine crystals) bring Vermillion clearly into the domain of the pallasite group of stony-irons and it has been so classified. And, in this respect, Vermillion's mineralogy is quite mainstream. The Fe-Ni metal is dominated by kamacite and taenite, olivine (Fa~88) is the predominant silicate, and normally expected accessories [e.g., chromite, merrillite] are present. However, there are a number of oddities about the Vermillion pallasite which have drawn extra attention from the meteoritical community. To begin with, about 5% of the silicate component is Mg-rich orthopyroxene —usually found along the rims of olivine crystals. The presence of olivine with pyroxene is quite common in both meteoritical and planetary bodies, but it is rare in pallasites. Indeed, the presence of even this seemingly small pyroxene component was sufficient for Vermillion to very quickly earn a semi-formal designation as a 'pyroxene pallasite.' Other anomalies have been discovered as well. The oxygen-isotope ratios of Vermillion are clearly distinguishable from both those of the main group pallasites (PMG) and from the smaller group of Eagle Station pallasites (PES). Furthermore, detailed studies of the major and trace elements within the Fe-Ni metal component make it clear that Vermillion metal does not share the relatively close affinities between PMG metal and IIIAB irons (Cf. Wasson & Choi, 2003). Vermillion, then, is one of 6 pallasites formally designated as 'ungrouped.' At the present time, the conventional assumption is that the PMG pallasites, the PES pallasites, and Vermillion are likely derived from 3 separate original parent bodies.

So why isn't Vermillion officially designated a 'pyroxene pallasite' [instead of a mere 'ungrouped pallasite']?' The answer is that the other ungrouped pallasites have unexplained variations from Vermillion in textural features, O-isotopes and/or metallic trace elements which would make such a grouping problematic. [One should note that the problem of pyroxene pallasites is eerily similar issue to the so-called 'missing olivine' problem for the pyroxene-rich diogenites. While our petrological expectations tell us that olivine and pyroxene are sequential products or co-products of igneous differentiation, the pallasites and diogenites appear to provide us with an olivine-rich meteorite group without the clearly associated pyroxene-rich cohort group that one would normally normally expect and, vice versa, a pyroxene-rich group without an associated olivine-rich group.]

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References