EVIDENCES OF WEATHERING OF IRON-NICKEL AND TROILITE IN THE GRUZ'KE Н4-TYPE CHONDRITE

Abstract

Terrestrial weathering of the Gruz'ke chondrite was investigated by considering its initial chemical, structural, and mineralogical characteristics. The spread of corrosion and degree of alteration were related to the high content of Fe-Ni metal, the phase inhomogeneity and the different concentration of Ni in the metal, as well as, the shock metamorphism features, and the residence time of the meteorite sample in a soil environment. The formation of secondary oxide veins and oxide rims, consisting of iron hydroxide pseudomorphs was caused by the selective corrosion of kamacite α-(Fe,Ni), compared to taenite γ-(Fe,Ni) and troilite FeS. A spatial relationship between shock metamorphism features, namely areas of plastic deformation, shock heating and weathering of the meteorite matter, has been confirmed. A secondary Ni-enriched phase, which contains 71.34 to 72.94 wt.% Ni, probably corresponds to awaruite (Ni2Fe to Ni3Fe) or native nickel. This phase is described for the first time as corrosion product of Fe-Ni grains in the Gruz'ke chondrite. We assume that this phase was formed during the alteration of Fe-Ni metal. Fe was removed from the meteorite metal during the corrosion process, while sulfur was introduced. The formation of iron hydroxide pseudomorphs caused changes in primary chemical and structural properties of some crystals of nickel-iron metal and troilite. Thus, the use of weathered mineral grains to interpret the pre-terrestrial conditions of the chondrite matter formation and evolution is limited. In spite of the changes in chemical and mineral composition, the corrosion products of nickel iron and troilite have consistent low chlorine content.Therefore we assume that the akaganeite β-FeO(OH,Cl) secondary phase is unlikely to form. It will prevent further alteration of the Fe-Ni metal and ensure long-term storage and preservation of the meteorite sample for further laboratory research and storage in the museum collection.