CHEMICAL COMPOSITION AND STRUCTURE OF THE WEATHERING PRODUCTS OF IRON-NICKEL OF THE GRUZ’KE CHONDRITE (H4)

Abstract

Secondary weathering microstructures such as oxide rims, oxide veins and pseudomorphs of the weathering products are genetically related to altered (weathered) iron-nickel grains (kamacite α-(Fe,Ni), taenite γ-(Fe,Ni), plessite α+γ-(Fe,Ni)) of the Gruz’ke ordinary chondrite (H4). Rare microglobules of corrosion products were found in the hollow space inside some pseudomorphs. Various weathering microstructures consist of the dispersed mixtures of secondary iron oxyhydroxides – wide-spread goethite α-FeO(OH) and hydrogoethite FeO(OH)·nH2O. They spread from the grain boundary into the interior of the metal grains. The presence of lepidocrocite γ-FeO(OH) is assumed. Iron hydroxides are characterized by: а) heterogeneous grey color; b) various structures, the presence of replaced areas of mottled, collomorphic and layered structure; c) porosity and zonal structure; d) variations in the FeO and NiO content. The oxide rim around a large grain of Fe-Ni metal consists of (the average content from 4 an., content limits are given in the brackets, wt. %): FeO 82.7 (81.6–83.8); NiO 10.9 (10.4–11.7); SO3 4.88 (4.23–5.92); CoO 0.79 (up to 1.31); CuO 0.14 (up to 0.57); MnO 0.14 (up to 0.54); MgO 0.16; K2O + CaO 0.13; Cl 0.17 (up to 0.26). Iron oxyhydroxide pseudomorph contains (the average of 3 an., limits – in the brackets. wt. %): FeO 92.6 (92.1–93.0); NiO 3.5 (2.72–4.02); SiO2 3.86 (3.32–4.96); Cl 0.08 (up to 0.24). The phase and chemical heterogeneity of Fe-Ni, low Ni concentration in α-(Fe,Ni) are the main causes of selective oxidation and replacement of kamacite and plessite, compared to taenite. The process of corrosion and replacement of Fe-Ni metal occurred in the presence of moisture, as evidenced by the direct spatial connection of iron-nickel with FeO(OH)·nH2O. Completely altered Fe-Ni grains, which have lost their primary structural and chemical characteristics, are not quite reliable for the interpretation of the pre-terrestrial processes of the meteorite matter formation. Iron hydroxides have a low chlorine content of Cl < 0,3 wt. %, which indicates their low corrosiveness. As a result, further corrosion and replacement of the Fe-Ni metal will be inhibited, which will prevent oxidation of the meteorite sample during laboratory research and storage.