Schöllhornite, ideally Na0.3(H2O)1[CrS2], occurs in the Norton County enstatite achondrite as thin bands a few μm wide in caswellsilverite, NaCrS2, and as individual grains up to 250 μm in size, adjacent to caswellsilverite. Other associated phases are daubreelite, titanoan troilite, ferromagnesian alabandite, oldhamite, kamacite and perryite. In reflected light, schöllhornite is gray in air and bluish gray in oil. It has distinct reflection pleochroism: in air, brownish gray (brighter) and bluish gray (darker); in oil, gray with an extremely faint yellowish tint (brighter) and bluish gray (darker). Anisotropism is strong and the phase is uniaxial (−). Reflectance at λ = 546 nm is Ro = 19.3 and RE = 16.0%. Electron microprobe and ion microprobe analyses show the mineral to be (ideally) Na0.3(H2O)[CrS2], with only minor Ti and Mn (avg. 0.17 wt.% each). Synthetic and natural schöllhornite have similar X-ray powder diffraction patterns with the following lines (in Å): 8.85 (vs)(003); 4.43 (w)(006); 2.80 (m)(102); 2.53 (m)(105); 2.21 (vvw)(00,12); 1.67 (m)(110). Because of the small amount of natural Schöllhornite available for X-ray studies, the weak (006) and very, very weak (00,12) lines could not be reliably measured. Lattice constants are a = 3.32Å and c = 26.6Å, based on a hexagonal setting. Possible space groups are R3m, R3m and R32. The measured density of synthetic schöllhornite is 2.70 g/cm3 and the calculated density is 2.74 g/cm3. Experimental work by us and others shows that sodium chromium sulfide hydrate (bilayered molecular water form) is readily produced by the partial hydration of caswellsilverite (NaCrS2) in aqueous solution at room temperature. The monolayered molecular water form, Schöllhornite, forms by subsequent partial dehydration of the bilayered phase. Based on its intergrowth with caswellsilverite, its composition, water content and structure and the lack of indigeneous water in the highly reduced Norton County enstatite achondrite, we propose that schöllhornite is the terrestrial weathering product of caswellsilverite. We conclude that caswellsilverite was altered to the bilayered sodium chromium sulfide hydrate through reaction with moisture while the meteorite specimens rested in the soil of Nebraska. During subsequent storage and drying in the arid atmosphere of New Mexico, the bilayered phase converted into the monolayered sodium chromium sulfide hydrate, i.e., into schöllhornite. We have named this new mineral after Robert Schöllhorn, in honor of his contributions to the study of synthetic hydrated layered chalcogenides.

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