Caswellsilverite, NaCrS2, occurs in the Norton County enstatite achondrite as anhedral grains up to 1 mm in size. It is associated with daubréelite, titanoan troilite, ferromagnesian alabandite, oldhamite, kamacite, perryite, and a dark-gray phase, also a new Na–Cr sulfide that contains 13% H2O and is the terrestrial weathering product of caswellsilverite. Caswellsilverite is yellowish-gray to light-gray in reflected light and exhibits strong bireflectance and anisotropism. Electron microprobe analysis shows that it is stoichiometric NaCrS2 and contains only minor Ti and Mn. Caswellsilverite has a NaHF2-type hexagonal rhombohedral structure (R3m), and its lattice parameters calculated from X-ray powder patterns are a = 3.55Å and c = 19.5Å, based on a hexagonal unit cell. Seven of the strongest lines in the X-ray powder pattern are: 6.49(7)(003), 2.60(10)(014), 2.07(8)(017), 1.910(8)(108), 1.779(8)(110), 1.465(6)(204) and 1.134(6)(01.16). Caswellsilverite formed at very low oxygen fugacities after most of the Norton County enstatite achondrite melt had already crystallized. This new mineral was named in honor of Dr. Caswell Silver, distinguished alumnus and benefactor of the Department of Geology and Institute of Meteoritics at the University of New Mexico.

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