The new mineral vyalsovite, FeS·Ca(OH)2·Al(OH)3, has been found in forsterite skarns at the lower contact of the Tatnakh gabbro-dolerite intrusion in Devonian dolomite. It occurs as 5- to 150-μm grains and veinlets in association with valleriite, diaspore, djerfisherite, serpentine, and magnetite. This assemblage formed as a result of the replacemont of minerals included in an earlier assemblage of chalcopyrite, forsterite, and spinel. Vyalsovite has an unusual composition and unique optical properties. It contains Fe 23.65,Ca 16.30, Al 10.94, S 13.17, Mn 0.23 wt%. In polished section, the mineral displays extreme reflection pleochroism from dark gray to crimson red. Reflectance in air at 540 nm is 8.15% for Ro and 11.4% for Re. Reflectance spectra of vyalsovite Ro and Re cross several times in the range from 250 to 800 nm. The optic sigr of vyalsovite changes four times within this range. No other known ore mineral possesses such optical properties.

According to X-ray and electron diffraction data, vyalsovite is orthorhombic (possible space groups Cmmm, C222, Cmm2, Cm2m, Cmma, C2ma) with a = 14.20(1), b = 20.98(2), c = 5A96(4) Å. The calculated density is 1.96 g/cm3 for the ideal composition and Z = 8.

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