Oldhamite
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Oldhamite is a calcium magnesium sulfide mineral with the chemical formula S. Ferrous iron may also be present in the mineral resulting in the chemical formula S. It is a pale to dark brown accessory mineral in meteorites. Wikipedia
Color: Pale chestnut-brown
Formula (repeating unit): (Ca,Mg)S
Mohs scale hardness: 4
Optical properties: Isotropic
Refractive index: n = 2.137