Wurtzite
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Wurtzite is a zinc and iron sulfide mineral with the chemical formula S, a less frequently encountered structural polymorph form of sphalerite. The iron content is variable up to eight percent. It is trimorphous with matraite and sphalerite. Wikipedia
Birefringence: δ = 0.022
Cleavage: and
Color: Brownish black, orange brown, reddish brown, black
Crystal class: Dihexagonal pyramidal (6mm); H-M symbol: (6mm)
Crystal habit: Radial clusters and colloform crusts and masses. Also as tabular crystals
Crystal system: Hexagonal
IMA symbol: Wur