Staurolite (nonfiction)

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Staurolite is a red brown to black, mostly opaque, nesosilicate mineral with a white streak. It crystallizes in the monoclinic crystal system, has a Mohs hardness of 7 to 7.5 and the chemical formula: Fe2+2Al9O6(SiO4)4(O,OH)2. Magnesium, zinc and manganese substitute in the iron site and trivalent iron can substitute for aluminum.

Staurolite often occurs twinned in a characteristic cross-shape, called cruciform penetration twinning. In handsamples, macroscopically visible staurolite crystals are of prismatic shape. The mineral often forms porphyroblasts.

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