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a silicate mineral with the chemical composition Fe2+Al4[Si04]2O2(OH)2. Admixtures of Co, Ni (lusakite variety), and Mn (nordmarkite variety) frequently occur.
Based on the structure of the crystal lattice, staurolite belongs to the silicate class having isolated tetrahedral [SiO4]4- groups joined through Fe2+ or Al3+ cations in fourfold or sixfold coordination; the excess bonding orbitals are compensated by additional O2- and (OH)- anions. Staurolite crystallizes in the mono-clinic (pseudorhombic) system, forming either separate short prismatic crystals dark brown in color or, more frequently, cruciform twins. Staurolite’s hardness on Mohs’ scale is 7–7.5, and the density is 3,650–3,770 kg/m3. The color may be dark red, brown, brown-black, or yellow-brown, and the luster is vitreous.
Staurolite is an accessory mineral in schists (together with kyanite, garnet, and andalusite) and gneisses. Its rare transparent crystals are sometimes used as gems.