staurolite

{{Infobox mineral | name = Staurolite | category = Nesosilicate | image = Staurolite-26463.jpg | alt = | caption = Staurolite from Pestsovye Keivy, Keivy Mountains, Kola Peninsula, Murmanskaja Oblast', Northern Region, Russia, 2.5 × 2.2 × 1 cm | formula = Fe2+2Al9O6(SiO4)4(O,OH)2 | IMAsymbol = St | molweight = | strunz = 9.AF.30 | dana = | system = Monoclinic | class = Prismatic (2/m) (same H-M symbol) | symmetry = C2/m | unit cell = a = 7.86 Å, b = 16.6 Å c = 5.65 Å; β = 90.45°; Z = 2 | color = Dark reddish brown to blackish brown, yellowish brown, rarely blue; pale golden yellow in thin section | habit = Commonly in prismatic crystals | twinning = Commonly as 60° twins, less common as 90° cruciform twins | cleavage = Distinct on {010} | fracture = Subconchoidal | tenacity = Brittle | mohs = 7 – 7.5 | luster = Subvitreous to resinous | streak = White to grayish | diaphaneity = Transparent to opaque | gravity = 3.74 – 3.83 meas. 3.686 calc. | density = | polish = | opticalprop = Biaxial (+) | refractive = nα = 1.736 – 1.747 nβ = 1.740 – 1.754 nγ = 1.745 – 1.762 | birefringence = δ = 0.009 – 0.015 | pleochroism = X = colorless; Y = pale yellow; Z = golden yellow | 2V = Measured: 88°, Calculated: 84° to 88° | dispersion = r > v; weak | extinction = | length fast/slow = | fluorescence = | absorption = | melt = | fusibility = | diagnostic = | solubility = | other = | alteration = | references = }}

Staurolite is a reddish 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 aluminium.