Staurolite

Staurolite
Staurolite from Pestsovye Keivy, Keivy Mountains, Kola Peninsula, Murmanskaja Oblast', Northern Region, Russia, 2.5 × 2.2 × 1 cm
General
Category	Nesosilicate
Formula (repeating unit)	Fe2+2Al9O6(SiO4)4(O,OH)2[1]
IMA symbol	St[2]
Strunz classification	9.AF.30
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/m
Unit cell	a = 7.86 Å, b = 16.6 Å c = 5.65 Å; β = 90.45°; Z = 2
Identification
Color	Dark reddish brown to blackish brown, yellowish brown, rarely blue; pale golden yellow in thin section
Crystal 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 scale hardness	7 – 7.5
Luster	Subvitreous to resinous
Streak	White to grayish
Diaphaneity	Transparent to opaque
Specific gravity	3.74 – 3.83 meas. 3.686 calc.
Optical properties	Biaxial (+)
Refractive index	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 angle	Measured: 88°, Calculated: 84° to 88°
Dispersion	r > v; weak
References	[3][4][5]

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.^[1]