Staurolite

Staurolite
Staurolite from Pestsovye Keivy, Keivy Mountains, Kola Peninsula, Murmanskaja Oblast', Northern Region, Russia, 2.5 x 2.2 x 1 cm
General
Category	Nesosilicate
Formula (repeating unit)	Fe2+2Al9O6(SiO4)4(O,OH)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

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 aluminium.

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.

In thin sections staurolite is commonly twinned and shows lower first order birefringence similar to quartz, with the twinning displaying optical continuity. It can be identified in metamorphic rocks by its swiss cheese appearance (with poikilitic quartz) and often mantled porphyroblastic character.

The name is derived from the Greek, stauros for cross and lithos for stone in reference to the common twinning.

Staurolite is a regional metamorphic mineral of intermediate to high grade. It occurs with almandine garnet, micas, kyanite; as well as albite, biotite, and sillimanite in gneiss and schist of regional metamorphic rocks.