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Staurolite

Mineral formula: Fe2+2Al9Si4O23(OH)

The formula can also be considered as alternating layers of kyanite and oxide, and so can be recast as:
4Al2O[SiO4].AlFe2+2O4H

Member of the Staurolite group

Crystal system: monoclinic

Crystal habit: elongated prisms; twinning is common, especially 60-degree twins on {231}. Twinning on {031} at 90-degrees is rare.

Cleavage: distinct/good on {010}

Fracture: sub-conchoidal

Color: dark brown, brownish-black, red-brown

Luster: sub-vitreous, resinous

Diaphaneity: opaque

Moh’s scale hardness: 7 – 7 ½

Streak: white to grayish

Specific gravity: 3.74 – 3.83

Named after: the Greek word "stavros", cross, alluding to the cruciform twins it displays in some cases.

Geological occurrence: in regionally metamorphosed rocks, especially micaceous schists.

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