STAUROLITE (small cabinet)

Staurolite is a nesosilicate mineral that ranges in color from reddish-brown to black and is primarily opaque, exhibiting a white streak. It crystallizes within the monoclinic system and possesses a Mohs hardness of 7 to 7.5, with the chemical formula Fe2+2Al9O6(SiO4)4(O,OH)2. Iron can be substituted by magnesium, zinc, and manganese, while trivalent iron may replace aluminum.

This mineral is frequently found in a distinctive cross-shaped twinning known as cruciform penetration twinning. In hand samples, staurolite crystals are typically prismatic and can form porphyroblasts. Under thin section examination, staurolite often displays twinning and exhibits lower first-order birefringence akin to quartz, with optical continuity in the twinning. It is identifiable in metamorphic rocks by its characteristic "swiss cheese" appearance, often accompanied by poikilitic quartz and a mantled porphyroblastic structure. The name staurolite is derived from the Greek words for cross and stone, reflecting its common twinning pattern.