Staurolite
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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 a rather complex chemical formula: (Fe,Mg,Zn)2Al9(Si,Al)4O20(OH)4. Iron, magnesium and zinc occur in variable ratios.
A special property of staurolite is that it often occurs twinned in a characteristic cross-shape. In handsamples, macroscopically visible staurolite crystals are of prismatic shape. They are often larger than the surrounding minerals and are then called 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 manteld 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 and other metamorphic minerals.
It is the official state mineral of Georgia.
Staurolite is one of the index minerals that are used to estimate the temperature, depth, and pressure at which a rock undergoes metamorphism.
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