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Staurolite
Chemical Formula	Fe2+2Al9O6(SiO4)4(O,OH)2[1]
Species	Silicates
Crystal System	Monoclinic
Mohs Scale	7
Specific Gravity	3.74-3.83
Color	Dark reddish brown to blackish brown, yellowish brown, rarely blue; pale golden yellow in thin section
Streak	White to grayish
Luster	Sub-Vitreous, Resinous
Refractive Index	n = 1.736 - 1.747 n = 1.740 - 1.754 n = 1.745 - 1.762
Diaphaneity	Translucent
Cleavage	Distinct/GoodDistinct on
Fracture	Sub-Conchoidal
Crystal Habit：Commonly in prismatic crystals
Geological Setting：Amphibolite grade of regionally metamorphosed pelitic rocks.

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: Fe²⁺₂Al₉O₆(SiO₄)₄(O,OH)₂. Magnesium, zinc and manganese substitute in the iron site and trivalent iron can substitute for aluminium.

Properties

         

A special property of staurolite is that it often occurs twinned in a characteristic cross-shape, called penetration twinning. 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 mantled porphyroblastic character.

Name

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

Occurrence

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.

It is the official state mineral of the U.S. state of Georgia and is also to be found in the Lepontine Alps in Switzerland.

Staurolite is also found in Fairy Stone State Park in Patrick County, Virginia. The park is named for a local name for staurolite from a legend in the area. Samples are also found in Taos, New Mexico, near Blanchard Dam in Minnesota and Selbu, Norway.

Use

Staurolite is one of the index minerals that are used to estimate the temperature, depth, and pressure at which a rock undergoes metamorphism.