Staurolite
A cross-shaped mineral used to gauge metamorphic conditions.
Staurolite is a nesosilicate mineral that ranges in color from reddish brown to black, is mostly opaque, and leaves a white streak. It belongs to the monoclinic crystal system, ranks 7 to 7.5 on the Mohs hardness scale, and has the chemical formula Fe2+2Al9O6(SiO4)4(O,OH)2. In its structure, magnesium, zinc, and manganese can replace iron, while trivalent iron can take the place of aluminum.
A key feature of staurolite is its frequent twinning, which forms a cross-shaped pattern known as cruciform penetration twinning. In hand samples, visible crystals are typically prismatic, and the mineral often appears as porphyroblasts. Under a microscope in thin sections, staurolite is commonly twinned and shows low first-order birefringence, similar to quartz, with the twinning exhibiting optical continuity. In metamorphic rocks, it can be recognized by a "Swiss cheese" look due to poikilitic quartz inclusions and its tendency to form mantled porphyroblasts.
The name comes from the Greek words *stauros* (cross) and *lithos* (stone), referencing the common cross-shaped twinning.
Staurolite forms in regional metamorphic rocks of intermediate to high grade. It is found alongside almandine garnet, micas, kyanite, albite, biotite, and sillimanite in gneiss and schist. It is the official state mineral of Georgia in the United States and also occurs in the Lepontine Alps of Switzerland. The most common location is Fannin County, Georgia. Other sites include Fairy Stone State Park in Patrick County, Virginia (named after a local legend about the mineral); Island Park, Idaho, near Henrys Lake; Taos, New Mexico; near Blanchard Dam in Minnesota; and Selbu, Norway.
Staurolite serves as an index mineral, helping geologists estimate the temperature, depth, and pressure conditions during metamorphism.
- chemical_formula
- Fe2+2Al9O6(SiO4)4(O,OH)2
- crystal_system
- monoclinic
- hardness
- 7 to 7.5
- streak
- white
- color
- reddish brown to black
- known_for
- cruciform twinning and use as an index mineral in metamorphic geology
Lore & Background
Staurolite is a reddish brown to black, mostly opaque nesosilicate mineral with a white streak. It crystallizes in the monoclinic crystal system and possesses a Mohs hardness of 7 to 7.5. Its chemical formula is Fe²⁺₂Al₉O₆(SiO₄)₄(O,OH)₂, with magnesium, zinc, and manganese able to substitute for iron, and trivalent iron capable of substituting for aluminium. The mineral is renowned for its frequent cruciform penetration twinning, which forms a characteristic cross-shape; the name itself derives from the Greek *stauros* (cross) and *lithos* (stone), referencing this common twinning. In hand samples, macroscopically visible staurolite crystals are prismatic, and the mineral often occurs as porphyroblasts. Under the microscope in thin sections, staurolite is commonly twinned and displays lower first-order birefringence similar to quartz, with the twinning showing optical continuity. It can be identified in metamorphic rocks by its distinctive “swiss cheese” appearance, caused by poikilitic quartz inclusions, and its often mantled porphyroblastic character. Staurolite is a regional metamorphic mineral of intermediate to high grade, found in gneiss and schist alongside almandine garnet, micas, kyanite, albite, biotite, and sillimanite. Notable occurrences include Fannin County, Georgia (where it is the official state mineral), Fairy Stone State Park in Virginia, the Lepontine Alps in Switzerland, and locations in Idaho, New Mexico, Minnesota, and Selbu, Norway. It serves as an index mineral for estimating the temperature, depth, and pressure of metamorphism.
Reader's Guide
Staurolite is a regional metamorphic mineral of intermediate to high grade, occurring with almandine garnet, micas, kyanite, albite, biotite, and sillimanite in gneiss and schist. Its significance lies in its role as one of the index minerals used to estimate the temperature, depth, and pressure at which a rock undergoes metamorphism. This makes it a key tool for geologists studying metamorphic processes. The mineral is the official state mineral of the U.S. state of Georgia and is also found in the Lepontine Alps in Switzerland. Notable localities include Fannin County, Georgia; Fairy Stone State Park in Patrick County, Virginia; Island Park, Idaho; Taos, New Mexico; near Blanchard Dam in Minnesota; and Selbu, Norway. The name derives from the Greek 'stauros' for cross and 'lithos' for stone, referencing its common twinning.
Did You Know?
- Staurolite is named from the Greek 'stauros' for cross and 'lithos' for stone, due to its common cruciform twinning.
- It is the official state mineral of the U.S. state of Georgia.
- Staurolite is used as an index mineral to estimate the temperature, depth, and pressure of metamorphism.
- It is most commonly found in Fannin County, Georgia.
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