Staurolite

staurolite, silicate mineral [(Fe,Mg,Zn)_3-4Al₁₈Si₈O₄₈H_2-4] produced by regional metamorphism in rocks such as mica schists, slates, and gneisses, where it is generally associated with other minerals such as kyanite, garnet, and tourmaline. Staurolite is a brittle, hard mineral that has a dull lustre. Its crystals are usually dark brown in colour and are often twinned in cruciform patterns (known as fairy crosses) that can be made into ornaments. The name is derived from stauros, the Greek word for cross. Staurolite occurs in Canada; North Carolina, Virginia, and Georgia, U.S.; Brazil; Brittany, France; and Switzerland, especially along the Saint Gotthard Pass. For chemical formula and detailed physical properties, see silicate mineral (table).