Staurolite (redirected from staurolitic)

staurolite

Silicate mineral produced by regional metamorphism in rocks such as mica schists, slates, and gneisses, where it is generally associated with other minerals. Staurolite is a brittle, hard mineral that has a dull lustre. Its crystals are usually dark brown and are often twinned in cruciform patterns (known as fairy crosses) that can be made into ornaments. Staurolite occurs especially in Canada, Brazil, France, Switzerland, and the U.S. (North Carolina, Virginia, and Georgia).

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staurolite [′stȯr·ə‚līt]

(mineralogy)

FeAl₄(SiO₄)₂(OH)₂A reddish-brown to black neosilicate mineral that crystallizes in the orthorhombic system, has resinous to vitreous luster, hardness is 7-7.5 on Mohs scale, and specific gravity is 3.7. Also known as cross-stone; fairy stone; grenatite; staurotide.

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Staurolite 

a silicate mineral with the chemical composition Fe²⁺Al₄[Si0₄]₂O₂(OH)₂. Admixtures of Co, Ni (lusakite variety), and Mn (nordmarkite variety) frequently occur.

Based on the structure of the crystal lattice, staurolite belongs to the silicate class having isolated tetrahedral [SiO4]^4- groups joined through Fe²⁺ or Al³⁺ cations in fourfold or sixfold coordination; the excess bonding orbitals are compensated by additional O^2- and (OH)^- anions. Staurolite crystallizes in the mono-clinic (pseudorhombic) system, forming either separate short prismatic crystals dark brown in color or, more frequently, cruciform twins. Staurolite’s hardness on Mohs’ scale is 7–7.5, and the density is 3,650–3,770 kg/m³. The color may be dark red, brown, brown-black, or yellow-brown, and the luster is vitreous.

Staurolite is an accessory mineral in schists (together with kyanite, garnet, and andalusite) and gneisses. Its rare transparent crystals are sometimes used as gems.