What is Staurolite?
Staurolite is a hard brown metamorphic silicate famous for cruciform penetration twins and its role as a pressure-temperature indicator.
Mineralogy and crystal structure
Reference formula: (Fe,Mg,Zn)2Al9Si4O23(OH). Staurolite is monoclinic and compositionally variable. Cross twins are characteristic but not every brown cross-shaped object is staurolite; the existing structure is retained with the locality and chemistry caveat. Staurolite is classified with silicates and has a monoclinic crystal system. Typical hardness is 7–7.5 on the Mohs scale and specific gravity is approximately 3.7–3.8.
Colour, lustre and habit
Typical colours include brown, reddish brown, black, yellow-brown. The usual prismatic, cross-twinned, granular habit and vitreous, resinous lustre help describe a specimen, but colour alone is not diagnostic.
Formation and notable localities
Staurolite forms in or is associated with medium- to high-grade regional metamorphism of aluminium-rich pelitic rocks. Important documented localities include Switzerland, Russia, Brazil, France, the United States, Scotland and Norway.
The natural cross-twin classic
Staurolite sells because crystal symmetry creates an immediately recognisable cross. Its best specimens combine clean twin geometry with a documented metamorphic host rock.
Collecting and care
Protect sharp twin arms from impact and clean with mild water. Avoid mistaking folklore for mineralogical identification.
