Двійник ставроліту з колекції мінералогічного музею Лазаренка Львівського національного університету — Photo: Helgi · CC BY-SA 3.0
Staurolite
Also known as: Fairy cross, Fairy stone.
A brown-to-black iron aluminosilicate — its cruciform twins were carried as 'fairy crosses' good-luck charms.
Intro adapted from Wikipedia (CC BY-SA).
Staurolite is an IMA-recognized mineral species in the silicate class, with the chemical formula Fe²⁺₂Al₉Si₄O₂₃(OH).
It crystallizes in the monoclinic crystal system (space group C2/m). It is a member of the Staurolite structural group.
Key properties: Mohs hardness 7–7.5; specific gravity 3.74–3.83; typically brown or black or red; White to grayish streak; Subvitreous to resinous luster; refractive index 1.747; optically biaxial.
IMA symbol St, grandfathered status — known before the IMA’s founding, named in 1792.
Common in pelitic schists, gneisses, and argillaceous rocks regionally metamorphosed to amphibolite grade; also a detrital mineral.
It is found associated with Almandine, sillimanite, kyanite, tourmaline, muscovite, quartz, chloritoid.
Derived from the Greek for a cross, describing the common twins.
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