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staurolite A member of the nesosilicates and an important metamorphic index mineral with the approximate composition (Fe2+,Mg)2(Al,Fe3+)9O6[Si4O16](O,OH)2; sp. gr. 3.74–3.85; hardness 7.5; monoclinic; crystals prismatic; shades of brown; occurs in regionally metamorphosed schists and gneisses, such as iron-rich pelites, with a high Fe3+/Fe2+ ratio at moderate grades of metamorphism and in association with garnet (almandine) and kyanite; it may develop from chloritoid as the metamorphic grade increases.