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sillimanite (fibrolite) A nesosilicate and one of an important group of three mineral polymorphs with the same composition Al2SiO5, the other two being andalusite and kyanite; sp. gr. 3.23; hardness 6.5–7.5; orthorhombic; occurs as elongate prismatic crystals with a diamond-shaped cross-section or as fibrous and felted masses; colourless; occurs under conditions of high-grade metamorphism either in the innermost zones of thermal aureoles (see CONTACT AUREOLE) or in high-grade regional metamorphic rocks (see REGIONAL METAMORPHISM) formed under high temperatures and pressures. It is named after the American mineralogist B. Silliman.