Also found in: Dictionary, Wikipedia.
a mineral of the silicate class. Sillimanite is a high-temperature, polymorphic modification of the composition Al[AlSiO5] and contains 1–1.5 percent Fe2O3 in admixture form. The sillimanite structure is made up of chains of alternating SiO4 and AlO4 tetrahedrons, connected by chains of Al octahedrons. The mineral crystallizes in the orthorhombic system, forming acicular crystals, compact radiated masses, or thin fibrous aggregates. It sometimes forms dispersed, filiform inclusions in other minerals (fibrolite variety). Sillimanite is gray, light brown, or pale green; it has a vitreous luster, a hardness of 6.5-7.5 on Mohs’ scale, and a density of 3,270 kg/m3. At high temperatures (approximately 1545°C), it decomposes into mullite and silica.
Sillimanite occurs in thermally and regionally metamorphosed clay rocks. It serves as a raw material in the production of refractories and acid-resistant products with a high alumina content. The largest known sillimanite deposits are in India (Khasi Hills and Pipri).