mineral

Sillimanite

Al2O3(SiO2)

Hardness: 6.5-7.5 on the Mohs scale, Color: Brown, grayish-white, pale green, or yellow, Luster: Vitreous to silky, Crystal Structure: Orthorhombic, Cleavage: Perfect in one direction, Specific Gravity: 3.23-3.25

Hardness: 6

Identified July 29, 2026More mineral →

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Physical properties

Formation & geological history

Forms primarily through regional metamorphism of argillaceous rocks at high temperatures and pressures, typically in pelitic schists and gneisses.

Uses & applications

Used in the manufacture of high-temperature porcelain and ceramics, particularly in furnace linings and spark plugs, as well as occasionally cut as a gemstone.

Geological facts

Named after the American chemist Benjamin Silliman, sillimanite is one of the three polymorphs of aluminum silicate, the other two being andalusite and kyanite.

Field identification & locations

Identified by its fibrous or prismatic crystal habits and association with high-grade metamorphic rocks. Commonly found in Brandywine Springs, Delaware, as well as regions in France, India, and the United States.