Sillimanite

Sillimanite is a mineral species in the aluminosilicate group with the chemical formula Al2SiO5. It is one of the three polymorphs of Al2SiO5, the others being andalusite and kyanite. Sillimanite crystallizes in the orthorhombic system and commonly occurs as slender fibers or prismatic crystals, often forming fibrous aggregates in high-temperature metamorphic rocks.

Formation and occurrence: Sillimanite forms at high temperatures in aluminous rocks during regional or contact metamorphism. It is stable at high temperatures and relatively low pressures, and its presence indicates high-grade metamorphic conditions. Under higher pressures, kyanite becomes stable, while at lower temperatures, andalusite forms. Sillimanite is typically found in pelitic and quartzofeldspathic rocks such as schists and granulites, and is commonly associated with garnet, cordierite, and quartz.

Naming and properties: The mineral is named after Benjamin Silliman, an American chemist and professor at Yale University. Sillimanite is colorless to white, occasionally brownish, with a vitreous luster. It has a hardness around 7 on the Mohs scale and can occur in massive, fibrous, or prismatic forms. Its high-temperature stability makes it of interest both in metamorphic petrology and in industrial applications.

Uses and significance: Sillimanite is used as a refractory material due to its high melting point and chemical stability, and it is encountered in ceramic and kiln linings. In petrography, the presence of sillimanite helps indicate high-temperature metamorphic conditions and aids in reconstructing metamorphic grade and P–T paths in rocks.