sillimanitt

Sillimanite, sometimes spelled sillimanitt in some languages, is an aluminosilicate mineral with the chemical formula Al2SiO5. It is one of the three polymorphs of Al2SiO5, the others being andalusite and kyanite. Sillimanite forms under high-temperature, relatively low-pressure conditions and is a hallmark of high-grade metamorphic rocks.

Sillimanite occurs predominantly in aluminous metamorphic environments such as pelites and granulites, where it develops during peak metamorphism. It can also appear in contact metamorphic zones and, less commonly, in pegmatites and hydrothermal veins. In rocks, it is often found together with garnet, quartz, cordierite, and other aluminum-rich minerals. The presence of sillimanite, in combination with andalusite and kyanite, helps geologists reconstruct pressure–temperature paths of metamorphism.

In terms of structure and appearance, sillimanite crystallizes in the orthorhombic system. It forms elongate prismatic crystals and fibrous aggregates known as fibrolite. The mineral is typically colorless to white, sometimes with brownish or yellowish tints, and has a vitreous luster. Mohs hardness is about 7 to 7.5, with cleavage that is generally imperfect or indistinct.

Industrially, sillimanite is valued as a source of alumina for high-temperature refractory materials, including bricks and crucibles used in steelmaking and glass production. At sufficient temperatures, sillimanite can transform into mullite (3Al2O3·2SiO2), a phase that enhances refractoriness. In metamorphic petrology, sillimanite serves as a diagnostic indicator of high-temperature, low- to intermediate-pressure conditions, aiding in the interpretation of metamorphic histories.

The mineral is named after the American mineralogist Benjamin Silliman Jr.