sillimanitebearing

Sillimanitebearing describes rocks or mineral assemblages that contain the aluminosilicate mineral sillimanite (Al2SiO5). Sillimanite is one of the three polymorphs of Al2SiO5, the others being andalusite and kyanite, and it forms under high-temperature metamorphic conditions. The term is commonly used to indicate a high-grade metamorphic history for pelitic rocks and quartzites, where sillimanite appears alongside minerals such as quartz, muscovite, garnet, and sometimes kyanite or andalusite.

In metamorphic petrology, sillimanite-bearing assemblages signal high-temperature conditions, typically associated with granulite facies metamorphism. The stability of sillimanite is favored by elevated temperatures and moderate to high crustal pressures, though exact stability fields depend on rock composition. Rocks described as sillimanitebearing often show metamorphic textures from deformation and recrystallization at high temperature, and sillimanite can occur as elongate crystals or fibrous prisms within the rock matrix.

Common rock types that host sillimanite include pelitic rocks (metapelitic schists and gneisses), quartzites, and granulites. Sillimanite-bearing rocks may display associations with other aluminous minerals such as garnet, cordierite, and muscovite, and can record complex pressure–temperature histories, including overprinting metamorphic events where sillimanite persists or reappears during cooling.

From a practical perspective, the presence of sillimanite is used as an index mineral to assess metamorphic grade and thermal history. Sillimanite-bearing rocks also have industrial relevance, as sillimanite is mined for refractory and ceramic materials due to its high-temperature stability and strength.