gehlenite

Gehlenite is a silicate mineral belonging to the series pyroxenite (a type of inosilicate). Its chemical formula is Ca2Al4Si2O10(OH)4. The mineral crystallizes in the orthorhombic crystal system and typically forms prismatic crystals, commonly nodular or irregular aggregates. Its crystal symmetry corresponds to the space group Pbnm.

On the Mohs scale gehlenite hardness ranges between 5.5 and 6. It has a vitreous to dull luster and occurs in a color spectrum of white to light gray, occasionally with a yellowish tint. The mineral is translucent to opaque and its fracture is irregular. The refractive indices are nα = 1.688, nβ = 1.692, and nγ = 1.699, giving a small birefringence of 0.0110. Its pleochroism is generally weak.

Gehlenite is typically found in high‑grade metamorphic rocks such as migmatites, granulites, and associated with orthite, anorthite, and sillimanite assemblages. It forms through metamorphism of aluminous silicate minerals and can be an indicator of high temperature conditions. Occurrence sites include the Fennoscandian Shield and the Canadian Shield, and it has been documented in the Ural Mountains, the Alps, and several locations in North America.

The mineral was first described in 1836 by Russian mineralogist Peter Gustav Lejeune Dirichlet. It was named in honor of German mineralogist Andreas von Gahlen, who contributed to early mineral classification studies. Gehlenite is occasionally used in research on metamorphic petrology and as a source rock for studying redox conditions during metamorphism. Because of its relatively low abundance and specific metamorphic occurrences, gehlenite is mainly of interest to specialists in mineralogy and petrology rather than commercial applications.