cordierite

Cordierite is a magnesium iron aluminum silicate mineral with the chemical formula (Mg,Fe)2Al4Si5O18. It belongs to the cordierite group and crystallizes in the orthorhombic system. It typically forms elongated prismatic crystals or fibrous aggregates and is usually colorless to pale gray or blue, displaying notable pleochroism under polarized light. It has a Mohs hardness of about 7 and a specific gravity around 2.5–2.7.

Cordierite occurs in metamorphic rocks, particularly aluminous granulites and granulite-grade metamorphism, and in some pegmatites and contact metamorphic zones. It forms under high-temperature metamorphism and is commonly associated with minerals such as sillimanite, quartz, muscovite, biotite, and garnet.

In industry, cordierite is valued for its low thermal expansion and high thermal shock resistance. It is used to manufacture ceramics and heating elements, and as the basis of cordierite glass-ceramics for cookware and electronic components.

The mineral name honors Louis Cordier, a French geologist who described it in the early 19th century.