CORDIERITE, COROACI, MINAS GERAIS, BRAZIL (thumbnail)

Cordierite, also known as iolite in gemology, is a cyclosilicate composed of magnesium, iron, and aluminum. It typically contains iron and exhibits a solid solution between magnesium-rich cordierite and iron-rich sekaninaite, represented by the series formula: (Mg,Fe)2Al3(Si5AlO18) to (Fe,Mg)2Al3(Si5AlO18). A high-temperature polymorph, indialite, shares a structure with beryl and features a random distribution of aluminum within the (Si,Al)6O18 rings. Cordierite is synthesized for high-temperature applications, including catalytic converters and pizza stones.