Cordierite (mineralogy) or iolite (gemology) is a magnesium iron aluminium cyclosilicate. Iron is almost always present and a solid solution exists between Mg-rich cordierite and Fe-rich sekaninaite with a series formula: (Mg,Fe)2Al3(Si5AlO18) to (Fe,Mg)2Al3(Si5AlO18). A high temperature polymorph exists, indialite, which is isostructural with beryl and has a random distribution of Al in the (Si,Al)6O18 rings. Cordierite ceramic is commonly used in catalytic converters. Cordierite is named after the French geologist P. L. A. Cordier (1777 - 1861).
Cordierite typically occurs in contact or regional metamorphism of argillaceous rocks. It is especially common in hornfels produced by contact metamorphism of pelitic rocks. Two common metamorphic mineral assemblages include sillimanite-cordierite-spinel and cordierite-spinel-plagioclase-orthopyroxene. Other associated minerals include garnet (cordierite-garnet-sillimanite gneisses) and anthophyllite. Cordierite also occurs in some granites, pegmatites, and norites in gabbroic magmas. Alteration products include mica, chlorite, and talc.
As the transparent variety iolite, it is often used as a gemstone. The name "iolite" comes from the Greek word for violet. Another old name is dichroite, a Greek word meaning "two-colored rock", a reference to cordierite's strong pleochroism. Gem quality iolite varies in colour from sapphire blue to blue violet to yellowish gray to light blue as the light angle changes. Iolite is found in Sri Lanka, Burma, Australia's Northern Territory, Namibia, Brazil, Tanzania, Madagascar, Connecticut, and the Yellowknife area of the Northwest Territories of Canada.
Closely related is Fahlunite, an altered variety of cordierite from the Falun Mine, Sweden.