{{Infobox mineral | name = Cordierite | category = | boxwidth = | boxbgcolor = | image = Cordierite.jpg | caption = Left: rough specimen showing dichroism; right: cut stone | formula = (Mg,Fe)₂Al₄Si₅O₁₈ | color = Blue, violet, yellow-brown; transparent to translucent | habit = Pseudo-hexagonal prismatic twins, as imbedded grains, and massive | system = Orthorhombic; 2/m 2/m 2/m | twinning = | cleavage = {010} poor | fracture = Conchoidal, uneven | mohs = 7 - 7.5 | luster = Greasy or vitreous | refractive = α=1.522 - 1.558 β=1.524 - 1.574 γ=1.527 - 1.578 Indices increase with Fe content. | opticalprop = Usually optically (-), sometimes (+); 2V = 0-90° | pleochroism = Strong, dichroic: brown-yellow, light and dark blue | streak = White | gravity = 2.57 - 2.66 | melt = | fusibility = on thin edges | diagnostic = Resembles quartz can be distinguished by pleochroism. Can be distinguished from corundum by its lower hardness | solubility = | other = Dana class: 61.2.1.1 }}

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 is named after the French geologist P. L. A. Cordier (1777 - 1861).

Occurrence

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. Cordierite occurs in the granite contact zone at Geevor Tin Mine in Cornwall.

Commercial use

Catalytic converters are commonly made from ceramics containing a large proportion of cordierite. The manufacturing process deliberately aligns the cordierite crystals to make use of the very low thermal expansion seen for one axis. This prevents thermal shock cracking from taking place when the catalytic converter is used.