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Cordierite
Left: rough specimen showing dichroism; right: cut stone
General
Category	Mineral
Chemical formula	(Mg,Fe)2Al4Si5O18
Identification
Color	Blue, violet, yellow-brown; transparent to translucent
Crystal habit	Pseudo-hexagonal prismatic twins, as imbedded grains, and massive
Crystal system	Orthorhombic; 2/m 2/m 2/m
Cleavage	{010} poor
Fracture	Conchoidal, uneven
Mohs Scale hardness	7 - 7.5
Luster	Greasy or vitreous
Refractive index	α=1.522 - 1.558 β=1.524 - 1.574 γ=1.527 - 1.578 Indices increase with Fe content.
Optical Properties	Usually optically (-), sometimes (+); 2V = 0-90°
Pleochroism	Strong, dichroic: brown-yellow, light and dark blue
Streak	White
Specific gravity	2.57 - 2.66
Fusibility	on thin edges
Diagnostic Features	Resembles quartz can be distinguished by pleochroism. Can be distinguished from corundum by its lower hardness
Other Characteristics	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).

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.

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.^{[citation needed]}

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 color 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.

• List of minerals
• List of minerals named after people

• Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., John Wiley and Sons, New York, p. 395-396, ISBN 0-471-80580-7
• Klein, Cornelius., 2002, The Manual of Mineral Science, 22nd ed., John Wiley & Sons, Inc. ISBN 0-471-25177-1
• Webmineral
• Mindat.org
• Mineral galleries
• http://www.gemstone.org/gem-by-gem/english/iolite.html