Left: rough specimen showing dichroism;
Right: cut stone.

- align=center bgcolor="#9966FF"	Cordierite		- align=center	- align=center bgcolor="#9966FF"	General		-	Category	Mineral	-	Chemical formula	(Mg,Fe)2Al4Si5O18	-	Dana class	61.2.1.1	- align="center" bgcolor="#9966FF"	Identification		-	Colour	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 distinguised by pleochroism. Can be distinguished from corundum by it's lower hardness.	-

Cordierite 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)₂Al₄Si₅O₁₈ to (Fe,Mg)₂Al₄Si₅O₁₈. A high temperature polymorph exists, indialite, which is isostructural with beryl and has a random distribution of Al in the (AlSi)₆O₁₈ rings. Cordierite ceramic is commonly used in catalytic converters. 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.

Gem variety

---

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 Fahlun Mine, Sweden.

References

---

• Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., John Wiley and Sons, New York, p. 395-396, ISBN 0471805807
• Klein, Cornelius., 2002, The Manual of Mineral Science, 22nd ed., John Wiley & Sons, Inc. ISBN 0-471-25177-1
• Webmineral
• Mindat.org
• Mineral galleries

Magnesium minerals | Iron minerals | Aluminium minerals | Silicate minerals

Cordierit | Cordiérite | Cordierite | Cordieriet | 菫青石 | Kordieriitti