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<tr> <td align="center" colspan="2">Image:Cordierite.jpg
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</tr> <tr><th align="center" style="background: #9966FF; color:white" colspan="2">General</th> </tr>

<tr> <td>Chemical formula</td><td>(Mg,Fe)₂Al₄Si₅O₁₈</td> </tr> <tr> <td>Color</td><td>Blue, violet, yellow-brown; transparent to translucent</td> </tr><tr> <td>Crystal habit</td><td>Pseudo-hexagonal prismatic twins, as imbedded grains, and massive</td> </tr><tr> <td>Crystal system</td><td>Orthorhombic; 2/m 2/m 2/m</td> </tr><tr> <td>Cleavage</td><td>{010} poor</td> </tr><tr> <td>Fracture</td><td>Conchoidal, uneven</td> </tr><tr> <td>Mohs Scale hardness</td><td>7 - 7.5</td> </tr><tr> <td>Luster</td><td>Greasy or vitreous</td> </tr><tr> <td>Refractive index</td><td>α=1.522 - 1.558 β=1.524 - 1.574 γ=1.527 - 1.578 Indices increase with Fe content.</td> </tr><tr> <td>Optical Properties</td><td>Usually optically (-), sometimes (+); 2V = 0-90°</td> </tr><tr> <td>Pleochroism</td><td>Strong, dichroic: brown-yellow, light and dark blue</td> </tr><tr> <td>Streak</td><td>White</td> </tr><tr> <td>Specific gravity</td><td>2.57 - 2.66</td> </tr><tr> <td>Fusibility</td><td>on thin edges</td> </tr><tr> <td>Diagnostic Features</td><td>Resembles quartz can be distinguished by pleochroism. Can be distinguished from corundum by its lower hardness</td> </tr><tr> <td>Other Characteristics</td><td>Dana class: 61.2.1.1</td> </tr>

Cordierite
Category	Mineral
Identification

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).

[edit] 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.

[edit] 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.

[edit] References

• 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 galleriesde:Cordierit

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