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Cordierite



Cordierite

Left: rough specimen showing dichroism; right: cut stone
General
CategoryMineral
Chemical formula(Mg,Fe)2Al4Si5O18
Identification
ColorBlue, violet, yellow-brown; transparent to translucent
Crystal habitPseudo-hexagonal prismatic twins, as imbedded grains, and massive
Crystal systemOrthorhombic; 2/m 2/m 2/m
Cleavage{010} poor
FractureConchoidal, uneven
Mohs Scale hardness7 - 7.5
LusterGreasy or vitreous
Refractive indexα=1.522 - 1.558 β=1.524 - 1.574 γ=1.527 - 1.578 Indices increase with Fe content.
Optical PropertiesUsually optically (-), sometimes (+); 2V = 0-90°
PleochroismStrong, dichroic: brown-yellow, light and dark blue
StreakWhite
Specific gravity2.57 - 2.66
Fusibilityon thin edges
Diagnostic FeaturesResembles quartz can be distinguished by pleochroism. Can be distinguished from corundum by its lower hardness
Other CharacteristicsDana 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).

Additional recommended knowledge

Contents

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. [1] This prevents thermal shock cracking from taking place when the catalytic converter is used.[citation needed]

References

  1. ^ Cybulski, A: "Structural Catyalysts and Reactors - Second Edition", page 34. CRC Press, 2005.

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 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.

See also

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 galleries
  • http://www.gemstone.org/gem-by-gem/english/iolite.html
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Cordierite". A list of authors is available in Wikipedia.
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