Cordierite
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| Cordierite | |
 Left: rough specimen showing dichroism; right: cut stone |
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| 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 |
| Streak | White |
| Specific gravity | 2.57 - 2.66 |
| Optical properties | Usually optically (-), sometimes (+); 2V = 0-90° |
| Refractive index | α=1.522 - 1.558 β=1.524 - 1.574 γ=1.527 - 1.578 Indices increase with Fe content. |
| Pleochroism | Strong, dichroic: brown-yellow, light and dark blue |
| 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)[1]. A high temperature polymorph exists, indialite, which is isostructural with beryl and has a random distribution of Al in the (Si,Al)6O18 rings.[2]
Cordierite is named after the French geologist Pierre Louis Antoine Cordier (1777 - 1861).
Contents |
[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.[3][4] 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.
[edit] 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.[5]
[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. It has also been called "water-sapphire" and "Vikings' Compass", because of its ability to determine the direction of the sun on overcast days. This works by determining the direction of polarization of the sky overhead. Light scattered by air molecules is polarized, and the direction of the polarization is at right angles to a line to the sun, even when the sun's disk itself is obscured by dense fog or lies just below the horizon. [6] 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 sometimes used as an inexpensive substitute for sapphire. It has a low price tag because it is much softer than sapphires[7] and is abundantly found in Sri Lanka, India, Burma, Australia's Northern Territory, Namibia, Brazil, Tanzania, Madagascar, Connecticut, and the Yellowknife area of the Northwest Territories of Canada.
[edit] See also
[edit] References
- ^ http://www.mindat.org/min-1128.html Mindat.org
- ^ Webmineral
- ^ 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
- ^ Cybulski, A: "Structural Catyalysts and Reactors - Second Edition"., p. 35, 2005, CRC Press.
- ^ (Geophysical Institute, University of Alaska Fairbanks) Oscar Noel and Sue Ann Bowling, "Polar Navigation and the Sky Compass", 1988.
- ^ Iolite as a gemstone
