Cordierite

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).[2] A high-temperature polymorph exists, indialite, which is isostructural with beryl and has a random distribution of Al in the (Si,Al)₆O₁₈ rings.[3]

Crystal structure of Cordierite. Green - Mg or Fe, blue - O, yellow - Si & Al.

"Praseolite" redirects here. For the green variety of quartz, see prasiolite.

Cordierite
General
Category	Cyclosilicate
Formula (repeating unit)	(Mg,Fe)2Al4Si5O18
Strunz classification	9.CJ.10
Dana classification	61.02.01.01 Cordierite group
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Cccm
Unit cell	a = 17.079 Å, b = 9.730 Å, c = 9.356 Å; Z = 4
Identification
Color	Blue, smoky blue, bluish violet; greenish, yellowish brown, gray; colorless to very pale blue in thin section in transmitted light
Crystal habit	Pseudo-hexagonal prismatic twins, as imbedded grains, and massive
Twinning	Common on {110}, {130}, simple, lamellar, cyclical
Cleavage	Fair on {100}, poor on {001} and {010}
Fracture	Subconchoidal
Tenacity	Brittle
Mohs scale hardness	7 - 7.5
Luster	Greasy or vitreous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.57 - 2.66
Optical properties	Usually optically (-), sometimes (+); 2V = 0-90°
Refractive index	nα = 1.527 - 1.560 nβ = 1.532 - 1.574 nγ = 1.538 - 1.578 Indices increase with Fe content.
Pleochroism	X = pale yellow, green; Y = violet, blue-violet; Z = pale blue
Fusibility	on thin edges
Diagnostic features	Resembles quartz, can be distinguished by pleochroism. Can be distinguished from corundum by its lower hardness
References	[1][2][3][4]

Name and discovery

Cordierite, which was discovered in 1813, in specimens from Níjar, Almería (Spain) is named after the French geologist Louis Cordier (1777–1861).[2]

Occurrence

Cordierite typically occurs in contact or regional metamorphism of pelitic 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.[4][5] Cordierite also occurs in some granites, pegmatites, and norites in gabbroic magmas. Alteration products include mica, chlorite, and talc. Cordierite occurs, for example, 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 synthetic cordierite. The manufacturing process deliberately aligns the cordierite crystals to make use of the very low thermal expansion along one axis. This prevents thermal shock cracking from taking place when the catalytic converter is used.[6]

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 usefulness in determining the direction of the sun on overcast days, the Vikings having used it for this purpose.[7] 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.[8]

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 is much softer than sapphires and is abundantly found in Australia (Northern Territory), Brazil, Burma, Canada (Yellowknife area of the Northwest Territories), India, Madagascar, Namibia, Sri Lanka, Tanzania and the United States (Connecticut). The largest iolite crystal found weighed more than 24,000 carats, and was discovered in Wyoming, US.[9]

• 
  Left: rough specimen showing dichroism; right: cut stone
• 
  Facet cut Iolite gemstone
• 
  Pleochroism of Cordierite

Another name for blue iolite is steinheilite, after Fabian Steinheil, the Russian military governor of Finland who observed that it was a different mineral from quartz.[10] Praseolite is another iolite variety which results from heat treatment. It should not be confused with prasiolite.[11]

See also

• List of minerals
• List of minerals named after people
• Sunstone (medieval)

References

1. "Cordierite" (PDF). Handbook of Mineralogy. RRUFF™ Project.
2. "Cordierite". Mindat.org.
3. http://webmineral.com/data/Cordierite.shtml Webmineral data
4. Dana, James Dwight; Klein, Cornelis; Hurlbut, Cornelius S. (1985). Manual of Mineralogy (20th ed.). New York: John Wiley and Sons. pp. 395–396. ISBN 0-471-80580-7.
5. Klein, Cornelis (2002). The Manual of Mineral Science (22nd ed.). John Wiley & Sons, Inc. ISBN 0-471-25177-1.
6. Cybulski, Andrzej; Moulijn, Jacob A., eds. (2005). Structured Catalysts and Reactors (Second ed.). CRC Press. p. 35. ISBN 978-0-8247-2343-9.
7. Guillot, Agnès; Meyer, Jean-Arcady (2010) [Published in French in 2008]. How To Catch a Robot Rat: When Biology Inspires Innovation [La bionique: Quand la science imite la Nature]. Translated by Susan Emanuel. The MIT Press. p. 212. ISBN 978-0-262-01452-6. Many insects and a few birds perceive polarized light. The Vikings used cordierite for this purpose, a stone that allowed them to reckon the position of the sun by observing the stone's changes in color.
8. Noel, Oscar; Bowling, Sue Ann (March 21, 1988). "Polar Navigation and the Sky Compass". Alaska Science Forum. Geophysical Institute, University of Alaska Fairbanks. Archived from the original on April 27, 2012. Retrieved March 25, 2009.
9. Topix Local News: Casper, WY, Wyoming is Most Gemstone-Rich State in US, Sept. 13, 2011
10. Sowerby, James (1811), Exotic mineralogy: or, Coloured figures of foreign minerals: as a supplement to British mineralogy, B. Meredith, p. 173.
11. "Prasiolite". Amethyst Galleries' Mineral Gallery.

External links

• Mineral galleries
• https://www.gemstone.org/education/gem-by-gem/222-iolite