Cordierite

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Cordierite
Cordierite-den07-06aa.jpg
General
Category Cyclosilicate
Formula
(repeating unit)		(Mg,Fe)2Al4Si5O18
IMA symbol Crd [1]
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
ColorBlue, 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 hardness7–7.5
Luster Greasy or vitreous
Streak White
Diaphaneity Transparent to translucent
Specific gravity 2.57–2.66
Optical propertiesUsually 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 featuresResembles quartz, can be distinguished by pleochroism. Can be distinguished from corundum by its lower hardness
References [2] [3] [4] [5]

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)2 Al 3(Si 5AlO 18) to (Fe,Mg)2Al3(Si5AlO18). [3] A high-temperature polymorph exists, indialite, which is isostructural with beryl and has a random distribution of Al in the (Si,Al)6O18 rings. [4] Cordierite is also synthesized and used in high temperature applications such as catalytic converters and pizza stones.

Contents

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

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). [3]

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. [5] [6] 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. [7]

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. [8] 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. [9]

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 (4,800 g), and was discovered in Wyoming, US. [10]

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. [11] Praseolite is another iolite variety which results from heat treatment. It should not be confused with prasiolite. [12]

See also

Related Research Articles

<span class="mw-page-title-main">Kyanite</span> Aluminosilicate mineral

Kyanite is a typically blue aluminosilicate mineral, found in aluminium-rich metamorphic pegmatites and sedimentary rock. It is the high pressure polymorph of andalusite and sillimanite, and the presence of kyanite in metamorphic rocks generally indicates metamorphism deep in the Earth's crust. Kyanite is also known as disthene or cyanite.

<span class="mw-page-title-main">Spinel</span> Mineral or gemstone

Spinel is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula MgAl
2O
4 in the cubic crystal system. Its name comes from the Latin word spinella, a diminutive form of spine, in reference to its pointed crystals.

<span class="mw-page-title-main">Garnet</span> Mineral, semi-precious stone

Garnets are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives.

Lustre or luster is the way light interacts with the surface of a crystal, rock, or mineral. The word traces its origins back to the Latin lux, meaning "light", and generally implies radiance, gloss, or brilliance.

<span class="mw-page-title-main">Pleochroism</span> Optical phenomenon

Pleochroism is an optical phenomenon in which a substance has different colors when observed at different angles, especially with polarized light.

<span class="mw-page-title-main">Actinolite</span> Mineral

Actinolite is an amphibole silicate mineral with the chemical formula Ca2(Mg4.5–2.5Fe2+0.5–2.5)Si8O22(OH)2.

<span class="mw-page-title-main">Andalusite</span> Aluminium nesosilicate mineral

Andalusite is an aluminium nesosilicate mineral with the chemical formula Al2SiO5. This mineral was called andalousite by Delamétehrie, who thought it came from Andalusia, Spain. It soon became clear that it was a locality error, and that the specimens studied were actually from El Cardoso de la Sierra, in the Spanish province of Guadalajara, not Andalusia.

<span class="mw-page-title-main">Sekaninaite</span> Mg, Fe, Al cyclosilicate mineral

Sekaninaite ((Fe+2,Mg)2Al4Si5O18) is a silicate mineral, the iron-rich analogue of cordierite.

<span class="mw-page-title-main">Hornfels</span>

Hornfels is the group name for a set of contact metamorphic rocks that have been baked and hardened by the heat of intrusive igneous masses and have been rendered massive, hard, splintery, and in some cases exceedingly tough and durable. These properties are caused by fine grained non-aligned crystals with platy or prismatic habits, characteristic of metamorphism at high temperature but without accompanying deformation. The term is derived from the German word Hornfels, meaning "hornstone", because of its exceptional toughness and texture both reminiscent of animal horns. These rocks were referred to by miners in northern England as whetstones.

<span class="mw-page-title-main">Sapphirine</span> Rare mineral, a silicate of magnesium and aluminium

Sapphirine is a rare mineral, a silicate of magnesium and aluminium, with the chemical formula 8(Al,Si)6O20. Named for its sapphire-like colour, sapphirine is primarily of interest to researchers and collectors: well-formed crystals are treasured and occasionally cut into gemstones. Sapphirine has also been synthesized for experimental purposes via a hydrothermal process.

<span class="mw-page-title-main">Andradite</span> Nesosilicate mineral species of garnet

Andradite is a mineral species of the garnet group. It is a nesosilicate, with formula Ca3Fe2Si3O12.

<span class="mw-page-title-main">Metamorphic facies</span> Set of mineral assemblages in metamorphic rocks formed under similar pressures and temperatures

A metamorphic facies is a set of mineral assemblages in metamorphic rocks formed under similar pressures and temperatures. The assemblage is typical of what is formed in conditions corresponding to an area on the two dimensional graph of temperature vs. pressure. Rocks which contain certain minerals can therefore be linked to certain tectonic settings, times and places in the geological history of the area. The boundaries between facies are wide because they are gradational and approximate. The area on the graph corresponding to rock formation at the lowest values of temperature and pressure is the range of formation of sedimentary rocks, as opposed to metamorphic rocks, in a process called diagenesis.

<span class="mw-page-title-main">Gedrite</span>

Gedrite is a crystal belonging to the orthorhombic ferromagnesian subgroup of the amphibole supergroup of the double chain inosilicate minerals with the ideal chemical formula Mg2(Mg3Al2)(Si6Al2)O22(OH)2.

<span class="mw-page-title-main">Metamorphic zone</span>

In geology, a metamorphic zone is an area where, as a result of metamorphism, the same combination of minerals occur in the bedrock. These zones occur because most metamorphic minerals are only stable in certain intervals of temperature and pressure.

<span class="mw-page-title-main">Edenite</span> Amphibole, double chain inosilicate mineral

Edenite is a double chain silicate mineral of the amphibole group with the general chemical composition NaCa2Mg5(Si7Al)O22(OH)2. Edenite is named for the locality of Edenville, Orange County, New York, where it was first described.

<span class="mw-page-title-main">Nambulite</span>

Nambulite is a lithium bearing manganese silicate mineral with the chemical formula (Li,Na)Mn4Si5O14(OH). It is named after the mineralogist, Matsuo Nambu (born 1917) of Tohoko University, Japan, who is known for his research in manganese minerals. The mineral was first discovered in the Funakozawa Mine of northeastern Japan, a metasedimentary manganese ore.

<span class="mw-page-title-main">Grandidierite</span>

Grandidierite is a rare mineral that was first discovered in 1902 in southern Madagascar. The mineral was named in honor of French explorer Alfred Grandidier (1836–1912) who studied the natural history of Madagascar.

S-type granites are a category of granites first proposed in 1974. They are recognized by a specific set of mineralogical, geochemical, textural, and isotopic characteristics. S-type granites are over-saturated in aluminium, with an ASI index greater than 1.1 where ASI = Al2O3 / (CaO + Na2O +K2O) in mol percent; petrographic features are representative of the chemical composition of the initial magma as originally put forth by Chappell and White are summarized in their table 1.

<span class="mw-page-title-main">Petrogenetic grid</span> Pressure-temperature diagram of mineral stability ranges

A petrogenetic grid is a geological phase diagram that connects the stability ranges or metastability ranges of metamorphic minerals or mineral assemblages to the conditions of metamorphism. Experimentally determined mineral or mineral-assemblage stability ranges are plotted as metamorphic reaction boundaries in a pressure–temperature cartesian coordinate system to produce a petrogenetic grid for a particular rock composition. The regions of overlap of the stability fields of minerals form equilibrium mineral assemblages used to determine the pressure–temperature conditions of metamorphism. This is particularly useful in geothermobarometry.

References

  1. ↑ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi: 10.1180/mgm.2021.43 . S2CID   235729616.
  2. ↑ "Cordierite" (PDF). Handbook of Mineralogy. RRUFF™ Project.
  3. 1 2 3 "Cordierite". Mindat.org.
  4. 1 2 http://webmineral.com/data/Cordierite.shtml Webmineral data
  5. 1 2 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.
  6. ↑ Klein, Cornelis (2002). The Manual of Mineral Science (22nd ed.). John Wiley & Sons, Inc. ISBN   0-471-25177-1.
  7. ↑ Cybulski, Andrzej; Moulijn, Jacob A., eds. (2005). Structured Catalysts and Reactors (Second ed.). CRC Press. p. 35. ISBN   978-0-8247-2343-9.
  8. ↑ 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.
  9. ↑ 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.
  10. ↑ Topix Local News: Casper, WY, Wyoming is Most Gemstone-Rich State in US, Sept. 13, 2011
  11. ↑ Sowerby, James (1811), Exotic mineralogy: or, Coloured figures of foreign minerals: as a supplement to British mineralogy, B. Meredith, p. 173.
  12. ↑ "Prasiolite". Amethyst Galleries' Mineral Gallery.
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