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Gehlenite

Gehlenite
Category	Sorosilicates, Calcium minerals, Aluminium minerals
Chemical formula	Ca₂Al[AlSiO₇]
Identification
Color	yellow-brown, green-grey, colourless
Crystal system	Tetragonal
Cleavage	Distinct/good
Mohs Scale hardness	5-6
Luster	vitreous, greasy
Birefringence	δ = 0.010
Streak	white, grey-white

Gehlenite, (Ca₂Al[AlSiO₇]), is a sorosilicate, Al-rich endmember of the melilite complete solid solution series with akermanite.^[1]^[2] The type locality is in the Monzoni Mountains, Fassa Valley in Trento Province in Italy,^[2] and is named after Adolf Ferdinand Gehlen by A.J. Fuchs in 1815.^[3]

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Geological Occurrence

Gehlenite is found in carbonaceous chondrites from which it condensed as a refractory mineral in the hotter stages (FU Ori) of the presolar nebula ^[4], and is found in diorite intruded carbonate rocks ^[5], and to a far lesser extent in uncompahgrites ^[6], melilitites ^[6], alnoites ^[7], lamprophyres and possibly kimberlite pipes.^[8]

Gehlenite has also been found on the comet 81P/Wild.^[9]

Crystallography, Composition and Physical Properties

Gehlenite is one of five, isostructural tetragonal crystal system minerals in the melilite group. The tetrahedral linkage within the structure is similar to that of an aluminosilicate framework structure^[10] and was once considered a feldspathoid-like mineral^[11] due to silica undersaturation.

Gehlenite has a Mohs hardness of 5-6, a vitreous to greasy lustre, distinct to good cleavage and is yellow brown, greenish grey or colourless. Its streak is white or grey-white. ^[2] It is uniaxial (-), has an anomolous nonzero 2V angle and has a characteristic 'ultrablue' birefringence.^[10]

References

^ Deer et al, 1993
^ ^a ^b ^c http://www.mindat.org/min-1668.html
^ Dana et al 1997
^ Grossman L (1972) Condensation in the primitive solar nebula, Geochemica et Cosmochemica Acta, 36, 597-619
^ Carmichael ISE, Turner FJ, Verhoogen J (1974) Igneous Petrology, 37
^ ^a ^b Le Maitre RW (2002), Igneous Rocks, 11, 153
^ Nixon PH (1987) Mantle Xenoliths, 102-103, 336, 450-451
^ Skinner EMW, Mahotkin IL, Grutter HS (1999) Melilite in Kimberlites, Proceedings of the International Kimberlite Conference, 7, 788-794
^ Mindat page for Comet Wild 2 (Comet 81P)
^ ^a ^b Louisnathan S (1969), Refinement of the crystal structure of gehlenite, Canadian Mineralogist, 10, 822-837
^ Best MG (2003), Igneous and Metamorphic Petrology (2nd Ed), 398, 693, 702-703

Categories: Silicate minerals | Sorosilicates | Aluminium minerals | Calcium minerals

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Gehlenite". A list of authors is available in Wikipedia.

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