Gehlenite

Infobox mineral
name = Gehlenite
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formula = Ca₂Al [AlSiO₇]
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color = yellow-brown, green-grey, colourless
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system = Tetragonal
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cleavage = Distinct/good
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mohs = 5-6
luster = vitreous, greasy
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birefringence = δ = 0.010
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streak = white, grey-white
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Gehlenite, (Ca₂Al [AlSiO₇] ), is a sorosilicate, Al-rich endmember of the melilite complete solid solution series with akermanite.Deer et al, 1993] [http://www.mindat.org/min-1668.html Gehlenite: Gehlenite mineral information and data ] ] The type locality is in the Monzoni Mountains, Fassa Valley in Trento Province in Italy, and is named after Adolf Ferdinand Gehlen by A.J. Fuchs in 1815.Dana et al 1997]

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 Grossman L (1972) Condensation in the primitive solar nebula, Geochemica et Cosmochemica Acta, 36, 597-619] , and is found in diorite intruded carbonate rocks Carmichael ISE, Turner FJ, Verhoogen J (1974) Igneous Petrology, 37 ] , and to a far lesser extent in uncompahgrites Le Maitre RW (2002), Igneous Rocks, 11, 153 ] , melilitites , alnoites Nixon PH (1987) Mantle Xenoliths, 102-103, 336, 450-451 ] , lamprophyres and possibly kimberlite pipes. Skinner EMW, Mahotkin IL, Grutter HS (1999) Melilite in Kimberlites, Proceedings of the International Kimberlite Conference, 7, 788-794 ]

Gehlenite has also been found on the comet 81P/Wild. [ [http://www.mindat.org/loc-133140.html Mindat page for Comet Wild 2 (Comet 81P)] ]

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 Louisnathan S (1969), Refinement of the crystal structure of gehlenite, Canadian Mineralogist, 10, 822-837 ] and was once considered a feldspathoid-like mineral Best MG (2003), Igneous and Metamorphic Petrology (2nd Ed), 398, 693, 702-703 ] 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. It is uniaxial (-), has an anomalous nonzero 2V angle and has a characteristic 'ultrablue' birefringence.

References