Gehlenite

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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]

[edit] 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]

[edit] 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 anomalous nonzero 2V angle and has a characteristic 'ultrablue' birefringence.[10]

[edit] References

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