Gehlenite, (Ca2AlAlSiO7), is a sorosilicate, Al-rich endmember of the melilite complete solid solution series with akermanite.[Deer et al., 1993][ Gehlenite: Gehlenite mineral information and data]
The type locality is in the Monzoni Mountains, Fassa Valley in Trentino in Italy,[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 was subsequently consumed in processes which created
enstatite and other more abundant minerals making it a remnant mineral from the early solar nebula (along with
corundum and
spinel). Its occurrence in the early condensation phase of the solar nebula was predicted by Harry Lord in the 1950s, but studies of carbonaceous chondrites did not support this claim until the Allende meteorite was discovered in 1969.
[ Exploring the Planets lecture by Archibald Reid. The University of Houston] It is also 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.[ 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.
