Gehlenite
{{Short description|Sorosilicate mineral}}
{{Distinguish|Galenite}}
{{Infobox mineral
| name = Gehlenite
| category = Sorosilicates
| image = Gehlenite-sea65a.jpg
| caption =
| formula = Ca2Al[AlSiO7]
| molweight =
| strunz = 9.BB.10
| system = Tetragonal
| class = Scalenohedral ({{overline|4}}2m)
H-M symbol: ({{overline|4}} 2m)
| symmetry = P{{overline|4}}21m
| colour = yellow-brown, green-grey, colourless
| habit =
| twinning =
| cleavage = Distinct/good
| fracture =
| tenacity =
| mohs = 5–6
| lustre = vitreous, greasy
| polish =
| refractive =
| opticalprop =
| birefringence = δ = 0.010
| dispersion =
| pleochroism =
| fluorescence=
| absorption =
| streak = white, grey-white
| gravity =
| density =
| melt =
| fusibility =
| diagnostic =
| solubility =
| diaphaneity =
| other =
| references =
}}
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 Trentino in Italy, and is named after the Adolf Ferdinand Gehlen (1775–1815) 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 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.[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 structureLouisnathan S (1969), Refinement of the crystal structure of gehlenite, Canadian Mineralogist, 10, 822–837 and was once considered a feldspathoid-like mineralBest 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
{{commonscat|Gehlenite}}
{{reflist|2}}