Sanidine - Puy de Sancy, Monts-Dore massif, Puy-de-Dôme, Auvergne, France. (5x4.5cm)
Category Feldspar
(repeating unit)
Strunz classification 9.FA.30
Dana classification
Crystal system Monoclinic
Crystal class Prismatic (2/m)
H-M symbol: (2/m)
Space group C2/m
Color Colorless to white
Crystal habit Tabular crystals, may be acicular
Twinning Carlsbad twinning common
Cleavage {001} perfect, {010} good
Fracture Uneven
Tenacity Brittle
Mohs scale hardness 6
Luster Vitreous, pearly on cleavage
Streak White
Diaphaneity Transparent to translucent
Specific gravity 2.52
Optical properties Biaxial (-)
Refractive index nα = 1.518 - 1.525 nβ = 1.523 - 1.530 nγ = 1.525 - 1.531
Birefringence δ = 0.007
2V angle Measured: 18° - 42° (low); 15° - 63° (high)
References [1][2][3]

Sanidine is the high temperature form of potassium feldspar with a general formula K(AlSi3O8).[1] Sanidine is found most typically in felsic volcanic rocks such as obsidian, rhyolite and trachyte. Sanidine crystallizes in the monoclinic crystal system. Orthoclase is a monoclinic polymorph stable at lower temperatures. At yet lower temperatures, microcline, a triclinic polymorph of potassium feldspar, is stable.

Due to the high temperature and rapid quenching, sanidine can contain more sodium in its structure than the two polymorphs that equilibrated at lower temperatures. Sanidine and high albite constitute a solid solution series with intermediate compositions termed anorthoclase. Exsolution of an albite phase does occur; resulting cryptoperthite can best be observed in electron microprobe images.


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