Sanidine

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Sanidine
Sanidine.jpg
Sanidine - Puy de Sancy, Monts-Dore massif, Puy-de-Dôme, Auvergne, France. (5x4.5cm)
General
CategoryFeldspar
Formula
(repeating unit)
(K,Na)(Si,Al)4O8
Dana classification76.01.01.02
Identification
ColorColorless to white
Crystal habitTabular crystals, may be acicular
Crystal systemMonoclinic - Prismatic H-M Symbol (2/m) Space Group: C 2/m
TwinningCarlsbad twinning common
Cleavage{001} perfect, {010} good
FractureUneven
TenacityBrittle
Mohs scale hardness6
LusterVitreous, pearly on cleavage
StreakWhite
DiaphaneityTransparent to translucent
Specific gravity2.52
Optical propertiesBiaxial (-)
Refractive indexnα = 1.518 - 1.525 nβ = 1.523 - 1.530 nγ = 1.525 - 1.531
Birefringenceδ = 0.007
2V angleMeasured: 18° - 42° (low); 15° - 63° (high)
References[1][2][3]
 
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Sanidine
Sanidine.jpg
Sanidine - Puy de Sancy, Monts-Dore massif, Puy-de-Dôme, Auvergne, France. (5x4.5cm)
General
CategoryFeldspar
Formula
(repeating unit)
(K,Na)(Si,Al)4O8
Dana classification76.01.01.02
Identification
ColorColorless to white
Crystal habitTabular crystals, may be acicular
Crystal systemMonoclinic - Prismatic H-M Symbol (2/m) Space Group: C 2/m
TwinningCarlsbad twinning common
Cleavage{001} perfect, {010} good
FractureUneven
TenacityBrittle
Mohs scale hardness6
LusterVitreous, pearly on cleavage
StreakWhite
DiaphaneityTransparent to translucent
Specific gravity2.52
Optical propertiesBiaxial (-)
Refractive indexnα = 1.518 - 1.525 nβ = 1.523 - 1.530 nγ = 1.525 - 1.531
Birefringenceδ = 0.007
2V angleMeasured: 18° - 42° (low); 15° - 63° (high)
References[1][2][3]

Sanidine is the high temperature form of potassium feldspar (K,Na)(Si,Al)4O8.[1] Sanidine most typically occurs 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.

References[edit]

  1. ^ a b http://rruff.geo.arizona.edu/doclib/hom/sanidine.pdf Handbook of Mineralogy
  2. ^ http://www.mindat.org/min-3521.html Mindat.org
  3. ^ http://www.webmineral.com/data/Sanidine.shtml Webmineral data