Sanidine

Sanidine
Sanidine from Puy de Sancy, Monts-Dore massif, Puy-de-Dôme, France. Size 5 cm × 4.5 cm (2.0 in × 1.8 in)
General
CategoryFeldspar
Formula
(repeating unit)
K(AlSi3O8)
IMA symbolSa[1]
Strunz classification9.FA.30
Dana classification76.01.01.02
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupC2/m
Identification
ColorColorless to white
Crystal habitTabular crystals, may be acicular
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[2][3][4]

Sanidine is the high temperature form of potassium feldspar with a general formula K(AlSi3O8).[2] 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.

  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ a b "The New IMA List of Minerals – A Work in Progress – Updated: March 2014" (PDF). Archived from the original (PDF) on March 25, 2014.
  3. ^ http://www.mindat.org/min-3521.html Mindat.org
  4. ^ http://www.webmineral.com/data/Sanidine.shtml Webmineral data