Andesine

Andesine
Andesine
General
Categoryplagioclase, feldspar, tectosilicate
Formula
(repeating unit)
(Ca,Na)(Al,Si)4O8, where Ca/(Ca + Na) is between 30–50%
Crystal systemTriclinic
Crystal classPinacoidal (1)
(same H-M symbol)
Space groupP1
Unit cella = 8.155 Å, b = 12.9 Å,
c = 9.16 Å; α = 93.917°,
β = 116.3333°, γ = 89.166°; Z = 8
Identification
ColorWhite, gray, green, yellow, flesh-red
Crystal habitCrystals rare, to 2 cm; commonly massive or granular
TwinningCommon following albite, pericline, and carlsbad twin laws
CleavagePerfect on {001}, good on {010}
FractureUneven to conchoidal
TenacityBrittle
Mohs scale hardness6 – 6.5
LusterSubvitreous to pearly
StreakWhite
DiaphaneityTransparent to translucent
Specific gravity2.66 – 2.68
Optical propertiesBiaxial (+/−)
Refractive indexnα = 1.543 – 1.554 nβ = 1.547 – 1.559 nγ = 1.552 – 1.562
Birefringenceδ = 0.009
2V angleMeasured: 76° to 83°
Diagnostic featuresRequires optical/chemical analysis
References[1][2][3][4]

Andesine is a silicate mineral, a member of the plagioclase feldspar solid solution series. Its chemical formula is (Ca, Na)(Al, Si)4O8, where Ca/(Ca + Na) (% anorthite) is between 30 and 50%. The formula may be written as Na0.7-0.5Ca0.3-0.5Al1.3-1.5Si2.7-2.5O8.[2]

The plagioclase feldspars are a continuous solid solution series and as such the accurate identification of individual members requires detailed optical study, chemical analysis or density measurements. Refractive indices and specific gravity increase directly with calcium content.[5]

It is sometimes used as a gemstone.[6]

  1. ^ Mineralienatlas
  2. ^ a b Handbook of Mineralogy
  3. ^ Webmineral data
  4. ^ Mindat.org
  5. ^ Klein, Cornelis, and Cornelius S. Hurlbut, Jr., Manual of Mineralogy, Wiley, 20th ed. 1985, p. 455 ISBN 0-471-80580-7
  6. ^ Tables of Gemstone Identification By Roger Dedeyne, Ivo Quintens p.118