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]

Name and discovery

Andesine was first described in 1841 for an occurrence in the Marmato mine, Marmato, Cauca, Chocó Department, Colombia.[3][4] The name is for the Andes due to its abundance in the andesite lavas in those mountains.[2][5]

In the early 2000s, red and green gemstones began to be marketed under the name of 'andesine'. After some controversy, these gemstones were subsequently discovered to have been artificially-colored.[7]

Occurrence

Andesine occurs in intermediate igneous rocks such as diorite, syenite, and andesite. It characteristically occurs in metamorphic rocks of granulite to amphibolite facies commonly exhibiting antiperthite texture. It also occurs as detrital grains in sedimentary rocks. It is commonly associated with quartz, potassium feldspar, biotite, hornblende, and magnetite.[2]

References

  1. ↑ Mineralienatlas
  2. 1 2 3 4 Handbook of Mineralogy
  3. 1 2 Webmineral data
  4. 1 2 Mindat.org
  5. 1 2 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
  7. ↑ "Three Occurrences of Oregon Sunstone | Gems & Gemology".
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