Andesine

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Andesine
01722 Andesine.jpg
Andesine
General
Category plagioclase, feldspar, tectosilicate
Formula
(repeating unit)
(Ca,Na)(Al,Si)4O8, where Ca/(Ca + Na) is between 30%-50%
Crystal symmetry Triclinic pinacoidal (1)
Unit cell a = 8.155 Å, b = 12.9 Å, c = 9.16 Å; alpha = 93.917°, beta = 116.3333°, gamma = 89.166°; Z = 6
Identification
Color White, gray, green, yellow, flesh-red
Crystal habit Crystals rare, to 2 cm; commonly massive or granular
Crystal system Triclinic pinacoidal H-M Symbol (1)
Twinning Common following albite, pericline, and carlsbad twin laws
Cleavage Perfect on {001}, good on {010}
Fracture Uneven to conchoidal
Tenacity Brittle
Mohs scale hardness 6 - 6.5
Luster Subvitreous to pearly
Streak White
Diaphaneity Transparent to translucent
Specific gravity 2.66 - 2.68
Optical properties Biaxial (+/-)
Refractive index nα = 1.543 - 1.554 nβ = 1.547 - 1.559 nγ = 1.552 - 1.562
Birefringence δ = 0.009
2V angle Measured: 76° to 83°
Diagnostic features Requires optical/chemical analysis
References [1][2][3]

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%-50%. The formula may be written as Na0.7-0.5Ca0.3-0.5Al1.3-1.5Si2.7-2.5O8.[1]

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.[4]

Name and discovery[edit]

Andesine was first described in 1841 for an occurrence in the Marmato mine, Marmato, Cauca, Chocó Department, Colombia.[2][3] The name is for the Andes due to its abundance in the andesite lavas in those mountains.[1][4]

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-coloured.[5]

Occurrence[edit]

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.[1]

References[edit]

  1. ^ a b c d http://rruff.geo.arizona.edu/doclib/hom/andesine.pdf Handbook of Mineralogy
  2. ^ a b http://webmineral.com/data/Andesine.shtml Webmineral data
  3. ^ a b http://www.mindat.org/min-220.html Mindat.org
  4. ^ a b Klein, Cornelis, and Cornelius S. Hurlbut, Jr., Manual of Mineralogy, Wiley, 20th ed. 1985, p. 455 ISBN 0-471-80580-7
  5. ^ http://www.gia.edu/gems-gemology/FA13-oregon-sunstone-pay

External links[edit]