Andesine

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A plagioclase feldspar with composition Ab₇₀An₃₀ to Ab₅₀An₅₀ (Ab = NaAlSi₃O₈; An = CaAl₂Si₂O₈). Andesine occurs primarily in igneous rocks, often in a glassy matrix as small, chemically zoned, lathlike crystals known as microlites. The rock types may be called andesinites (if dominantly feldspar), andesites, andesitic basalts (or olivine-bearing andesites, as in Hawaiian lava flows), or pyroxene-, hornblende- or biotite-andesites (all are volcanic). See also Andesite; Feldspar; Igneous rocks.

The symmetry of andesine is triclinic, hardness on the Mohs scale 6, specific gravity 2.69, melting point ∼1210°C (2210°F). If quenched at very high temperatures, andesine has an albitelike structure, with aluminum (Al) and silicon (Si) essentially disordered in the structural framework of the crystals. But, in the course of cooling, most natural andesines develop an Al-Si ordered structure called e-plagioclases. See also Albite; Crystal structure.

Calcic andesines and labradorites (Ab₅₅An₄₅-Ab₄₀An₆₀) may exsolve into two distinctly intergrown lamellar phases whose regularity of stacking produces beautiful interference colors like those in the feathers of a peacock. Polished specimens of this material are called spectrolite in the gem trade, and at some localities (notably eastern Finland) crystals up to 10 in. (25 cm) are mined by hand. Smaller crystals are made into cabochons for jewelry. They may be abundant enough in the host rock to be valued as a decorative stone. See also Gem; Labradorite.

Andesine
Andesine
General
Category	plagioclase, feldspar, tectosilicate
Chemical formula	(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 Na_0.7-0.5Ca_0.3-0.5Al_1.3-1.5Si_2.7-2.5O₈.^[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]

Contents 1 Name and discovery 2 Occurrence 3 References 4 External links

Name and discovery

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]

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.^[1]

References

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

External links

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