Andesine

Andesine
Andesine
General
Category	plagioclase, feldspar, tectosilicate
Formula (repeating unit)	(Ca,Na)(Al,Si)4O8, where Ca/(Ca + Na) is between 30–50%
Crystal system	Triclinic
Crystal class	Pinacoidal (1) (same H-M symbol)
Space group	P1
Unit cell	a = 8.155 Å, b = 12.9 Å, c = 9.16 Å; α = 93.917°, β = 116.3333°, γ = 89.166°; Z = 8
Identification
Color	White, gray, green, yellow, flesh-red
Crystal habit	Crystals rare, to 2 cm; commonly massive or granular
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

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 Na_0.7-0.5Ca_0.3-0.5Al_1.3-1.5Si_2.7-2.5O₈.

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.

It is sometimes used as a gemstone.

Name and discovery

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

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.

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.