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%. It is a red feldspar gemstone, sometimes with greenish hints and a slight labradorescent effect. 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.
Andesine was first described in 1841 as an occurrence in the Marmato mine, Colombia. It is named after the Andes mountains of South America. It is found in a range of colors, from red, honey-red to orange, yellow, champagne and green. Refractive indices and specific gravity increase directly with calcium content.
General Information
- Category: plagioclase, feldspar, tectosilicate
- Formula: (Ca, Na)(Al, Si)4O8, where Ca/(Ca + Na) is between 30%-50%
- Crystal system: Triclinic
- Crystal class: Pinacoidal (1) (same H-M symbol)
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.
Properties
- 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
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.
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