Andesine is a silicate mineral, a member of the plagioclase feldspar solid solution series. Its chemical formula is (Ca, Na)(Al, Si)<sub>4</sub>O<sub>8</sub>, where the ratio of albite to anorthite (Na:Ca) is 50:50 to 70:30. The formula may be written as Na<sub>0.5-0.7</sub>Ca<sub>0.3-0.5</sub>Al<sub>1.3-1.5</sub>Si<sub>2.5-2.7</sub>O<sub>8</sub>.
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.
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. A Tibetan source of bona fide (untreated) red andesine, however, was eventually verified by a number of independent groups of well-respected gemologists.