Andesine - Encyclopedia

    Class : Silicates
    Subclass : Tectosilicates
    Crystal System : Triclinic
    Chemistry : (Na,Ca)(Al,Si)₄O₈
    Rarity : Very common

Andesine belongs to the group of plagioclase feldspars, and constitutes an intermediate sodi-calcic term (Ab30-An70 to Ab50-An50). It is a very unusual constituent of basic volcanic rocks (andesites) and plutonic rocks (diorites), it is also present in certain metamorphic rocks (amphibolites and granulites). It is a mineral which owes its name to the Andean chain because it was discovered in the lavas of Marmato (Colombian Andes). Its crystals are tabular to lamellar, and very commonly appear as the typical polysynthetic twin of plagioclase feldspars. Andesine most often occurs in cleavable, granular or compact masses, whitish to greyish in color, rarely greenish or pinkish. It is a mineral that is sometimes used in jewelry.

The twins are common around [010] or perpendicular to {010}, giving polysynthetic streaks on {001} or {010} ; many other single and multiple contact twins exist.

The gem market saw the appearance in the 2010's of red-orange andesines notified as coming from the Congo. Unfortunately, it later turned out that the color of these andesines was due to copper diffusion treatment. Currently most red-orange andesines occur from Gu Yang in Inner Mongolia (China). They are originally pale yellow and systematically treated. The only deposit proven to produce natural red-orange andesines is that of the village of Zha Lin and the Yu Lin Gu Valley in Tibet. Unfortunately, it is very difficult to tell the difference between treated andesines and untreated andesines even with advanced laboratory techniques.