andesine

Andesine is a plagioclase feldspar mineral in the feldspar group, part of the plagioclase solid-solution series between albite and labradorite. It has an intermediate sodium–calcium composition (approximately An30–An50 in many classifications). The chemical formula is often written as (Na,Ca)(Al,Si)4O8, reflecting substitution of Na+ by Ca2+ in the crystal structure.

Andesine crystallizes in the triclinic system and shows two directions of cleavage at about 90 degrees. It has a hardness of about 6–6.5 on the Mohs scale and a vitreous to pearly luster. The mineral is typically colorless to white or gray, but it can appear pinkish or pale green in some specimens; surface striations on cleavage faces are common.

Occurrence and geology: Andesine is a common constituent in felsic to intermediate igneous rocks, including andesite, diorite, granodiorite, and granite. It can also occur in metamorphic rocks. As a member of the plagioclase series, it forms part of the early crystallizing minerals in magmatic differentiation and appears alongside other feldspars in many continental crust rocks.

Etymology and usage: The name andesine derives from andesite, the rock in which the mineral is often found, and its position in the albite–labradorite series. In addition to its role as a rock-forming mineral, transparent to translucent andesine crystals are sometimes cut as gemstones, with colors ranging from pale to greenish; some varieties marketed as andesine-labradorite may be enhanced or misidentified in the gemstone trade.