​ANDESINE, a member of the group of minerals known as plagioclase felspars, occupying a position in the isomorphous series about midway between albite (NaAlSi₃O₈) and anorthite (CaAl₂Si₂O₈); its chemical composition and physical characters are therefore intermediate between those of the two extremes of the series. Distinctly developed crystals or crystallized specimens are rarely met with, the mineral usually occurring as embedded crystals and grains in the igneous and gneissic rocks, of which it forms a component part. It occurs, for example, in the andesite of the Andes, from whence it derives its name.