plagioclase

{{rfc-def}} {{mineralogy}} Any of a group of feldspar minerals that form a solid solution series from albite (sodium aluminum silicate) to anorthite (calcium aluminum silicate). The plagioclase feldspars are found in igneous, plutonic, and metamorphic rocks. They all have the triclinic-pinacoidal crystal habit, but only Albite commonly occurs in well crystallized specimens.