Any of the → feldspar minerals consisting of a mixture of → sodium and → calcium → aluminium → silicates in triclinic crystalline form. Members of the plagioclase group are the most common rock-forming minerals. They are important or dominant minerals in most → igneous rocks of the → Earth's crust.
From Ger. Plagioclase, from Gk. plagio- a combining form meaning "oblique," + clase a suffix used in the formation of compound words that denote minerals with a particular cleavage, as specified by the initial element.