An Etymological Dictionary of Astronomy and Astrophysics
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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.