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usually uncountable, plural plagioclases
(mineralogy) Any of a group of aluminum silicate feldspathic minerals ranging in their ratio of calcium to sodium. quotations examples
These crystals are of one of the most abundant minerals in nature – feldspar; furthermore, they are of a particular kind of feldspar rich in the element calcium, termed plagioclase.
2004, Richard Fortey, The Earth, Folio Society, published 2011, page 51