any of the feldspar minerals varying in composition from acidic albite, NaAlSi 3 O 8 , to basic anorthite, CaAl 2 Si 2 O 8 , found in most igneous rocks: shows twinning striations on good cleavage surfaces.
Also called soda-lime feldspar.

1865–70; plagio- + -clase

plagioclastic [pley-jee-uh-klas-tik] , adjective Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
plagioclase (ˈpleɪdʒɪəʊˌkleɪz)
a series of feldspar minerals consisting of a mixture of sodium and calcium aluminium silicates in triclinic crystalline form: includes albite, oligoclase, and labradorite

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Science Dictionary
plagioclase   (plā'jē-ə-klās')  Pronunciation Key 
Any of a series of common feldspar minerals, consisting of differing mixtures of sodium and calcium aluminum silicates. Plagioclase is typically white, yellow, or reddish-gray, but it can also be blue to black. It occurs in igneous rocks. The minerals albite, oligoclase, andesine, labradorite, bytownite, and anorthite are all plagioclases. Chemical formula: (Na,Ca)Al(Si,Al)Si2O8.
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