[pley-jee-uh-kleys] 
| any of the feldspar minerals varying in composition from acidic albite, NaAlSi3O8, to basic anorthite, CaAl2Si2O8, found in most igneous rocks: shows twinning striations on good cleavage surfaces. |
| pla·gi·o·clase (plā'jē-ə-klās', -klāz', plāj'ē-) n. Any of a common rock-forming series of triclinic feldspars, consisting of mixtures of sodium and calcium aluminum silicates. Also called oligoclase. [Greek plagio-, plagio- + Greek klasis, breaking (from its oblique cleavage) (from klān, to break).] |
| 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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