- 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.
Origin of plagioclase
Examples from the Web for plagioclase
Historical Examples of plagioclase
Sometimes 584 the striae on crystal-faces are due to repeated lamellar twinning, as in the plagioclase felspars.
Many dolerites are porphyritic and carry phenocrysts of olivine, augite and plagioclase felspar (or of one or more of these).
More rarely biotite and augite exhibit the same relations to the plagioclase.
The former consist mainly of plagioclase, leucite and augite, while the latter contain olivine in addition.
Their essential minerals are pyroxene, plagioclase and garnet: chemically they resemble the gabbros.
- a series of feldspar minerals consisting of a mixture of sodium and calcium aluminium silicates in triclinic crystalline form: includes albite, oligoclase, and labradorite
- 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.