plagioclase

American  

[pley-jee-uh-kleys] / ˈpleɪ dʒi əˌkleɪs /

noun

1. 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.

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plagioclase British  

/ ˈpleɪdʒɪəʊˌkleɪz, ˌpleɪdʒɪəʊˈklæstɪk /

noun

1. a series of feldspar minerals consisting of a mixture of sodium and calcium aluminium silicates in triclinic crystalline form: includes albite, oligoclase, and labradorite

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plagioclase Scientific  

/ plā′jē-ə-klās′ /

1. 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)Si ₂ O ₈ .

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Other Word Forms

• plagioclastic adjective

Etymology

Origin of plagioclase

First recorded in 1865–70; plagio- + -clase

Example Sentences

To overcome this challenge, the researchers used high-precision techniques to examine untouched portions of plagioclase feldspar crystals.

From Science Daily • Jan. 10, 2026

Because of twinning in the crystal, striations show up on one of the two cleavage faces of the plagioclase crystal.

From Textbooks • Jan. 1, 2017

The actual cleavage angle in plagioclase is slightly different than 90o and the alternating mirror images in these twinned zones produce a series of parallel lines on one of plagioclase’s two cleavage faces.

From Textbooks • Jan. 1, 2017

This occurs when calcium-rich early- forming plagioclase crystals become coated with progressively more sodium-rich plagioclase as the magma cools.

From Textbooks • Jan. 1, 2015

Sections of the finer schist in polarized light show many small areas of quartz and plagioclase and numerous crystals of epidote, magnetite, and chlorite, the whole having a marked parallel arrangement.

From History and Comprehensive Description of Loudoun County, Virginia by Head, James William