plagioclase

American  

[play-jee-uh-klays] / 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 /

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

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Etymology

Origin of plagioclase

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

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Example Sentences

See Examples For:

plagioclase

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

From Science Daily ● Jan. 10, 2026

It usually contains roughly-equal amounts of light and dark minerals, including light grains of plagioclase feldspar and dark minerals like amphibole.

From Textbooks ● Jan. 1, 2017

See the cleavage surfaces on galena above or plagioclase below.

From Textbooks ● Jan. 1, 2017

As it cools further, much of the remaining silica goes into calcium-rich plagioclase, and any silica left may be used to convert some of the olivine to pyroxene.

From Textbooks ● Jan. 1, 2015

These diabases, as they will be called generically, are usually composed of plagioclase feldspar, and diallage or augite; additional and rarer minerals are quartz, olivine, hypersthene, magnetite, ilmenite, and hornblende.

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

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