plagioclase

American

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

noun

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.

plagioclase British

/ ˈpleɪdʒɪəʊˌkleɪz, ˌpleɪdʒɪəʊˈklæstɪk /

noun

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" 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

plagioclase Scientific

/ plā′jē-ə-klās′ /

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 .

Other Word Forms

Derived Forms

plagioclastic

adjective

Etymology

Origin of plagioclase

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

Example Sentences

Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.

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

From Science Daily • Jan. 10, 2026

Intermediate rocks can have up to 25% quartz and 50% to 75% plagioclase.

From Textbooks • Jan. 1, 2015

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

As the temperature drops, and providing that there is sodium left in the magma, the plagioclase that forms is a more sodium-rich variety.

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

Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023