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.

---

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

---

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 .

---

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

See the cleavage surfaces on galena above or plagioclase below.

From Textbooks • Jan. 1, 2017

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

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

They consist essentially of plagioclase felspar, with hornblende or augite, and may contain also biotite, quartz and a limited amount of orthoclase.

From Encyclopaedia Britannica, 11th Edition, Volume 2, Slice 2 "Anjar" to "Apollo" by Various