Definition of plagioclase in English:


(also plagioclase feldspar)


  • A form of feldspar consisting of aluminosilicates of sodium and/or calcium, common in igneous rocks and typically white.

    • ‘Feldspar is dominated by microcline, but plagioclase and perthitic feldspar are also present.’
    • ‘Coexisting MI phases in these rocks include garnet, plagioclase, quartz, tourmaline and fluorite.’
    • ‘The semi-pelites usually consist of biotite, quartz, plagioclase and untwinncd feldspar, and garnet forms abundant porphyroblasts in some varieties.’
    • ‘It consists of clasts of angular to rounded quartz, plagioclase, tourmaline and rock fragments, up to 3 mm in diameter.’
    • ‘It is a reddish brown sandstone formed by detrital quartz, plagioclase, feldspar and muscovite, with subordinate apatite, zircon and opaque minerals.’
    • ‘All monazite grains are located in the rock matrix, which consists of quartz, muscovite, biotite, epidote, chlorite, plagioclase, potassium feldspar and garnet with a granoblastic texture.’
    • ‘It consists of quartz, K-feldspar, plagioclase, biotite and white mica, and in places displays a graphic texture.’
    • ‘Strain shadows around feldspar porphyroclasts contain recrystallized potassium feldspar, plagioclase and quartz.’
    • ‘The sample is strongly foliated and consists of garnet, biotite, quartz, plagioclase and cordierite.’
    • ‘The Bakfjord Granite contains quartz, K-feldspar, biotite, plagioclase, epidote, allanite, muscovite and occasional garnet.’
    • ‘The siltstones contain poorly sorted angular fragments of quartz, and angular clasts and cleavage fragments of plagioclase and alkali feldspar.’
    • ‘Like P133, it consists mainly of quartz, feldspar and white mica; plagioclase can also be distinguished under the microscope.’
    • ‘The metabasites typically contain coarse-grained hornblende, biotite and plagioclase with accessory titanite.’
    • ‘Like zircon, quartz, plagioclase, biotite and hornblende, major phenocryst phases, also show variable resorbtion effects.’
    • ‘The main constituent phases of these rocks are plagioclase, hornblende, pyroxene, quartz and K-feldspar, and accessory phases include magnetite, sphene, zircon and apatite.’
    • ‘The mineral assemblage comprises quartz, K-feldspar, albitic plagioclase, white mica, apatite, tourmaline and garnet.’
    • ‘The rock consists of plagioclase, K-feldspar, quartz, clinopyroxene and biotite, with abundant apatite, and ilmenite.’
    • ‘Aligned igneous minerals of euhedral potassium feldspar, plagioclase, biotite and hornblende in an undeformed quartz matrix define a magmatic foliation.’
    • ‘The first phase of diagenetic albite constitutes an impure, turbid variety including albitized grains of original plagioclase and alkali feldspar.’
    • ‘This granulite consists of garnet, biotite, muscovite, quartz, plagioclase and cordierite.’


Mid 19th century: from plagio- + Greek klasis cleavage (because originally characterized as having two cleavages at an oblique angle).