plagioclastic /play'jee euh klas"tik/, adj.
/play"jee euh klays'/, n.
any of the feldspar minerals varying in composition from acidic albite, NaAlSi3O8, to basic anorthite, CaAl2Si2O8, found in most igneous rocks: shows twinning striations on good cleavage surfaces. Also called soda-lime feldspar.
[1865-70; PLAGIO- + -CLASE]

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Any member of the series of abundant feldspar minerals that usually occur as light-to medium-grey-coloured, transparent to translucent grains or crystals.

Plagioclase ranges in composition from albite to anorthite. It is used in the manufacture of glass and ceramics; iridescent varieties are valued as gemstones. The primary importance of plagioclase, however, derives from its role as a rock-forming mineral.

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       Composition of the plagioclase mineralsany member of the series of abundant feldspar minerals usually occurring as light-coloured, glassy, transparent to translucent, brittle crystals. Plagioclase is a mixture of albite (Ab), or sodium aluminosilicate (NaAlSi3O8), and anorthite (An), or calcium aluminosilicate (CaAl2Si2O8); the two intermingle and form a continuous chemical range (called a solid-solution series) between albite and anorthite (the end-members of the series). Intermediate members are homogeneous mixtures of the pure end-members. Divided according to their composition, the members of the plagioclase series are named in the Table (Composition of the plagioclase minerals).

      Most of the crystals of the members of this series exhibit microscopic, repeated lamellar albite twinning; this produces the fine parallel striations observed on some surfaces. The twinning sometimes causes an iridescence, usually blue or green, that may arise either from reflection or diffraction at the edges of the lamellae, or from diffusion by adjoining areas with different optical properties.

      Plagioclase is used in the manufacture of glass and ceramics; the iridescent varieties peristerite and labradorite are valued as gemstones or ornamental material. The primary importance of plagioclase, however, derives from its role in rock formation. oligoclase, the most common plagioclase, occurs in granite, diorite, and other felsic igneous rocks and in some metamorphic rocks; notable occurrences are at Aust-Agder, Norway, and Fine, N.Y., United States. Andesine, less common, occurs in many granular and volcanic rocks with intermediate silica content, as in Marmato, Colom., and Bodenmais, Bavaria, Ger. The rarest plagioclase is bytownite, which occurs in basic igneous rocks and in stony meteorites. For detailed physical properties of the plagioclase series, see feldspar (table).

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Universalium. 2010.

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Look at other dictionaries:

  • Plagioclase — A photomicrograph of a plagioclase crystal under cross polarized light. The plagioclase crystal shows a distinct banding effect called a Polysynthetic twinning. (unknown scale) General Category …   Wikipedia

  • plagioclase — [ plaʒjoklaz ] n. m. • 1899; du gr. plagios « oblique » et clasis « cassure » ♦ Minér. Feldspath contenant du calcium et du sodium, mais pas de potassium. ● plagioclase nom masculin (grec plagios, oblique) Aluminosilicate naturel de sodium et de… …   Encyclopédie Universelle

  • Plagioclase — Pla gi*o*clase, n. [Gr. pla gios oblique + kla^n to break.] (Min.) A general term used of any triclinic feldspar. See the Note under {Feldspar}. [1913 Webster] …   The Collaborative International Dictionary of English

  • plagióclase — s. f. [Geologia] Silicato de alumínio, sódio e cálcio, do grupo dos feldspatos. = PLAGIOCLÁSIO   ‣ Etimologia: grego plágios, a, on, oblíquo, enviesado + grego klásis, eos, quebra, fratura …   Dicionário da Língua Portuguesa

  • plagioclase — [plā′jē ō klās΄] n. [Ger plagioklas < Gr plagios (see PLAGIO ) + klasis, a cleaving, fracture < klaein, to break: see CLASTIC] any of a series of triclinic feldspars, ranging in composition from albite to anorthite and found in many rocks …   English World dictionary

  • Plagioclase — Grains fins de plagioclases dans roche volcanique Les plagioclases sont une importante série de silicates minéraux dans la famille des feldspaths. Plutôt que de se référer à un minéral particulier avec une composition chimique spécifique, le… …   Wikipédia en Français

  • plagioclase — noun A group of feldspar minerals that form a solid solution series from albite (sodium aluminum silicate) to anorthite (calcium aluminum silicate). The plagioclase feldspars are found in igneous, plutonic, and metamorphic rocks. They all have… …   Wiktionary

  • plagioclase — [ pleɪdʒɪə(ʊ)kleɪz, plagɪəʊ ] (also plagioclase feldspar) noun a form of feldspar consisting of aluminosilicates of sodium and/or calcium, common in igneous rocks and typically white. Origin C19: from plagio + Gk klasis cleavage (because orig.… …   English new terms dictionary

  • plagioclase series — ▪ Table The plagioclase series mineral     percent albite     percent anorthite albite 100–90 0–10 oligoclase 90–70 10–30 andesine 70–50 30–50 labradorite 50–30 50–70 bytownite 30–10 70–90 anorthite 10–0 90–100 See as table: * * * …   Universalium

  • plagioclase — noun Etymology: Greek plagios oblique + klasis breaking, from klan to break more at clast Date: circa 1868 a triclinic feldspar; especially one having calcium or sodium in its composition …   New Collegiate Dictionary

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