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A plagioclase feldspar mineral with a composition ranging from Ab90 An10 to Ab70 An30, where Ab = NaAlSi3O8 and An = CaAl2O8.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



a rock-forming mineral of the feldspar (plagio-clase) group; an isomorphous mixture of 10–30 percent anorth-ite, Ca[Al2Si2O8], and 70–90 percent albite, Na[AlSi3O8].

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
References in periodicals archive ?
The XRD patterns showed formation of oligoclase, albite, and zeolites.
The trachyandesite of Unit D is distinctive in that the groundmass plagioclase laths are oligoclase, simply carlsbad twinned and bluntly terminated, rather than tapered; and the groundmass is mostly intersertal, with pale brown to clear glass which is sometimes altered to brown smectite.
The plagioclase generally ranges in composition from An26 to An48, (oligoclase to andesine).
In May 2001, one of the many unusual skarn outcrops around the remote village of Imilchil produced several handfuls of loose, opaque beige to gray-white crystals of oligoclase, a member of the plagioclase series which only very rarely is found as euhedrons.
Andesine to oligoclase ([An.sub.27-30]) predominate over sodic sanidine and scarce clinopyroxene and titanomagnetite in the matrix of fresh trachyandesite facies.
Alkali-feldspar and plagioclase are modelled as K-feldspar and oligoclase, and smectite is regarded as a combination of 50% smectite-Na and smectite-Ca referring to previous studies [16, 17].
Additionally, limited exchange of xenocrysts (labradorite, edenitic amphibole, and bronzite in dacites, oligoclase and andesine in andesitic enclaves) might have occurred during this entrainment stage.
The specific surface area in all size fractions increased in order: quartz (no cleavage), olivine (less regular cleavage {001}), albite < oligoclase, bytownite (cleavage of feldspars {00} and {010}) < hornblende, diopside (stark crossing cleavage {110}).
Unaltered crystals of orthoclase and plagioclase (probably oligoclase), including Carlsbad-law twins, have been found rarely as phenocrysts in the volcanic porphyry.
Free plagioclase is oligoclase in composition and it is often zoned or twinned.
In thin section, large perthitic K-feldspar megacrysts are in a hypidiomorphic granular matrix of normal to oscillatory zoned oligoclase, perthitic orthoclase-microcline, quartz, dark brown to orange-brown biotite, pinitized cordierite, and minor muscovite.
It should be noted that alkali feldspar is represented as K-feldspar, plagioclase is represented as an ideal solid solution of oligoclase, and smectite is divided into Na-smectite and Ca-smectite equally by volume fraction referring to previous studies [46-48].