Oligoclase is a rock-forming mineral belonging to the plagioclase feldspars. In chemical composition and in its crystallographic and physical characters it is intermediate between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8).[1] The albite:anorthite molar ratio of oligoclase ranges from 90:10 to 70:30.
Oligoclase is a high sodium feldspar crystallizing in the triclinic system. The Mohs hardness is 6 to 6.5 and the specific gravity is 2.64 to 2.66. The refractive indices are: nα = 1.533–1.543, nβ = 1.537–1.548, and nγ = 1.542–1.552. In color it is usually white, with shades of grey, green, or red.[1]
Oligoclase is a common mineral in the more silica-rich varieties of igneous rock and in many metamorphic rocks.
^ abcOne or more of the preceding sentences incorporates text from a publication now in the public domain: Spencer, Leonard James (1911). "Oligoclase". In Chisholm, Hugh (ed.). Encyclopædia Britannica. Vol. 20 (11th ed.). Cambridge University Press. p. 82.
Oligoclase is a rock-forming mineral belonging to the plagioclase feldspars. In chemical composition and in its crystallographic and physical characters...
Sunstone is a microcline or oligoclase feldspar, which when viewed from certain directions exhibits a spangled appearance. It has been found in Southern...
silica and related volcanics such as andesite. Oligoclase is common in granite and monzonite. The name oligoclase is derived from the Greek olígos ('small,...
Osumilite Otavite Ottrelite Otwayite Varieties that are not valid species: Oligoclase (a mixture of albite and anorthite) Onyx (a monochromatic banded variety...
(/ˈmʌɡiəraɪt/) is a type of oligoclase-bearing basalt, comprising olivine, apatite, and opaque oxides. The main feldspar in mugearite is oligoclase. Mugearite is a...
murchisonite, Ceylon opal, hecatolite — a decorative variety of albite (oligoclase) of white or light gray color with a distinct iridescence effect. By composition...
plagioclase is typically intermediate to sodic in composition, andesine to oligoclase. Quartz is present in significant amounts. Biotite and/or hornblende constitute...
produced by hydrothermal alteration of basalt, and composed of albite or oligoclase, together with chlorite, epidote, calcite, and actinolite. Spilite is...
variety of tonalite in which the plagioclase is mostly in the form of oligoclase. Trondhjemites that occur in the oceanic crust or in ophiolites are usually...
an element of the ground-mass. The plagioclase in dacite ranges from oligoclase to andesine and labradorite. Sanidine occurs, although in small proportions...
sometimes are present as phenocrysts. The plagioclase is usually sodium-rich (oligoclase or andesine). Cristobalite and trydimite are sometimes present along with...
with phaneritic texture. Feldspar is present as plagioclase (typically oligoclase or andesine) with 10% or less potassium feldspar. Quartz is present at...
(coarse-grained) texture. Feldspar is present as plagioclase (typically oligoclase or andesine) with alkali feldspar making up less than 10% of the total...
Notes: Five Senses on Erebus Mountain, E.D. (17 March 1925). "Potash-oligoclase from Mt. Erebus, Antarctic, and anorthoclase from Mt. Kenya, East Africa"...
igneous rock – A form of tonalite where plagioclase-group feldspar is oligoclase Tuff – Rock consolidated from volcanic ash Websterite – Ultramafic and...
are common accessory minerals. The plagioclase is typically sodium-rich oligoclase. The alkali feldspar is typically also sodium-rich sanidine (anorthoclase)...
rocks, the common feldspars are orthoclase, perthite, microcline, and oligoclase—all having much silica and alkalis. In the mafic rocks labradorite, anorthite...
microcline and is often perthitic. The plagioclase is typically sodium-rich oligoclase. Phenocrysts are usually alkali feldspar. Granitic rocks are classified...
plagioclase in andesite varies widely in sodium content, from anorthite to oligoclase, but is typically andesine, in which anorthite makes up about 40 mol%...
little or no quartz, but much sanidine or anorthoclase and sometimes oligoclase feldspar, with occasional biotite, augite, and hornblende. In weathering...
granite: pink coarse-grained leucocratic; contains quartz, perthite, oligoclase, biotite and accessory apatite and zircon; veins of aplite and pegmatite...
varieties between them (given in order from sodium- to calcium-rich): oligoclase, andesine, labradorite, and bytownite. Other examples of series include...
biotite (sp. gr. 3.1), muscovite (sp. gr. 2.85), quartz (sp. gr. 2.65), oligoclase (sp. gr. 2.64), and orthoclase (sp. gr. 2.56), the crushed minerals float...
2008). "Albitization of granitic rocks: The mechanism of replacement of oligoclase by albite". The Canadian Mineralogist. 46 (6): 1401–1415. doi:10.3749/canmin...
of orthoclase are enveloped within rinds of sodic plagioclase such as oligoclase. In shallow intrusives or volcanic flows phenocrysts which formed before...
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