Oligoclase

Oligoclase
Oligoclase from Chihuahua, Mexico
General
Category	plagioclase, feldspar, tectosilicate
Chemical formula	(Ca,Na)(Al,Si)4O8, where Ca/(Ca + Na) (% anorthite) is between 10%–30%
Identification
Color	usually white, with shades of grey, green, or red.
Mohs scale hardness	6 to 6.5
Specific gravity	2.64 to 2.66
Refractive index	nα=1.533–1.543; nβ=1.537–1.548; nγ=1.542–1.552
Birefringence	1st order

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). The albite:anorthite molar ratio 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.

Name and discovery

The name oligoclase was given by August Breithaupt in 1826 from the Greek oligos, little, and clasein, to break, because the mineral was thought to have a less perfect cleavage than albite. It had previously been recognized as a distinct species by J. J. Berzelius in 1824, and was named by him soda-spodumene (Natron-spodumen), because of its resemblance in appearance to spodumene.

Occurrence

Perfectly colorless and transparent glassy material found at Bakersville, North Carolina has occasionally been faceted as a gemstone. Another variety more frequently used as a gemstone is the aventurine-feldspar or sun-stone found as reddish cleavage masses in gneiss at Tvedestrand in southern Norway; this presents a brilliant red to golden metallic glitter, due to the presence of numerous small scales of hematite oriented within the feldspar structure.

Oligoclase occurs, often accompanying orthoclase, as a constituent of plutonic igneous rocks such as granite, syenite, and diorite. It occurs in porphyry and diabase dikes and sills as well as in the volcanic rocks andesite and trachyte. It also occurs in gneiss. The best developed and largest crystals are those found with orthoclase, quartz, epidote, and calcite in veins in granite at Arendal in Norway. The distinctive texture of rapakivi granite is due to oligoclase rims on orthoclase phenocrysts. Oligoclase is also found in metamorphic rocks formed under transitional greenschist to amphibolite facies conditions.

Schiller iridescence

Some examples show Schiller iridescence due to the presence of exsolution lamellae on cooling in the peristerite miscibility gap, ~An₅-An₁₈.

References

•  This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press. 
• Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., Wiley, ISBN 0-471-80580-7
• Webmineral data
• Mineral galleries
• Mindat

This article about a specific silicate mineral is a stub. You can help Wikipedia by expanding it.v · Categories: Tectosilicates Sodium minerals Calcium minerals Feldspar Triclinic minerals Silicate mineral stubs Wikimedia Foundation. 2010. Игры ⚽ Нужна курсовая? William Bernard O'Donoghue Marlene Hagge Look at other dictionaries: Oligoclase — Catégorie IX : silicates[1] Oligoclase Chihuahua, Mexico …   Wikipédia en Français Oligoclase — Ol i*go*clase, n. [Oligo + Gr. ? fracture, fr. ? to break.] (Min.) A triclinic soda lime feldspar. See {Feldspar}. [1913 Webster] …   The Collaborative International Dictionary of English oligoclase — ● oligoclase nom féminin Feldspath appartenant à la série des plagioclases, très répandu dans les roches cristallines …   Encyclopédie Universelle oligoclase — [äl′i gōklās΄] n. [Ger oligoklas < Gr oligos (see OLIGO ) + klasis, a fracture (see CLASTIC), in contrast to ORTHOCLASE, because cleavage differs slightly from 90°] a plagioclase feldspar whose composition corresponds to 70 to 90 percent… …   English World dictionary oligoclase — noun A plagioclase feldspar, the second member of the Albite Anorthite solid solution series. Primarily found as small crystals in impure marble. Oligoclase contains a small amount of calcium substituting for some of the sodium in its formula.… …   Wiktionary oligoclase — oligoklazas statusas T sritis chemija apibrėžtis Mineralas. formulė (Na,Ca)[(Si,Al)AlSi₂O₈] atitikmenys: angl. oligoclase rus. олигоклаз …   Chemijos terminų aiškinamasis žodynas oligoclase — noun Etymology: German Oligoklas, from olig olig + Greek klasis breaking, from klan to break more at clast Date: 1832 a mineral of the plagioclase series …   New Collegiate Dictionary oligoclase — /ol i goh klays /, n. Mineral. a kind of plagioclase feldspar occurring commonly in white crystals, sometimes shaded with gray, green, or red. [1825 35; OLIGO + CLASE] * * *       the most common variety of the feldspar mineral plagioclase (q.v.) …   Universalium oligoclase — (entrée créée par le supplément) (o li go kla z ) s. f. Terme de minéralogie. Silicate double d alumine et de soude. C est une substance blanchâtre ou gris verdâtre, qui se présente en petites masses lamellaires …   Dictionnaire de la Langue Française d'Émile Littré oligoclase — [ ɒlɪgə(ʊ)kleɪz] noun a feldspar mineral common in siliceous igneous rocks, consisting of a sodium rich plagioclase. Origin C19: from oligo + Gk klasis breaking (because thought to have a less perfect cleavage than albite) …   English new terms dictionary 18+ © Academic, 2000-2024 Contact us: Technical Support, Advertising Dictionaries export, created on PHP, Joomla, Drupal, WordPress, MODx. Mark and share Search through all dictionaries Translate… Search Internet Share the article and excerpts Direct link … Do a right-click on the link above and select “Copy Link”