World Library  
Flag as Inappropriate
Email this Article

Oligoclase

Article Id: WHEBN0001681238
Reproduction Date:

Title: Oligoclase  
Author: World Heritage Encyclopedia
Language: English
Subject: Mugearite, Feldspar, Tonalite, Haast Schist, Spilite
Collection:
Publisher: World Heritage Encyclopedia
Publication
Date:
 

Oligoclase

Oligoclase
Oligoclase from Chihuahua, Mexico
General
Category plagioclase, feldspar, tectosilicate
Formula
(repeating unit)
(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

  • Public Domain This article incorporates text from a publication now in the public domain
  • Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., Wiley, ISBN 0-471-80580-7
  • Webmineral data
  • Mineral galleries
  • Mindat
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
 
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
 
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.
 



Copyright © World Library Foundation. All rights reserved. eBooks from World eBook Library are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.