Griffen, Dana T. Department of Geology, Brigham Young University, Provo, Utah.
Last reviewed:December 2019
- Additional Reading
A plagioclase feldspar with composition in the range Ab90An10 to Ab70An30, where Ab represents the composition of albite, NaAlSi3O8 and An represents the composition of anorthite, CaAl2Si2O8. The diagnostic properties are hardness on Mohs scale, 6–6.5; density, 2.65 g/cm3; two good cleavages that intersect at approximately 90°; and color usually white or colorless, transparent to translucent. The presence of minute, mutually parallel inclusions of hematite (Fe2O3) causes a golden play of color in the variety of oligoclase called aventurine or sunstone. Repeated twinning is common and results in closely spaced striations visible with a hand-held magnifier. Mean refractive index is 1.545; the mineral may be optically positive or negative, depending on composition. Oligoclase is triclinic. The mineral is common in both plutonic and volcanic silicic igneous rocks, as well as in quartzofeldspathic and pelitic metamorphic rocks. See also: Albite; Anorthite; Crystal structure
The content above is only an excerpt.
for your institution. Subscribe
To learn more about subscribing to AccessScience, or to request a no-risk trial of this award-winning scientific reference for your institution, fill in your information and a member of our Sales Team will contact you as soon as possible.
to your librarian. Recommend
Let your librarian know about the award-winning gateway to the most trustworthy and accurate scientific information.
AccessScience provides the most accurate and trustworthy scientific information available.
Recognized as an award-winning gateway to scientific knowledge, AccessScience is an amazing online resource that contains high-quality reference material written specifically for students. Contributors include more than 10,000 highly qualified scientists and 46 Nobel Prize winners.
MORE THAN 8700 articles covering all major scientific disciplines and encompassing the McGraw-Hill Encyclopedia of Science & Technology and McGraw-Hill Yearbook of Science & Technology
115,000-PLUS definitions from the McGraw-Hill Dictionary of Scientific and Technical Terms
3000 biographies of notable scientific figures
MORE THAN 19,000 downloadable images and animations illustrating key topics
ENGAGING VIDEOS highlighting the life and work of award-winning scientists
SUGGESTIONS FOR FURTHER STUDY and additional readings to guide students to deeper understanding and research
LINKS TO CITABLE LITERATURE help students expand their knowledge using primary sources of information