Ribbe, Paul H. Formerly, Department of Geological Sciences, Virginia Polytechnic Institute, Blacksburg, Virginia.
- Links to Primary Literature
- Additional Readings
The calcium-rich plagioclase feldspar with composition Ab0An100 to Ab10An90 (Ab = NaAlSi3O8; An = CaAl2Si2O8), occurring in olivine-rich igneous rocks and rare volcanic ejecta (for example, at Mount Vesuvius, Italy, and Miyakejima, Japan). Hardness is 6 on Mohs scale, specific gravity 2.76, melting point 1550°C (2822°F). The crystal structure of Ab0An100 (triclinic space group Pī) consists of an infinite three-dimensional array of corner-sharing [AlO4] and [SiO4] tetrahedra, alternately linked together in a framework of [Al2Si2O8]∞2− composition in which charge-balancing calcium (Ca2+) cations occupy four distinct, irregular cavities. The crystal structure of anorthite inverts to higher symmetry (Iī) with each of the following: (1) heating above 242°C (468°F), (2) adding ∼10 mole % Ab component (substituting Na+ + Si4+ ↔ Ca2+ + Al3+), (3) increasing hydrostatic pressure above ∼2.6 gigapascals (26,000 atmospheres), and (4) effectively disordering the aluminum (Al) and silicon (Si) atoms at very high temperatures. Each inversion is evidenced by certain types of structurally out-of-step (antiphase) domains that can be imaged by transmission electron microscopy. Natural anorthite has no commercial uses, but the synthetic material [CaO · Al2O3 · 2SiO2 (known as CAS2)] is important in the ceramic industry and in certain composite materials with high-temperature applications. See also: Crystal structure; Feldspar; Igneous rocks
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 9000 highly qualified scientists and 43 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