Article
Article
Anorthoclase
Article By:
Laves, Fritz H. Formerly, Eidg. Technische Hochschule, Institut für Kristallographie und Petrographie, Zurich, Switzerland.
Last reviewed:October 2019
DOI:https://doi.org/10.1036/1097-8542.037100
The name usually given to alkali feldspars which have a chemical composition ranging from Or40Ab60 to Or10Ab90± up to approximately 20 mole % An (Or, Ab, An = KAlSi3O8, NaAlSi3O8, CaAl2Si2O8) and which deviate in one way or another from monoclinic symmetry tending toward triclinic symmetry. When found in nature, they usually do not consist of a single phase but are composed of two or more kinds of K- and Na-rich domains mostly of submicroscopic size. In addition, they are frequently polysynthetically twinned after either or both of the albite and pericline laws. It appears that they originally grew as the monoclinic monalbite phase inverting and unmixing in the course of cooling during geological times. They are typically found in lavas or high-temperature rocks. See also: Feldspar; Igneous rocks
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