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Revision as of 14:07, 25 October 2011
The feldspathoids are a group of tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content. They occur in rare and unusual types of igneous rocks.
Foid, contraction of the term feldspathoid, is applied to any igneous rock containing up to 60% modal feldspathoid minerals. For example, a syenite with significant nepheline present can be termed a ‘(nepheline)-bearing syenite’, or a ‘(nepheline)-syenite’, with the term (nepheline) replaceable by any 'foid' mineral. Such terminology is used on the Streckeisen (QAPF) classification of igneous rocks.[1]
Feldspathoid Minerals
References
- ^ A Dictionary of Earth Sciences, Ailsa Allaby and Michael Allaby, Oxford University Press 1999
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