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 in the Streckeisen (QAPF) classification of igneous rocks.
- A Dictionary of Earth Sciences, Ailsa Allaby and Michael Allaby, Oxford University Press 1999
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