Feldspathoid

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, and are not found in rocks containing primary quartz. 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.