:Feldspathoid

{{short description|Mineral grouping of silica-poor tectosilicates}}

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 usually not found in rocks containing primary quartz. A notable exception where feldspathoids and quartz-bearing rocks are found together is the Red Hill Syenite.{{cite web |url=http://www.galleries.com/minerals/silicate/feldspat.htm |title=The Feldspathoid Group of Minerals |publisher=Amethyst Galleries, Inc. |website=Amethyst Galleries' Mineral Gallery |access-date=6 Jul 2015}}

Foid, a 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 nepheline syenite, with the term nepheline replaceable by any foid mineral. Such terminology is used in the Streckeisen (QAPF) classification of igneous rocks.{{cite book |title=A Dictionary of Earth Sciences |first1=Ailsa |last1=Allaby |first2=Michael |last2=Allaby |publisher=Oxford University Press |year=1999}}