The feldspathoids are a group of
tectosilicateminerals 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.[1]
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.[2]