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.
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.
- Afghanite – Tectosilicate mineral
- Analcime – Tectosilicate mineral
- Cancrinite – Feldspathoid mineral
- Kalsilite – Vitreous white to grey feldspathoidal mineral
- Leucite – Potassium and aluminium tectosilicate mineral
- Nepheline – Silica-undersaturated aluminosilicate mineral
- Petalite – Silicate mineral, used in ceramic glazing
- Sodalite Group