Feldspathoid
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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.
Feldspathoid minerals
- Afghanite– Tectosilicate mineral in the feldspathoid group
- Analcime– Zeolite mineral
- Cancrinite– Feldspathoid mineral
- Kalsilite– Vitreous white to grey feldspathoidal mineral
- Leucite– Potassium and aluminium tectosilicate mineral
- Nepheline– Silica-undersaturated aluminosilicate mineral
- Sodalite Group Hauyne– Rare feldspathoid mineral in the sodalite group Lazurite– Alumino-silicate mineral whose blue colour is due to a sulfide species and not copper Nosean– Mineral of the feldspathoid group Sodalite– Blue tectosilicate mineral Tugtupite– Feldspathoid mineral in the sodalite group