A family of rock-forming minerals consisting of aluminosilicates of sodium, potassium, or calcium and having too little silica to form feldspar. There is considerable structural variation, so it is not a true group. Feldspathoids take the places of feldspars in igneous rocks that are undersaturated with respect to silica or that contain more alkalis and aluminium than can be accommodated in the feldspars. Feldspathoids commonly occur with feldspars, and also with amphiboles, olivine and pyroxenes, but never with quartz or other polymorphs of silica.
