A group of alkali aluminosilicate minerals that are similar in chemical composition to the feldspars but are relatively deficient in silica and richer in alkalis. The structure consists of a framework of (Si,Al)O4 tetrahedra with aluminium and silicon atoms at their centres. The feldspathoids occur chiefly with feldspars but do not coexist with free quartz (SiO2) as they react with silica to yield feldspars. The chief varieties of feldspathoids are: nepheline, KNa3(AlSiO4)4; leucite, KAlSi2O6; analcime, NaAlSi2O6.H2O; cancrinite, Na8(AlSiO4)6(HCO3)2; and the sodalite subgroup comprising: sodalite, 3(NaAlSiO4).NaCl; nosean, 3(NaAlSiO4).Na2SO4; haüyne, 3(NaAlSiO4).CaSO4; lazurite (Na,Ca)8(Al,Si)12O24(S,SO4) (see lapis lazuli).