A member of the feldspathoid group of silicate minerals with a complex formula, approximately (Na,Ca,K)_6–8[AlSiO₄]₆(CO₃,SO₄,Cl)_1–2.1–5H₂O, it may be an alteration product of nepheline, and is related to vishnevite, the sulphate-rich variety; normally massive; occurs in nepheline-syenites.