rare feldspathoid mineral, an aluminosilicate that contains sodium and calcium carbonate and occurs as an alteration product of nepheline and feldspar in nepheline-syenite and related rocks. It also is found in metamorphic rocks and in contact zones between limestone and igneous intrusives. Famous localities are Alnö, Sweden; Fen district, Norway; Iivaara, Finland; and Iron Hill, Colo., U.S. For chemical formula and detailed physical properties, see feldspathoid (table).