The heptahydrate, Na2C03.7H20, is obtained by crystallizing a warm saturated solution in a vacuum; it appears to be dimorphous.
The compound FeS 2 is dimorphous, and the modern practice is to distinguish the cubic forms as pyrites and the orthorhombic as marcasite (q.v.).
In association with antimonious and arsenious sulphides, silver sulphide forms many important minerals, which sometimes present dimorphous forms, reflecting the dimorphism of silver sulphide; moreover, the corresponding arsenious and antimonious compounds are frequently isomorphous.
Lead monoxide is dimorphous, occurring as cubical dodecahedra and as rhombic octahedra.
Calcium carbonate, CaCO 3, is of exceptionally wide distribution in both the mineral and animal kingdoms. It constitutes the bulk of the chalk deposits and limestone rocks; it forms over one-half of the mineral dolomite and the rock magnesium limestone; it occurs also as the dimorphous minerals aragonite (q.v.) and calcite (q.v.).