BaSO4

Isomorphism I`so*mor"phism, n. [Cf. F. isomorphisme.] 1. (Crystallog.) A similarity of crystalline form between substances of similar composition, as between the sulphates of barium ({BaSO4}) and strontium ({SrSO4}). It is sometimes extended to include similarity of form between substances of unlike composition, which is more properly called {hom[oe]omorphism}.

2. (Math.) A one-to-one mapping of one set onto another set which preserves the relations between the elements of the domains of the sets. [PJC]