The term isomorphism of minerals refers to a similarity in crystal structure between two or more distinct substances. Many solids have a tendency to crystallize in definite geometric frameworks. Table salt, for example, has a characteristically cubic crystalline shape that can be observed with the naked eye. Quartz and gypsum are other familiar examples of crystalline structures. Crystalline solids such as these have a three-dimensional pattern that can be represented as a coordinate system or lattice. Like a network of interconnecting cubes, the crystalline lattice is composed of regularly arranged subunits. Lattices of ionic crystals, like table salt, consist of alternating ions, or charged atoms. The attraction between alternate cations (positive charge) and anions (negative charge) stabilizes the crystalline structure. Likewise, metallic crystals consist of lattices of positively charged metal ions regularly arranged in three dimensions among a virtual sea of electrons. The lattice structures of isomorphic substances are comparable. Therefore, they form crystals that appear to be nearly identical. Two or more isomorphic substances sometimes crystallize together to form a solid solution with a singular geometric configuration. Isomorphous substances usually have similar chemical formulas, and the relative distances between anions and cations are generally alike.Sodium nitrate and calcium sulfate are isomorphous, as are the sulfates of barium, strontium, and lead. Isomorphism was discovered by Eilhard Mitscherlich (1794-1863) in the early nineteenth century.