Isomorphous substitution is one of the examples. It is isomorphous with potassium ferricyanide. The salt is isomorphous with [Mg(HO)]SO. Cerussite crystallizes in the orthorhombic system and is isomorphous with aragonite. The salt is isomorphous with ferrous sulfate heptahydrate. The molecule has D symmetry, being isomorphous with other octahedral tris(acetylacetonate)s. Early demonstrations of isomorphous replacement in crystallography come from Cork, John Monteath Robertson, and others. It is isomorphous with Pd(acac) and Cu(acac). Other members of this isomorphous group of minerals are adamite, ZnAsOOH, and eveite, MnAsOOH. Isomorphous with heulandite is the strontium and barium zeolite brewsterite. The group is an isomorphous mixture of the meionite and marialite endmembers. In modern usage isomorphous crystals belong to the same space group. Small amounts of Fe and Zr also may be isomorphous with Th. Common impurities include Mg, Mn, Ni, Pb, and Zn, many of which form isomorphous salts. The two are, in fact, isomorphous.