The mineral hgcro4 (hexahydrocomancheite) is a new hg-bearing mineral that forms when 10-4 M HgO and 10-4 M chromic acid are combined under hydrothermal conditions. The crystal structure of a-hgcro4 has been re-determined and consists of distorted [HgO7] pentagonal bipyramids, containing tetrahedral chromate anions. The monohydrate b-hgcro4 adopts the CrVO4 (b-CrPO4) structure type and is composed of slightly distorted [HgO6] octahedra. The bond-valence parameters for N3- have been determined, enabling the discrimination between Hg2+-N3- and Hg2+-O2- bonds based on the valence-sum rule. The Raman spectra of the two polymorphs display different vibration patterns, permitting them to be discriminated on the basis of their Raman band shapes.