The chemical formula for mercury (II) oxide is HgO. Also referred to as mercuric oxide, it is solid at room pressure and temperature. There is a rarely found mineralogical form of HgO called montroydite.
A red form of HgO is created by heating mercury and oxygen. A yellow form is made by precipitation of aqueous mercury with a base. The different colors are a result of the particle size, as both forms have the same structure. Mercuric oxide is the material commonly used for cathodes on mercury batteries. In 1774, Joseph Priestley discovered the oxygen released by heating mercuric oxide.