Under atmospheric pressure mercuric oxide has two crystalline forms: one is called montroydite (orthorhombic, 2/m 2/m 2/m, Pnma), and the second is analogous to the sulfide mineral cinnabar (hexagonal,
hP6, P3221); both are characterized by Hg-O chains. At pressures above 10 GPa both structures convert to a tetragonal form.