RHOMBOHEDRAL CLASS Parallel-faced hemihedral.) The only elements of symmetry are the triad axis and a centre of symmetry. The general form { hkl } is a rhombohedron, and is a hemihedral form, with parallel faces, of the scalenohedron. The form { hkl 1, where h-2k -}-1 = o, is also a rhombohedron, being the hemihedral form of the hexagonal bipyramid. The dihexagonal prism { hkl} of the holosymmetric class becomes here a hexagonal prism. The rhombohedra {hkk }, hexagonal prisms {21 I) and { 'oil, and the basal pinacoid {III} are geometrically the same in this class as in the holosymmetric class.
Fig. 76 represents a crystal of dioptase with the fundamental rhombohedron r {too} and the hexagonal prism of the second order rn { i o i } combined with the rhombohedron s { 03i1.
Examples of minerals which crystallize in this class are phena kite, dioptase, willemite, dolomite, ilmenite and pyrophanite: amongst artificial substances is ammonium diperiodate ((NH4)4I2Os•3H2O).