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}, where h–2k+l=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 12'1] and {IOT and the basal pinacoid 1111} are geometrically the same in this class as in the holosymmetric class . Fig . 76 represents a crystal of dioptase with the fundamental rhombohedron r{Ioo) and the hexagonal prism of the second order m toll combined with the rhombohedron s 1o3Y} . Examples of minerals which crystallize in this class are phenacite,581 dioptase, willemite, dolomite, ilmenite and pyrophanite: amongst artificial substances is ammonium periodate ((NH4)4I209.3H20) .