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RHOMBOHEDRAL CLASS (Parallel-faced hemihedral)." The only elements of symmetry are the triad See also: axis and a centre of symmetry
.
The general See also: 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 See also: 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 See also: dioptase with the fundamental rhombohedron r{See also: Ioo) and the hexagonal prism of the second See also: order m See also: toll combined with the rhombohedron s 1o3Y}
.
Examples of minerals which crystallize in this class are See also: phenacite,581
dioptase, See also: willemite, See also: dolomite, See also: ilmenite and pyrophanite: amongst artificial substances is ammonium periodate ((NH4)4I209.3H20)
.
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