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RHOMBOHEDRAL

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Originally appearing in Volume V07, Page 581 of the 1911 Encyclopedia Britannica.
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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 See also: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 See also: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).

End of Article: RHOMBOHEDRAL

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