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Encyclopedia Britannica - Main :: RAY-RHU |
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RHOMBOHEDRAL CLASS (Parallel-faced hemihedral)." The only elements of symmetry are the triad axis
general form {hkl} is a rhombohedron, and is a hemihedral form, with parallel faces, of the scalenohedron. The form {hkl}, where h2k+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 {IOTand 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
Examples of minerals which crystallize in this class are phenacite
dioptase, willemite
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