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Encyclopedia Britannica - Main :: TOO-TUM |
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TRIGONAL BIPYRAMIDAL CLASS Here there is a plane of symmetry perpendicular to the triad axis
Dioptase. Here there are three similar planes of sym- metry intersecting in the triad axis
vertical planes with the horizontal
The general form is bounded by twelve scalene triangles and is a ditrigonal bipyramid. Like the general form of the last class, this has two sets of indices {hkl, pp}, (hid) for faces above the equatorial plane of symmetry and (pr) for faces below: with hexagonal axes there would be only one set of indices. The hexagonal bipyramids, the hexagonal prism 1103} and the basal pinacoid 1111} are geometrically the same in this class as in the holosymmetric class. The trigonal prism 12YYj and ditrigonal prisms fall are the same as in the ditrigonal pyramidal class. The only representative of this type of symmetry is the mineral
benitoite
Hexagonal Division. In crystals of this division of the hexa- FIG.77.Dihexagonal gonal system
principal
axis of symmetry. Hexagonal axes of reference are used: if rhombohedral axes be used many of the simple forms will have two sets of indices. End of Article: TRIGONAL BIPYRAMIDAL If you wish, you can link directly to this article.
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