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TRIGONAL BIPYRAMIDAL

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

BIPYRAMIDAL CLASS Here there is a See also:plane of symmetry perpendicular to the triad See also:axis. The trigonal pyramids of the last class are here trigonal bipyramids (fig. 75) ; the prisms are all trigonal prisms, and parallel to the plane of symmetry is the basal pinacoid. No example is known for this class. See also:Dioptase. Here there are three similar planes of sym- metry intersecting in the triad axis, and perpendicular to them is a See also:fourth plane of symmetry; at the intersection of the three See also:vertical planes with the See also:horizontal plane are three similar dyad axes; there is no centre of symmetry. The See also:general See also: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 See also: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 See also:prism 1103} and the basal pinacoid 1111} are geometrically the same in this class as in the See also: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 See also:mineral See also:benitoite (q.v.). Hexagonal See also:Division.

In crystals of this division of the hexa- FIG.77.—Dihexagonal gonal See also:

system the See also:principal axis is a hexad Bipyramid. axis of symmetry. Hexagonal axes of reference are used: if See also:rhombohedral axes be used many of the See also:simple forms will have two sets of indices.

End of Article: TRIGONAL BIPYRAMIDAL

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