Online Encyclopedia


Online Encyclopedia
Originally appearing in Volume V07, Page 581 of the 1911 Encyclopedia Britannica.
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TRIGONAL BIPYRAMIDAL CLASS Here there is a plane of symmetry perpendicular to the triad 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. Dioptase. Here there are three similar planes of sym- metry intersecting in the triad axis, and perpendicular to them is a fourth plane of symmetry; at the intersection of the three vertical planes with the horizontal plane are three similar dyad axes; there is no centre of symmetry. 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 (q.v.). Hexagonal Division. In crystals of this division of the hexa- FIG.77.—Dihexagonal gonal system the principal axis is a hexad Bipyramid. 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.
TRIGLYPH (Gr. rpelis, three, and yXv4 , an incision...
TRIGONAL PYRAMIDAL CLASS (Hemimorphic-tetartohedral...

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