Online Encyclopedia


Online Encyclopedia
Originally appearing in Volume V07, Page 581 of the 1911 Encyclopedia Britannica.
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HOLOSYMMETRIC CLASS (Holohedral; Dihexagonal bipyramidal). Intersecting in the hexad axis are six planes of symmetry of two kinds, and perpendicular to them is an equatorial plane of symmetry. Perpendicular to the hexad axis are six dyad axes of two kinds and each perpendicular to a vertical plane of symmetry. The seven simple forms are: Dihexagonal bipyramid, bounded by twenty-four scalene triangles (fig. 77; v in fig. 8o). The indices are {2131}, &c., or in general {hikl}. This form may be considered as a combination of two scalenohedra, a direct and an inverse. Hexagonal bipyramid of the first order, bounded by twelve isosceles triangles (fig. 70; p and u in fig. 8o); indices 110111, (202I} . . . (holll). The hexagonal bipyramid so common in quartz is geometrically similar to this form, but it really is a combination of two rhombohedra, a direct and an inverse, the faces of whit.h differ in surface characters and often also in size.
HOLOSYMMETRIC CLASS (Holohedral (ass, whole)

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