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 .