(Hemimorphic-hemihedral).
Here there are three similar planes of symmetry intersecting in the triad axis; there are no dyad axes and no centre of symmetry. The triad axis is uniterminal and polar, and the crystals are differently developed at the two ends; crystals of this class are therefore pyro-electric. The forms are all open forms:—
| Fig. 73.—Crystal of Tourmaline. |
Trigonal pyramid {hkk}, consisting of the three faces which correspond to the three upper or the three lower faces of a rhombohedron of the holosymmetric class.
Ditrigonal pyramid {hkl}, of six faces, corresponding to the six upper or lower faces of the scalenohedron.
Hexagonal pyramid (hkl) where (h − 2k + l = 0), of six faces, corresponding to the six upper or lower faces of the hexagonal bipyramid.
Trigonal prism {211} or {211}, two forms each consisting of three faces parallel to principal axis and perpendicular to the planes of symmetry.
Hexagonal prism {101}, which is geometrically the same as in the last class.
Ditrigonal prism {hkl} (where h + k + l = 0), of six faces parallel to the principal axis, and with two sets of angles between them.
Basal pedion (111) or (111), each consisting of a single plane perpendicular to the principal axis.