DITRIGONAL PYRAMIDAL  CLASS (

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: Trigonal pyramid lhkk}, consisting of 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=o), of six faces, corresponding to the six upper or lower faces of the hexagonal bipyramid . Trigonal prism {211} or {III}, two forms each consisting of three faces parallel to principal axis and perpendicular to the planes of symmetry . Hexagonal prism 1.1011, which is geo-FIG . 73.—Crystal of metrically the same as m the last class . Tourmaline . Ditrigonal prism {hkl) (where h+k+l=o), of six faces parallel to the principal axis, and with two sets of angles between them . Basal pedion (III) or (TM, each consisting of a single plane perpendicular to the principal axis .

Fig . 73 represents a crystal of tourmaline with the trigonal prism (2YY), hexagonal prism (IOY), and a trigonal pyramid at each end . Other substances crystallizing in, this class are

pyrargyrite, proustite, iodyrite (AgI), greenockite, zincite, spangolite, sodium lithium sulphate, tolylphenylketone .