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 { hkk } , consisting of three faces which corre spond to the three upper or the three lower faces of a rhombo hedron of the holosymmetric class.
Ditrigonal pyramid { hkl I, of six faces, corresponding to the six upper or lower faces of the scaleno hedron.
Hexagonal pyramid { hkl } (w here h-2k-H=o), of six faces, corresponding to the six upper or lower faces of the hexagonal bipyramid.
Trigonal prism { 2 1 i } or Pr' } , two forms each consisting of three faces paral lel to the principal axis and perpendicular to the planes of symmetry.
Hexagonal prism { Ioi } which is geo metrically the same as in the last class.
Ditrigonal prism {hkl}, (where h+k -}-1 = o), of six faces parallel to the principal axis, and with two sets of angles between them.
Basal pedion {III } or { I I i } , each con sisting of a single plane perpendicular to the principal axis.
Fig. 73 represents a crystal of tourmaline with the trigonal prism { 2I I } hexagonal prism { Tor } and a trigonal pyramid at each end. Other substances crystallizing in this class are pyrargy rite, proustite, iodyrite (AgI), greenockite, zincite, spangolite, sodium lithium sulphate, tolylphenylketone.