(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:--
[Illustration: 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 {21'1'} or {2'11}, 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 {hk'l'} (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 (1'1'1'), each consisting of a single plane
perpendicular to the principal axis.
Fig. 73 represents a crystal of tourmaline with the trigonal prism
(21'1'), hexagonal prism (101'), 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.