TRAPEZOHEDRAL CLASS (Trapezohedral-hemihedral.) Here there are three similar dyad axes inclined to one another at 60° and perpendicular to the triad axis. There are no planes or centre of symmetry. The dyad axes are uniterminal, and are pyro electric axes. Crystals of most substances of this class rotate the plane of polarization of a beam of light.
In this class the rhombohedra { hkk } , the hexagonal prism { 2 i 1 } and the basal pinacoid { III} are geometrically the same as in the holosymmetric class; the trigonal prism { 'oil and the ditrigonal prisms are as in the ditrigonal pyramidal class. The remaining simple forms are : TrigonalTrig trapezohedron (fig. 74), bounded by six trapezoidal faces. There are two complementary and enantiomor phous trapezohedra, { hk! } and { Mk), de rivable from the scalenohedron.
Trigonal bipyramid (fig. 75), bounded by six isosceles triangles ; the indices are { hkl } , where h-2k+l =o, as in the hexagonal bipyramid.
The only minerals crystallizing in this class are quartz (q.v.) and cinnabar, both of which rotate the plane of a beam of polar ized light transmitted along the triad axis. Other examples are dithionates of lead calcium and strontium, and of potassium benzil, matico-stearoptene.