(Hemihedral; Domatic).
Crystals of this class are symmetrical only with respect to a single
plane. The only form which is here geometrically the same as in the
holosymmetric class is the clino-pinacoid {010}. The forms
perpendicular to the plane of symmetry are all pedions, consisting of
single planes with the indices {100}, {1'00}, {001}, {001'}, {hol},
&c. The remaining forms, {hko}, {okl} and {hkl}, are domes or
"gonioids" ([Greek: gonia], an angle, and [Greek: eidos], form),
consisting of two planes equally inclined to the plane of symmetry.
Examples are potassium tetrathionate (K2S4O6), hydrogen trisodium
hypophosphate (HNa3P2O6.9H2O); and amongst minerals, clinohedrite
(H2ZnCaSiO4) and scolectite.