PLAGIHEDRAL CLASS (Plagihedral-hemihedral ; Pentagonal-icositetrahedral; Gyroidal.) In this class there are the full number of axes of symmetry (three tetrad, four triad and six dyad), but no planes of symmetry and no centre of symmetry.
Pentagonal icositetrahedron (fig. 40).—This is the only simple form in this class which differs geometrically from those of the holosymmetric class. By suppressing either one or other set of alternate faces of the hexakis-octahedron two pentagonal rahedra { hkl } and{ khl } are derived. These are each bounded by twenty-four irregular pentagons, and although similar to each other they are respectively right- and left-handed, one being the mirror image of the other; such similar but nonsuperposable forms are said to be enantiomorphous (Evavrios, opposite, and µop4», form), and crystals showing such forms sometimes tate the plane of polarization of polarized light. Faces of a pentagonal itetrahedron with high indices have been very rarely observed on crystals of cuprite, potassium chloride and ammonium ride, but none of these are circular ing; nor is the internal structure, as mined by means of X-rays, in agreement with this external development of the faces.
