Online Encyclopedia


Online Encyclopedia
Originally appearing in Volume V07, Page 576 of the 1911 Encyclopedia Britannica.
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PLAGIHEDRALI CLASS (Plagihedral-hemihedral; Pentagonal icositetrahedral; Gyroidal 2). 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 icositetrahedra {hkl) and (khl) are derived. These are each bounded by twenty-four irregular Icositetrahedron. Dodecahedron. 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 µop¢$, form), and crystals showing such forms sometimes rotate the plane of polarization of plane-polarized light. Faces of a pentagonal icositetrahedron with high indices have been very rarely observed on crystals of cuprite, potassium chloride and ammonium chloride, but none of these are circular polarizing.

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