Online Encyclopedia


Online Encyclopedia
Originally appearing in Volume V07, Page 581 of the 1911 Encyclopedia Britannica.
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DITRIGONAL PYRAMIDAL CLASS (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: Trigonal pyramid lhkk}, consisting of 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=o), of six faces, corresponding to the six upper or lower faces of the hexagonal bipyramid. Trigonal prism {211} or {III}, two forms each consisting of three faces parallel to principal axis and perpendicular to the planes of symmetry. Hexagonal prism 1.1011, which is geo-FIG. 73.—Crystal of metrically the same as m the last class. Tourmaline. Ditrigonal prism {hkl) (where h+k+l=o), of six faces parallel to the principal axis, and with two sets of angles between them. Basal pedion (III) or (TM, each consisting of a single plane perpendicular to the principal axis. Fig. 73 represents a crystal of tourmaline with the trigonal prism (2YY), hexagonal prism (IOY), 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.

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