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Ch. 1: Form of Minerals

Ch. 1: Form of Minerals Page of 515 Ch. 1: Form of Minerals Text size:minus plus Restore normal size   Mail page  Print this page
FORM OF MINERALS.
27
hedron or cube (l) is produced, designated by the crystal-lographic sign; expressing the proportion of the
parameters of its faces, orIf a plane is sup-
posed placed in each edge parallel to one axis, and cut­ting the two other axes at equal distances, the resulting figure is the rhombic dodecahedron (3), designated by the sign0, the proportion of the parameters of its faces be­ing: 1 : 1. The triakisoctahedron (5) arises when on each edge of the octahedron planes are placed cutting the axis not belonging to that edge at a distance from the cen­tre rn which is a rational number greater than 1. The proportion of its parameters is therefore m : 1 : 1, and its sign rnO; the most common varieties being20, and
30. When, on the other hand, from a similar distance m in each two semiaxes prolonged, a plane is drawn to the other semiaxis, or to each angle, an ikositetrahedron (6) is formed ; the parameters of its faces have consequently the proportion m : 1: m, and its sign is mOm; the most com­mon varieties being 202 and 303, the former very frequent in leucite, analcime, and garnet. When, again, planes are drawn from each angle, or the end of one semiaxis of the octahedron, parallel to a second axis, and cutting the third at a distance n, greater than 1, then the tetrakishexahedron (4) is formed, the parameter of its facesits sign
and the most common varieties in nature andFinally, if in each semiaxis of the octa-
hedron two distances, m and n, be tak'en, each greater than 1, and m also greater than n, and planes be drawn from each angle to these points, so that the two planes lying over each edge cut the second semiaxis belonging to that edge, at the smaller distance n, and the third axis at the greater distance m, then the hexakisoctahedron (7) is pro­duced, the parameters of which are m : n : 1, its sign mOn, and the most common varieties
Ch. 1: Form of Minerals Page of 515 Ch. 1: Form of Minerals
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