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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
FORI! OF MIXERALS.                                    45
plex combination of five forms is represented in the crystal of calc-spar, fig. 46, its sign with, the letters on the faces beingTetartohedric
combinations are seen most distinctly in pure quartz or rock-crystal, the pyramids of the first kind appearing as rhom-bohedrons, those of the second kind as trigonal pyramids, the dihexahedral prisms as ditrigonal prisms, and the prism oo P2 as a trigonal prism. Most of these forms, however, occupy but a very subordinate place in the combinations which consist essentially of the prismand the rhom-
bohedron
IV. Jthombie System.—The rhombic system is charac­terized by three axes, all unequal, but at right angles to each other. One of these is assumed as the chief axis, when the others are named subordinate. The plane pass­ing through the secondary axes, or the basis, forms a rhomb, and from this the name is derived. This system comprises only a few varieties of forms that are essentially distinct, and its relations are consequently very simple.
The closed forms are,—(1st.) The rhombic pyramids (figs. 47, 48), bounded by eight scalene triangles, whose
Ch. 1: Form of Minerals Page of 515 Ch. 1: Form of Minerals
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