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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
52                     A POPULAR TREATISE ON GEMS.
Fig. 01 is a crystal of com mon felspar or orthoclase, composed of the clinopinacoid ithe prism the basal pinacoid OP
(P), and the hemidomesto which, in fig. 62 of
the same mineral, the hemipyramid P(o), and the clino-domeare added.
VI. Triclinohedric System.—This is the least regular of all the systems, and departs the most widely from symmetry of form. The axes are all unequal, and inclined at angles none of which are right angles, so that to determine any crystal or series of forms the proportion of the axes a:b:c, and also their angles, or those of the inclination of the chief sections, must be known. As in the previous system, one axis is chosen as the principal axis, and the two others dis­tinguished as the macrodiagonal and brachydiagonal axes. In consequence of the oblique position of the principal sec­tions, this system consists entirely of partial forms wholly independent on each other, and each composed only of two parallel faces. The complete pyramid is thus broken up into four distinct quarter pyramids, and the prism into two hemiprisms. Each of these partial forms is thus nothing more than a pair of parallel planes, and the various forms consequently mere individual faces. This circumstance
Ch. 1: Form of Minerals Page of 515 Ch. 1: Form of Minerals
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