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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
48
A POPULAR TREATISE ON GEMS.
eropinacoidon the other, numerous brachyprisms
with the limit formor the brachypinacoid.
In figs. 50, 51, the two domes are shown in their relation to the primitive pyramid.
The pyramids seldom occur independent, or even as the predominant forms in a combination,—sulphur, however, being an exception. Prisms or pinacoids usually give the general character to the crystal, which then appears either in a columnar or tabular, or even in a rectangular pyramidal form. The determination of the position of these crystals,
as vertical or horizontal, depends on the choice of the chief axis of the fundamental form. In the topaz crystal (fig. 52) the brachyprism and the pyramid are the predominant ele­ments, associated with the prism, its sign and letters being
Fig. 53 of stilbite is another ex­ample, the macropinacoidor 31, being combined with the pyramid P(r), the brachypinacoid(T), and the basal pinacoid OP (P). Another instance is fig. 54 of a lievrite crystal, where the brachyprism and pyramid com­bine with the macrodome, orThe follow­ing figures are very common forms of barytes; figs. 55 and
Ch. 1: Form of Minerals Page of 515 Ch. 1: Form of Minerals
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