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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
54
A POPULAR TREATISE ON GEMS.
vitriol, and axinite, show great incompleteness and want of symmetry. In the latter case the determination of the forms is often difficult and requires great attention. As specimens, we may notice the albite crystal (fig. 64), in which Ρ is the basal pinacoidOP; ilfthe brachydiagonal pinacoidthe upper right pyramid P' ; I the right
hemiprismΤ the left hemiprismj and a; the
hemidomtFigures 65 and 66 are crystals of axinite,
the former from Dauphiné, the latter very common in Corn­wall, of whose faces the following is the development :—r the macropinacoidthe left hemiprism
the left upper quarter pyramid 'P ; I the left upper quarter pyramid 2'P ; s the left upper partial form of the macro-pyramid 3'P3 ; and χ the hemidome
Imperfections of Crystals.
In the foregoing description of the forms of crystals the planes have been supposed smooth and even, the faces equal and uniform, or at the same distance from the centre or point of intersection of the axes, and each crystal also perfect or fully formed and complete on every side. In nature, however, these conditions are rarely if ever real­ized, and the edges of crystals are seldom straight lines, or the faces mathematical plane surfaces. A very interesting variety of these irregularities, which pervades all the sys­tems except the tesserai, is named Iiemimorphism. In this
Ch. 1: Form of Minerals Page of 515 Ch. 1: Form of Minerals
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