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54
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A POPULAR TREATISE ON GEMS.
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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 pinacoid
hemiprism
hemidomt
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the former from Dauphiné, the latter very common in Cornwall, of whose faces the following is the development :—r the macropinacoid
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
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