at
first sight to have no similarity of form. One can soon become
familiar with these variations, however, and by making due allowance
for them learn to recognize crystal forms quickly and accurately.
In
addition to an empirical familiarity with the forms of crystals, some
knowledge of the general groups of crystals is desirable, since there
are thus expressed relations which characterize not only the external
form, but internal structure.
The
forms into wbich a mineral, or any substance of definite chemical
composition, may crystallize, are divided into six systems. These are
known as the isometric, tetragonal, hexagonal, orthorhombic,
monoclinic, and tri-clinic systems. By some a seventh, called the
rhombohedral system, is added, though here it is considered a
subdivision of the hexagonal. In the discrimination of crystal forms,
the relations of the planes can best be expressed by referring them to
a series of three or more imaginary axes within the crystal. One of
these, known as a, is supposed to run from front to back; another, known as b, from right to left; and the third, known as c, vertically
The latter is known as the vertical axis, and the two former are
designated as lateral axes. In the hexagonal system, the existence of
three lateral axes is assumed.
The differences between the six systems can be stated in terms of these axes as follows:
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