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ones
and is the hexad axis at right angles to them. It has seven planes of
symmetry, a single hexad axis of symmetry, six dyad axes of symmetry,
and a centre of symmetry. Beryl, emerald, ruby and sapphire, quartz and
tourmaline crystallize in this system.
4. The orthorhombic system.—This
system has crystals with three unequal axes but all of them are
perpendicular to one another. It has three planes of symmetry, three
dyad axes of symmetry and a centre of symmetry. Topaz and olivine
crystallize in this system.
5. Monoclinic system.—There
are three unequal axes of which two intersect at an oblique angle,
while the third is perpendicular to them. It has one plane of symmetry,
one dyad axis of symmetry, and a centre of symmetry.
6. The triclinic system.—In
this system, the three axes are unequal and all are inclined. It has
only a centre of symmetry, e.g. labradorite and moonstone.
The
symmetry gradually diminishes with each system in the order given above
and each system has also several classes in which the symmetry is only
partial, i.e. only one-half or one-fourth of the full number of faces
is developed. These are known as hemihedral and tetarto-hedral forms as
distinct from the holohedral or full faced forms. There are 32 of these
crystal classes and only a few of them include gem-stones.
The
morphological classification of crystals into systems is not at all
artificial. Optical properties form also a second natural basis for
classification, which divide the crystals into three groups: (1) cubic,
(2) tetragonal and hexagonal, and (3) orthorhombic, monoclinic and
triclinic. These are dealt with in detail in a later chapter.
Crystal forms
1. Cubic system.—The
common forms are the cube which has six square faces and is similar to
the geometrical cube; the octahedron has eight triangular faces, the
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