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Constancy of crystal angles
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The
angles between similar faces remain constant regardless of the size
or, the shape of the crystal for the same substance. This is a very
important property of crystals, whereas sizes and shapes of faces may
be distorted by irregular growth. There are instruments known as
goniometers for correct measurements of crystal angles.
Size of Crystals
Crystals
exhibit variations from microscopic dimensions to those weighing
several tons. Large crystals of spodumene and beryl have been found to
weigh several tons.
Crystal systems
All
crystals fall into six groups according to the length and inclination
of imaginary lines known as the crystallo-graphic axes which are
fundamental to their structure.
1. The cubic system.—Crystals
of the cubic system have three mutually perpendicular and equal axes.
The forms known as cube, octahedron, dodecahedron and tetrahedron
belong to this system. Diamond, spinel, and garnet are examples of this
system. The cubic system has the highest symmetry, e.g. it has three
cubic planes of symmetry, six other planes of symmetry and has 13 axes
of symmetry.
2. The tetragonal system.—-In
this system, crystals have three axes which'intersect at right angles,
but the vertical axis is longer or shorter than the horizontal axes.
Zircon and Vesuvianite occur in this system. This system is
characterized by five planes of symmetry, one tetrad axis of symmetry,
six dyad axes of symmetry and a centre of symmetry.
3. The hexagonal system.—Hexagonal
crystals have four axes—three are horizontal and equal, and intersect
at 60°. The fourth is longer or shorter than the horizontal
13
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