The study of the growth, shape, and geometric characteristics of crystals is called crystallography. When the conditions are right, each chemical element and compound can crystallize in a definite and characteristic form.
Thirty-two classes of crystals are theoretically possible, almost all common minerals fall into one of about twelve classes, and some classes have never been seen. The thirty-two classes are grouped into six crystal systems, based on the length and position of the crystal axes. Crystal axes are imaginary lines passing through the center of the crystals. Minerals in each system share certain proportions and crystal form and many important optical properties.
The six crystal systems are very important to a mineralogists and geologists; specification of the system is necessary in the description of each crystal system.
Isometric
This system comprises crystals with three axes, all perpendicular to one another and all have equal length.
Tetragonal
This system comprises crystals with three axes, all perpendicular to one another; but only two are equal in length.
Orthorhombic
This system comprises crystals with three mutually perpendicular axes, all of different lengths.
Monoclinic
This system comprises crystals with three axes, all unequal in length, two o which are not perpendicular to another, but both of which are perpendicular to the third.
Triclinic
This system comprises crystals with three axes, all unequal in length and is not perpendicular to one another.
Hexagonal
This system comprises crystals with four axes. Three of these axes are in a single plane, proportionally spaced, and of equal length. The fourth axis is perpendicular to the other three. Some crystallographers split the hexagonal in two, calling the seventh system trigonal or rhombohedral.
Formation of Crystals
Crystals are formed when a liquid becomes solid or when a vapor or liquid solution becomes supersaturated. Some substances tend to form seed crystal. If a solution like this is cooled slowly, a few seeds grow into large ones; but if it is cooled rapidly, numerous seeds form and grow only into tiny crystals. Table salt, purified at a factory by recrystallization, is composed of lots of cubed crystals, which are barely visible with the naked eye; rock salt, formed in a really long time, contains enormous crystals of the same cubed form.
