The Probert Encyclopaedia of Rocks & Minerals

CRYSTAL

In mineralogy, a crystal is any body which, by the mutual attraction of its particles, has assumed the form of some one of the regular geometric solids, being bounded by a certain number of plane surfaces. The chemist procures crystals either by fusing the bodies by heat and then allowing them gradually to cool, or by dissolving them in a fluid and then abstracting the fluid by slow evaporation. The method of describing and classifying crystals now universally adopted is based upon certain imaginary lines drawn through the crystal, and called its axes. The classes are as follow:

1st, The monometric, regular, or cubic system in which the axes are equal and at right angles to one another;
2nd, The square prismatic or dimetric system in which the axes are at right angles to each other, and while two are equal, the third is longer or shorter;
3rd, The right prismatic, rhombic, or trimetric system in which the axes are at right angles to each other, but all are of different lengths;
4th, The hexagonal or rhomhohedral system which has four axes, three in one plane inclined to each other at 60 degrees, the fourth perpendicular to this plane;
5th, The monoclinic or oblique system in which two axes are at right angles and the third is inclined to their plane;
6th, The diclinic or doubly oblique system in which two axes are at right angles, the third oblique to both;
7th, The triclinic system in which the three axes are inclined to each other at any angle other than a right angle.

A crystal consists of three parts. 1st, Plane surfaces, called faces, which are said to be similar when they are equal to one another and similarly situated; dissimilar, when they are unequal or have a different position. 2nd, Edges, formed by the meeting of two faces. They are said to be similar when formed by similar faces; dissimilar, by dissimilar faces. Equal edges are formed when the faces are inclined at the same angle to one another; unequal, when they are inclined at different angles. 3rd, Solid angles, formed by the meeting of three or more faces; and in this case also there are similar and dissimilar, equal and unequal solid angles, according as they are formed by similar or dissimilar faces, and equal or unequal angled edges. The angles of crystals are measured by an instrument called the goniometer.
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