“Besides external symmetry of form, crystallization produces also regularity of internal structure, and often of fracture. This regularity of fracture, or tendency to break or cleave along certain planes, is called cleavage. The surface afforded by cleavage is often smooth and brilliant (see specimens 17, 18, and 21), and is always parallel with some external plane of the crystal. It should be understood that the cleavage lamellæ are not in any sense present before they are made to appear by fracture.”—(Dana.)
Crystals are arranged in six systems, based upon the number and relations of the axes, as follows:—
Isometric System.—Three equal axes crossing at right angles. Example, cube.
Tetragonal System.—Two axes equal, third unequal, all crossing at right angles. Example, square prism.
Orthorhombic System.—Three unequal axes, but intersections all at right angles. Example, rhombic prism.
Monoclinic System.—Three unequal axes, one intersection oblique. Example, oblique rhombic prism.
Triclinic System.—Three unequal axes, all crossing obliquely. Example, oblique rhomboidal prism.
Hexagonal System.—Three equal axes lying in one plane and intersecting at angles of 60°, and a fourth axis crossing each of these at right angles and longer or shorter. Example, hexagonal prism.
By the truncation and bevelment of the angles and edges of these fundamental forms a vast variety of secondary forms are produced. The limits of the guide will not permit us to follow this topic farther; but it may be added that for the proper elucidation of even the simpler crystalline forms the teacher should be provided with a set of wooden crystal models and Dana’s “Text-Book of Mineralogy.”
The crystallization of a mineral may be manifested in two ways: first, by the regularity of its internal structure or molecular arrangement, as shown by cleavage and the polarization of transmitted light; and, second, by the regularity of external form which follows, under favorable conditions, as a necessary consequence of symmetry in the arrangement of the molecules.