Of the simple undecomposable substances which chemists call elements, and of which rather more than seventy have been discovered, only sixteen enter at all largely into the composition of the earth's crust, so far as this is accessible to observation. It is estimated that 98 % of the crust is made up of the following eight elements, arranged in the order of abundance, with the percentages as calculated by F. W. Clarke.

Oxygen............47.07

Silicon....... . . . . . 28.06

Aluminium......... 7.90

Iron.............. 4.43

Calcium............ 3.44

Magnesium.......... 2.40

Sodium.............2.43

Potassium...........2.45

The remaining eight elements, titanium, carbon, sulphur, hydrogen, chlorine, phosphorus, manganese, and barium, are far less abundant, but still of considerable importance.

Only two of these elements, carbon and sulphur, are found in a more or less impure state as minerals or rock masses; the others occur as compounds, formed by the union of two or more of them.

A mineral is a natural, inorganic substance, which has a homogeneous structure, definite chemical composition and physical properties, and usually a definite crystal form.

Crystals are solids of more or less regular and symmetrical shape, bounded, usually, by plane surfaces. The number of known crystal forms is very great, and yet they may be all grouped in six systems, which are characterized by the relations of their axes. The axes of a crystal are imaginary lines, which connect the centres of opposite faces, or opposite edges, or opposite solid angles, and which intersect one another at a point in the interior of the crystal.

The Systems of Crystal Forms have received many names, the following being those which are most generally used in this country: -

I. Isometric System (Monometric, Cubical, Regular)

In this system the three axes are of equal length and intersect one another at right angles.

II. Tetragonal System (Dimetric, Pyramidal)

The axes intersect at right angles, but while the lateral axes are of equal length, the vertical axis is longer or shorter than the laterals.

III. Hexagonal System

Here four axes are employed, three equal lateral axes intersecting at angles of 6o°, and a vertical axis, which is perpendicular to and longer or shorter than the laterals.

IV. Orthorhombic System (Rhombic, Trimetric)

The three axes intersect at right angles and are all of different lengths.

V. Monoclinic System (Monosymmetric, Oblique)

All three axes are of different lengths; the two laterals are at right angles to each other, while the third is oblique to one of the former.

VI. Triclinic System (Anorthic, Asymmetric)

Three axes of unequal lengths and oblique to one another.

It is important to bear in mind the relations which the forms sustain toward one another. For example, a regular octahedron may be derived from a cube by evenly paring off the eight solid angles, until the planes thus produced intersect one another, the centres of the faces of the cube becoming the apices of the solid angles of the octahedron. Conversely, a cube may be formed from an octahedron by symmetrically truncating the angles, until the planes thus formed intersect. By slicing away the twelve edges of a cube or an octahedron a dodecahedron will result. These crystal forms are, therefore, so related as to be all derivable one from another, and the relations of their axes remain unchanged; all three forms may be assumed by the same mineral, and they thus properly belong in the same system. Similar relations may be observed between the crystal forms of the other systems.

It might be supposed that the crystal systems and the relations of their imaginary axes were merely mathematical devices to reach a convenient classification of forms. Such a conclusion would, however, be a very erroneous one. Crystalline form is an expression of molecular structure, and the physical properties of minerals are closely related to their mathematical figure. It is clear that these physical properties are not inherent in the molecules of the mineral, but are conditioned by the way in which the molecules are built up into the crystal. Amorphous substances refract light equally in all directions, and are thus called isotropic; but when an amorphous substance crystallizes, it assumes the qualities proper to its crystal form. Thus water is isotropic, while the hexagonal crystals of ice are singly refractive in only one direction, doubly refractive in all others. The same substance may, under different circumstances, crystallize in different systems, and will then display the properties appropriate to each system.

Not only the refractive powers of a crystal, but also its mode of expansion when heated, and its conductivity of electricity and heat are controlled by the molecular structure which determines its shape.

The crystals of the isometric system, which have their three axes of equal length, are singly refractive in all directions, expand equally when heated, and conduct heat and electricity equally in all directions. Those of the tetragonal and hexagonal systems, which have one axis longer or shorter than the others, are doubly refractive along the lateral axes, expand equally when heated, and show equal conductivity along these axes. Along the principal axis they are singly refractive, expand to a different degree when heated, and display a different conductivity along this axis than along the others. In the orthorhombic, monoclinic, and triclinic systems, which have all the axes of unequal lengths, the crystals are singly refractive in two directions; they expand unequally and conduct differently along all their axes.

The optical properties of minerals are of great value in the study of rocks, and by the aid of the polarizing microscope very minute crystals may be identified.

Cleavage (see p. II) is still another physical property, the dependence of which upon crystal form is very clear.

Most inorganic substances which are solid under any circumstances are capable of assuming a crystal form, so that solidification and crystallization are usually identical. For the formation of large and regular crystals, it is necessary that the process be gradual and that space be given for the individual crystals to grow. Usually crystallization begins at many points simultaneously, and the crystals crowd upon one another, resulting in a mass of more or less irregular crystalline grains. The same substance which, when very rapidly solidified, forms an amorphous glass, will give rise to distinct crystals, if slowly solidified.

Crystallization requires that the molecules be free to move upon each other, and thus to arrange themselves in a definite fashion. It may take place either by the deposition of a solid from solution, by cooling from a state of fusion, or by solidification from the condition of vapour. In all cases the size and regularity of the crystals depend upon the time and space allowed for their growth. In a manner not yet understood, amorphous solids may be converted into crystalline aggregates. This has been observed in the case of certain glassy volcanic rocks, which, though amorphous when first solidified, have gradually become crystalline, without losing their solidity, and a similar change has been observed in certain artificial glasses. This process is called devitrification.

The actual steps of crystallization may be observed by slowly evaporating a solution of some crystalline salt under the microscope. The first visible step in the process is the appearance of innumerable dark points in the fluid, which rapidly grow, until their spherical shape is made apparent. The globules then begin to move about rapidly and arrange themselves in straight lines, like strings of beads, and next suddenly coalesce into straight rods. The rods arrange themselves into layers, and thus build up the crystals so rapidly, that it is hardly possible to follow the steps of change. In certain glassy rocks, which solidified too quickly to allow crystallization to take place, the incipient stages of crystals, in the form of globules and hair-like rods, may be detected with the microscope.