Mineralogy

Previously, crystals were defined as bodies bounded by usually flat surfaces, arranged in a regular manner expressing the internal arrangement of the atoms. The study of the arrangement of atoms within a crystal, that is, of atomic structure, has been made possible in recent years by new methods of analysis in which X-rays are employed. This advance dates from the discovery by Laué and others, in 1912, of the diffraction of X-rays by crystals. The first analysis was made in 1913 by W. L. Bragg, on crystals of sodium chloride (common salt).

X-rays are somewhat like light waves but have a much shorter wave-length (see p. 149), this being comparable to the distances between atoms in a crystalline solid. When a beam of X-rays falls on a crystal, it is scattered or diffracted by the layers of atoms within the crystal, in the same way that light waves are diffracted by an optical grating. In making an analysis of a crystal structure, the diffracted X-rays are allowed to fall on a photographic plate, and the resulting photograph shows a series of spots or lines which form a more or less symmetrical pattern. From measurements made on the photograph, the arrangement of the atoms in the crystal can be deduced and also the distances between them. Distances are expressed in Angstrom units; one Angstrom (Ã…)=10-8 em. The several methods of taking X-ray photographs of crystals or of powdered minerals cannot be discussed here, but the principle is broadly that outlined above; details can be found in books on the subject. We are more concerned with the results of X-ray analysis, as these have thrown a flood of light on the structure of crystals, and in particular of minerals, and have confirmed the classes of symmetry worked out in the past by crystallographers from a study of external form, as previously. X-ray analysis has been especially helpful in connection with the big group of mineral silicates, and in this field the work of many investigators has given us a considerable, though not yet complete, knowledge of their structures.

The Unit Cell

Every crystal consists of certain atoms or groups of atoms arranged in a three-dimensional pattern, which is repeated throughout the crystal. The smallest complete unit of pattern is called the unit cell, and the whole pattern is formed by stacking unit cells together. To take a simple example, in crystals of sodium chloride (NaCI) the atoms of N a and CI are arranged at the corners of a series of cubes, as shown in Fig. 68. The unit cell of sodium chloride contains four atoms of Na and four of CI, whose arrangement is exactly similar to that in every other unit cell of the substance. It is to be noted that the number of atoms in the unit cell of a particular mineral is not necessarily the same as in its formula, but is usually some simple multiple; for NaCI this multiple is four. Verify by counting the atoms in Fig. 68, allowing for some atoms belonging half to the unit cell shown there and half to adjacent cells. The array of points in space at which the pattern repeats is called the lattice.

Atomic Bonds

There are four main kinds of bond which hold together the atoms in different crystal structures; they are known respectively as the ionic or polar bond, the homopolar or co-valent bond, the metallic bond, and the residual or van de Waals bond. Correspondingly, crystals may be divided into four classes, each characterized mainly by one of the above four types of bond. The van der Waals bond is very weak in character and is present in all crystals. Nearly all the common minerals have ionic bonding.