Beautiful Gems

Many crystals enclose a certain number of foreign particles during their growth. These inclusions often make a clear mineral appear cloudy, or may change the colour or some other characteristic of the mineral. Occasionally, gaseous or liquid remnants from the mother liquor are enclosed in the crystal. Quartz, for instance, may appear milky, due to minute liquid inclusions which are invisible to the naked eye. Other varieties of quartz, such as rock-crystal, may contain quite large, clearly visible crystals of actinolite, tourmaline or rutile.

The properties of any particular mineral are determined not so much by its chemical composition as by the arrangement of its smallest particles. This regular arrangement in space of atoms and ions is called a lattice, and the smallest complete unit of the pattern is known as the unit cell. Every crystalline substance is made up of such a lattice of atoms and ions, and by combining the basic lattice structures according to the laws of symmetry one obtains 230 distinct lattice types into which all the many different crystal lattices can be grouped. The mineral with the simplest crystal lattice is rock-salt (sodium chloride). In its lattice the ions of sodium and chlorine are arranged at the corners of a series of cubes, so that every chlorine ion is linked to six sodium ions, and every sodium ion to six chlorine ions. The unit cell thus consists of four sodium and four chlorine ions, and the distance between the centres of adjacent ions of the same type is 5- 64 x 10-8 cm or 5.64 ten-millionths of a millimetre. Along the edge of a cube of rock-salt one cubic millimetre in size there are thus almost two million unit cells. The determination of the atomic structure of minerals by X-rays is now one of the most important fields of crystallography.