Rock Crystal: its Structure and Uses 2

Abstract

ROCK crystal, quartz, the common crystallised form of dioxide of silicon SiO2, is, from many points of view, the most interesting of all minerals and the most instructive example of crystalline structure known to us. “What is a crystal?” The evidence is now overwhelmingly complete that a crystal is a homogeneous structure built up on the plan of a space-lattice, each of the unit cells of which is the habitat of a chemical molecule of the substance of which the crystal is composed, and of which it is the most highly organised solid form. It was shown by Frankenheim and Bravais that there are fourteen such space-lattices possible, all of which exhibit the full symmetry of one or other of the seven crystal systems, the cubic, trigonal, tetragonal, hexagonal, rhombic, monoclinic, or triclinic. As a typical example of a space-lattice, and the one of most general form, the triclinic space-lattice is shown in Fig. 1. FIG. Triclinic Space-lattice.