ABSTRACT
INTRODUCTION A century ago, the study of crystals was only concerned with examinations of their external form and with the physical properties, as these varied along the different crystal directions. The regularity of the appearance and of the external form of naturally occurring crystals led observers to believe that these materials were formed by the regular repetition of identical building blocks. It was deduced that when a crystal grows in a constant environment, the shape remains unchanged while the mass is built up from the continuous addition of fundamental units. Mineralogists made the important discovery that the faces of a crystal could be indexed to exact integers, and in 1784 Haüy showed that only the arrangement of identical particles in a threedimensional periodic array could account for the law of rational indices (1). It was subsequently suggested by Seeber that the elementary building blocks of crystals were small spheres, held together in a lattice array through the balance of attractive and repulsive forces (2). Different crystal structures were believed to arise from the various ways spheres could be packed into alternate motifs. Shrewd guesses about the structures of a number of crystals were made by Barlow, who argued from considerations of symmetry and the packing of spheres (3).
