gems, topaz, optical properties of topaz, crystallography

In my view, the normal structure of a crystal, as an example of the association of structural particles under the constraint of electrical forces of attraction and repulsion, may be regarded as a primarily stressed system, analogous to the association of particles in free atoms and molecules. Under thermal influence, the structural forces of compression and tension within such a “leptonic association” generally undergo continuous changes and, in some cases, superimposed abrupt periodic changes; these structural transformations consist of continuous or discontinuous atomic rearrangements and atomic deformations. Changes in volume and, where applicable, in angles are indications of the former, while optical changes are indications of the latter. These signs of structural processes are particularly striking in the abrupt changes just mentioned, especially during transformations between crystal modifications. In such cases X-ray diffraction patterns, particularly Laue patterns, also make the discontinuous atomic rearrangements very evident. Quartz provides an excellent example of this.