The structure of topaz [Al (F, OH)]2 SiO4

Abstract

Abstract 1. In a recent paper on the structure of certain silicates, Prof. W. L. Bragg and one of the authors discussed in considerable detail the role which the oxygen atoms—due to their relatively large size—played in these compounds. The structures examined fell roughly into two classes: one in which the O atoms were throughout packed as closely together as possible, the other in which close packing was local only. Many examples of structures which had actually been worked out were given, and it was suggested that topaz might possibly prove a further example of a close-packed arrangement of large ions. An attempt to solve the structure of topaz on these lines proved unexpectedly difficult, and a detailed general quantitative examination was therefore carried out. A full account of this examination, which has now been completed, will be published later in the 'Zeitschrift für Kristallographie.’ We propose here to give but a general outline of the structure, desiring rather to draw attention to an interesting new type or variant of close packing which the study has revealed, and which suggests important considerations in the elucidation of complex structures still awaiting analysis.