The structure of beryl, Be 3 Al 2 Si 6 O 18

Abstract

1. The structure of beryl, described in this paper, has some interesting features. In the first place, the ratio of silicon to oxygen atoms in the molecule is that of a metasilicate. As far as we know, no other example of a metasilicate has as yet been completely analysed (Wyckoff has made some measurements on diopside MgCa(SiO 3 ) 2 , but has only succeeded in determining its space group). Although the ratio of silicon to oxygen is as one to three, we find no SiO 3 groups in beryl. We find instead an arrangement of oxygen atoms around a silicon atom of exactly the same conformation as the groups (SiO 4 ) == in typical orthosilicates such as olivine and garnet. In this metasilicate the ratio of one silicon to three oxygen atoms is effected by a sharing of two oxygen atoms of each tetrahedral SiO 4 group with neighbouring groups. Thus the structure forms an intermediate stage between the orthosilicates and the structures of quartz and christobalite where every oxygen atom is shared by silicon atoms. This interesting feature may throw some light on the structure of other metasilicates. In the second place, we have analysed the structure by making careful quantitative measurements of the absolute intensity of X-ray reflexion, and interpreting these measurements by the formulæ which have proved to hold for simpler cases. The structure is one of moderate complexity, depending on seven parameters. We hope to show that all these parameters can be directly and accurately fixed. The analysis is greatly simplified when quantitative data are available, and there appears to be no reason why structures with many more parameters should not be attacked in the same direct manner. 2. Beryl belongs to the holosymmetric class of the hexagonal system. Groth ('Chemische Krystallographie’) quotes the axial ratio— a : c = 1 : 0·4989, and gives the density as being between 2·6 and 2·7. We have found the following values for the axes, measured by the X-ray spectrometer— c = 9·17 ± 0·01 Å. a = 9·21 ± 0·01 Å.