The incorporation of alkalis in beryl; a multinuclear MAS NMR and crystal-structure study

The stereochemical details of the incorporation of alkali ions into the beryl structure axe examined by multi-nuclear MAS NMR spectroscopy and single*rystal structure refinement. The 2eSi signal is narrow in alkali-poor beryl, and broadens with increasing alkali content, a result of the different short-range next-n€arest-neighbor configurations produced by the incorporation of alkalis into the structnre; we designate this substitutional broadening, and calculate its value from the proposed local configurations and the method of Sherriff & Grundy (1988). Substitutional broadening is observed in the 27AI spectra that also show only [6]-coordinate Al to be present, in agreement with the current and earlier X-ray results. Two 7Li signals are observed, indicating that Li occurs both as a framework constituent and as a channel species. The crystal structures of the four samples were refined, to R indices of 2-390 using MoKcr X radiation, The distance expands linearly with increasing Li :+ Be substitution at this site, but the O-Be-O bond angles remain unchanged. This substitution produces a bond-valence deficiency at the O(2) anion. This is compensated by lengthening of Si-O(l) and Si-O(l)a, and contraction of Si-O(2) with increasing Li + Be substitution. All samples of alkali-bearing beryl examined show residual electron density in the channels due to occupancy by alkali ions; all show more density at the 2a site than at the 2b site. Comparison of these residual densities with known alkali contents confinns previous work tlut assigned Cs to the 2a position. The electron density observed at the 2b position correlates well with the Na contents and is iniufficient to account for the H2O; thu$ we assign Na to 2b and H2O to 24. Local stereochemistry and analytical rerults suggest tlat Na is bonded to two H2O groups in the channel.