A reconsideration of the structure of datolite, CaBSiO4(OH)

Abstract

The crystal structure of datolite, CaBSiO4 (OH), has been refined by the method of least squares with allowance for anisotropic vibrations using Ito and Mori's (1953) three-dimensional data. Refinement to an R of 12.8% with 525 reflections has led to the mean dimensions: Si–O(br) = 1.668 ± 0.007, Si–O(per) = 1.563 ± 0.012 Å and Si–O(br)–B = 125°; bond lengths in the boron tetrahedra vary from 1.449 ± 0.022 to 1.561 ± 0.027 Å. The Ca…O links play a significant role in the structure. The Pauling-Zachariasen method of the balancing of valences gives an excellent qualitative description of the variations in the Si–O, B–O and Ca…O lengths, π bonding maybe the mechanism for the variations in the Si–O lengths. The structures of herderite, CaBePO4F which is isotypic with datolite, and gadolinite are also discussed on the basis of the balancing of valences.