Crystal chemistry and low-temperature behavior of datolite: A single-crystal X-ray diffraction study

The crystal chemistry of six natural datolites from different localities was investigated by electron microprobe analysis in the wavelength dispersive mode and single-crystal X-ray diffraction. The chemical analyses show no significant site substitution. The single-crystal structure refinements confirm the structural model of datolite previously reported (with a similar to 4.83, b similar to 7.61, c similar to 9.63 angstrom, and beta similar to 90.15 degrees, space group P2(1)/c). Intra-polyhedral bond distances and angles show common features in all samples at room T: (1) the Si-tetrahedron is strongly deformed, with Si-O distances ranging between similar to 1.57 and similar to 1.66 angstrom and O-Si-O angles ranging between similar to 105.4 and similar to 115.3 degrees; (2) the B-tetrahedron is almost regular; (3) the Ca-polyhedron is significantly distorted, with bond distances ranging between similar to 2.28 and similar to 2.67 angstrom; (4) only one independent H-site occurs and its refined position suggests a bifurcated hydrogen bonding scheme with O5 as donor and O4 and O2 as acceptors [with O5-H similar to 0.8 angstrom and (1) O5 ... O4 similar to 2.99 angstrom, H ... O4 similar to 2.33 angstrom, and O5-H ... O4 similar to 140 degrees, and (2) O5 ... O2 similar to 2.96 angstrom, H ... O2 similar to 2.36 angstrom, and O5-H ... O2 similar to 131 degrees]. Low-temperature diffraction measurements between 300 and 100 K show that the thermal expansion of datolite is mainly governed by the axial response along [100] and [010], whereas the c-axis length is almost unchanged in this temperature interval. The volume thermal expansion coefficient (alpha(v) = V-1 partial derivative V/partial derivative T) between 100 and 280 K is alpha(v) = 1.5(2).10(-5) K-1. The higher thermal expansion of the a-axis is due to the layered nature of the structure of datolite: the Ca-O bond distances are the most compressible and expandable, and govern the contraction, upon cooling, along the direction perpendicular to the polyhedral layers. The tetrahedral layer is significantly more rigid and no changes of the tetrahedral tilts are observed from 300 to 100 K.