High-temperature single-crystal neutron diffraction study of natural chondrodite

The H-atom environment in a Tilly Foster chondrodite was analyzed using single-crystal neutron-diffraction data collected at 500, 700 and 900 K and previously published low temperature data collected at 10, 100 and 300 K on the same crystal (Mg4.64Fe0.28Mn0.014 Ti-0.023(Si1.01O4)(2)F-1.16(OH)(0.84); Friedrich et al. in Am Mineral 86: 981 - 989, 2001). The full mean square displacement matrix Sigma of the O - H pair was determined from the temperature dependence of the anisotropic displacement parameters, enabling a proper correction of the O - H bond for thermal vibration without assumptions about the correlation of O and H movements. The results show that the perpendicular O - H motions in chondrodite are intermediate between the riding and the independent motion models. The corrected O - H bond lengths do not change with temperature whereas the corrected H center dot center dot center dot F distances show an increase of similar to 0.02 angstrom with temperature, as do the Mg - O distances. This result shows that spectroscopic observations on the strength of the covalent O - H bond cannot be interpreted unambiguously in terms of a corresponding behaviour of the associated H center dot center dot center dot O/F hydrogen bond.