Single-crystal neutron-diffraction study of pyrope in the temperature range 30-1173 K

Single-crystal neutron-diffraction data were collected over the temperature range 30-1173 K on a pyrope crystal from the Dora Maira Massif, Western Alps, Italy. The study expands the structure analysis of synthetic end-member pyrope previously performed with X-ray single-crystal diffraction over a wide temperature range. The neutron nuclear scattering allows a more accurate definition of the atomic displacement parameters and their temperature dependence. The problems involved in the characterization of the atomic displacement parameters in garnet minerals, and especially in pyrope, have implication for the correct interpretation of the physical and thermodynamic properties of the garnet structure. The present analysis of the atomic displacement properties by the higher cumulants model in the Gram-Charlier series approximation shows that the anharmonic contribution to the dynamic part of the magnesium site displacement is significant above 700 K. No evidence is found for static site disorder in the dodecahedral Mg site by refinement with split-atom models, and the intercept at 0 K resulting from linear interpolation of the high-temperature harmonic displacement coefficients is not statistically significant. The refinement of high-order displacement-tenser components results in significantly lower figures of merit in the whole explored temperature range.