The premelting and high temperature behaviour of minerals

This thesis presents the results of the computer simulation and high-temperature crystallographic studies of geologically important materials and their analogues. The minerals neighborite (NaMgF3) and perovskite (CaTiO3) are generally considered to be two of the best analogue materials for MgSiO3. Classical molecular dynamics has been used in this thesis to show that NaMgF3 transforms directly from an orthorhombic (Pbnm) structure to the ideal cubic perovskite structure (Pm3m). The possibility of superionic conductivity has been observed at temperatures very close to the melting point of NaMgF3. Earlier reports of anomalous Mg-F bond lengths have been shown to arise from the very anisotropic thermal motion of the F ion. This thermal motion is seen to be sufficient to cause the unit cell volume at high temperatures to be up to 5% less than would be expected in the absence of such vibrations. High-resolution neutron powder diffraction has shown that the sequence of phase transitions in CaTiO3 is Pbnm - I4/mcm - Pm3m. The orthorhombic to tetragonal transition is suggested to be first-order in nature, whilst the tetragonal to cubic tetragonal transition is seen to be second-order. Accurate crystal structures have been determined at various temperatures in the range 298 - 1800K. Lattice parameter measurements have shown that both phase transitions are describable by mean field theory with critical exponents for the order parameter of 1/2 and 1/4 respectively. (Mg,Fe)2SiO4 olivine is the major constituent of the Earth's upper mantle and is found in a wide variety of igneous and metamorphic rocks. The cell parameters and site occupancies for the metal cations in this mineral have been determined by neutron powder diffraction in the range 298 - 1823K It is shown that the use of the partitioning coefficient as a means for determining cooling histories of olivine bearing rocks is not possible without further understanding of the effects of composition on this coefficient. The crystal structure of Li2SiO3 has been determined by neutron powder diffraction in the temperature range 373 - 1323K and that of Na2SiO3 determined from 298 - 873K. At 873K Na2SiO3 undergoes a second-order phase transition to a structure which, unusually, appears to be of lower symmetry. It has not been possible to determine the structure of this mineral above 873K