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The equation of state and the crystal structure evolution with pressure were determined for two single crystals of pure natural MgAl2O4 spinels with different degrees of order. The two samples studied were cut from a larger single crystal and one of them was experimentally disordered at high temperature. The two crystals, showing an inversion parameter x of 0.27 and 0.15 at ambient conditions, were loaded together in a diamond anvil cell and their unit-cell edge was measured up to about 7.5 GPa at 14 different pressures. The unit-cell volume, V0, the bulk modulus, KT0, and its first pressure derivative, K', were simultaneously refined using a third-order Birch-Murnaghan equation of state, giving the following coefficients: V0 = 529.32(2) ų, KT0 = 193(1) GPa, K' = 5.6(3) for the ordered sample and V0 = 528.39(2) ų, KT0 = 192(1) GPa, K' = 5.4(3) for the disordered one. Complete intensity data were collected at 0, 0.44, 2.92, 7.34, and 8.03 GPa in a separate experiment. For the ordered and disordered samples the oxygen atomic coordinate u remains practically unchanged inside the investigated pressure range with an average value of 0.2633(5) and 0.2614(2), respectively. As a consequence, the polyhedral compressibilities are similar and are not influenced by the Mg/Al distribution over the two crystallographic sites. This also suggests that pressure has little or no influence on the degree of order in the MgAl2O4 spinel. [ABSTRACT FROM AUTHOR] more...

The effects of cation substitution and ordering in the otavite (CdCO3)-magnesite (MgCO3) solid solution have been investigated on samples synthesized at 1 GPa in the temperature range 500.800 °C for run durations up to 120 h. A complete, disordered solid solution, with R3̄c symmetry, was obtained at 800 °C, whereas Mg and Cd show partial ordering within the cadmium dolomite stability field, with R3̄ symmetry, at intermediate compositions in the temperature range 500-650 °C. Rietveld refinements for X-ray diffraction data show that variation of the a-axis is linear as a function of composition, independent of the degree of order, whereas the c-axis shows a positive deviation from linearity as a function of composition, decreasing with increasing degree of order. Octahedral bond distances of the 800 °C series vary linearly with composition. Site occupancies were used to determine the long-range order parameter, Q, for samples with R3̄ symmetry. Q4 varies linearly as a function of temperature, suggesting a tricritical phase transition with a critical transition temperature Tc of 702(10) °C. EDX-TEM compositional microanalyses of samples within two-phase regions are in good agreement with Rietveld refinements, and allow better constraint of phase boundaries. [ABSTRACT FROM AUTHOR] more...