- A macroscopic and microscopic investigation of the magnesite – otavite solid soluition (2004)
- The magnesite (MgCO3) – calcite (CaCO3) system is the most geologically important of the carbonate minerals. At intermediate compositions a distinct ordered phase exists, dolomite [MgCa(CO3)2], which has symmetry. A complete, disordered ( c symmetry) solid solution exists, but at temperatures in excess of 1400°C. Unmixing and ordering processes that occur at high temperatures in the magnesite – calcite system make experimental work difficult. However, the use of an analogue system allows syntheses under more accessible experimental conditions. The effects of cation substitution and ordering in the magnesite (MgCO3) – otavite (CdCO3) solid solution have, therefore, been investigated using X-ray powder diffraction and Hard Mode Infrared Spectroscopy (HMIS). The results from Rietveld refinements of the X-ray powder diffraction data show that the variation of the a-axis is linear as a function of composition and is not affected by different degrees of order at intermediate compositions. However, the c-axis shows a positive deviation from linearity as a function of composition for the 800°C series. The observed deviation decreases with increasing degree of order for samples of composition Mg0.5Cd0.5CO3. Oxygen-carbon-oxygen bond angles for samples with symmetry showed a decrease in bond angle from 120° (constrained by symmetry in samples with the c structure) with increase in degree of order. Site occupancies were used to determine the long-range order parameter, Q, for samples with symmetry. The square of the order parameter varies linearly as a function of temperature, suggesting a second-order phase transition, and a critical transition temperature of Tc = 719°C. Intensity ratios between reflections present only in the low symmetry phase and reflections present in both the low and high symmetry phases can be used to determine a value proportional to the square of the long-range order parameter, Q. Intensity ratios, between the (101) and (202) reflections for samples of composition Mg0.5Cd0.5CO3, were determined from X-ray powder diffraction patterns and show a linear variation as a function of temperature, with a critical transition temperature, Tc = 716°C. The local mixing and ordering behaviour of the magnesite – otavite solid soluition has been characterised using infrared powder absorption spectroscopy. Phonon bands due to cadmium-oxygen translations, and magnesium-oxygen translations were both observed in the IR spectra for samples of intermediate composition. Band positions for cadmium-oxygen translations remained constant as a function of composition, whereas band positions for magnesium-oxygen translations showed a decrease in wavenumber as a function of composition. Vibrational bands in other spectral regions (libration and translation of carbonate groups and doubly degenetrate in-plane bending and out-of-plane bending of carbonate groups) vary linearly as a function of composition and no effect of order on band position is observed. At high frequencies the samples with c symmetry (800°C series) show a linear shift in band position as a function of composition, whereas, samples in the cadmium dolomite stability field, with symmetry, show a marked increase in frequency compared to their c symmetry counterparts. Cation substitution, or disordering causes broadening of IR vibrational bands for a given material. In this study an alternative method was used, which makes use of the autocorrelation function to establish average line widths for six spectral regions. The autocorrelation results, delta corr, can be interpreted in terms of local strain fields present in the structure due to cation substitution and disordering. In the low frequency region, the delta corr values show a positive deviation from linearity as a function of composition. For samples of composition Mg0.5Cd0.5CO3 the delta corr values are virtually all the same, independent of the degree of order. Over all the spectral regions, the addition of 10mol% MgCO3 (1 Mg atom every 2 unit cells) to the otavite results in a large increase in line width. It is, therefore, suggested that the strain fields surrounding the substituting magnesium atoms into the otavite structure are on the order of 30Å. In the low MIR and at a composition of Mg0.5Cd0.5CO3, an effect of order is observed as a reduction in delta corr values with respect to the sample with c symmetry. The difference in delta corr values between the ordered and disordered samples was used to determine the local-order parameter, q, which scales linearly with the long-range order parameter, Q.