In situ measurements of cation disorder in dolomite and some spinels at high pressure and temperature: Effect on elasticity and stability

This study has shown experimentally that cation disorder has significant effects on the stability and elasticity of lower crustal and mantle minerals using in situ experiments. In particular, cation disorder causes a significant change in sound velocities in magnesioferrite. Some members of the spinel-group (magnesioferrite, MgFe2O 4; qandilite, TiMg2O4; and GeMg2O 4) and a dolomite from the carbonate-group were selected for detailed investigations because they were expected to show significant changes in order-disorder with pressure and temperature, and this is now confirmed experimentally in the present study. In this study, state-of-the-art experimental capabilities were used to obtain good in situ structural and ultrasonic data under high-PT conditions. The present study shows that cations in spinels and dolomite-type minerals disordered as T is increased. Inverse spinels with 2-3 cation charges tend to become more ordered with increasing P at constant T.; The present study has investigated cation disorder in magnesioferrite, MgFe2O4; qandilite, TiMg2O4; and GeMg2O4 at room P and high T. Unlike qandilite and GeMg2O4, magnesioferrite shows considerable disorder up to about 1250 K. Qandilite breaks down to MgO and MgTiO3 at 1250 K. Pressure enhances cation ordering in magnesioferrite. These results were used in both O'Neill and Navrotsky (1983) and Landau thermodynamic models for equilibrium cation ordering with P and T. Both models fit the present experimental data equally well. Elasticity data on magnesioferrite indicate that acoustic anomalies are associated with cation and magnetic disordering processes. Increasing cation disorder lowers the sound velocity in magnesioferrite.; There is interest in the properties of carbonate minerals under high- PT conditions because of their important implications for the earth's carbon cycle, oxygen fugacity, and stability of other minerals in the mantle. Dolomite, CaMg(CO3)2, structure contains alternating planes of Ca2+ and Mg2+ cations, and these cations can be disordered with P and T. The most important aim in this part of the study is to test whether the stability of dolomite arises from cation disordering. The present study has experimentally confirmed that the stability of dolomite arises from cation disordering, which was thermodynamically modeled using a new modified Bragg-William model. Generally, cation disordering significantly affects mineral properties.