Thermodynamic Studies of Minerals under Mantle Conditions.

Thermodynamic Calculation;Heat Capacity;High Pressure Phase Equilibria;Geology and Earth Sciences;Science;Geology

Yong, WenjunZhang, Youxue ;

University of Michigan

关键词: Thermodynamic Calculation;    Heat Capacity;    High Pressure Phase Equilibria;    Geology and Earth Sciences;    Science;    Geology;   

Others  :  https://deepblue.lib.umich.edu/bitstream/handle/2027.42/61681/wenjuny_1.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

This dissertation is a contribution to the thermodynamic properties and phase equilibria of minerals under mantle conditions. For the first time, the heat capacity data of K-hollandite (KAlSi3O8), Si-wadeite (K2Si4O9) and γ-Fe2SiO4 in the temperature range of 5−303 K were measured using a Physical Properties Measurement System (PPMS). These high pressure phases were synthesized with multi-anvil and piston cylinder equipment at University of Minnesota. They were sent to Austria for the low-temperature heat capacity measurements after sample characterization with EMPA and XRD. These low-temperature heat capacity data are integrated to obtain the entropies of these phases. The entropy of K-hollandite at standard state is 166.2 ± 0.2 Jmol-1K-1, including an 18.7 Jmol-1K-1 contribution from the configurational entropy due to disorder of Al and Si in the octahedral sites. The entropy of Si-wadeite at standard state calculated from the measured heat capacity data is 253.8 ± 0.6 Jmol-1K-1, which is considerately larger than some of the previous estimates. Phase equilibria related to K-hollandite and Si-wadeite were calculated under mantle conditions with the help of the newly measured heat capacity data as well as other available thermodynamic properties. The calculated phase transition boundaries in the system K2O−Al2O3−SiO2 are generally consistent with previous experimental results. Thermodynamic calculation together with seven multi-anvil experiments at 1,400 K and 6.0−7.7 GPa suggests no stability field for kalsilite + coesite, in contrast to previous predictions. The low-temperature heat capacity of γ-Fe2SiO4 measured by PPMS shows a broad λ-transition at 11.8 K, which is presumably due to a paramagnetic−antiferromagnetic transition that represents the Néel temperature. The entropy and Gibbs free energy of γ-Fe2SiO4 at standard pressure and temperature are calculated to be 140.2 ± 0.4 Jmol-1K-1 and -1369.3 ± 2.7 Jmol-1K-1 based on the measured heat capacity and available enthalpy data. The phase boundary for the fayalite−γ-Fe2SiO4 transition at 298 K based on current thermodynamic data is located at 2.4 ± 0.6 GPa with a slope of 25.4 bars/K, consistent with extrapolated results of previous experimental studies.