[SMP36-08] Redetermination of enthalpy and reassessment of thermodynamic data set for SiO2 stishovite
キーワード：SiO2 スティショバイト、熱力学、エンタルピー、熱容量、状態方程式、相平衡境界線
SiO2 stishovite (St) is a high pressure polymorph of SiO2 and believed to be one of major constituent minerals of the subducted slab. Thermodynamic parameters of SiO2 St used for thermodynamic examination of the stability field of SiO2 St or a phase assemblage containing it have not yet been constrained well. In particular, standard enthalpy of formation largely affects a position of a calculated phase boundary. Since previous enthalpy measurement of SiO2 St by Akaogi et al. (1995) was performed using the differential drop-solution calorimetry, measured enthalpy included heat effect from not only SiO2 St sample but also silica capillary tubes as a container and Pt chips as a weight. In this study, heat effect of only SiO2 St was measured by the drop-solution calorimetry. Furthermore, to give an internally-consistent thermodynamic data set, high temperature heat capacity of SiO2 St was reassessed based on an equation of state (EoS).
SiO2 St samples for the drop-solution calorimetry were synthesized by heating reagent grade SiO2 quartz (Qz) at 15 GPa and 1673 K using a Kawai-type high-pressure apparatus at Gakushuin University. It was confirmed that synthesized samples were single phase of SiO2 St by micro-focused X-ray diffraction method. The drop-solution calorimetry was performed using a Calvet-type high-temperature calorimeter (SETARAM, HT-1000). A sintered SiO2 St sample with weight of 3-6 mg was dropped directly into lead borate solvent (2PbO·B2O3) placed in the calorimeter at 978 K. Drop-solution enthalpy (ΔHd-s), which is the summation of heat content from room temperature to 978 K and solution enthalpy at 978 K was measured. To hasten the solution of the samples, the solvent was stirred by bubbles of Ar gas.
In the reassessment of isobaric heat capacity (Cp) of SiO2 St, isochoric heat capacity (Cv) was calculated using the Kieffer model with adjusting a VDoS model to reproduce low-temperature Cp data measured by Akaogi et al. (2011) and Yong et al. (2012). The anharmonic effect was calculated using isothermal bulk modulus, its temperature derivative and thermal expantivity derived from the EoS of Wang et al. (2012).
From the average of six data, ΔHd-s (SiO2 St) was determined to be 3.8±0.4 kJ/mol. This value is slightly larger than 3.0±0.9 kJ/mol measured by Akaogi et al. (1995), though within the experimental errors. The difference between ΔHd-s (SiO2 Qz) [40.1±0.4 kJ/mol, Akaogi et al. (1995)] and the present ΔHd-s (SiO2 St) gives the phase transition enthalpy from Qz to St of 36.2±0.5 kJ/mol which provides the standard enthalpy of formation from elements of -874.5±0.5 kJ/mol. The calculated Cp of SiO2 St agrees well with high-temperature Cp data measured by Akaogi et al. (1995). However, our Cp extrapolated to higher temperature region than 700 K shows slightly larger value (5% at 2000 K) than that by Cp equation of Akaogi et al. (2011). Thermodynamically calculated coesite-stishovite phase equilibrium boundary using the reassessed thermodynamic data set for SiO2 St is consistent with the experimentally determined phase boundaries by Zhang et al. (1996) and Ono et al. (2017a). When the new thermodynamic data set for SiO2 St is applied to thermodynamic calculation of the Mg2SiO4 ringwoodite (Rw) + SiO2 St = 2MgSiO3 akimotoite boundary, the phase transition pressure at 1000 K is obtained to be 18 GPa and it is about 3 GPa lower than experimentally determined boundaries (e.g., Ono et al., 2017b), implying that the meta-stable region of Rw + St assembly is relatively extensive.
SiO2 St samples for the drop-solution calorimetry were synthesized by heating reagent grade SiO2 quartz (Qz) at 15 GPa and 1673 K using a Kawai-type high-pressure apparatus at Gakushuin University. It was confirmed that synthesized samples were single phase of SiO2 St by micro-focused X-ray diffraction method. The drop-solution calorimetry was performed using a Calvet-type high-temperature calorimeter (SETARAM, HT-1000). A sintered SiO2 St sample with weight of 3-6 mg was dropped directly into lead borate solvent (2PbO·B2O3) placed in the calorimeter at 978 K. Drop-solution enthalpy (ΔHd-s), which is the summation of heat content from room temperature to 978 K and solution enthalpy at 978 K was measured. To hasten the solution of the samples, the solvent was stirred by bubbles of Ar gas.
In the reassessment of isobaric heat capacity (Cp) of SiO2 St, isochoric heat capacity (Cv) was calculated using the Kieffer model with adjusting a VDoS model to reproduce low-temperature Cp data measured by Akaogi et al. (2011) and Yong et al. (2012). The anharmonic effect was calculated using isothermal bulk modulus, its temperature derivative and thermal expantivity derived from the EoS of Wang et al. (2012).
From the average of six data, ΔHd-s (SiO2 St) was determined to be 3.8±0.4 kJ/mol. This value is slightly larger than 3.0±0.9 kJ/mol measured by Akaogi et al. (1995), though within the experimental errors. The difference between ΔHd-s (SiO2 Qz) [40.1±0.4 kJ/mol, Akaogi et al. (1995)] and the present ΔHd-s (SiO2 St) gives the phase transition enthalpy from Qz to St of 36.2±0.5 kJ/mol which provides the standard enthalpy of formation from elements of -874.5±0.5 kJ/mol. The calculated Cp of SiO2 St agrees well with high-temperature Cp data measured by Akaogi et al. (1995). However, our Cp extrapolated to higher temperature region than 700 K shows slightly larger value (5% at 2000 K) than that by Cp equation of Akaogi et al. (2011). Thermodynamically calculated coesite-stishovite phase equilibrium boundary using the reassessed thermodynamic data set for SiO2 St is consistent with the experimentally determined phase boundaries by Zhang et al. (1996) and Ono et al. (2017a). When the new thermodynamic data set for SiO2 St is applied to thermodynamic calculation of the Mg2SiO4 ringwoodite (Rw) + SiO2 St = 2MgSiO3 akimotoite boundary, the phase transition pressure at 1000 K is obtained to be 18 GPa and it is about 3 GPa lower than experimentally determined boundaries (e.g., Ono et al., 2017b), implying that the meta-stable region of Rw + St assembly is relatively extensive.