Low temperature heat capacity of the high-pressure-phase of SiO2, coesite, and calculation of theα-quartz-to-coesite equilibrium boundary

Thermophysical properties and phase diagrams in the system MgO–SiO2–FeO at upper mantle and transition zone conditions derived from a multiple-Einstein method

“…We showed that low-temperature heat capacity of coesite determined by Atake et al (2000) is better consistent with the phase boundary between coesite and α-quartz than that determined by Hemingway et al (1998) and Holm et al (1967). Measured values for the heat capacity of β-quartz are too large to be reconciled with the phase boundary between coesite and β-quartz.…”

“…Although Fig. 3 shows that the differences between heat capacity curves derived from the datasets of Atake et al (2000) and Hemingway et al (1998) are small, the smaller value for ambient entropy of the former is sufficient to arrive at a phase boundary between coesite and quartz with a positive Clapeyron slope, representing all experimental datasets. The resulting enthalpy difference between coesite and quartz at room temperature is in that case in agreement with all experimental values in Table 4 with the exception of that established by Holm et al (1967).…”

“…Lower-left: adiabatic bulk and shear modulus of olivine calculated at compositions 10 and 12.5 mol% Fe 2 SiO 4 . Properties at 12.5 mol% were calculated at 300 K, 1000 K, 1500 K and 1700 K. Lowerright: volume-temperature behavior of orthopyroxene for different compositions indicated by the mole fraction numbers close to the volume axis constrained our model for coesite by heat capacity data of Atake et al (2000). Although Fig.…”