Christophe Tequi

Thermodynamic properties of quartz, cristobalite and amorphous SiO2: drop calorimetry measurements between 1000 and 1800 K and a review from 0 to 2000 K

We report relative enthalpy measurements on quartz, cristobalite and amorphous SiO2 between 1000 and 1800 K. We have observed a glass transition around 1480 K for amorphous SiO2. From our results and available Cp, relative enthalpy, and enthalpy of solution data we have derived a consistent set of thermodynamic data for these phases. Our calculated enthalpies of fusion are 8.9 ± 1.0 kJ mole−1 for cristobalite at 1999 K and 9.4 ± 1.0 kJ mole−1 at 1700 K for quartz.

High-temperature heat capacity of grossular (Ca3Al2Si3O12), enstatite (MgSiO3), and titanite (CaTiSiO5)

We performed experiments at 3.0 GPa and 1530–1565 °C to investigate the effects of crystal composition on trace element partitioning between garnet and anhydrous silicate melt. Bulk compositions along the pyrope (Py: Mg3Al2Si3O12)-grossular (Gr: Ca3Al2Si3O12) join, doped with a suite of trace elements (Li, B, K, Sc, Ti, Sr, Y, Zr, Nb, Cd, In, REE, Hf, Ta, Th, and U) produced homogeneous garnets, ranging in composition from Py84Gr16 to Py9Gr91, in equilibrium with melt. Trace element partition coefficients (D-values), measured by SIMS, depend greatly on the Mg/ (Mg + Ca) of garnet. For example, from Py84 to Py9, DLa increases from 0.004 to 0.2, whereas DU increases from 0.029 to 0.42. These variations can be explained by the lattice strain model of Blundy and Wood (1994), which describes trace element partitioning of an element i in terms of the ionic radius of i (ri), the size of the lattice site on which i partitions (r0), the Young’s modulus of the site (E), and the (theoretical) partition coefficient D0 for an ion of radius r0. For trivalent cations substituting in the garnet X-site (Y, REE, Sc, and In), apparent values of r0 fitted to our data vary systematically from 0.935 ± 0.004 Å (Py84) to 0.99 ± 0.01 Å (Py9), a trend consistent with variations in the size of the X-site. Values of D0 show an increase from Py9 (D0 = 2.8 ± 0.1) to Py84 (4.8 ± 0.1) and Young’s modulus E varies from 257 ± 20 GPa for Py60 to 590 ± 40 GPa for Py84. These results allow a quantitative assessment of the influence of crystal chemistry on garnet-melt D-values, thereby forming the basis for a predictive model similar to that recently developed for clinopyroxene-melt partitioning by Wood and Blundy (1997). Our new data emphasize the importance of taking into account crystal composition when modeling trace element behavior in natural systems.

Melting and thermodynamic properties of pyrope (Mg3Al2Si3O12)

Abstract The heat capacity of Mg3Al2Si3O12 glass has been measured from 10 to 1000 K by adiabatic and differential scanning calorimetry. The heat capacity of crystalline pyrope has been determined from drop-calorimetry measurements between 820 and 1300 K. From these and previously published results a consistent set of thermodynamic data is presented for pyrope and Mg3Al2Si3O12 glass and liquid for the interval 0–2000 K. The enthalpy of fusion at 1570 ± 30 K, the metastable congruent 1-bar melting point, is 241 ± 12 kJ/mol .

Thermodynamics of open networks: ordering and entropy in NaAlSiO4 glass, liquid, and polymorphs

The thermodynamic properties of carnegieite and NaAlSiO4 glass and liquid have been investigated through Cp determinations from 10 to 1800 K and solution-calorimetry measurements. The relative entropies S298-S0 of carnegieite and NaAlSiO4 glass are 118.7 and 124.8 J/mol K, respectively. The low-high carnegieite transition has been observed at 966 K with an enthalpy of transition of 8.1±0.3 kJ/mol, and the enthalpy of fusion of carnegieite at the congruent melting point of 1799 K is 21.7±3 kJ/mol. These results are consistent with the reported temperature of the nepheline-carnegieite transition and available thermodynamic data for nepheline. The entropy of quenched NaAlSiO4 glass at 0 K is 9.7±2 J/mol K and indicates considerable ordering among AlO4 and SiO4 tetrahedra. In the liquid state, progressive, temperature-induced Si, Al disordering could account for the high configurational heat capacity. Finally, the differences between the entropies and heat capacities of nepheline and carnegieite do not seem to conform to current polyhedral modeling of these properties

Thermodynamic study of alkali sulphates: Calculation of enthalpy, entropy and Gibbs free energy

Abstract Joined with measures of specific heat at low temperature , a calorimetric study of alkali sulphates from ordinary temperature to 1500 K has allowed the calculation of enthalpy, entropy and Gibbs free energy of these salts. The value of entropy at melting point is compared with the value assessed by an acoustical method.