Geology topics

P. Richet

Publications and source records attributed to P. Richet.

4 recordsLinked to original sources

USGS1993

Entropy and structure of silicate glasses and melts

Low-temperature adiabatic C p measurements have been made on NaAlSi 2 O 6 , MgSiO 3 , Ca 3 Al 2 Si 3 O 12 and Ca 1.5 Mg 1.5 Al 2 Si 3 O 12 glasses. Above about 50 K, these and previous data show that the heat capacity is an additive function of composition to within ±1% throughout the investigated glassforming part of the system CaO-MgO-Al 2 O 3 -SiO 2 . In view of the determining role of oxygen coordination polyhedra on the low-temperature entropy, this is interpreted as indicating that Si and Al are tetrahedrally coordinated in all these glasses, in agreement with structural data; whereas Ca and Mg remain octahedrally coordinated. In contrast, heat capacities and entropies are not additive functions of composition for alkali aluminosilicates, indicating increases in the coordination numbers of alkali elements from about six to nine when alumina is introduced. A thermochemical consequence of additivity of vibrational entropies of glasses is that entropies of mixing are essentially configurational for calcium and magnesium aluminosilicate melts. For alkali-bearing liquids, it is probable that vibrational entropies contribute significantly to entropies of mixing. At very low temperatures, the additive nature of the heat capacity with composition is less well followed, likely as a result of specific differences in medium-range order.

Geochimica et Cosmochimica Acta↗

USGS1991

Melting and thermodynamic properties of pyrope (Mg3Al2Si3O12)

The heat capacity of Mg 3 Al 2 Si 3 O 12 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 Mg 3 Al 2 Si 3 O 12 glass and liquid for the interval 0–2000 K. The enthalpy of fusion at 1570 ± 30 "> 1570 ± 30 K, the metastable congruent 1-bar melting point, is 241 ± 12 kJ/mol "> 241 ± 12kJ/mol .

Geochimica et Cosmochimica Acta↗

USGS1990

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 ?? 1990 Springer-Verlag.

Physics and Chemistry of Minerals↗

USGS1986

Low-temperature heat capacity of diopside glass (CaMgSi2O6): A calorimetric test of the configurational-entropy theory applied to the viscosity of liquid silicates

Heat-capacity measurements have been made between 8 and 370 K on an annealed and a rapidly quenched diopside glass. Between 15 and 200 K, Cp does not depend significantly on the thermal history of the glass. Below 15 K Cp is larger for the quenched than for the annealed specimen. The opposite is true above 200 K as a result of what is interpreted as a secondary relaxation around room temperature. The magnitude of these effects, however, is small enough that the relative entropies S(298)−S(0) of the glasses differ by only 0.5 J/mol K, i.e. , a figure within the combined experimental uncertainties. The insensitivity of relative entropies to thermal history supports the assumption that the configurational heat capacity of the liquid may be taken as the heat capacity difference between the liquid and the glass (Δ Cp ). Furthermore, this insensitivity allows calculation of the residual entropies at 0 K of diopside glasses as a function of the fictive temperature from the entropy of fusion of diopside and the heat capacities of the crystalline, glassy and liquid phases. For a glass with a fictive temperature of 1005 K, for example, this calorimetric residual entropy is 24.3 ± 3 J /mol K, in agreement with the prediction made by RICHET (1984) from an analysis of the viscosity data with the configurational-entropy theory of relaxation processes of Adam and Gibbs (1965). In turn, all the viscosity measurements for liquid diopside, which span the range 0.5-4· 10 13 poise, can be quantitatively reproduced through this theory with the calorimetrically determined entropies and Δ Cp data. Finally, the unclear significance of “activation energies” for structural interpretations of viscosity data is emphasized, and the importance of Δ Cp and glass-transition temperature systematics for determining the composition and temperature dependences of the viscosity is pointed out.

Geochimica et Cosmochimica Acta↗