Bruce S. Hemingway

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 (ΔCp). 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· 1013 poise, can be quantitatively reproduced through this theory with the calorimetrically determined entropies and ΔCp data. Finally, the unclear significance of “activation energies” for structural interpretations of viscosity data is emphasized, and the importance of ΔCp and glass-transition temperature systematics for determining the composition and temperature dependences of the viscosity is pointed out.

Heat capacity and thermodynamic properties for coesite and jadeite, reexamination of the quartz-coesite equilibrium boundary

Single-crystal and powder X-ray diffraction data were collected to characterize the macroscopic solid-solution and cation-ordering behavior in the system augite-jadeite (low acmite content). We examined 28 natural pyroxenes with compositions on the join augitejadeite and with different degrees of order. Annealing experiments were carried out to obtain crystals with different degrees of order (P2/n) and complete disorder (C2/c) at compositions between 35 and 60% Jd. Three synthetic C2/c pyroxenes with composition Di80Jd20, Di60Jd40, and Di50Jd50 were also examined. The long-range order parameters QM1 and QM2 of the M1 and M2 sites were obtained by a minimization procedure combining single-crystal X-ray diffraction data and chemical analyses. For both C2/c and P2/n pyroxenes, the a, b, c lattice parameters and unit-cell volume, as well as tetrahedral and octahedral mean bond distances depend linearly on composition. Only the angle b of ordered omphacites slightly deviates from the linear trend of the C2/c samples. The out-of-plane tilting of the basal face of tetrahedra is sensitive to the different degrees of order.

Determination of melanterite-rozenite and chalcanthite-bonattite equilibria by humidity measurements at 0.1 MPa

Abstract Melanterite (FeSO4·7H2O)-rozenite (FeSO4·4H2O) and chalcanthite (CuSO4·5H2O)-bonattite (CuSO4·3H2O) equilibria were determined by humidity measurements at 0.1 MPa. Two methods were used; one is the gas-flow-cell method (between 21 and 98 °C), and the other is the humiditybuffer method (between 21 and 70 °C). The first method has a larger temperature uncertainty even though it is more efficient. With the aid of humidity buffers, which correspond to a series of saturated binary salt solutions, the second method yields reliable results as demonstrated by very tight reversals along each humidity buffer. These results are consistent with those obtained by the first method, and also with the solubility data reported in the literature. Thermodynamic analysis of these data yields values of 29.231 ± 0.025 and 22.593 ± 0.040 kJ/mol for standard Gibbs free energy of reaction at 298.15 K and 0.1 MPa for melanterite-rozenite and chalcanthite-bonattite equilibria, respectively. The methods used in this study hold great potential for unraveling the thermodynamic properties of sulfate salts involved in dehydration reactions at near ambient conditions.

Thermodynamic tabulations for selected phases in the system CaO‐Al2O3‐ SiO2‐H2 at 101.325 kPa (1 atm) between 273.15 and 1800 K

The standard thermodynamic properties of phases in the lime‐alumina‐silica‐ water system between 273.15 and 1800 K at 101.325 kPa (1 atm) were evalated from published experimental data. Phases included in the compilation are boehmite, diaspore, gibbsite, kaolinite, dickite, halloysite, andalusite, kyanite, sillimanite, Ca‐Al cliniopyroxene, anorthite, gehlenite, grossular, prehnite, zoisite, margarite, wollastonite, cyclowollastonite ( = pseudowollastonite), larnite, Ca olivine, hatrurite, and rankinite. The properties include heat capacity, entropy, relative enthalpy, and the Gibbs energy function of the phases and the enthalpies, Gibbs energies, and equilibrium constants for formation both from the elements and the oxides. Tabulated values are given at 50 K intervals with the 2‐sigma confidence limit at 250 K intervals. Summaries for each phase give the temperature‐ dependent functions for heat capacity, entropy, and relative enthalpy and the experimental data used in the final evaluation.

Low-temperature molar heat capacities and entropies of MnO2 (pyrolusite), Mn3O4 (hausmanite), and Mn2O3 (bixbyite)

Abstract Pyrolusite (MnO2), hausmanite (Mn3O4), and bixbyite (Mn2O3), are important ore minerals of manganese and accurate values for their thermodynamic properties are desirable to understand better the {p(O2), T} conditions of their formation. To provide accurate values for the entropies of these important manganese minerals, we have measured their heat capacities between approximately 5 and 380 K using a fully automatic adiabatically-shielded calorimeter. All three minerals are paramagnetic above 100 K and become antiferromagnetic or ferrimagnetic at lower temperatures. This transition is expressed by a sharp λ-type anomaly in Cpmo for each compound with Neel temperatures TN of (92.2±0.2), (43.1±0.2), and (79.45±0.05) K for MnO2, Mn3O4, and Mn2O3, respectively. In addition, at T ≈ 308 K, Mn2O3 undergoes a crystallographic transition, from orthorhombic (at low temperatures) to cubic. A significant thermal effect is associated with this change. Hausmanite is ferrimagnetic below TN and in addition to the normal λ-shape of the heat-capacity maxima in MnO2 and Mn2O3, it has a second rounded maximum at 40.5 K. The origin of this subsidiary bump in the heat capacity is unknown but may be related to a similar “anomalous bump” in the curve of magnetization against temperature at about 39 K observed by Dwight and Menyuk.(1) At 298.15 K the standard molar entropies of MnO2, Mn3O4, and Mn2O3, are (52.75±0.07), (164.1±0.2), and (113.7±0.2) J·K−1·mol−1, respectively. Our value for Mn3O4 is greater than that adopted in the National Bureau of Standards tables(2) by 14 per cent.

Specific Heats of Lunar Surface Materials from 90 to 350 Degrees Kelvin

The specific heats of lunar samples 10057 and 10084 returned by the Apollo 11 mission have been measured between 90 and 350 degrees Kelvin by use of an adiabatic calorimeter. The samples are representative of type A vesicular basalt-like rocks and of finely divided lunar soil. The specific heat of these materials changes smoothly from about 0.06 calorie per gram per degree at 90 degrees Kelvin to about 0.2 calorie per gram per degree at 350 degrees Kelvin. The thermal parameter γ=(kpC-� for the lunar surface will accordingly vary by a factor of about 2 between lunar noon and midnight.

The heat-capacity of ilmenite and phase equilibria in the system Fe-T-O

Abstract Low temperature adiabatic calorimetry and high temperature differential scanning calorimetry have been used to measure the heat-capacity of ilmenite (FeTiO3) from 5 to 1000 K. These measurements yield S2980 = 108.9 J/(mol · K). Calculations from published experimental data on the reduction of ilmenite yield Δ2980(I1) = −1153.9 kJ/(mol · K). These new data, combined with available experimental and thermodynamic data for other phases, have been used to calculate phase equilibria in the system Fe-Ti-O. Calculations for the subsystem Ti-O show that extremely low values of ƒO 2 are necessary to stabilize TiO, the mineral hongquiite reported from the Tao district in China. This mineral may not be TiO, and it should be re-examined for substitution of other elements such as N or C. Consideration of solid-solution models for phases in the system Fe-Ti-O allows derivation of a new thermometer/oxybarometer for assemblages of ferropseudobrookite-pseudobrookitess and hematite-ilmenitess. Preliminary application of this new thermometer/oxybarometer to lunar and terrestrial lavas gives reasonable estimates of oxygen fugacities, but generally yields subsolidus temperatures, suggesting re-equilibration of one or more phases during cooling.

Heat capacities of kaolinite from 7 to 380 K and of DMSO-intercalated kaolinite from 20 to 310 K; the entropy of kaolinite Al 2 Si 2 O 5 (OH) 4

The heat capacities of kaolinite (7 to 380 K) and of dimethyl sulfoxide (DMSO) intercalated kaolinite (20 to 310 K) were measured by adiabatically shielded calorimetry. The third law entropy of kaolinite, S298, is 200.9 ± 0.5 J½mol−1K−1.The “melting point” of the DMSO in the intercalate, 288.0 ± 0.2 K, is 3.7 K lower than that of pure DMSO, 291.67 K. The heat capacity of DMSO in the intercalate above 290 K is approximately 5.2 J·mol−1·K−1 smaller than that of pure liquid DMSO at the same temperature.

The entropy and Gibbs free energy of formation of the aluminum ion

A reevaluation of the entropy and Gibbs free energy of formation of Al3+(aq) yields −308 ± 15 J/K·mol and 489.4 ± 1.4kj/mol for S0298 and ΔG0ƒ,298 respectively. The standard electrode potential for aluminum is 1.691 ± 0.005 volts.

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 .