I. A. Kiseleva

Thermochemistry and phase equilibria in calcium zeolites

Abstract Thermodynamic properties of the natural calcium zeolites laumontite, leonhardite, dehydrated leonhardite (metaleonhardite), wairakite, and yugawaralite were studied by calorimetry in lead borate solvent at 975 K. Enthalpies of formation from the elements at 298 K are as follows: -7251.0 ± 8.5 kJ/mol for laumontite, CaAl2Si4O12 · 4H2O; -7107.3 ± 5.6 kJ/mol for leonhardite, CaAl2Si4O12 · 3.5H2O; - 5964.3 ± 5.1 kJ/mol for metaleonhardite, CaAl2Si4O12; -6646.7 ± 6.3 kJ/mol for wairakite, CaAl2Si4O12 · 2H2O; and -9051.3 ± 10.4 kJ/mol for yugawaralite, CaAl2Si4O12 · 4H2O. The value for leonhardite is in good agreement with early values from acid calorimetry (Barany 1961) but not with revised values from Hemingway and Robie (1977). The enthalpy of dehydration of leonhardite is 140.2 ± 6.7 kJ/mol, and the loss of one mole of H2O is associated with an endothermic effect of about 40 kJ. Standard entropies, S0298, of wairakite [400.7 J/(mol· K)] and yugawaralite [609.8 J/(mol·K)] were derived from our new enthalpy data combined with reversed P-Tphase equilibria (Liou 1971; Zeng and Liou 1982). The upper limit ofwairakite stability, the univariant curve for equilibrium of wairakite with anorthite, quartz, and fluid, was calculated from these values of enthalpy and entropy. Good agreement between thermodynamic calculations and reversed phase equilibria supports the reliability of the new thermodynamic data.

Thermochemistry of natural potassium sodium calcium leonhardite and its cation-exchanged forms

Abstract Leonhardite, a partially dehydrated laumontite, and its alkali variety, primary leonhardite, have been studied by high-temperature calorimetry. The enthalpies offormation from oxides and elements at 298 K are - 306.7 ±7.1 and -14214.6 ± 11.2 kJ/mol, respectively, forleonhardite, Ca2Al4Si8O24 · 7H2O, and - 521.2 ± 10.5 and -14253.7 ± 13.5 kll mol, respectively, for primary leonhardite of composition Ca1.3Na0.6K0.8Al4Si8O24 · 7H2O. The values for primary leonhardite are significantly more negative. New calorimetric data for sodium and potassium oxides were obtained on the basis of thermochemical cycles involving carbonates. The enthalpies of drop solution are -113.10 ± 0.83 kJ/mol for Na2O and -193.68 ± 1.10 kJ/mol for K2O, giving enthalpies of solution of -170.78 ± 0.90 kJ/mol for Na20 and -260.98 ± 1.20 kJ/mol for K2O. The effects of exchange cations (K,Na) on energetics and dehydration were studied using cation-exchanged samples. Alkali substitution decreases thermal stability (decomposition on heating in air) but increases thermodynamic stability with respect to the oxides and elements. Equilibrium relations between leonhardite and alkali-feldspar, calculated on the basis of these data, show that primary leonhardite can form only from geothermal solutions having rather high ratios of alkali ions to Ca.

Thermochemical study of calcium zeolites—heulandite and stilbite

Abstract Calorimetric measurements were made on natural samples of heulandite having the composition Ca0.86Na0.37K0.06Al2.14Si6.86O18 ·6.1H2O and stilbite Ca1.01Na0.12Al2.12Si6.88O18 · 7.27H2O both from the same locality (Nidym River, E. Siberia, Russia). Enthalpies of formation and hydration were studied by calorimetry in lead borate solvent at 975 K. The enthalpies of formation from oxides and elements at 298 K of heulandite are: -238.7 ± 4.9 kJ/mol, and -10656.3 ± 8.6 kJ/mol, and of stilbite are -232.0 ± 8.3 kJ/mol and -11017.9 ± 10.9 kJ/mol respectively. The integral hydration enthalpies including the enthalpies of phase transitions during heulandite and stilbite dehydration are -209.4 ± 5.9 kJ/mol and -229.2 ± 7.4 kJ/mol at 298 K, respectively. The molar Gibbs free energies of formation for idealized calcium-sodium and pure calcium heulandites and stilbites were calculated by combining these new calorimetric data with thermodynamic quantities from the literature. Equilibrium temperatures for the reaction: Ca-stilbite = Ca-heulandite + H2O, calculated on the basis of these thermodynamic data, agree with experimental phase equilibria.

Calorimetric determination of the enthalpy of formation for pyrophyllite

A calorimetric study of the natural pyrophyllite was performed by high-temperature melt calorimetry on a Tian-Calvet calorimeter. Based on experimentally determined in this work for pyrophyllite and gibbsite, as well as previously obtained for corundum and quartz, the total value of the enthalpy increment for the sample heated from room temperature to 973 K and the enthalpy of dissolution at 973 K by Hess’s law, the enthalpy of formation of pyrophyllite of Al2[(OH)2/Si4O10] composed of elements was calculated at 298.15 K: ΔfHelo(298.15 K) = −5639.8 ± 5.7 kJ/mol.

Thermodynamic properties of strontianite-witherite solid solution (Sr,Ba)CO3

Structural parameters and thermodynamic properties of strontianite — witherite solid solutions have been studied by X-ray powder diffraction, heat flux Calvet calorimetry and cation-exchange equilibria technique. X-ray study of the synthetic samples have shown linear and quadratic (for c-parameter) composition dependencies of the lattice constants in the carbonate solid solution. The thermodynamic energy parameters demonstrate the non-ideal character of strontianite — witherite solid solutions. Enthalpies of solution of the samples have been measured in 2PbO*B2O3 at 973 K. The new data on the enthalpy of formation ΔHf,298.150of SrCO3 and BaCO3 were obtained: -1231.4±3.2 and -1209.9±5.8 kJ*mol-1 respectively. The enthalpy of mixing of the solid solution was found to be positive and asymmetric with maximum at XBa (carbonate)=0.35. The composition dependence of the enthalpy of mixing may be described by two — parametric Margules model equation: ΔHmix=XBa✻XSr✻[(4.40±3.91)✻XBa+(28.13±3.91)✻XSr] kJ✻mol−1 Cation-exchange reactions between carbonates and aqueous SrCl2-BaCl2 supercritical solutions (fluids) were carried out at 973 and 1073 K and 2 kbar. Calculated Margules model parameters of the excess free energy are: for orthorhombic carbonate solid solutions WSr=WBa=11.51±0.40 kJ✻mol−1 (973 K) and WSr=WBa=12.09±0.95 kJ✻mol− (1073 K) for trigonal carbonate solid solutions WSr=WBa=13.55±0.40 kJ✻mol− (1073 K).

Thermodynamic properties of copper carbonates — malachite Cu2(OH)2CO3 and azurite Cu3(OH)2(CO3)2

The thermodynamic properties of the copper carbonates malachite and azurite have been studied by adiabatic calorimetry, by heat-flux Calvet Calorimetry, by differential thermal analysis (DTA) and by thermogravimetrie (TGA) analysis. The heat capacities, Cp0of natural malachite and azurite have been measured between 3.8 and 300 K by low-temperature adiabatic calorimetry. The heat capacity of azurite exhibits anomalous behavior at low temperatures. At 298.15 K the molar heat capacities Cp0and the third law entropies S298.150are 228.5±1.4 and 254.4±3.8 J mol−1 K−1 for azurite and 154.3±0.93 and 166.3±2.5 J mol−1 K−1 for malachite. Enthalpies of solution at 973 K in lead borate 2PbO·B2O3 have been measured for heat treated malachite and azurite. The enthalpies of decomposition are 105.1±5.8 for azurite and 66.1±5.0 kJ mol− for malachite. The enthalpies of formation from oxides of azurite and malachite determined by oxide melt solution calorimetry, are −84.7±7.4 and −52.5±5.9 kJ mol−1, respectively. On the basis of the thermodynamic data obtained, phase relations of azurite and malachite in the system Cu2+-H2O-CO2 at 25 and 75 °C have been studied.

A study of dachiardite, a natural zeolite of the mordenite group

Dachiardite of the composition (Na2.21K0.35Ca0.66Mg0.10)[Al4.41Si19.67O48] · 11.8H2O (Tedzami, Georgia), a natural zeolite of the mordenite group, was studied using a Tian-Calvet high-temperature microcalorimeter. Melt solution calorimetry was used to determine the enthalpy of formation of the mineral from oxides (−613±45 kJ/mol) and elements (−26595±50kJ/mol). The obtained experimental and literature data were used to calculate the Gibbs energy of formation of dachiardite from elements. The thermodynamic properties of the hypothetical limiting members of the isomorphous series (Na, K, Ca)[Al4Si20O48] · 13H2O were estimated.

Thermochemical study of natural montmorillonite

The paper reports results of an experimental thermochemical study (in a heat-flux Tian-Calvet microcalorimeter) of montmorillonite from (I) the Taganskoe and (II) Askanskoe deposits and (III) from the caldera of Uzon volcano, Kamchatka. The enthalpy of formation ΔfHel0 (298.15 K) of dehydrated hydroxyl-bearing montmorillonite was determined by melt solution calorimetry: −5677.6 ± 7.6 kJ/mol for Na0.3Ca0.1(Mg0.4Al1.6)[Si3.9Al0.1O10](OH)2 (I), −5614.3 ± 7.0 kJ/mol for Na0.4K0.1(Ca0.1Mg0.3Al1.5Fe0.13+)[Si3.9Al0.1O10](OH)2 (II), −5719 ± 11 kJ/mol for K0.1Ca0.2Mg0.2(Mg0.6Al1.3Fe0.13+) [Si3.7Al0.3O10](OH)2 (III), and −6454 ± 11 kJ/mol for water-bearing montmorillonite (I) Na0.3Ca0.1(Mg0.4Al1.6)[Si3.9Al0.1O10](OH)2 · 2.6H2O. The paper reports estimated enthalpy of formation for the smectite end members of the theoretical composition of K-, Na-, Mg-, and Ca-montmorillonite and experimental data on the enthalpy of dehydration (14 ± 2 kJ per mole of H2O) and dehydroxylation (166 ± 10 kJ per mole of H2O) for Na-montmorillonite.

Calorimetric Determination of the Enthalpy of Formation of Partheite, a Calcium Zeolite

A thermochemical study of partheite of composition (Ca1.96Mg0.04Na0.01K0.01) · [(Al4.04Fe0.013+)Si3.95O14.97(OH)2.03] · 4.2H2O, a natural calcium zeolite extracted from gabbro pegmatites of the Denezhkin Kamen’ deposit (North Ural, Russia), was performed. The enthalpies of formation of partheite from the constituent oxides, (ΔfH°ox(298.15 K) = −359 ± 21), and elements, (ΔfH°el(298.15 K) = −10108 ± 21), were determined by means of high-temperature in-melt-dissolution calorimetry. On the basis of the experimental data obtained, the enthalpy of formation of partheite of theoretical composition Ca2[Al4Si4O15(OH)2] · 4H2O from the elements was evaluated, −10052 ± 21 kJ/mol.

Natural sepiolite: Enthalpies of dehydration, dehydroxylation, and formation derived from thermochemical studies

Abstract Sepiolite is widely used in various fields due to its unique colloidal-rheological and physicochemical properties. The first experimental thermochemical study of natural sepiolite Mg8Si12O30(OH)4(H2O)4·nH2O from Akkermanovskoe field (Southern Ural, Russia) was performed utilizing the high-temperature heat-flux Tian-Calvet microcalorimeter. X-ray powder diffraction, thermal analysis, and FTIR spectroscopy methods were used to characterize sepiolite. Processes of dehydration, dehydroxylation, and various water types’ removal enthalpies were studied using thermochemical methods. The values of Ddehydr.H0(298.15 K) of adsorbed, zeolitic, and bound water calculated per 1 mol of released H2O, were as follows: 15 ± 4, 28 ± 8, and 39 ± 15 kJ/mol, respectively. The enthalpy of dehydroxylation of sepiolite was found as 145 ± 14 kJ/(mol H2O). Obtained data point at different binding strengths of water in the structure of sepiolite. The enthalpies of formation from the elements ΔH0f (298.15 K) were derived by melt solution calorimetry for sepiolite with various content of different water types: -18 773 ± 28 kJ/mol for Mg8Si12O30(OH)4(H2O)4·4H2O and -16 426 ± 21 kJ/mol for Mg8Si12O30(OH)4(H2O)4