L. P. Ogorodova

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.

Cancrinite and cancrisilite in the Khibina-Lovozero alkaline complex: Thermochemical and thermal data

The paper presents the results of a thermochemical and thermal study of cancrinite, (Na6.93Ca0.545K0.01)Σ7.485[(Si6.47Al5.48Fe0.05)Σ12O24](CO3)1.25 · 2.30 H2O, and cancrisilite, (Na7.17 Ca0.01)Σ7.18[(Si7.26Al4.70Fe0.04)Σ12O24][(CO3)1.05(OH)0.21(PO4)0.04(SO4)0.01] · 2.635 H2O, from the Khibina-Lovozero Complex, Kola Peninsula, Russia. Stages of the thermal decomposition of these minerals were studied using IR spectroscopy. The enthalpies of formation of the minerals from elements were determined by melt drop solution calorimetry: ΔfHel0 (298.15 K) = −14 490 ± 16 kJ/mol for cancrinite and −14302 ± 17 kJ/mol for cancrisilite. The values of ΔfHel0 (298.15 K), So(298.15 K), and ΔfHel0 (298.15 K) are determined for cancrinite and cancrisilite of theoretical composition.

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

Enthalpy of formation for natural paulingite zeolite

Natural paulingite zeolite of the composition (K4.48Na0.56Ca2.95Ba0.87Mg0.06)[Al11.04Fe0.043+Si30.49O84] · 35.25H2O (Vinariská Hora, Czechia) was studied calorimetrically on a high-temperature Tian-Calvet microcalorimeter. Enthalpies of formation from oxides (−1969 ± 128 kJ/mol) and elements (−52393 ± 132 kJ/mol) were determined for this mineral by melt dissolution calorimetry. The obtained experimental results and the literature data were used to calculate the Gibbs free energy of formation from elements.

Thermochemical study of polymorphic modifications of Al(OH)3: Gibbsite and nordstrandite

90 Four natural polymorphic modifications of alumi num trihydroxide Al(OH)3 (gibbsite, nordstrandite, bayerite, and doyleite) and one cubic phase synthesized at high pressure are presently known [1]. Among them, only gibbsite is relatively widespread and comprehen sively studied mineral, whereas other modifications are relatively rare in abundance. Nordstrandite is triclinic aluminum trihydroxide. Up to 1962, this mineral has been known only as synthesized compound, and only later it was found in natural conditions, well identified, and studied in detail by diverse methods [2–4]. In Rus sia, nordstrandite was found for the first time in the bauxites of the Sokolovo–Sarbai deposit [5] and later was described in the bauxite weathering zones of the Urals, as well as in the nordstrandite–siliceous terrige nous rocks of the Kuznetsk Basin [6]. Aluminum trihy droxides rarely occur as individual aggregates. Usually, they form mixtures with other aluminum and iron tri hydroxides, kaolinite, other minerals, and are incorpo rated in bauxites—the main economic ore for obtaining alumina and aluminum. As was shown in [6], the nord strandite bearing rocks may be of practical interest as aluminous raw material alternative to bauxites. Alumi num trihydroxides are the low temperature minerals of supergene zone, but sometimes they are precipitated from low temperature hydrothermal solutions.