Thermodynamic data from redox reactions at high temperatures. II. The MnO-0Mn3O4 oxygen buffer,and implications for the thermodynamic properties of MnO and Mn3O4

The \(\mu _{O_2 } \) defined by the reaction 6 MnO+O₂ =2 Mn₃O₄ has been determined from 917 to 1,433 K using electrochemical cells (with calcia-stabilized zirconta, CSZ) of the type: Steady emfs were achieved rapidly at all temperatures on both increasing and decreasing temperature, indicating that the MnO-Mn₃O₄ oxygen buffer equilibrates relatively easily. It therefore makes a useful alternative choice in experimental petrology to Fe₂O₃-Fe₃O₄ for buffering oxygen potentials at oxidized values. The results are (in J/mol, temperature in K, reference pressure 1 bar); \(\mu _{O_2 } \) (±200)=-563,241+1,761.758T-220.490T inT+0.101819T ² with an uncertainty of ±200 J/mol. Third law analysis of these data, including a correction for the deviations in stoichiometry of MnO, impliesS _298.15 for Mn₃O₄ of 166.6 J/K · mol, which is 2.5 J/K · mol higher than the calorimetric determination of Robie and Hemingway (1985). The low value of the calorimetric entropy may be due to incomplete ordering of the magnetic spins. The third law value of Δ _r H _298.15 ⁰ is-450.09 kJ/mol, which is significantly different from the calorimetric value of-457.5±3.4 kJ/mol, calculated from Δ _f H _298.15 ⁰ of MnO and Mn₃O₄, implying a small error in one or both of these latter.     

Thermodynamic properties of compounds in the PdO-H2O system at 25°C

Literature thermodynamic data on species and particles existing in the heterogeneous PdO-H₂O system were checked for consistency, and the equilibrium constants for dissolution of palladium oxide and hydroxide in water and for Pd²⁺ (aq) hydrolysis were recommended. Δ _f G _298.15 ^° obtained in this work for Pd²⁺(aq) sharply differs (no less than by 6 kJ/mol) from values that are reported in fundamental thermodynamic reference books and based on experimentally measured palladium electrode potential at 25°C. Detailed examination of literature data on the thermodynamic properties of compounds in the Cl-Pd(aq) system is required to account for revealed inconsistency.     

Thermodynamic properties of Fe-rich smectite-nontronite

The results of thermochemical studies are reported for nontronite samples from the Pinares-de-Majari (Eastern Cuba) (Sample I) and Kempirsai serpentine massif (South Urals, Kazakhstan) (Sample II). The enthalpies of formation of dehydrated hydroxyl-bearing nontronites from elements were determined by melt dissolution calorimetry using high-temperature heat-flux Tiana-Calvet microcalorimeter: Δ _f H _el ^o (298.15 K): ?4958 ± 13 kJ/mol for Mg_0.4(Fe _1.5 ³⁺ Mg_0.4Ni_0.1)[Si_3.7Al_0.3O₁₀](OH)₂ (I) and ?5003.6 ± 8.0 kJ/mol for Mg_0.3Na_0.1Ca_0.1(Fe _1.4 ³⁺ Mg_0.5Ni_0.1)[Si_3.7Al_0.3O₁₀](OH)₂ (II). It was determined experimentally that the enthalpy of dehydration (removal of molecular adsorption and interlayer water) of the studied nontronites is 6 ± 2 kJ per 1 mole H₂O. The enthalpy of formation of nontronite of theoretical composition Mg_0.15Fe ₂ ³⁺ [Si_3.7Al_0.3]O₁₀(OH)₂ was estimated at ?4750 kJ/mol. The Gibbs free energies of formation of the nontronites were calculated.     

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 Δ _f H _el ⁰ (298.15 K) of dehydrated hydroxyl-bearing montmorillonite was determined by melt solution calorimetry: ?5677.6 ± 7.6 kJ/mol for Na_0.3Ca_0.1(Mg_0.4Al_1.6)[Si_3.9Al_0.1O₁₀](OH)₂ (I), ?5614.3 ± 7.0 kJ/mol for Na_0.4K_0.1(Ca_0.1Mg_0.3Al_1.5Fe _0.1 ³⁺ )[Si_3.9Al_0.1O₁₀](OH)₂ (II), ?5719 ± 11 kJ/mol for K_0.1Ca_0.2Mg_0.2(Mg_0.6Al_1.3Fe _0.1 ³⁺ ) [Si_3.7Al_0.3O10](OH)₂ (III), and ?6454 ± 11 kJ/mol for water-bearing montmorillonite (I) Na_0.3Ca_0.1(Mg_0.4Al_1.6)[Si_3.9Al_0.1O₁₀](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.     

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, C _p ⁰ of 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 C _p ⁰ and the third law entropies S _298.15 ⁰ are 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.     

Calorimetric Study of Natural Basic Copper Phosphate—Pseudomalachite

The thermochemical study of a natural basic copper phosphate, pseudomalachite Cu₅(PO₄)₂(OH)₄ (Virneberg deposit, Germany), was carried out using high-temperature melt solution calorimetry method with a Tian–Calvet microcalorimeter. The enthalpy of formation of the mineral from elements was obtained to be Δ _f H_el(298.15 K) =–3214 ± 13 kJ/mol. The value of the Gibbs energy of pseudomalachite formation calculated using literature data on its standard entropy is Δ _f H_el^°(298.15 K) =–2812 ± 13 kJ/mol.     

Determination of the enthalpies of formation of some platinum antimonides and their phase diagrams under standard conditions

Using the method of direct synthesis calorimetry, we determined the standard enthalpy of formation of PtSb (stumpflite), Δ _f H°_298.15 (PtSb, cr) =–105.16 ± 0.84 kJ/mol and PdSb₂ (geversite), Δ _f H°_298.15 (PtSb₂,cr) =–160.92 ± 0.84 kJ/mol. Isothermal (298.15 K, p = 1 bar) phase diagrams were computed for the Pt–Sb–S and Pt–Sb–O ternary systems in the coordinates composition of the Pt–Sb binary system versus fugacity of a gaseous volatile component (O₂, S₂).     

High temperature calorimetry of sulfide systems

Enthalpies of solution of synthetic pentlandite Fe_4.5Ni_4.5S₈, natural violarite (Fe_0.2941Ni_0.7059)₃S₄ from Vermillion mine, Sudbury, Ontario, synthetic pyrrhotite, FeS, synthetic high temperature NiS, synthetic vaesite, NiS₂, synthetic pyrite, FeS₂, Ni and Fe have been measured in a Ni_0.6S_0.4 melt at 1,100 K. Using these data and the standard enthalpies of formation of binary sulfides, given in literature, standard enthalpies of formation of pentlandite and violarite were calculated. The following values are reported: ΔH _f ^{o, Pent} =?837.37±14.59 kJ mol^?1 and ΔH _f ^{o, Viol} =?378.02±11.81 kJ mol^?1. While there are no thermo-chemical data for pentlandite with which our new value can be compared, an equilibrium investigation of stoichiometric violarite by Craig (1971) gives a significantly less negative enthalpy of formation. It is suggested that the difference may be due to the higher degree of order in the natural sample.     

Ephesite,Na(LiAl2) [Al2Si2O10] (OH)2: I. thermal stability and standard state thermodynamic properties

Ephesite, Na(LiAl₂) [Al₂Si₂O₁₀] (OH)₂, has been synthesized for the first time by hydrothermal treatment of a gel of requisite composition at 300≦T(° C)≦700 and \(P_{H_2 O}\) upto 35 kbar. At \(P_{H_2 O}\) between 7 and 35 kbar and above 500° C, only the 2M₁ polytype is obtained. At lower temperatures and pressures, the 1M polytype crystallizes first, which then inverts to the 2M₁ polytype with increasing run duration. The X-ray diffraction patterns of the 1M and 2M₁ poly types can be indexed unambiguously on the basis of the space groups C2 and Cc, respectively. At its upper thermal stability limit, 2M₁ ephesite decomposes according to the reaction (1) $$\begin{gathered} {\text{Na(LiAl}}_{\text{2}} {\text{) [Al}}_{\text{2}} {\text{Si}}_{\text{2}} {\text{O}}_{{\text{10}}} {\text{] (OH)}}_{\text{2}} \hfill \\ {\text{ephesite}} \hfill \\ {\text{ = Na[AlSiO}}_{\text{4}} {\text{] + LiAl[SiO}}_{\text{4}} {\text{] + }}\alpha {\text{ - Al}}_{\text{2}} {\text{O}}_{\text{3}} {\text{ + H}}_{\text{2}} {\text{O}} \hfill \\ {\text{nepheline }}\alpha {\text{ - eucryptite corundum}} \hfill \\ \end{gathered}$$ Five reversal brackets for (1) have been established experimentally in the temperature range 590–750° C, at \(P_{H_2 O}\) between 400 and 2500 bars. The equilibrium constant, K, for this reaction may be expressed as (2) $$log K{\text{ = }}log f_{{\text{H}}_{\text{2}} O}^* = 7.5217 - 4388/T + 0.0234 (P - 1)T$$ where \(f_{H_2 O}^* = f_{H_2 O} (P,T)/f_{H_2 O}^0\) (1,T), with T given in degrees K, and P in bars. Combining these experimental data with known thermodynamic properties of the decomposition products in (1), the following standard state (1 bar, 298.15 K) thermodynamic data for ephesite were calculated: H _f,298.15 ⁰ =-6237372 J/mol, S _298.15 ⁰ =300.455 J/K·mol, G _298.15 ⁰ =-5851994 J/mol, and V _298.15 ⁰ =13.1468 J/bar·mol.     

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, (Na_6.93Ca_0.545K_0.01)_Σ7.485[(Si_6.47Al_5.48Fe_0.05)_Σ12O₂₄](CO₃)_1.25 · 2.30 H₂O, and cancrisilite, (Na_7.17 Ca_0.01)_Σ7.18[(Si_7.26Al_4.70Fe_0.04)_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: Δ _f H _el ⁰ (298.15 K) = ?14 490 ± 16 kJ/mol for cancrinite and ?14302 ± 17 kJ/mol for cancrisilite. The values of Δ _f H _el ⁰ (298.15 K), S ^o(298.15 K), and Δ _f H _el ⁰ (298.15 K) are determined for cancrinite and cancrisilite of theoretical composition.     

Thermodynamic Properties of Natural Cuspidine

The paper reports pioneering data on the calorimetrically determined enthalpy of formation from elements of cuspidine, Ca fluordiorthosilicate Ca₄Si₂O₇F₂, from the Tyrny-Auz Mo–W deposit in Kabardino- Balkaria, Russia. The data were obtained by high-temperature melt solution calorimetry. The determined value is ΔfH_el° (298.15 K) =–5190 ± 13 kJ/mol. The paper reports estimated S°(298.15 K) and Δ_fG_el° (298.15 K) of cuspidine.     

The solubility of Gd<Subscript>2</Subscript>Ti<Subscript>2</Subscript>O<Subscript>7</Subscript> and the hydrolysis of Gd<Superscript>3+</Superscript> in acidic solutions at 250°C and saturation vapor pressure

The solubility of Gd₂Ti₂O₇ ceramic in acidic solutions (HCl and HClO₄) was studied at 250°C and saturation vapor pressure within pH 2.5–5.2. The dissolution process occurs mainly via two reactions: 0.5 Gd₂Ti₂O_7(cr) + 3H⁺ = Gd³⁺ + TiO_2(cr) + 1.5 H₂O at pH < 3 and 0.5Gd₂Ti₂O₇(cr) + H⁺ + 0.5H₂O = Gd(OH) ₂ ⁺ TiO₂(cr) at pH 3–5. The thermodynamic equilibrium constants were calculated at the 0.95 confidence level as log K ₍₁₎ ^o = 4.12 ± 0.47; = ?0.97 ± 0.16 at 250°C. It was shown that Gd³⁺ undergoes hydrolysis in solutions with pH > 3, and the species Gd(OH) ₂ ⁺ dominates up to at least pH 5. At pH < 3, Gd occurs in solutions as Gd³⁺. The second constant of Gd³⁺ hydrolysis was determined at 250°C as K ^o = ?5.09 ± 0.5, and the thermodynamic characteristics of the initial Gd₂Ti₂O₇ solid phase were determined: S _298.15 ^o = 251.4 J/(mol K) and ΔfG _298.15 ^o = ?3630 ± 10 kJ/mol.     

The lattice energy of forsterite. Charge distribution and formation enthalpy of the SiO 4 4− ion

In the lattice energy expression of forsterite, based on a Born-Mayer (electrostatic+repulsive+dispersive) potential, the oxygen charge z _o, the hardness parameter ρ and the repulsive radii r _Mg and r _Si appear as unknown parameters. These were determined by calculating the first and second partial derivatives of the energy with respect to the cell edges, and equalizing them to quantities related to the crystal elastic constants; the overdetermined system of equations was solved numerically, minimizing the root-mean-square deviation. To test the results obtained, the SiO ₄ ^4? ion was assumed to move in the unit-cell, and the least-energy configuration was sought and compared with the experimental one. By combining the two methods, the optimum set of parameters was: z _o=?1.34, ρ=0.27 Å, r _Mg=0.72 Å, r _Si=0.64 Å. The values ?8565.12 and ?8927.28 kJ mol^?1 were obtained, respectively, for the lattice energy E _Land for its ionic component E _L ⁰ ,which accounts for interactions between Mg²⁺ and SiO ₄ ^4? ions only. The charge distribution calculated on the SiO ₄ ^4? ion was discussed and compared with other results. Using appropriate thermochemical cycles, the formation enthalpy and the binding energy of SiO ₄ ^4? were estimated to be: ΔH _f(SiO ₄ ^4? )=2117.6 and E(SiO ₄ ^4? )=708.6 kJ mol^?1, respectively.     

Thermodynamics of minerals of variable composition: Andradite-grossularite and pistacite-clinozoisite solid solutions

Exchange-mineral equilibria with Al and Fe³⁺ aqueous chloride solutions (aq.), Andr + AlCl _aq ³ = FeCl _aq ³ + Gros, (1) Psc + AlCl _aq ³ = FeCl _aq ³ + Czo, (2) were studied under the following experimental conditions: 500°;C and 580°;C and 1 and 2 kbar, respectively, with an overall concentration of metals in the aqueous solutions of about 0.5 M and pH 3. The mixing functions of the components in garnet and epidote were calculated from the experimental data. Thermodynamic treatment of experimental evidence for reaction (1) led to the conclusion that, within the accuracy of experiment, garnet in the andradite-grossularite series was an ideal solid solution. However, epidote solid solution markedly departed from the ideal, as was shown by concentration and pressure-temperature (PT) dependencies of Gibbs's molar excess energies and by mixing-volume concentration dependence.     

On the nonstoichiometry and point defects of olivine

This paper presents the point-defect thermodynamics for fayalite and olivine solid solutions (Fe _x Mg_1?x )₂SiO₄. By means of thermogravimetry, the metal-to-oxygen ratio of these silicates has been determined as a function of oxygen potential, compositionx and temperature. Experiments were performed in the range of 1,000° C≦T≦1,280° C and 0.2≦x≦1.0. It is found that V _Me ^″ , Fe _Me ^· and the associate {Fe′ _Si ^′ Fe _Me ^· } are the majority defects. With this knowledge it is possible to calculate the nonstoichiometry at given temperature as a function of \(p_{O_2 } \) and \(a_{SiO_2 } \) . The cation vacancy concentration shows a \(p_{O_2 }^{1/5} \) -dependence (forx≧0.2) and increases at givenT and \(p_{O_2 } \) almost exponentially with compositionx. In the composition range studied here, the silicates show an oxygen excess, and FeO is more soluble in the olivine than SiO₂.     

Calorimetric Study of Natural Anapaite

The thermochemical study of natural hydrous calcium and iron phosphate, anapaite Ca₂Fe(PO₄)₂ · 4H₂O (Kerch iron ore deposit, Crimea, Russia), was carried out using high-temperature melt solution calorimetry with a Tian-Kalvet microcalorimeter. The enthalpy of formation of the mineral from elements was obtained to be Δ _f H_el°(298.15 K) =–4812 ± 16 kJ/mol. The values of the standard entropy and the Gibbs energy of anapaite formation are S°(298.15 K) = 404.2 J/K mol and Δ _f G_el°(298.15 K) =–4352 ± 16 kJ/mol, respectively.     

The system Fe-Si-O: Oxygen buffer calibrations to 1,500K

The five solid-phase oxygen buffers of the system Fe-Si-O, iron-wuestite (IW), wuestite-magnetite (WM), magnetite-hematite (MH), quartz-iron-fayalite (QIF) and fayalite-magnetite-quartz (FMQ) have been recalibrated at 1 atm pressure and temperatures from 800°–1,300° C, using a thermogravimetric gas mixing furnace. The oxygen fugacity, \(f_{{\text{O}}_{\text{2}} }\) was measured with a CaO-doped ZrO₂ electrode. Measurements were made also for wuestite solid solutions in order to determine the redox behavior of wuestites with O/Fe ratios varying from 1.05 to 1.17. For FMQ, additional determinations were carried out at 1 kb over a temperature range of 600° to 800° C, using a modified Shaw membrane. Results agree reasonably well with published data and extrapolations. The reaction parameters K, ΔG _r ^o , ΔH _r ^o , and ΔS _r ^o were calculated from the following log \(f_{{\text{O}}_{\text{2}} }\) /T relations (T in K): $$\begin{gathered} {\text{IW }}\log f_{{\text{O}}_{\text{2}} } = - 26,834.7/T + 6.471\left( { \pm 0.058} \right) \hfill \\ {\text{ }}\left( {{\text{800}} - 1,260{\text{ C}}} \right), \hfill \\ {\text{WM }}\log f_{{\text{O}}_{\text{2}} } = - 36,951.3/T + 16.092\left( { \pm 0.045} \right) \hfill \\ {\text{ }}\left( {{\text{1,000}} - 1,300{\text{ C}}} \right), \hfill \\ {\text{MH }}\log f_{{\text{O}}_{\text{2}} } = - 23,847.6/T + 13.480\left( { \pm 0.055} \right) \hfill \\ {\text{ }}\left( {{\text{1,040}} - 1,270{\text{ C}}} \right), \hfill \\ {\text{QIF }}\log f_{{\text{O}}_{\text{2}} } = - 27,517.5/T + 6.396\left( { \pm 0.049} \right) \hfill \\ {\text{ }}\left( {{\text{960}} - 1,140{\text{ C}}} \right), \hfill \\ {\text{FMQ }}\log f_{{\text{O}}_{\text{2}} } = - 24,441.9/T + 8.290\left( { \pm 0.167} \right) \hfill \\ {\text{ }}\left( {{\text{600}} - 1,140{\text{ C}}} \right). \hfill \\ \end{gathered}$$ These experimentally determined reaction parameters were combined with published 298 K data to determine the parameters G_f, H_f, and S_f for the phases wuestite, magnetite, hematite, and fayalite from 298 K to the temperatures of the experiments. The T? \(f_{{\text{O}}_{\text{2}} }\) data for wuestite solid solutions were used to obtain activities, excess free energies and Margules mixing parameters. The new data provide a more reliable, consistent and complete reference set for the interpretation of redox reactions at elevated temperatures in experiments and field settings encompassing the crust, mantle and core as well as extraterrestrial environments.     

Thermodynamics of plagioclase II: Temperature evolution of the spontaneous strain at the I\bar 1 \leftrightarrow P\bar 1 phase transition in anorthite

The temperature dependence of the lattice parameters of pure anorthite with high Al/Si order reveals the predicted tricritical behaviour of the \(I\bar 1 \leftrightarrow P\bar 1\) phase transition at T _c ^* =510 K. The spontaneous strain couples to the order parameter Q° as x _i∝S _xQ ⁱ Q°² with S _xQ ¹ =4.166×10^?3, S _xQ ² =0.771×10^?3, S _xQ ³ =?7.223×10^?3 for the diagonal elements. The temperature dependence of Q° is $$Q^{\text{o}} = \left( {1 - \frac{T}{{510}}} \right)^\beta ,{\text{ }}\beta = \tfrac{{\text{1}}}{{\text{4}}}$$ A strong dependence of T _c ^* , S _xQ ⁱ and β is predicted for Al/Si disordered anorthite.     

Thermodynamic parameters of CaMgSi2O6-Mg2Si2O6 pyroxenes based on regular solution and cooperative disordering models

Thermodynamic parameters for the reaction: $$\begin{gathered} Mg_2 Si_2 O_6 = Mg_2 Si_2 O_6 \hfill \\ enstatite clinopyroxene \hfill \\ \end{gathered} $$ in the system CaO-MgO-SiO₂ have been deduced from phase equilibrium and enthalpy of solution data. From the regular solution theory, the seventeen currently available reversed experimental compositions of coexisting enstatite and clinopyroxene, presumed to be ordered diopside, lead, by a statistical regression, to the following best fit parameters: ΔH ^o=6.80 kJ ΔS ^o=2.75 J/K W _H ^Cpx =24.47 kJ (regular solution enthalpy parameter) W _V ^Cpx =0.105 J/bar (regular solution volume parameter). The derived parameters are not significantly affected by the (necessary) choice of W ^Opx in the range 20–50 kJ. The above values are in very good agreement with deductions from the solution calorimetry on synthetic CaMgSi₂O₆-Mg₂Si₂O₆ clinopyroxenes of Newton et al. (1979), which also places bounds on possible departures from the optimal values of these parameters. The calorimetric data may also be interpreted in terms of a Bragg-Williams cooperative-disordering model (Navrotsky and Loucks, 1977), in which diopside-structure clinopyroxene and a ‘relaxed’ low-Ca clinopyroxene (‘Fe-free pigeonite’) approach each other in composition, structural state, and stability with increasing temperature. The ΔH ^o parameter deduced from the regular solution theory is reinterpreted as the enthalpy change of enstatite to Mg₂Si₂O₆ pigeonite; the ΔH ^o of the transformation of enstatite to the diopside structure would, in this case, be considerably larger than 6.8 kJ. The curvature of the enthalpy of solution data, explained by the regular solution theory in terms of M2-site energetics (involving W _H ^cpx ), is reinterpreted as due to disordering and ‘relaxation’ in the Navrotsky-Loucks model. Although the regular solution theory with the best-fit parameters accounts for all of the reversed enstatite and diopside compositions to within 18 ° C, and is a convenient representation of the phase equilibria for purposes of geothermometry, the disordering model is, at the present level of knowledge, equally valid and allows for a region of stability of two coexisting clinopyroxenes.     

Uranium(VI) in aqueous solutions at 25°C and a pressure of 1 bar: Insight from experiments and calculations

The behavior of the 0.1 mNaCl + 0.002 mHCl + 1.9 × 10^?5 mUO₂(NO₃)₂ solution was studied at pH from 2.7 to 11.0, 25°C, and 1 bar in an argon atmosphere. The curve of variations in U concentration exhibits two minima at pH = 6.6 ± 0.7 and 10.0 ± 0.5. These minima are related to the precipitation of schoepite and clarkeite, respectively. The experimental data were used to refine the stability constants of U(VI) (hydroxo) complexes. For the polymer species of U(VI) with charges from +2 to ?1, the method of additivity of thermochemical increments was used, and increments of the linear relation were determined for the calculation of the Gibbs free energies of formation (Δ_fG _298.15 ⁰ ) of respective homologue series. The proposed method was applied to calculate the Δ_fG _298.15 ⁰ of formation of U(VI) (hydroxo)complexes containing from one to five uranium atoms.