Enthalpy of water adsorption and surface enthalpy of lepidocrocite (γ-FeOOH)

Lepidocrocite (γ-FeOOH) appears to be thermodynamically metastable with respect to goethite (α-FeOOH) and yet the former phase forms and persists both in nature and laboratory. Here we show that the thermodynamic factors relevant to these observations cannot be dismissed, although kinetics undoubtedly plays a significant role in the formation and preservation of metastable phases. To understand the relationships of the FeOOH polymorphs in the bulk and nanoscale, we investigated the energetics of lepidocrocite nanoparticles. We measured enthalpy of water adsorption and enthalpy of formation of lepidocrocite samples with surface area of 42-103 m²/g. Having both quantities measured allowed us to calculate the surface enthalpy for a water-free surface of this phase as 0.62 ± 0.14 J/m² and the energy of a relaxed (hydrated) surface as 0.40 ± 0.16 J/m². Our measurements show that a portion of the adsorbed water (∼40% under laboratory conditions) is chemisorbed (strongly bound) with enthalpy of adsorption of −65.8 ± 2.6 kJ/mol of H₂O relative to vapor (or −21.8 ± 2.6 kJ/mol relative to liquid water). The standard enthalpy of formation from elements for a hypothetical lepidocrocite with nominal composition FeOOH and zero surface area is −552.0 ± 1.6 kJ/mol. Our results demonstrate that when considering the thermodynamic properties of iron oxides in the environment, a conclusive statement about their stability cannot be made without specifying the particle size of individual phases.     

Synthesis, characterization and thermochemistry of a Pb-jarosite

The enthalpy of formation from the elements of a well-characterized synthetic Pb-jarosite sample corresponding to the chemical formula (H₃O)_0.74Pb_0.13Fe_2.92(SO₄)₂(OH)_5.76(H₂O)_0.24 was measured by high temperature oxide melt solution calorimetry. This value ( = −3695.9 ± 9.7 kJ/mol) is the first direct measurement of the heat of formation for a lead-containing jarosite. Comparison to the thermochemical properties of hydronium jarosite and plumbojarosite end-members strongly suggests the existence of a negative enthalpy of mixing possibly related to the nonrandom distribution of Pb²⁺ ions within the jarosite structure. Based on these considerations, the following thermodynamic data are proposed as the recommended values for the enthalpy of formation from the elements of the ideal stoichiometric plumbojarosite Pb_0.5Fe₃(SO₄)₂(OH)₆:  = −3118.1 ± 4.6 kJ/mol,  = −3603.6 ± 4.6 kJ/mol and S° = 376.6 ± 4.5 J/(mol K). These data should prove helpful for the calculation of phase diagrams of the Pb-Fe-SO₄-H₂O system and for estimating the solubility product of pure plumbojarosite. For illustration, the evolution of the estimated solubility product of ideal plumbojarosite as a function of temperature in the range 5-45 °C was computed (Log(K_sp) ranging from −24.3 to −26.2). An Eh-pH diagram is also presented.     

Thermochemistry of yavapaiite KFe(SO4)2: Formation and decomposition

Yavapaiite, KFe(SO₄)₂, is a rare mineral in nature, but its structure is considered as a reference for many synthetic compounds in the alum supergroup. Several authors mention the formation of yavapaiite by heating potassium jarosite above ca. 400°C. To understand the thermal decomposition of jarosite, thermodynamic data for phases in the K-Fe-S-O-(H) system, including yavapaiite, are needed. A synthetic sample of yavapaiite was characterized in this work by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and thermal analysis. Based on X-ray diffraction pattern refinement, the unit cell dimensions for this sample were found to be a = 8.152 ± 0.001 Å, b = 5.151 ± 0.001 Å, c = 7.875 ± 0.001 Å, and β = 94.80°. Thermal decomposition indicates that the final breakdown of the yavapaiite structure takes place at 700°C (first major endothermic peak), but the decomposition starts earlier, around 500°C. The enthalpy of formation from the elements of yavapaiite, KFe(SO₄)₂, ΔH°_f = −2042.8 ± 6.2 kJ/mol, was determined by high-temperature oxide melt solution calorimetry. Using literature data for hematite, corundum, and Fe/Al sulfates, the standard entropy and Gibbs free energy of formation of yavapaiite at 25°C (298 K) were calculated as S°(yavapaiite) = 224.7 ± 2.0 J.mol⁻¹.K⁻¹ and ΔG°_f = −1818.8 ± 6.4 kJ/mol. The equilibrium decomposition curve for the reaction jarosite = yavapaiite + Fe₂O₃ + H₂O has been calculated, at pH₂O = 1 atm, the phase boundary lies at 219 ± 2°C.     

MgSiO3 ilmenite: Heat capacity,thermal expansivity,and enthalpy of transformation

A new determination, using high temperature drop-solution calorimetry, of the enthalpy of transformation of MgSiO₃ pyroxene to ilmenite gives H ₂₉₈ = 59.03 ±4.26 kJ/mol. The heat capacity of the ilmenite and orthopyroxene phases has been measured by differential scanning calorimetry at 170–700 K; C_p of MgSiO₃ ilmenite is 4–10 percent less than that of MgSiO₃ pyroxene throughout the range studied. The heat capacity differences are consistent with lattice vibrational models proposed by McMillan and Ross (1987) and suggest an entropy change of -18 ± 3 J-K^-1 ·mol^-1, approximately independent of temperature, for the pyroxene-ilmenite transition. The unit cell parameters of MgSiO₃ ilmenite were measured at 298–876 K and yield an average volume thermal expansion coefficient of 2.44 × 10^-5 K^-1. The thermochemical data are used to calculate phase relations involving pyroxene, -Mg₂SiO₄ plus stishovite, Mg₂SiO₄ spinel plus stishovite, and ilmenite in good agreement with the results of high pressure studies.     

The ZnSiO3 clinopyroxene-ilmenite transition: Heat capacity,enthalpy of transition,and phase equilibria

ZnSiO₃ clinopyroxene stable above 3 GPa transforms to ilmenite at 10–12 GPa, which further decomposes into ZnO (rock salt) plus stishovite at 20–30 GPa. The enthalpy of the clinopyroxene-ilmenite transition was measured by high-temperature solution calorimetry, giving ΔH⁰=51.71 ±3.18 kJ/mol at 298 K. The heat capacities of clinopyroxene and ilmenite were measured by differential scanning calorimetry at 343–733 and 343–633 K, respectively. The C _p of ilmenite is 3–5% smaller than that of clinopyroxene. The entropy of transition was calculated using the measured enthalpy and the free energy calculated from the phase equilibrium data. The enthalpy, entropy and volume changes of the pyroxene-ilmenite transition in ZnSiO₃ are similar in magnitude to those in MgSiO₃. The present thermochemical data are used to calculate the phase boundary of the ZnSiO₃ clinopyroxene-ilmenite transition. The calculated boundary,

Hydration-dehydration behavior and thermodynamics of chabazite

Equilibrium in the chabazite-H₂O system was investigated by isothermal thermogravimetric analysis over a large range of temperatures (from 23 to 315°C) and H₂O-vapor pressures (from 0.03 to 28 mbar). Thermodynamic analysis of the phase-equilibrium data revealed the existence of three energetically distinct types of H₂O, referred to as S-1, S-2, and S-3. At 23°C and 26 mbar of H₂O-vapor pressure, chabazite has maximum H₂O occupancies of 8.2, 11.1, and 3.1 wt.% for S-1, S-2, and S-3, respectively. During dehydration, S-1 H₂O is lost first, followed by S-2 H₂O and then S-3 H₂O, with significant overlap for S-1 and S-2 as well as S-2 and S-3. The thermodynamics of chabazite-H₂O were modeled using three independent equilibrium formulations for S-1, S-2, and S-3. These formulations yielded standard-state molar Gibbs free energy of hydration of −21.8 ± 0.6, −52.1 ± 1.8, and −111.7 ± 6.7 kJ/mol for S-1, S-2, and S-3. Standard-state molar enthalpies of hydration for each type of H₂O are −65.6 ± 0.5, −100.1 ± 1.6, and −156.9 ± 6.2 kJ/mol, respectively. Integral molar values for the Gibbs free energy of hydration for each type of H₂O are −19.0 ± 0.7, −40.1 ± 2.1, and −76.9 ± 9.6 kJ/mol, respectively. Integral molar values for the enthalpy of hydration for each type of H₂O are −62.8 ± 0.6, −88.1 ± 1.9, and −122.2 ± 9.3 kJ/mol, respectively. Integration of the predicted total partial molar enthalpy of hydration for all three types of H₂O over the full H₂O content of chabazite gave an integral molar enthalpy of −39.65 ± 9.3 kJ/mol relative to liquid water. The thermodynamic data obtained for the hydration of natural chabazite were used to predict the hydration state of chemically similar chabazites under various temperatures and P_H2O, ranging from 25 to 400°C and from 10⁻⁵ to 10⁴ bars.     

Thermochemistry of the new silica polymorph moganite

Samples of microcrystalline silica varieties containing variable amounts of the new silica polymorph moganite (up to R~82 wt.%) have been studied by a combination of high temperature solution calorimetry using lead borate (2 PbO · B₂O₃) solvent and transposed temperature drop calorimetry near 977 K, in order to investigate the thermochemical stability of this new silica mineral. The enthalpy of solution at 977 K and the heat content (H₉₇₇ — H₂₉₈) of “pure” moganite phase were estimated to be -7.16 ± 0.35 kJ/mol and 43.62 ± 0.50 kJ/mol, respectively. The standard molar enthalpy of formation is-907.3 ± 1.2 kJ/mol. Thus, calorimetry strongly supports results of previous X-ray and Raman spectroscopic studies that moganite is a distinct silica polymorph. Its thermochemical instability relative to quartz at 298 K of 3.4 ± 0.7 kJ/mol is marginally higher than those of cristobalite and tridymite. Structurally, this instability may be related to the presence of distorted 4-membered rings of SiO₄ tetrahedra, which are not found in the quartz structure. The metastability relative to quartz may also explain the apparent scarcity of moganite in altered rocks and in rocks that are older than 130 my.     

Thorite versus huttonite: stability, electronic properties and X-ray emission spectra from first-principle calculations

The structural, electronic properties and stability of thorium orthosilicate ThSiO₄ polymorphs: thorite and huttonite are investigated by means of the full-potential linearized augmented-plane-wave method with the generalized gradient approximation for the exchange-correlation potential (FLAPW-GGA). The forbidden gaps of thorite and huttonite are estimated at about 7.8 and 7.6 eV, respectively. It is found that Th5f states in ThSiO₄ partially overlap with occupied O2p bands. The data obtained showed that thorite is more stable than huttonite; in turn both ThSiO₄ polymorphs are unstable with respect to their constituent binary oxides (thorianite ThO₂ and α-quartz SiO₂) in agreement with the experiments. The theoretical shapes of X-ray emission (XES) (Si,O)Kα,β spectra for thorite, huttonite as well as for SiO₂ and ThO₂ are calculated and discussed. We show that the XES spectroscopy near the (Si,O)K edge may be very useful technique not only for detailed investigation of the bulk-electronic structure of Th silicates but also for the phase analysis of complex mineral samples containing these species.     

The enthalpy of fusion of anorthite

The high-temperature enthalpies of liquid and glassy CaAl₂Si₂O₈ were measured by drop calorimetry using a diphenyl ether drop calorimeter. These data are combined with published values of the high-temperature enthalpy of crystalline anorthite and the enthalpy of vitrification of anorthite to obtain the enthalpy of fusion of anorthite. Analysis of the data yields the following preferred values (enthalpy in kcal/mol, uncertainty limits correspond to two standard deviations):enthalpy of vitrification at 985 K, _v H _{v 985}=18.6±0.6; enthalpy of the liquid at 1,830 K, H ₁₈₃₀ ^l ₃₀₀ ^g =130.4±1.2; enthalpy of the glass at 985 K, H ₉₈₅ ^g -H ₃₀₀ ^g =46.7±0.4; enthalpy of crystalline anorthite between 985 and 1,830 K, H ₁₈₃₀ ^c -H ₉₈₅ ^c =69.9±1.4; calculated enthalpy of fusion of anorthite at 1,830 K, _f H ₁₈₃₀= 32.4±2.1.The average heat capacity of supercooled liquid CaAl₂Si₂O₈ between the glass transition (T _g 1,086 K) and the melting point (T _f7=1,830 K) is 102 ± 2 cal/mol/K. The large difference between the enthalpy of fusion and the enthalpy of vitrification for the minerals anorthite and diopside is emphasized. The practice of assuming _fH _vH should be discontinued for silicate compounds for which T _f T _g.     

Experimental determination of the activity of chromite in multicomponent spinels

We determined activity-composition relationships in Pt-Cr and Pt-Fe-Cr alloys at 1300°C experimentally and used the results to constrain the thermodynamic properties of chromite-picrochromite spinels. The Pt-Cr binary is characterized by strong negative deviations from ideality throughout the investigated composition range and the activity-composition relationship can be fit by a four-suffix asymmetric regular solution with three binary interaction parameters. The ternary alloy was modeled as a four-suffix asymmetric regular solution; the three ternary interaction parameters in this model were constrained by combining interaction parameters for the three bounding binaries taken from this and previous work with results for a set of experiments in which the activity of Cr in Pt-Fe-Cr-alloys was fixed by coexisting Cr₂O₃ at known f_O2.The free energy of formation of FeCr₂O₄ at 1300°C was determined using the activities of Fe and Cr in Pt-alloys in equilibrium with oxide mixes of FeCr₂O₄ and Cr₂O₃. The free energy of formation of chromite from Fe+Cr₂O₃+O₂ is −202.7 ± 0.4 kJ/mol (1σ), indistinguishable from literature values. The corresponding free energy of formation of FeCr₂O₄ from the elements is −923.5 ± 2.1 kJ/mol (1σ), and the enthalpy of formation at 298 K is −1438 kJ/mol. The activity-composition relationship for the chromite component in (Fe,Mg)Cr₂O₄ solid solutions was determined from a set of experiments in which Pt-alloys were equilibrated with spinel + Cr₂O₃. (Fe,Mg)Cr₂O₄ spinels are nearly ideal at 1300°C; modeling our data with a one-site symmetric regular solution yields an interaction parameter of +2.14 ± 0.62 kJ/mol (1σ), similar to values based on data from the literature.     

Synthesis, characterization, and thermochemistry of K-Na-H3O jarosites

The thermochemistry of well-characterized synthetic K-H₃O, Na-H₃O and K-Na-H₃O jarosites was investigated. These phases are solid solutions that obey Vegard’s law. Electron probe microanalyses indicated lower alkali and iron contents than predicted from the theoretical end-member compositions, in agreement with thermal analyses, suggesting the presence of hydronium and “additional” water. The standard enthalpies of formation (ΔH°_f) of K-H₃O, Na-H₃O and K-Na-H₃O jarosites were determined by high-temperature oxide melt solution calorimetry. These enthalpies vary linearly with the K/H₃O, Na/H₃O and K/Na ratio, respectively. The enthalpy of formation of pure hydronium jarosite was also determined experimentally (ΔH°_f = −3741.6 ± 8.3 kJ.mol⁻¹), and it was used to evaluate ΔH°_f for the end-members KFe₃(SO₄)₂(OH)₆ (ΔH°_f = −3829.6 ± 8.3 kJ.mol⁻¹) and NaFe₃(SO₄)₂(OH)₆ (ΔH°_f = −3783.4 ± 8.3 kJ.mol⁻¹). Finally, enthalpies of dehydration (loss of the “additional” water) of some jarosites were determined and found to be near the enthalpy of vaporization of water, suggesting that the “additional” water is weakly bonded in the structure.     

Calorimetric measurements of fusion enthalpies for Ni2SiO4 and Co2SiO4 olivines and application to olivine-liquid partitioning

Calorimetric measurements of fusion enthalpies for Ni₂SiO₄ and Co₂SiO₄ olivines were carried out using a high-temperature calorimeter, and Ni and Co partitioning between olivine and silicate liquid was analyzed using the measured heats of fusion. The fusion enthalpy of Co₂SiO₄ olivine measured by transposed-temperature drop calorimetry was 103 ± 15 kJ/mol at melting point (1688 K). The fusion enthalpy of Ni₂SiO₄ olivine was calculated based on the enthalpies of liquids in the system An₅₀Di₅₀-Ni₂SiO₄ measured by transposed-temperature drop calorimetry at 1773 K, and was 221 ± 26 kJ/mol at its metastable melting point (1923 K). The fusion enthalpy of Ni₂SiO₄ is the largest among those of olivine group, this is caused by the large crystal field stabilization energy of six-coordinated Ni²⁺ in olivine. The larger fusion enthalpy of Ni₂SiO₄ can account for the large and variable partition coefficient of Ni between olivine and silicate liquid. Based on the comparison between partition coefficients calculated from thermodynamic data and those observed in partition experiments, it is considered that the magnitude of partition coefficients is primarily dependent on the heats of fusion of the components. Furthermore, the activity coefficients for Ni-, Co- and Mn-bearing components in magmatic liquid are nearly of the same magnitude.     

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).     

Self-diffusion of Si and O in diopside-anorthite melt at high pressures

Self-diffusion coefficients for Si and O in Di₅₈An₄₂ liquid were measured from 1 to 4 GPa and temperatures from 1510 to 1764°C. Glass starting powders enriched in ¹⁸O and ²⁸Si were mated to isotopically normal glass powders to form simple diffusion couples, and self-diffusion experiments were conducted in the piston cylinder device (1 and 2 GPa) and in the multianvil apparatus (3.5 and 4 GPa). Profiles of ¹⁸O/¹⁶O and ^29,30Si/²⁸Si were measured using secondary ion mass spectrometry. Self-diffusion coefficients for O (D(O)) are slightly greater than self-diffusion coefficients for Si (D(Si)) and are often the same within error. For example, D(O) = 4.20 ± 0.42 × 10⁻¹¹ m²/s and D(Si) = 3.65 ± 0.37 × 10⁻¹¹ m²/s at 1 GPa and 1662°C. Activation energies for self-diffusion are 215 ± 13 kJ/mol for O and 227 ± 13 kJ/mol for Si. Activation volumes for self-diffusion are −2.1 ± 0.4 cm³/mol and −2.3 ± 0.4 cm³/mol for O and Si, respectively. The similar self-diffusion coefficients for Si and O, similar activation energies, and small, negative activation volumes are consistent with Si and O transport by a cooperative diffusion mechanism, most likely involving the formation and disassociation of a high-coordinated intermediate species. The small absolute magnitudes of the activation volumes imply that Di₅₈An₄₂ liquid is close to a transition from negative to positive activation volume, and Adam-Gibbs theory suggests that this transition is linked to the existence of a critical fraction (∼0.6) of bridging oxygen.     

Energetics of anhydrite, barite, celestine, and anglesite: a high-temperature and differential scanning calorimetry study

The thermochemistry of anhydrous sulfates (anglesite, anhydrite, arcanite, barite, celestine) was investigated by high-temperature oxide melt calorimetry and differential scanning calorimetry. Complete retention and uniform speciation of sulfur in the solvent was documented by (a) chemical analyses of the solvent (3Na₂O · 4MoO₃) with dissolved sulfates, (b) Fourier transform infrared spectroscopy confirming the absence of sulfur species in the gases above the solvent, and (c) consistency of experimental determination of the enthalpy of drop solution of SO₃ in the solvent. Thus, the principal conclusion of this study is that high-temperature oxide melt calorimetry with 3Na₂O · 4MoO₃ solvent is a valid technique for measurement of enthalpies of formation of anhydrous sulfates. Enthalpies of formation (in kJ/mol) from the elements (ΔH_f^o) were determined for synthetic anhydrite (CaSO₄) (−1433.8 ± 3.2), celestine (SrSO₄) (−1452.1 ± 3.3), anglesite (PbSO₄) (−909.9 ± 3.4), and two natural barite (BaSO₄) samples (−1464.2 ± 3.7, −1464.9 ± 3.7). The heat capacity of anhydrite, barite, and celestine was measured between 245 and 1100 K, with low- and high-temperature Netzsch (DSC-404) differential scanning calorimeters. The results for each sample were fitted to a Haas-Fisher polynomial of the form C_p(245 K < T < 1100 K) = a + bT + cT⁻² + dT^−0.5 + eT². The coefficients of the equation are as follows: for anhydrite a = 409.7, b = −1.764 × 10⁻¹, c = 2.672 × 10⁶, d = −5.130 × 10³, e = 8.460 × 10⁻⁵; for barite, a = 230.5, b = −0.7395 × 10⁻¹, c = −1.170 × 10⁶, d = −1.587 × 10³, e = 4.784 × 10⁻⁵; and for celestine, a = 82.1, b = 0.8831 × 10⁻¹, c = −1.213 × 10⁶, d = 0.1890 × 10³, e = −1.449 × 10⁻⁵. The 95% confidence interval of the measured C_p varies from 1 to 2% of the measured value at low temperature up to 2 to 5% at high temperature. The measured thermochemical data improve or augment the thermodynamic database for anhydrous sulfates and highlight the remaining discrepancies.     

Interdiffusion of Mg–Fe in olivine at 1,400–1,600 °C and 1 atm total pressure

The interdiffusion coefficient of Mg–Fe in olivine (D _Mg–Fe) was obtained at 1,400–1,600 °C at the atmospheric pressure with the oxygen fugacity of 10^?3.5–10^?2 Pa using a diffusion couple technique. The D _Mg–Fe shows the anisotropy (largest along the [001] direction and smallest along the [100] direction), and its activation energy (280–320 kJ/mol) is ~80–120 kJ/mol higher than that estimated at lower temperatures. The D _Mg–Fe at temperatures of >1,400 °C can be explained by the cation-vacancy chemistry determined both by the Fe³⁺/Fe²⁺ equilibrium and by the intrinsic point defect formation with the formation enthalpy of 220–270 kJ/mol depending on the thermodynamical model for the Fe³⁺/Fe²⁺ equilibrium in olivine. The formation enthalpy of 220–270 kJ/mol for the point defect (cation vacancy) in olivine is consistent with that estimated from the Mg self-diffusion in Fe-free forsterite. The increase in the activation energy of D _Mg–Fe at >1,400 °C is thus interpreted as the result of the transition of diffusion mechanism from the transition metal extrinsic domain to the intrinsic domain at the atmospheric pressure.     

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.     

Thermochemistry of carbonate-pyroxene equilibria

The enthalpies of drop solution of calcite, magnesite, dolomite, wollastonite and diopside have been measured in a lead borate solvent at 977 K in a Calvettype microcalorimeter. The carbonate calorimetry was done under flowing gas atmosphere. Both natural and synthetic samples were used. From these calorimetric data, the enthalpies of several reactions of carbonate with quartz were calculated. The enthalpies of these reactions (kJ/mol) at 298 K are: calcite+quartzwollastonite+CO₂, 92.3±1.0; magnesite+quartzenstatite+CO₂, 82.9±2.8; dolomite+quartzdiopside+CO₂, 163.0±1.9. These values generally are in agreement with those calculated from Robie et al., Helgeson et al., Berman and Holland and Powell. The enthalpy of dolomite-quartz reaction overlaps marginally with those from Berman and Holland and Powell. The enthalpy of formation of dolomite from magnesite and calcite (-11.1±2.5 kJ/mol) was also derived from the measured enthalpies, and this value is consistent with that from acid solution calorimetric measurements as shown by Navrotsky and Capobianco, but different from values in the earlier literature. These results support the premise that drop-solution of carbonates into molten lead borate results in a well-defined final state consisting of dissolved oxide and evolved CO₂. This was also confirmed by weight change experiments. Thus, oxide melt calorimetry is applicable to carbonates.     

The hydrolysis of gold(I) in aqueous solutions to 600°c and 1500 bar

The solubility of gold has been measured in aqueous solutions at temperatures between 300 and 600°C and pressures from 500 to 1500 bar to determine the stability and stoichiometry of the hydroxy complexes of gold(I) in hydrothermal solutions. The experiments were carried out using a flow-through autoclave system. The solubilities, measured as total dissolved gold, were in the range 1.2 × 10⁻⁸ to 2.0 × 10⁻⁶ mol kg⁻¹ (0.002 to 0.40 mg kg⁻¹), in solutions of total dissolved sodium between 0.0 and 0.5 mol kg⁻¹, and total dissolved hydrogen between 4.0 × 10⁻⁶ and 4.0 × 10⁻⁴ mol kg⁻¹. At constant hydrogen molality, the solubility of gold increases with increasing temperature and decreases with increasing pressure. The solubilities were found to be independent of pH but increased with decreasing hydrogen molality at constant temperature and pressure. Consequently, gold dissolves in aqueous solutions of acidic to alkaline pH according to the reactionAu(s)+H₂O(l)=AuOH(aq)+0.5H₂(g) K_s,1The solubility constant, logK_s,1, increases with increasing temperature from a minimum of −8.76 (±0.18) at 300°C and 500 bar to a maximum of −7.50 (±0.11) at 500°C and 1500 bar and decreases to −7.61 (±0.08) at 600°C and 1500 bar. From the equilibrium solubility constant and the redox potential of gold, the formation constant to form AuOH(aq) was calculated. At 25°C the complex formation is characterised by an exothermic enthalpy and a positive entropy. With increasing temperature and decreasing pressure, the formation reaction becomes endothermic and is accompanied by a large positive entropy, indicating a greater electrostatic interaction between Au⁺ and OH⁻.     

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.