A thermochemical study of monohydrocalcite

The thermodynamic properties of monohydrocalcite, CaCO₃ · H₂O, have been obtained using a well-characterized natural specimen. Equilibration of the solid with water at 25°C under 0.97 atm CO₂ led to an activity product [Ca²⁺][CO₃^2?] = 10^?7.60±0.03 and a free energy of formation ΔG_f^o = ?325,430 ± 270 calmol^?. The enthalpy of solution of monohydrocalcite in 0.1 N HCl at 25°C led to a standard enthalpy of formation ΔH_f^o = ?358,100 ± 280 cal mol^?1. Estimates of the variation of ΔG_f with temperature and pressure showed monohydrocalcite to be metastable with respect to calcite and aragonite.     

Dissociation constants of calcite and CaHC03+ from 0 to 50°C

From conductance measurements, the negative logarithm of the dissociation constant of the CaHCO₃₊ ion pair, pK(CaHCO₃⁺), is 0.7, 1.0 and 1.35 within ±0.05 units at 0, 25 and 60°C, respectively. A revaluation of published and unpublished data yields pK(CaCO₃⁰) ≈ 3.2 at 25°C. Use of these pK's to compute the dissociation constant of calcite (Kc) from published calcite solubility measurements in pure water gives pKc values which increase markedly with ionic strength. However, if the ion pairs are ignored, computed pKc values are nearly constant with ionic strength. All reasonable attempts to eliminate the trend in pKc by adjusting ion activity coefficients, and/or values of K(CaCO₃⁰) failed, so the dilemma remains. Kc values computed from the most reliable published calcite solubility data are in good agreement with such values based on solubility data measured in this study at 5, 15, 35 and 50°C. Study results ignoring ion pairs are accurately represented by the equation log Kc = 13.870 — (3059/T) ?0.04035T, and correspond to ?8.35, ?8.42, and ?8.635 at 0, 25 and 50°C, respectively. The logarithmic expression leads to ΔH_r^o = ?2420 ± 300 cal/mol, ΔCp = ?110 ± 2 cal/deg mol, and ΔS_r^o = ?46.6 ± 1.0 cal/deg mol for the calcite dissociation reaction at 25°C. The dependence of Kc on temperature when CaCO₃⁰ and CaHCO₃⁺ are assumed, is described by log Kc = 13.543 ? (3000/T) ? 0.0401T which yields ?8.39, ?8.47, and -8.70 at 0, 25 and 50°C. This gives ΔH_r^o = ?2585 ± 300 cal/mol, ΔCp = ?109 ± 2 cal/deg mol, and ΔS_r⁰ = ?47.4 ± 1.0 cal/deg mol at 25°C.     

The Mg2GeO4 olivine-spinel phase transition

Enthalpies and entropies of transition for the Mg₂GeO₄ olivine-spinel transformation have been determined from self-consistency analyses of Dachille and Roy's (1960), Hensen's (1977) and Shiota et al.'s (1981) phase boundary studies. When all three data sets are analyzed simultaneously,ΔH ₉₇₃ andΔS ₉₇₃ are constrained between ?14000 to ?15300 J mol^?1 and ?13.0 to ?14.1·J mol^?1 K^?1, respectively. High-temperature solution calorimetric experiments completed on both polymorpha yield a value of ?14046±1366 J mol^?1 forΔH ₉₇₃. Kieffer-type lattice vibrational models of Mg₂GeO₄ olivine and spinel based on newly-measured infrared and Raman spectra predict a value of ?13.3±0.6 J mol^?1 K^?1 forΔS ₁₀₀₀. The excellent agreement between these three independent determinations ofΔH andΔS suggests that the synthesis runs of Shiota et al. (1981) at high pressures and temperatures bracket equilibrium conditions. In addition, no configurational disorder of Mg and Ge was needed to obtain the consistent parameters quoted. The Raman spectrum and X-ray diffractogram show that little disorder, if any, is present in Mg₂GeO₄ spinel synthesized at 0.2 GPa and 973–1048 K.     

CO<Subscript>2</Subscript> content of andesitic melts at graphite-saturated upper mantle conditions with implications for redox state of oceanic basalt source regions and remobilization of reduced carbon from subducted eclogite

We have performed experiments to determine the effects of pressure, temperature and oxygen fugacity on the CO₂ contents in nominally anhydrous andesitic melts at graphite saturation. The andesite composition was specifically chosen to match a low-degree partial melt composition that is generated from MORB-like eclogite in the convective, oceanic upper mantle. Experiments were performed at 1–3 GPa, 1375–1550?°C, and fO₂ of FMQ ?3.2 to FMQ ?2.3 and the resulting experimental glasses were analyzed for CO₂ and H₂O contents using FTIR and SIMS. Experimental results were used to develop a thermodynamic model to predict CO₂ content of nominally anhydrous andesitic melts at graphite saturation. Fitting of experimental data returned thermodynamic parameters for dissolution of CO₂ as molecular CO₂: ln(K ⁰) = ?21.79?±?0.04, ΔV ⁰?=?32.91?±?0.65 cm³mol^?1, ΔH ⁰?=?107?±?21 kJ mol^?1, and dissolution of CO₂ as CO₃ ^2?: ln(K ⁰ ) = ?21.38?±?0.08, ΔV ⁰?=?30.66?±?1.33 cm³ mol^?1, ΔH ⁰?=?42?±?37 kJ mol^?1, where K ⁰ is the equilibrium constant at some reference pressure and temperature, ΔV ⁰ is the volume change of reaction, and ΔH ⁰ is the enthalpy change of reaction. The thermodynamic model was used along with trace element partition coefficients to calculate the CO₂ contents and CO₂/Nb ratios resulting from the mixing of a depleted MORB and the partial melt of a graphite-saturated eclogite. Comparison with natural MORB and OIB data suggests that the CO₂ contents and CO₂/Nb ratios of CO₂-enriched oceanic basalts cannot be produced by mixing with partial melts of graphite-saturated eclogite. Instead, they must be produced by melting of a source containing carbonate. This result places a lower bound on the oxygen fugacity for the source region of these CO₂-enriched basalts, and suggests that fO₂ measurements made on cratonic xenoliths may not be applicable to the convecting upper mantle. CO₂-depleted basalts, on the other hand, are consistent with mixing between depleted MORB and partial melts of a graphite-saturated eclogite. Furthermore, calculations suggest that eclogite can remain saturated in graphite in the convecting upper mantle, acting as a reservoir for C.     

Thermodynamic properties of synthetic calcium-free carbonate cancrinite

Calcium-free carbonate cancrinite with formula unit Na_8.28[Al_5.93Si_6.07O₂₄](CO₃)_0.93(OH)_0.49·3.64H₂O (CAN) has been synthesized under hydrothermal conditions. The product has been characterized by the methods of scanning electronic microscopy and energy dispersive X-ray analysis, Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis with FTIR of evolved gases (TGA–FTIR), and X-ray powder diffraction. The heat capacity of CAN has been measured from 6 to 259 K via low-temperature adiabatic calorimetry. A linear combination of Einstein functions has been used to approximate the obtained data on the heat capacity. The thermal contributions to the entropy and enthalpy of CAN in the temperature range 0–300 K have been calculated from these data. The heat capacity and third-law absolute entropy of CAN at 298.15 K are 1,047 ± 30 and 1,057 ± 35 J mol^?1 K^?1, respectively. High-temperature oxide-melt solution calorimetry has been used to determine the enthalpy of formation from elements of CAN at 298.15 K; the value equals ?14,684 ± 50 kJ mol^?1. The Gibbs energy of formation from elements at 298.15 K has been calculated and totaled ?13,690 ± 51 kJ mol^?1.     

Solubility and equilibrium constants of lead in carbonate solutions (25°C,I = 0.3 mol dm−3)

The solubilities of PbCO₃(s), 2PbCO₂·Pb(OH)₂(s), and of 3PbCO₃ 2Pb(OH)₂(s) have been studied at 25°C ± 0.1°C in solutions of the constant ionic strength I = 0.3 mol/dm³, consisting primarily of sodium perchlorate. A few experiments with hydrocerussite were performed in solutions of 0.1 M KNO₃. The concentrations of lead and hydrogen ions have been determined in solution in contact with the solid phase. From experimental data the following values for equilibrium constants are obtained: log [Pb²⁺]·pCO₂·[H⁺]^?2 = 5.20log [Pb²⁺]·pCO^0.67₂·[H⁺]^?2 = 6.80log [Pb₂₊]³·[CO^2?₃]²·[OH^?]² = ?44.08 (and ?44.8 forI = 0.1 M)log [PbCO⁰₃]·[Pb²⁺]^?1·[CO^2?₃]^?1 = 5.40log [Pb(CO₃)^2?₂]·[Pb₂₊]^?1·[CO^2?₃]^?2 = 8.86 The data indicate that hydrocerussite is the most stable solid phase in natural waters. Comparison with the literature and needs for further research are also presented.     

In situ observation of crystal growth in a basalt melt and the development of crystal size distribution in igneous rocks

The speciation of CO₂ in dacite, phonolite, basaltic andesite, and alkali silicate melt was studied by synchrotron infrared spectroscopy in diamond anvil cells to 1,000 °C and more than 200 kbar. Upon compression to 110 kbar at room temperature, a conversion of molecular CO₂ into a metastable carbonate species was observed for dacite and phonolite glass. Upon heating under high pressure, molecular CO₂ re-appeared. Infrared extinction coefficients of both carbonate and molecular CO₂ decrease with temperature. This effect can be quantitatively modeled as the result of a reduced occupancy of the vibrational ground state. In alkali silicate (NBO/t = 0.98) and basaltic andesite (NBO/t = 0.42) melt, only carbonate was detected up to the highest temperatures studied. For dacite (NBO/t = 0.09) and phonolite melts (NBO/t = 0.14), the equilibrium CO₂ + O^2? = CO₃ ^2? in the melt shifts toward CO₂ with increasing temperature, with ln K = ?4.57 (±1.68) + 5.05 (±1.44) 10³ T ^?1 for dacite melt (ΔH = ?42 kJ mol^?1) and ln K = ?6.13 (±2.41) + 7.82 (±2.41) 10³ T ^?1 for phonolite melt (ΔH = ?65 kJ mol^?1), where K is the molar ratio of carbonate over molecular CO₂ and T is temperature in Kelvin. Together with published data from annealing experiments, these results suggest that ΔS and ΔH are linear functions of NBO/t. Based on this relationship, a general model for CO₂ speciation in silicate melts is developed, with ln K = a + b/T, where T is temperature in Kelvin and a = ?2.69 ? 21.38 (NBO/t), b = 1,480 + 38,810 (NBO/t). The model shows that at temperatures around 1,500 °C, even depolymerized melts such as basalt contain appreciable amounts of molecular CO₂, and therefore, the diffusion coefficient of CO₂ is only slightly dependent on composition at such high temperatures. However, at temperatures close to 1,000 °C, the model predicts a much stronger dependence of CO₂ solubility and speciation on melt composition, in accordance with available solubility data.     

The effect of pressure on the ionization of boric acid in sodium chloride and seawater from molal volume data at 0 and 25°C

The apparent molal volume, φ_V of boric acid, B(OH)₃ and sodium borate, NaB(OH)₄, have been determined in 35%. salinity seawater and 0·725 molal NaCl solutions at 0 and 25°C from precise density measurements. Similar to the behavior of nonelectrolytes and electrolytes in pure water, the φ_V of B(OH)₃ is a linear function of added molality and the φ_V of NaB(OH)₄ is a linear function of the square root of added molarity in seawater and NaCl solutions. The partial molal volumes, $\text{V}\text{?}\text{1}$, of B(OH)₃ and NaB(OH)₄ in seawater and NaCl were determined from the φ_V's by extrapolating to infinite dilution in the medium. The $\text{V}\text{?}\text{1}$ of B(OH)₃ is larger in NaCl and seawater than pure water apparently due to the ability of electrolytes to dehydrate the nonelectrolyte B(OH)₃. The $\text{V}\text{?}\text{1}$ of NaB(OH)₄ in itself, NaCl and seawater is larger than the expected value at 0·725 molal ionic strength due to ion pair formation [Na⁺ + B(OH)₄^? → NaB(OH)₄⁰]. The volume change for the formation of NaB(OH)₄⁰ in itself and NaCl was found to be equal to 29·4 ml mol^?1 at 25°C and 0·725 molal ionic strength. These large $\text{Δ}\text{V}\text{?}\text{1}$'s indicate that at least one water molecule is released when the ion pair is formed [Na⁺ + B(OH)₄^? → H₂O + NaOB(OH)₂⁰]. The observed $\text{V}\text{?}\text{1}$ in seawater and the $\text{Δ}\text{V}\text{?}\text{1}$ (NaB⁰) in water and NaCl were used to estimate $\text{Δ}\text{V}\text{?}\text{1 (MgB}{}^{\mathrm{+}}\text{) = Δ}\text{V}\text{?}\text{1 (CaB}{}^{\mathrm{+}}\text{) = 38·4 ml mol}{}^{\mathrm{?1}}$ for the formation of MgB⁺ and CaB⁺. The volume change for the ionization of B(OH)₃ in NaCl and seawater was determined from the molal volume data. Values of $\text{Δ}\text{V}\text{?}\text{1 = ?29·2 and ?25·9 ml mol}{}^{\mathrm{?1}}$ were found in seawater and $\text{Δ}\text{V}\text{?}\text{1 = ?21·6 and ?26·4}$ in NaCl, respectively, at 0 and 25°C. The effect of pressure on the ionization of B(OH)₃ in NaCl and seawater at 0 and 25°C determined from the volume change is in excellent agreement with direct measurements in artificial seawater (culberson and Pytkowicz, 1968; Disteche and Disteche, 1967) and natural seawater (Culberson and Pytkowicz, 1968).     

Complex formation in the ternary system Mg(II)-CO2-H2O

The carbonato and hydrogencarbonato complexes of Mg²⁺ were investigated at 25 and 50° in solutions of the constant ClO₄^? molality (3 M) consisting preponderantly of NaClO₄. The experimental data could be explained assuming the following equilibria: Mg²⁺ + CO_2B + H₂O ag MgHCO⁺₃ + H⁺, $\text{log}{}^1\text{β}{}_1\text{= ?7.64}{}_4\text{± 0.01}{}_7\text{(25°), ?7.46}{}_2\text{± 0.01}{}_1\text{(50°)}$, Mg²⁺ + 2 CO_2g + 2 H₂Oag Mg(HCO₃)⁰₂ ± 2 H⁺, $\text{log}{}^1\text{β}{}_2\text{= ?15.00 ± 0.14 (25°)}$, ?15.37 ± 0.39 (50°), Mg²⁺ + CO_2g + H₂Oag MgCO⁰₃ + 2 H⁺, $\text{log}{}^1\text{k}{}_1\text{= ?15.64 ± 0.06 (25°)}$,?15.23 ± 0.02 (50°), with the assumption γ_MgCO3⁰ = γ_Mg(HCO3)⁰2, ΔG⁰(I = 0) for the reaction MgCO⁰₃ + CO_2g + H₂O = Mg(HCO₃)⁰₂ was estimated to be ?3.9₁ ± 0.8₆ and 0.6 ± 2.4 kJ/mol at 25 and 50°C, respectively. The abundance of carbonate linked Mg(II) species in fresh water systems is discussed.     

The solubility of BaCO3(cr) (witherite) in CO2-H2O solutions between 0 and 90°C,evaluation of the association constants of BaHCO3+(aq) and BaCO30(aq) between 5 and 80°C,and a preliminary evaluation of the thermodynamic properties of Ba2+(aq)

One hundred and fifty new measurements of the solubility of witherite were used to evaluate the equilibrium constant of the reaction BaCO₃(cr) = Ba²⁺(aq) + CO₃²⁻(aq) between 0 and 90°C and 1 atm total pressure. The temperature dependence of the equilibrium constant is given by logK = 607.642 + 0.121098T − 20011.25/T − 236.4948 logT where T is in degrees Kelvin. The logK of BaCO₃(cr), the Gibbs energy, the enthalpy and entropy of the reaction at 298.15 K are −8.562, 48.87 kJ · mol⁻¹, 2.94 kJ · mol⁻¹ and −154.0 J · mol⁻¹ · K⁻¹, respectively. The equilibrium constants are consistent with an aqueous model that includes the ion pairs BaHCO₃⁺(aq) and BaCO₃⁰(aq) Three different methods were used to evaluate the association constant of BaHCO₃⁺(aq), and all yielded similar results. The temperature dependence of the association constant for the reaction Ba²⁺(aq) + HCO₃⁻(aq) = BaHCO₃⁺(aq) is given by logK_BaHCO3⁺ = −3.0938 + 0.013669T.The log of the association constant, the Gibbs energy, the enthalpy and entropy of the reaction at 298.15°K are 0.982, −5.606 kJ · mol⁻¹, 23.26 kJ · mol⁻¹ and 96.8 J · mol⁻¹ · K⁻¹, respectively. The temperature dependence of the equilibrium constant for the reaction Ba²⁺(aq) + CO²⁻₃(aq) = BaCO₀³(aq) is given by logK_BaCO3⁰ = 0.113 + 0.008721T.The log of the association constant, the Gibbs energy, the enthalpy and entropy of the reaction at 298.15° K are 2.71, −15.49 kJ · mol⁻¹, 14.84 kJ · mol⁻¹ and 101.7 J· mol⁻¹ · K⁻¹.The above model leads to reliable calculations of the aqueous speciation and solubility of witherite in the system BaCO₃-CO₂-H₂O from 0 to more than 90°C. Literature data on witherite solubility were re-evaluated and compared with the results of this study.Problems in the thennodynamic selections of Ba compounds are considered. Newer data require the revision of Δ_fH° and Δ_fG° of Ba²⁺(aq) to −532.5 and −555.36 kJ · mol⁻¹, respectively, for agreement with solubility data.     

Solubility of Ca6[Al(OH)6]2(CrO4)3·26H2O,the chromate analog of ettringite; 5–75°C

Ca₆[Al(OH)₆]₂(CrO₄)₃·26H₂O, the chromate analog of the sulfate mineral ettringite, was synthesized and characterized by X-ray diffraction, Fourier transform infra-red spectroscopy, thermogravimetric analyses, energy dispersive X-ray spectrometry, and bulk chemical analyses. The solubility of the synthesized solid was measured in a series of dissolution and precipitation experiments conducted at 5–75°C and at initial pH values between 10.5 and 12.5. The ion activity product (IAP) for the reaction Ca₆[Al(OH)₆]₂(CrO₄)₃·26H₂O⇌6Ca²⁺+2Al(OH)⁻₄+3CrO²⁻₄+4OH⁻+26H₂O varies with pH unless a CaCrO_4(aq) complex is included in the speciation model. The log K for the formation of this complex by the reaction Ca²⁺+CrO²⁻₄=CaCrO_4(aq) was obtained by minimizing the variance in the IAP for Ca₆[Al(OH)₆]₂(CrO₄)₃·26H₂O. There is no significant trend in the formation constant with temperature and the average log K is 2.77±0.16 over the temperature range 5–75°C. The log solubility product (log K_SP) of Ca₆[Al(OH)₆]₂(CrO₄)₃·26H₂O at 25°C is −41.46±0.30. The temperature dependence of the log K_SP is log K_SP=A−B/T+D log(T) where A=498.94±48.99, B=27,499±2257, and D=−181.11±16.74. The values of ΔG⁰_r,298 and ΔH⁰_r,298 for the dissolution reaction are 236.6±3.9 and 77.5±2.4 kJ mol⁻¹. the values of ΔC⁰_P,r,298 and ΔS⁰_r,298 are −1506±140 and −534±83 J mol⁻¹ K⁻¹. Using these values and published standard state partial molal quantities for constituent ions, ΔG⁰_f,298=−15,131±19 kJ mol⁻¹, ΔH⁰_f,298=−17,330±8.6 kJ mol⁻¹, ΔS⁰₂₉₈=2.19±0.10 kJ mol⁻¹ K⁻¹, and ΔC⁰_Pf,298=2.12±0.53 kJ mol⁻¹ K⁻¹, were calculated.     

Thermodynamic functions of eskolaite Cr<Subscript>2</Subscript>O<Subscript>3</Subscript>(c) at 0–1800 K

The heat capacity of eskolaite Cr₂O₃(c) was determined by adiabatic vacuum calorimetry at 11.99–355.83 K and by differential calorimetry at 320–480 K. Experimental data of the authors and data compiled from the literature were applied to calculate the heat capacity, entropy, and the enthalpy change of Cr₂O₃ within the temperature range of 0–1800 K. These functions have the following values at 298.15 K: C _p ⁰ (298.15) = 121.5 ± 0.2 J K⁻¹mol⁻¹, S ⁰(298.15) = 80.95 ± 0.14 J K⁻¹mol⁻¹, and H ⁰(298.15)-H ⁰(0) = 15.30±0.02 kJ mol⁻¹. Data were obtained on the transitions from the antiferromagnetic to paramagnetic states at 228–457 K; it was determined that this transition has the following parameters: Neel temperature T _N = 307 K, Δ _tr S = 6.11 ± 0.12 J K⁻¹mol⁻¹ and δ _tr H = 1.87 ± 0.04 kJ mol⁻¹.     

Quantitative analysis of H2O and CO2 in cordierite using polarized FTIR spectroscopy

We report a FTIR (Fourier transform infrared) study of a set of cordierite samples from different occurrence and with different H₂O/CO₂ content. The specimens were fully characterized by a combination of techniques including optical microscopy, single-crystal X-ray diffraction, EMPA (electron microprobe analysis), SIMS (secondary ion mass spectrometry), and FTIR spectroscopy. All cordierites are orthorhombic Ccmm. According to the EMPA data, the Si/Al ratio is always close to 5:4; X _Mg ranges from 76.31 to 96.63, and additional octahedral constituents occur in very small amounts. Extraframework K and Ca are negligible, while Na reaches the values up to 0.84 apfu. SIMS shows H₂O up to 1.52 and CO₂ up to 1.11 wt%. Optically transparent single crystals were oriented using the spindle stage and examined by FTIR micro-spectroscopy under polarized light. On the basis of the polarizing behaviour, the observed bands were assigned to water molecules in two different orientations and to CO₂ molecules in the structural channels. The IR spectra also show the presence of small amounts of CO in the samples. Refined integrated molar absorption coefficients were calibrated for the quantitative microanalysis of both H₂O and CO₂ in cordierite based on single-crystal polarized-light FTIR spectroscopy. For H₂O the integrated molar coefficients for type I and type II water molecules (ν₃ modes) were calculated separately and are ^[I]ε?=?5,200?±?700?l?mol^?1?cm^?2 and ^[II]ε?=?13,000?±?3,000?l?mol^?1?cm^?2, respectively. For CO₂ the integrated coefficient is $ \varepsilon_{{{\text{CO}}_{ 2} }} $ ?=?19,000?±?2,000?l?mol^?1?cm^?2.     

The effect of cobalt ion on nucleation of calcium-carbonate polymorphs

Cobalt, like Mg, may cause the precipitation of aragonite rather than calcite in aqueous solutions due to the adsorption and crystal poisoning of calcite by a hydrated ion. Solutions containing NaCl and CaCl₂, having the ionic strength and Ca content of seawater (35‰ salinity), were spiked with known amounts of CoCl₂. Calcium carbonate was precipitated by the addition of 0.7 ml of 1 M Na₂CO₃. All experimental runs were made at 25°C, and all products were examined by X-ray diffraction. At low concentrations of Co (< 5·^?4M) calcite and vaterite formed. At concentrations from 5·10^?4 M to 2·10^?3M, the products consisted of combinations of calcite and vaterite; aragonite and calcite; aragonite and vaterite; calcite, vaterite and aragonite. In solutions of 3·10^?3M CoCl₂, most precipitates were aragonite with only one sample containing a small amount of calcite. All precipitates from 5·10^?3M CoCl₂ solutions either contained aragonite or were amorphous. Solutions with concentrations of 1 · 10^?2M CoCl₂ produced only amorphous precipitates. All precipitates contained an amorphous violet phase, assumed to be basic cobaltous carbonate (2CoCO₃·Co(OH)₂·H₂O).     

High temperature calorimetry of sulfide systems. I. Thermochemistry of liquid and solid phases of Ni + S

An experimental arrangement suitable for application of high temperature calorimetry to liquid sulfide systems has been developed. Using this approach, we have measured the integral enthalpies of mixing of Ni + NiS at 1100 K to form liquid alloys with compositions from X_Nis = 0.576 to X_Nis = 0.754. Partial enthalpies of the two components also were measured. After correcting for the enthalpies of fusion of Ni and NiS at 1100 K, the results of all measurements can be represented by an analytical expression which reflects subregular behavior of the mixing enthalpies ΔH_mix^l−1 = X_Nis²X_NiA + X_NisX_NiS²B with A = −97.712 kJ mol⁻¹ and B = −4.772 kJ mol⁻¹.The standard enthalpies of formation of the high and low temperature forms of NiS were evaluated from the calorimetrically measured enthalpy change associated with the reaction between nickel and sulfur at 1021 K. The standard enthalpies of formation of Ni₃S₂ (heazlewoodite), Ni₇S₆ and Ni_0.958S were determined from the enthalpy changes of reactions in which the compounds were formed from NiS and Ni at 873 K and 833 K. The standard enthalpy of formation of NiS₂(vaesite) was obtained from the enthalpy change of the reaction of NiS₂ with Ni to give NiS at 873 K. The following values are reported for the standard enthalpies of formation of the phases studied (in kJ mol⁻¹): ΔH_f,NiS(HT)⁰ = −88.1 ± 1.0 ΔH_{f, Ni0.958S}⁰ = −93.2 ± 0.7ΔH_f,Ni7S6⁰ = −582.8 ± 5.7 ΔH_f,NiS(LT)⁰ = −91.0 ± 1.0ΔH_f,Ni3S2(LT)⁰ = −217.2 ± 1.6 ΔH_f,NiS2⁰ = −124.9 ± 1.0.     

The high-temperature heat capacity of natural calcite (CaCO3)

The heat capacity (C _P) of a natural sample of calcite (CaCO₃) has been measured from 350 to 775 K by differential scanning calorimetry (DSC). Heat capacities determined for a powdered sample and a single-crystal disc are in close agreement and have a total uncertainty of ±1 percent. The following equation for the heat capacity of calcite from 298 to 775 K was fit by least squares to the experimental data and constrained to join smoothly with the low-temperature heat capacity data of Staveley and Linford (1969) (C _P in J mol^?1 K^?1, T in K): $$\begin{gathered} C_p = - 184.79 + 0.32322T - 3,688,200T^{ - 2} \hfill \\ {\text{ }} - (1.2974{\text{ }} \times {\text{ 10}}^{ - {\text{4}}} )T^2 + 3,883.5T^{ - 1/2} \hfill \\ \end{gathered} $$ Combining this equation with the S ₂₉₈ ⁰ value from Staveley and Linford (1969), entropies for calcite are calculated and presented to 775 K. A simple method of extrapolating the heat capacity function of calcite above 775 K is presented. This method provides accurate entropies of calcite for high-temperature thermodynamic calculations, as evidenced by calculation of the equilibrium: CaCO_{3 (s)}=CaO_(s)+CO_{2 (s)}.     

Standard Gibbs free energy of formation for Cu2O,NiO, CoO,and FexO: High resolution electrochemical measurements using zirconia solid electrolytes from 900–1400 K

Galvanic cells with oxygen-specific solid electrolytes made of calcia-stabilized zirconia have been used to make equilibrium measurements of the standard Gibbs free energy of formation, Δ_fG⁰_m,(T), for copper (I) oxide (Cu₂O), nickel (II) oxide (NiO), cobalt (II) oxide (CoO), and wüstite (Fe_xO) over the temperature range from 900–1400 K. The measured values of Δ_fG⁰_m at 1300 K are −73950, −123555, −142150, and −179459 J · mol⁻¹ for Cu₂O, NiO, CoO, and Fe_0.947O, respectively. The precision of these measurements is ± 30–60 J · mol⁻¹, and their absolute accuracy is estimated to be ± 100–200 J·mol⁻¹. Using values of –76.557, −94.895, −79.551, and −71.291 J · K⁻¹ · mol⁻¹ for the entropies of formation, Δ_fS_m⁰, (298.15 K), the calculated enthalpies of formation, Δ_fH_m⁰, (298.15 K), are −170508, −240110, −237390, and −266458 J · mol⁻¹ for Cu₂O, NiO, CoO, and Fe_0.947O, respectively. These values of Δ_fS_m⁰ (298.15 K) and Δ_fH_m⁰ (298.15 K) are in good agreement with the best available calorimetric measurements.     

The heat capacity of a natural monticellite and phase equilibria in the system CaO-MgO-SiO2-CO2

The heat capacity of a natural monticellite (Ca_1.00Mg_.09Fe_.91Mn_.01Si_0.99O_3.99) measured between 9.6 and 343 K using intermittent-heating, adiabatic calorimetry yields C_p⁰(298) and S₂₉₈⁰ of 123.64 ± 0.18 and 109.44 ± 0.16 J · mol⁻¹K⁻¹ respectively. Extrapolation of this entropy value to end-member monticellite results in an S⁰₂₉₈ = 108.1 ± 0.2 J · mol⁻¹K⁻¹. High-temperature heat-capacity data were measured between 340–1000 K with a differential scanning calorimeter. The high-temperature data were combined with the 290–350 K adiabatic values, extrapolated to 1700 K, and integrated to yield the following entropy equation for end-member monticellite (298–1700 K): S_T⁰(J · mol⁻¹K⁻¹) = S₂₉₈⁰ + 164.79 In T + 15.337 · 10⁻³T + 22.791 · 10⁵T⁻² − 968.94. Phase equilibria in the CaO-MgO-SiO₂ system were calculated from 973 to 1673 K and 0 to 12 kbar with these new data combined with existing data for akermanite (Ak), diopside (Di), forsterite (Fo), merwinite (Me) and wollastonite (Wo). The location of the calculated reactions involving the phases Mo and Fo is affected by their mutual solid solution. A best fit of the thermodynamically generated curves to all experiments is made when the S⁰₂₉₈ of Me is 250.2 J · mol⁻¹ K⁻¹ less than the measured value of 253.2 J · mol⁻¹ K⁻¹.A best fit to the reversals for the solid-solid and decarbonation reactions in the CaO-MgO-SiO₂-CO₂ system was obtained with the ΔG⁰₂₉₈ (kJ · mole⁻¹) for the phases Ak(−3667), Di(−3025), Fo(−2051), Me(−4317) and Mo(−2133). The two invariant points − Wo and −Fo for the solid-solid reactions are located at 1008 ± 5 K and 6.3 ± 0.1 kbar, and 1361 ± 10 K and 10.2 ± 0.2 kbar respectively. The location of the thermodynamically generated curves is in excellent agreement with most experimental data on decarbonation equilibria involving these phases.     

Electron spin resonance of Mn2+ in sodalite

Electron spin resonance of allowed (Δm=0) and forbidden (Δm=±1) hyperfine transitions of Mn²⁺ in sodalite, Na₈(Al₆Si₆O₂₄)Cl₂, is reported. No fine structure other than the central M=∣+1/2>?∣?1/2> transition is observed. From intensity ratios of forbidden to allowed transitions and doubling of allowed lines in powder spectra the crystal field parameter |D| was estimated as equal to (8±5) 10^?3 cm^?1. The g-value for the spectrum was obtained as equal to 2.0033±0.0005. The hyperfine structure constant |A| was 83±1 gauss, equal to (77±1) 10^?4 cm^?1.     

Thio complexes of gold and the transport of gold in hydrothermal ore solutions

The solubility of gold in aqueous sulphide solutions has been determined from pH_20°C ≈ 4 to pH_20°C ≈ 9.5 in the presence of a pyrite-pyrrhotite redox buffer at temperatures from 160 to 300°C and 1000 bar pressure. Maximum solubilities were obtained in the neutral region of pH as, for example, with m_NaHS = 0.15 m, pH_20°C = 5.96, T = 309°C, P = 1000 bar where a gold solubility of 225 mg/kg was obtained. It was concluded that three thio gold complexes contributed to the solubility. The complex Au₂(HS)₂S^2? predominated in alkaline solution, the Au(HS)₂^? complex occurred in the neutral pH region, and in the acid pH region, it was concluded with less certainty that the Au(HS)° complex was present. Formation constants calculated forAu₂(HS)₂S^2? and Au (HS)₂^? emphasize their high stability. In the temperature range from 175 to 250°C, values of pβ for Au₂(HS)₂S^2? vary from ?53.0 to 47.9 (±1.6) and from ?23.1 to ?19.5 ( ± 1.5) for Au(HS)₂^?. Equilibrium constante for the dissolution reactions, $\text{Au° + H}{}_2\text{S + HS}{}^{\mathrm{?}}\text{? Au(HS)}{}_2{}^{\mathrm{?}}\text{+}\text{1}\text{2}\text{H}{}_2$ and 2Au° + H₂S + 2H8^? ? Au(HS)₂^? + H₂ vary from pK_m = +2.4 to +2.55 (±0.10) for Au₂(HS)₂S^2? and from pK_n = + 1.29 to + 1.19 (±0.10) for Au(HS)₂^? over the temperature range 175 to 250°C. Enthalpies of these dissolution reactions were calculated to be ΔH_m° = ?5.2 ±2.0 kcal/mol and ΔH_n° = +1.7 ±2.0 kcal/mol respectively. It was concluded that gold is probably transported in hydrothermal ore solutions as both thio and chloro complexes and may be deposited in response to changes in temperature, pressure, pH, oxidation potential of the system and total sulphur concentration.