Thermochemistry of forsterite-fayalite olivine solutions

The enthalpies of solution of synthetic Mg₂SiO₄-Fe₂SiO₄ olivine solid solutions have been measured in Pb₂B₂O₅ melt at 970 K. The heat of solution of forsterite was found to be 15.62 ± 0.3 kcal mol^?1 and that of fayalite 9.39 ± 0.14 kcal mol^?1. Solid solutions between these end-members exhibit small positive deviations from mixing ideality, asymmetric towards the Fe end-member. In terms of the sub-regular solution model, excess enthalpies of intermediate olivine are adequately represented by the equation H^xs = 2(1000 + 1000X_Fe) X_FeX_MgThe enthalpies of solution at 970 K are consistent with high temperature phase equilibrium measurements of activity-composition relationships in the olivine series. Excess entropy terms are not needed to relate the phase equilibrium data to the calorimetric data presented here.The enthalpy of solution of FeSiO₃ ferrosilite at 970 K was found to be 4.36 ± 0.10 kcal mol^?1. This value, when taken together with calorimetric measurements on fayalite and quartz, is consistent with phase equilibrium investigations of the reaction: 2FeSiO₃ = Fe₂SiO₄ + SiO₂ Ferrosilite Fayalite QuartzThese provide a check on the internal consistency of the calorimetric data presented here.     

Amorphous silica solubilities—I. Behavior in aqueous sodium nitrate solutions; 25–300°C, 0–6 molal

The solubility of amorphous silica was obtained in aqueous sodium nitrate solutions up to six molal and at temperatures from 25 to 300°C. It was expected that solubilities in aqueous sodium chloride solutions would be similar. At 25°C, the solubility of amorphous silica is lowered from that in water to 0.00086 m in 6.12 m sodium nitrate, or a decrease of 60%. At 300°C, the corresponding decrease is only 27% from a solubility of 0.0269 m in H₂O. From the change in solubility with temperature at a given constant molality of sodium nitrate, the molal heat of solution over the range, 100 to 300°C, increases from + 2.93 kcal mol^?1 in water to + 3.64 kcal mol^?1 in 6m sodium nitrate. The value approaches a constant of +3.8 kcal mol^?1 as sodium nitrate approaches saturation at 10.8 molal.     

The thermodynamic properties and phase relations of some minerals in the system CaO-AI2O3-SiO2-H2O

The heat capacities of lawsonite, margante, prehnite and zoisite have been measured from 5 to 350 K with an adiabatic-shield calorimeter and from 320 to 999.9 K with a differential-scanning calorimeter. At 298.15 K, their heat capacities, corrected to end-member compositions, are 66.35, 77.30, 79.13 and 83.84 cal K^?1 mol^?1; their entropies are 54.98, 63.01, 69.97 and 70.71 cal K^?1 mol^?1, respectively. Their high-temperature heat capacities are described by the following equations (in calories, K, mol): Lawsonite (298–600 K): Cp° = 66.28 + 55.95 × 10^?3T ? 15.27 × 10⁵T^?2 Margarite (298–1000 K): Cp° = 101.83 + 24.17 × 10^?3T ? 30.24 × 10⁵T^?2 Prehnite (298–800 K): Cp° = 97.04 + 29.99 × 10^?3T ? 25.02 × 10⁵T^?2 Zoisite (298–730 K): Cp° = 98.92 + 36.36 × 10^?3T ? 24.08 × 10⁵T^?2 Calculated Clapeyron slopes for univariant equilibria in the CaO-Al₂O₃-SiO₂-H₂O system compare well with experimental results in most cases. However, the reaction zoisite + quartz = anorthite + grossular + H₂O and some reactions involving prehnite or margarite show disagreements between the experimentally determined and the calculated slopes which may possibly be due to disorder in experimental run products. A phase diagram, calculated from the measured thermodynamic values in conjunction with selected experimental results places strict limits on the stabilities of prehnite and assemblages such as prehnite + aragonite, grossular + lawsonite, grossular + quartz, zoisite + quartz, and zoisite + kyanite + quartz. The presence of this last assemblage in eclogites indicates that they were formed at moderate to high water pressure.     

Enthalpy effects associated with Al/Si ordering in anhydrous Mg-cordierite

Measurements of the heats of solution (ΔH_soln) in molten Pb₂B₂O₅ at 708°C of anhydrous magnesian cordierites, prepared with a range of structural states, show that the enthalpy effect associated with Al/Si ordering is substantial (? 9.76 ± 1.56 kcal mole^?1). Differences in the state of order between synthetic cordierites used in phase equilibrium studies and cordierites in the natural environment could lead to significant errors in the estimation of palaeo-pressures and temperatures. A continuous change of ΔH_soln with annealing time supports the suggestion of putnis (1980) that the hexagonal → orthorhombic transformation in cordierite, which can occur via a modulated structure, is truly continuous under metastable conditions. In addition, a linear relation between ΔH_soln and the logarithm of annealing time has been found, which provides some insight into the nature of the ordering mechanisms at an atomic level. Al and Si exchanges occur continuously between neighbouring tetrahedral sites with a net drift towards increasing order. No kinetic or thermochemical distinction can be made between the development of long range and short range order.The enthalpy of vitrification (~ 12 kcal mole^?1) for a metastable stuffed β-quartz polymorph of cordierite composition is similar to that for pure quartz (on a per two oxygen basis), while the heat of vitrification for even the most disordered cordierite seen in this study is more than a factor of three greater (~40 kcal mole^?1). This is consistent with the view that cordierite glass resembles the quartz structure more closely than the crystalline cordierite structure, and that crystallisation of the glass below ~900°C is controlled by a tetrahedral framework.     

Oxygen diffusion in amphiboles

The kinetics of oxygen isotope self-diffusion in natural samples of hornblende, tremolite, and richterite have been measured. Samples were run under hydrothermal conditions using ¹⁸O enriched water. Profiles of ${}^{18}\text{O}\text{(}{}^{16}\text{O +}{}^{18}\text{O)}$vs depth into the crystal were obtained using an ion microprobe; the depths of sputtered holes were measured using an optical interferometer. At 1000 bars (100 MPa) water pressure, the activation energies (Q) and pre-exponential factors (D₀) for diffusion parallel to c are: D₀(cm²/sec) Q (kcal/gm-atom) T (°C) Hornblende 1⁺²⁰_?1 × 10^?741 ± 6 650–800 Tremolite 2⁺³⁰_?2× 10^?8 39 ± 5 650–800 Richterite 3⁺⁵_?2 × 10^?4 57 ± 2 650–800The diffusion coefficient (D) for hornblende at 800°C and 1000 bars water pressure measured parallel to the c crystallographic direction is at least ten times greater than that parallel to the a or b directions. An increase in water pressure from 200 to 2000 bars increases D by a factor of 2.7 for diffusion parallel to c at 800°C. The D value for hornblende at 800°C is about 0.01 that for quartz and 0.001 that for anorthite. As a result, closure temperatures for oxygen exchange in natural primary amphiboles are significantly higher than for quartz or feldspars. It is unlikely that amphiboles will exchange oxygen isotopes by diffusion under most crustal conditions.     

Thermodynamics and crystal chemistry of the hematite–corundum solid solution and the FeAlO3 phase

High-temperature oxide-melt calorimetry and Rietveld refinement of powder X-ray diffraction patterns were used to investigate the energetics and structure of the hematite–corundum solid solution and ternary phase FeAlO₃ (with FeGaO₃ structure). The mixing enthalpies in the solid solution can be described by a polynomial ΔH_mix=WX _hem(1?X _hem) with W=116 ± 10 kJ mol^?1. The excess mixing enthalpies are too positive to reproduce the experimental phase diagram, and excess entropies in the solid solution should be considered. The hematite–corundum solvus can be approximately reproduced by a symmetric, regular-like solution model with ΔG _excess=(W _H ?TW _S )X _hem X _cor, where W _H= 116 ± 10 kJ mol^?1 and W _S =32 ± 4 J mol^?1 K^?1. In this model, short-range order (SRO) of Fe/Al is neglected because SRO probably becomes important only at intermediate compositions close to Fe:Al=1:1 but these compositions cannot be synthesized. The volume of mixing is positive for Al-hematite but almost ideal for Fe-corundum. Moreover, the degree of deviation from Vegard's law for Al-hematite depends on the history of the samples. Introduction of Al into the hematite structure causes varying distortion of the hexagonal network of oxygen ions while the position of the metal ions remains intact. Distortion of the hexagonal network of oxygen ions attains a minimum at the composition (Fe_0.95Al_0.05)₂O₃. The enthalpy of formation of FeAlO₃ from oxides at 298 K is 27.9 ± 1.8 kJ mol^?1. Its estimated standard entropy (including configurational entropy due to disorder of Fe/Al) is 98.9 J mol^?1 K^?1, giving the standard free energy of formation at 298 K from oxides and elements as +19.1 ± 1.8 and ?1144.2 ± 2.0 kJ mol^?1, respectively. The heat capacity of FeAlO₃ is approximated as C _p (T in K)= 175.8 ? 0.002472T ? (1.958 × 10⁶)/T ²? 917.3/T ^0.5+(7.546 × 10^?6) T ² between 298 and 1550 K, based on differential scanning calorimetric measurements. No ferrous iron was detected in FeAlO₃ by Mössbauer spectroscopy. The ternary phase is entropy stabilized and is predicted to be stable above about 1730 ± 70 K, in good agreement with the experiment. Static lattice calculations show that the LiNbO₃-, FeGaO₃-, FeTiO₃-, and disordered corundum-like FeAlO₃ structures are less stable (in the order in which they are listed) than a mechanical mixture of corundum and hematite. At high temperatures, the FeGaO₃-like structure is favored by its entropy, and its stability field appears on the phase diagram.     

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.     

Comparison of isoleucine epimerization in a model dipeptide and fossil protein

The dipeptide isoleucyl-glycine in aqueous solution has activation energies for isoleucine epimerization and hydrolysis of 23.1 and 20.9 kcal mol^?1, respectively. The activation energy for epimerization of NH₂-terminal isoleucine is 5–8 kcal mol^?1 less than activation energies reported for epimerization of isoleucine in the free state or in various protein systems. Furthermore, the rate of isoleucine epimerization in the NH₂-terminal position exceeds the rate of hydrolysis at temperatures between 0 and 152°C. Consequently, a high rate of epimerization in fossils should only occur when isoleucine is in the terminal position. Partially epimerized isoleucine will be converted through hydrolysis from the terminal form to the slower epimerizing free isoleucine. Over the course of the total isoleucine epimerization reaction, either in dipeptides or proteins, the activation energy will increase and the reaction rate will decrease as terminal isoleucine is converted to free isoleucine. This process may explain the non-linear kinetics observed for isoleucine epimerization in carbonate fossils.The degree of isoleucine epimerization is low in different molecular weight fractions of a fossil protein from Mercenaria in all fractions greater than 500 mol. wt, where most isoleucine is likely to be in the interior position. In the less than 500 mol. wt peptide fraction, where a considerable portion of the isoleucine is likely to be in the terminal position, the degree of epimerization is significantly greater. These analytical results from fossil protein support the interpretation of isoleucine kinetics obtained by study of dipeptides.     

Comparative study of the absorptive potential of raw and activated carbon <Emphasis Type="Italic">Acacia nilotica</Emphasis> for Reactive Black 5 dye

Acacia nilotica was used for the adsorption of Reactive Black 5 (RB5) dye from an aqueous solution. Both the raw and activated (with H₃PO₄) carbon forms of Acacia nilotica (RAN and ANAC, respectively) were used for comparison. Various parameters (including dye concentration, contact time, temperature, and pH) were optimized to obtain the maximum adsorption capacity. RAN and ANAC were characterized using Fourier transform infrared spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The maximum experimental adsorption capacities for RAN and ANAC were 34.79 and 41.01 mg g^?1, respectively, which agreed with the maximum adsorption capacities predicted by the Langmuir, Freundlich, and Dubinin–Radushkevich equilibrium isotherm models. The adsorption data of ANAC showed a good fit to the isotherm models based on the coefficient of determination (R ²): Langmuir type II (R ² = 0.99) > Freundlich (R ² = 0.9853) > Dubinin–Radushkevich (R ² = 0.9659). This result suggested monolayer adsorption of RB5 dye. The adsorption of RB5 dye followed pseudo-second-order kinetics. The RAN adsorbent reflected an exothermic reaction (enthalpy change, ΔH = ?0.006 kJ mol^?1) and increased randomness (standard entropy change, ΔS = 0.038 kJ mol^?1) at the solid–solution interface. In contrast, ANAC reflected both exothermic [?0.011 kJ mol^?1 (303–313 K)] and endothermic [0.003 kJ mol^?1 (313–323 K)] reactions. However, the ΔS value of ANAC was lower when the RB5 adsorption increased from 313 to 323 K. The negative values for the Gibbs free energy change at all temperatures indicated that the adsorption of RB5 dye onto RAN and ANAC was spontaneous in the forward direction.     

Enthalpy of diaplectic labradorite glass

Enthalpies of solution in molten 2PbO·B₂O₃ at ～988 K have been measured for diaplectic labradorite glass from the Manicouagan impact crater and a fused glass formed from the same material. The enthalpies of solution of the diaplectic and fusion-formed glasses are 4,347 and 2,023 cal mol^?1, respectively. The more endothermic enthalpy of solution of the diaplectic glass indicates a greater relative energetic stability of about 2.3 kcal mol^?1. The data are consistent with Diemann and Arndt's (1984) structural model that suggests the diaplectic glass is more ordered than fusion-formed glass and with the presence of crystallites. Comparison of data to enthalpies of solution of crystalline labradorite (Carpenter et al. 1985) indicates a maximum percentage of crystalline relics of ～15–18%, also consistent with Diemann and Arndt's (1984) estimate of <17%. Thus the diaplectic glass is intermediate in thermochemical properties between normal glass and crystal (much closer to glass) and does not represent any state more unstable than normal fusion-formed glass.     

Cu and Fe chalcopyrite leach activation energies and the effect of added Fe

The leaching kinetics of chalcopyrite (CuFeS₂) concentrate in sulfuric acid leach media with and without the initial addition of Fe³⁺ under carefully controlled solution conditions (E_h 750 mV SHE, pH 1) at various temperatures from 55 to 85 °C were measured. Kinetic analyses by (i) apparent rate (not surface area normalised), and rate dependence using (ii) a shrinking core model and (iii) a shrinking core model in conjunction with Fe³⁺ activity, were performed to estimate the activation energies (E_a) for Cu and Fe dissolution.The E_a values determined for Cu and Fe leaching in the absence of added Fe³⁺ are within experimental error, 80 ± 10 kJ mol⁻¹ and 84 ± 10 kJ mol⁻¹, respectively (type iii analyses E_a are quoted unless stated otherwise), and are indicative of a chemical reaction controlled process. On addition of Fe³⁺ the initial Cu leach rate (up to 10 h) was increased and Cu was released to solution preferentially over Fe, with the E_a value of 21 ± 5 kJ mol⁻¹ (type ii analysis) suggestive of a transport controlled rate determining process. However, the rate of leaching rapidly decreased until it was consistently slower than for the equivalent leaches where Fe³⁺ was not added. The resulting E_a value for this leach regime of 83 ± 10 kJ mol⁻¹ is within experimental error of that determined in the absence of added Fe³⁺. In contrast to Cu release, Fe release to solution was consistent with a chemical reaction controlled leach rate throughout. The Fe release E_a of 76 ± 10 kJ mol⁻¹ is also within experimental error of that determined in the absence of added Fe³⁺. Where type (ii) and (iii) analyses were both successfully carried out (in all cases except for Cu leaching with added Fe³⁺, <10 h) the E_a derived are within experimental error. However, the type (iii) analyses of the leaches in the presence of added Fe³⁺ (>10 h), as compared to in the absence of added Fe³⁺, returned a considerably smaller pre-exponential factors for both Cu and Fe leach analyses commensurate with the considerably slower leach rate, suggestive of a more applicable kinetic analysis.XPS examination of leached chalcopyrite showed that the surface concentration of polysulfide and sulfate was significantly increased when Fe³⁺ was added to the leach liquor. Complementary SEM analysis revealed the surface features of chalcopyrite, most likely due to the nature of the polysulfide formed, are subtly different with greater surface roughness upon leaching in the absence of added Fe³⁺ as compared to a continuous smooth surface layer formed in the presence of added Fe³⁺. These observations suggest that the effect of Fe³⁺ addition on the rate of leaching is not due to the change in the chemical reaction controlled mechanism but due to a change in the available surface area for reaction.     

Reaction overstepping and re-evaluation of peak P‒T conditions of the blueschist unit Sifnos,Greece: implications for the Cyclades subduction zone

ABSTRACT

Equilibrium thermodynamic modelling, quartz in garnet (QuiG) Raman geobarometry, and modelling of garnet nucleation at overstepped conditions were applied to three garnet-bearing blueschists from a 1.5 km-long transect across the eclogite-blueschist unit in Sifnos, Greece, in order to evaluate the accuracy of P?T conditions calculated via equilibrium thermodynamics. QuiG barometry uses the Raman shift of quartz inclusions in garnet to estimate the pressure of garnet nucleation and is independent of chemical equilibrium. Garnet nucleation temperatures were estimated by determining the stability field of the palaeo-assemblage inferred from garnet inclusion suites on mineral assemblage diagrams calculated in the MnNCKFMASH system and on temperatures obtained from Zr in rutile thermometry. These conditions were then compared to P?T conditions calculated at the equilibrium garnet isograd, and the method of intersecting isopleths. The P?T conditions calculated with intersecting garnet isopleths over- and underestimated the temperature of nucleation in samples SPH99-1a and SPH99-7, respectively, whereas they significantly underestimated nucleation pressure in SPH99-5. Nucleation of garnet in SPH99-1a at 12 kbar and ~484°C requires overstepping of ~6 kbar and a reaction affinity of 2.2 kJ mol^?1 O. SPH99-5 requires overstepping of ~8 kbar with garnet reaction affinities of at least 2.0 kJ mol^?1 O at 15 kbar and ~520°C. SPH99-7 requires overstepping of approximately 15 kbar and affinities of about 2.0–2.4 kJ mol^?1 O at ~23 kbar and ~530°C. The geotherms calculated from SPH99-7 (~6.7°C km^?1) and SPH99-5 (9.8°C km^?1) are in accordance with previous studies. The geotherm calculated from SPH99-1a, however, is warmer (11.3°C km^?1), and could reflect changes in the rate of subduction or differences in structural position within the down-going slab. The 10 kbar pressure difference between SPH99-7 and SPH99-1a can be explained by thrusting and accretion of thin slices of underplated wedge material facilitated by slab rollback and gravitational collapse.     

Gibbsite solubility and thermodynamic properties of hydroxy-aluminum ions in aqueous solution at 25°C

Solubility curves were determined for a synthetic gibbsite and a natural gibbsite (Minas Gerais, Brazil) from pH 4 to 9, in 0.2% gibbsite suspensions in 0.01 M NaNO₃ that were buffered by low concentrations of non-complexing buffer agents. Equilibrium solubility was approached from oversaturation (in suspensions spiked with Al(NO₃)₃ solution), and also from undersaturation in some synthetic gibbsite suspensions. Mononuclear Al ion concentrations and pH values were periodically determined. Within 1 month or less, data from over-and undersaturated suspensions of synthetic gibbsite converged to describe an equilibrium solubility curve. A downward shift of the solubility curve, beginning at pH 6.7, indicates that a phase more stable than gibbsite controls Al solubility in alkaline systems. Extrapolation of the initial portion of the high-pH side of the synthetic gibbsite solubility curve provides the first unified equilibrium experimental model of Al ion speciation in waters from pH 4 to 9.The significant mononuclear ion species at equilibrium with gibbsite are Al³⁺, AlOH²⁺, Al(OH)⁺₂ and Al(OH)^?₄, and their ion activity products are $\text{1K}{}_{50}\text{= 1.29 × 10}{}^8$, $\text{1K}{}_{\mathrm{s1}}\text{= 1.33 × 10}{}^3$, $\text{1K}{}_{\mathrm{s2}}\text{= 9.49 × 10}{}^{\mathrm{?3}}$ and $\text{1K}{}_{\mathrm{s4}}\text{= 8.94 × 10}{}^{\mathrm{?15}}$. The calculated standard Gibbs free energies of formation (ΔG°_f) for the synthetic gibbsite and the A1OH²⁺, Al(OH)⁺₂ and Al(OH)^?₄ ions are ?276.0, ?166.9, ?216.5 and ?313.5 kcal mol^?1, respectively. These ΔG°_f values are based on the recently revised ΔG°_f value for Al³⁺ (?117.0 ± 0.3 kcal mol_?1) and carry the same uncertainty. The ΔG°_f of the natural gibbsite is ?275.1 ± 0.4 kcal mol^?, which suggests that a range of ΔG°_f values can exist even for relatively simple natural minerals.     

The behavior of apatite during crustal anatexis: Equilibrium and kinetic considerations

The solubility and dissolution kinetics of apatite in felsic melts at 850°–1500°C have been examined experimentally by allowing apatite crystals to partially dissolve into apatite-undersaturated melts containing 0–10 wt% water. Analysis of P and Ca gradients in the crystal/melt interfacial region enables determination of both the diffusivities and the saturation levels of these components in the melt. Phosphorus diffusion was identified as the rate-limiting factor in apatite dissolution. Results of four experiments at 8 kbar run in the virtual absence of water yield an activation energy (E) for P diffusion of 143.6 ± 2.8 kcal-mol^?1 and frequency factor (D₀) of 2.23^+2.88_?1.26 × 10⁹cm²-sec^?1. The addition of water causes dramatic and systematic reduction of both E and D₀ such that at 6 wt% H₂O the values are ~25 kcal-mol^?1 and 10^?5 cm²-sec^?1, respectively. At 1300°C, the diffusivity of P increases by a factor of 50 over the first 2% of water added to the melt, but rises by a factor of only two between 2 and 6%, perhaps reflecting the effect of a concentration-dependent mechanism of H₂O solution. Calcium diffusion gradients do not conform well to simple diffusion theory because the release of calcium at the dissolving crystal surface is linked to the transport rate of phosphorus in the melt, which is typically two orders of magnitude slower than Ca. Calcium chemical diffusion rates calculated from the observed gradients are about 50 times slower than calcium tracer diffusion.Apatite solubilities obtained from these experiments, together with previous results, can be described as a function of absolute temperature (T) and melt composition by the expression: In D^apatite/melt_P = [(8400 + ((SiO₂ ? 0.5)2.64 × 10⁴))/T] ? [3.1 + (12.4(SiO₂ ? 0.5))] where SiO₂ is the weight fraction of silica in the melt. This model appears to be valid between 45% and 75% SiO₂, 0 and 10% water, and for the range of pressures expected in the crust.The diffusivity information extracted from the experiments can be directly applied to several problems of geochemical interest, including I) dissolution times for apatite during crustal anatexis, and 2) pileup of P, and consequent local saturation in apatite, at the surfaces of growing major-mineral phases.     

The solubility of calcite and aragonite in seawater of 35%. salinity at 25°C and atmospheric pressure

The solubilities of synthetic, natural and biogenic aragonite and calcite, in natural seawater of 35%. salinity at 25°C and 1 atm pressure, were measured using a closed system technique. Equilibration times ranged up to several months. The apparent solubility constant determined for calcite of 4.39(±0.20) × 10^?7 mol² kg^?2 is in good agreement with other recent solubility measurements and is constant after 5 days equilibration. When we measured aragonite solubility we observed that it decreased with increasing time of equilibration. The value of 6.65(±0.12) × 10^?7 mol² kg^?2, determined for equilibration times in excess of 2 months, is significantly less than that found in other recent measurements, which employed equilibration times of only a few hours to days. No statistically significant difference was found among the synthetic, natural and biogenic material. Solid to solution ratio, contamination of aragonite with up to 10 wt% calcite and recycling of the aragonite made no statistically significant difference in solubility when long equilibration times were used.Measured apparent solubility constants of aragonite and calcite are respectively 22( ± 3)% and 20( ± 2)% less than apparent solubility constants calculated from thermodynamic equilibrium constants and seawater total activity coefficients. These large differences in measured and calculated apparent solubility constants may be the result of the formation of surface layers of lower solubility than the bulk solid.     

Selective leaching of antimony and arsenic from mechanically activated tetrahedrite,jamesonite and enargite

In this study, the changes in surface area, morphology and leachability of antimony and arsenic from tetrahedrite, jamesonite and enargite mechanically activated by a high-energy planetary mill were investigated. It appears that the leaching of antimony from tetrahedrite and jamesonite and arsenic from enargite in alkaline solution of sodium sulphide is temperature-sensitive reaction. The temperature dependencies of all reactions were investigated in the interval 313–363 K. Resulting experimental activation energies were E_a = 111–182 kJ mol^− 1 for mechanically activated minerals. The values of E_a are characteristic for processes controlled by surface chemical reactions.     

Lead orthophosphates—II. Stability of cholopyromophite at 25°c

The conversion of secondary lead orthophosphate [PbHPO₄] into chloropyromorphite [Pb₅(PO₄)₃Cl] in ca. 10^?1 M NaCl solutions has been investigated at 25°C. From the composition of the supernatant solutions, the solubility product constant for Pb₅(PO₄)₃Cl has been calculated to be 10^?84.4±0.1, corresponding to $\text{ΔG}{}_{\mathrm{?}}\text{°}$ of ?906.2 kcal mol^?1. The solution equilibria and phase relationships in the system PbCl₂-PbO-P₂O₈-H₂O are discussed along with the geological implications.     

Insight into adsorption equilibrium,kinetics and thermodynamics of lead onto alluvial soil

In the present study, adsorption of lead (II) ions from aqueous solution by alluvial soil of Bhagirathi River was investigated under batch mode. The influence of solution pH, sorbent dose, initial lead (II) concentration, contact time, stirring rate and temperature on the removal process were investigated. The lead adsorption was favored with maximum adsorption at pH 6.0. Sorption equilibrium time was observed in 60 min. The equilibrium adsorption data were analyzed by the Freundlich, Langmuir, Dubinin–Radushkevich and Temkin adsorption isotherm models. The kinetics of lead (II) ion was discussed by pseudo first-order, pseudo second-order, intra-particle diffusion, and surface mass transfer models. It was shown that the adsorption of lead ions could be described by the pseudo second-order kinetic model. The activation energy of the adsorption process (E _a) was found to be ?38.33 kJ mol^?1 using the Arrhenius equation, indicating exothermic nature of lead adsorption onto alluvial soil. Thermodynamic parameters, such as Gibbs free energy (?G ⁰), the enthalpy (?H ⁰), and the entropy change of sorption (?S ⁰) have also been evaluated and it has been found that the adsorption process was spontaneous, feasible, and exothermic in nature. The results indicated that alluvial soil of Bhagirathi River can be used as an effective and low cost adsorbent to remove lead ions from aqueous solutions.     

Solubility and near-equilibrium dissolution rates of quartz in dilute NaCl solutions at 398-473 K under alkaline conditions

The dissolution-precipitation of quartz controls porosity and permeability in many lithologies and may be the best studied mineral-water reaction. However, the rate of quartz-water reaction is relatively well characterized far from equilibrium but relatively unexplored near equilibrium. We present kinetic data for quartz as equilibrium is approached from undersaturation and more limited data on the approach from supersaturated conditions in 0.1 molal NaCl + NaOH + NaSiO(OH)₃ solutions with pH 8.2-9.7 at 398, 423, 448, and 473 K. We employed a potentiometric technique that allows precise determination of solution speciation within 2 kJ mol⁻¹ of equilibrium without the need for to perturb the system through physical sampling and chemical analysis. Slightly higher equilibrium solubilities between 423 and 473 K were found than reported in recent compilations. Apparent activation energies of 29 and 37 kJ mol⁻¹ are inferred for rates of dissolution at two surface sites with different values of connectedness: dissolution at Q¹ or Q² silicon sites, respectively. The dissolution mechanism varies with ΔG such that reactions at both sites control dissolution up until a critical free energy value above which only reactions at Q¹ sites are important. When our near-equilibrium dissolution rates are extrapolated far from equilibrium, they agree within propagated uncertainty at 398 K with a recently published model by Bickmore et al. (2008). However, our extrapolated rates become progressively slower than model predictions with increasing temperature. Furthermore, we see no dependence of the postulated Q¹ reaction rate on pH, and a poorly-constrained pH dependence of the postulated Q² rate. Our slow extrapolated rates are presumably related to the increasing contribution of dissolution at Q³ sites far from equilibrium. The use of the potentiometric technique for rate measurement will yield both rate data and insights into the mechanisms of dissolution over a range of chemical affinity. Such measurements are needed to model the evolution of many natural systems quantitatively.     

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.