Lattice dynamics of BaFBr

A normal coordinate and force constant calculation for BaFBr was carried out on the basis of the rigid ion model. There is a strong anisotropy of the effective dynamical charges of the barium, fluoride, and bromide ions, which are 1.57e, ?0.78e, and ?0.79e for the degenerate vibrations and 1.04e, ?0.71e, and ?0.33e for the vibrations in the z-direction. In accordance with the ionic nature of BaFBr the short range force constants have only small values.     

Surface complexation modeling of Co(II) adsorption on mixtures of hydrous ferric oxide, quartz and kaolinite

Co sorption was measured as a function of pH, ionic strength (0.001-0.1 M NaNO₃) and sorbate/sorbent ratio on pure quartz, HFO and kaolinite and on binary and ternary mixtures of the three solids. Sorption data measured for the pure solids were used to derive internally-consistent diffuse layer surface complexation model (DLM) stability constants for Co sorption. Co sorption on HFO could be adequately modeled over a broad range of ionic strengths and sorbate/sorbent ratios with a two variable-charge site model. Fits based on a single variable-charge site model were reasonable, but were improved by using ionic-strength dependent stability constants. A single variable-charge site model with an additional permanent ion exchange site produced the best fit to Co edges measured on kaolinite over a range of ionic strength and sorbate/sorbent ratios. These DLM fits were also improved by using ionic-strength dependent stability constants. The DLM approach could not adequately describe the slope of Co sorption edges on quartz. This study demonstrates that for accurate prediction of Co sorption over wide ranges of ionic strength and sorbate/sorbent ratio, the DLM may require ionic-strength dependent stability constants. DLM stability constants for Co sorption derived for the pure solids were used to predict sorption as a function of pH and solid concentration on binary and ternary mixtures of the three solids. Discrepancies between predictions and measurements were quantitatively similar to those observed for the pure mineral systems. Thus, a simple component additivity approach provides useful predictions of metal sorption in the mixed solid systems.     

Dielectric constants of apatite,epidote, vesuvianite,and zoisite,and the oxide additivity rule

The dielectric constants and dielectric loss values of 4 Ca-containing minerals were determined at 1 MHz using a two-terminal method and empirically determined edge corrections. The results are: vesuvianitel κ′ _a=9.93 tan δ=0.006 κ′ _c=9.79 tan δ=0.005 vesuvianitel κ′ _a=10.02 tan δ=0.002 κ′ _c=9.85 tan δ=0.003 zoisite1 κ′ _a =10.49 tan δ=0.0006 κ′ _b =15.31 tan δ=0.0008 κ′ _c=9.51 tan δ=0.0008 zoisite2 κ′ _a =10.55 tan δ=0.0011 κ′ _b =15.45 tan δ=0.0013 κ′ _c=9.39 tan δ=0.0008 epidote κ′ ₁₁= 9.52 tan δ=0.0008 κ′ ₂₂=17.1 tan δ=0.0009 κ′ ₃₃= 9.37 tan δ=0.0006 fluorapatite1 κ′ _a =10.48 tan δ=0.0008 κ′ _c = 8.72 tan δ=0.0114 fluorapatite2 κ′ _a =10.40 tan δ=0.0010 κ′ _c=8.26 tan δ=0.0178 The deviation (δ) between measured dielectric polarizabilities as determined from the Clausius-Mosotti equation and those calculated from the sum of oxide polarizabilities according to α _D (mineral)=∑ α _D (oxides) for vesuvianite is ～ 0.5%. The large deviations of epidote and zoisite from the additivity rule with Δ=+ 10.1 and + 11.7%, respectively, are attributed to “rattling” Ca ions. The combined effects of both a large F thermal parameter and possible F-ion conductivity in fluorapatite are believed to be responsible for Δ=+2–3%. Although variation of oxygen polarizability with oxygen molar volume (V_o) is believed to affect the total polarizabilities, the variation of V_o in these Ca minerals is too small to observe the effect.     

Surface complexation modelling of Cd(II), Cu(II), Ni(II), Pb(II) and Zn(II) adsorption onto kaolinite

Proton binding constants for the edge and basal surface sites of kaolinite were determined by batch titration experiments at 25 °C in the presence of 0.1 M, 0.01 M and 0.001 M solutions of NaNO₃ and in the pH range 3-9. By optimizing the results of the titration experiments, the ratio of the edge sites to the basal surface sites was found to be 6:1. The adsorption of Cd(II), Cu(II), Ni(II), Zn(II) and Pb(II) onto kaolinite suspensions was investigated using batch adsorption experiments and results suggested that in the lower pH range the metallic cations were bound through non-specific ion exchange reactions on the permanently charged basal surface sites (X⁻). Adsorption on these sites was greatly affected by ionic strength. With increasing pH, the variable charged edge sites (SOH) became the major adsorption sites and inner-sphere specifically adsorbed monodentate complexes were believed to be formed. The effect of ionic strength on the extent of adsorption of the metals on the variable charged edge sites was much less than those on the permanently charged sites. Two binding constants, log K_(X2Me) and log K_(SOMe), were calculated by optimizing these constants in the computer program FITEQL. A model combining non-specific ion exchange reactions and inner-sphere specific surface complexations was developed to predict the adsorption of heavy metals onto kaolinite in the studied pH range. Linear free energy relationships were found between the edge site binding constants and the first hydrolysis constants of the metals.     

The stability of palladium(II) hydroxide and hydroxy–chloride complexes: an experimental solubility study at 25–85°C and 1 bar

Solubility methods were employed to determine conditional equilibrium constants for the formation of hydroxide and mixed hydroxy–chloride complexes of Pd(II). Measurements were made over a temperature range of 25–85°C, a pH range from 0 to 12, and ionic strengths of 0.1, 0.2, 0.5 and 1.0 molal in both KCl and NaClO₄ media. Several speciation models were fit to the data using nonlinear regression, and the model yielding the best fit with the fewest number of species was accepted for each temperature and ionic strength. The conditional equilibrium constants were then fit to a function of ionic strength and temperature (including a Debye–Hückel term) to facilitate interpolation and extrapolation to infinite dilution. The following species were found to be important in KCl solutions: PdCl₄²⁻, PdCl₃(OH)²⁻, and Pd(OH)₂⁰. The relative proportions of the species are dependent on pH and ionic strength (chloride concentration). In perchlorate media the predominant species were Pd(OH)₃⁻, Pd(OH)₂⁰, PdOH⁺ and Pd²⁺, depending on pH. Conditional stability constants determined in this study agree well with those reported in previous work for the simple chloride and hydroxide complexes, but our results suggest that mixed complexes may be more important than previously thought, and that PdCl₃(OH)²⁻ may be the dominant species in seawater, followed by Pd(OH)₂⁰.     

Theoretical studies of charge relaxation effects on the statics and dynamics of oxides

Inclusion of spherical charge relaxation in response to the long-range electrostatic potential (potential induced breathing, or PIB) gives improved results for the static and dynamic properties of oxides. PIB is a Gordon-Kim type model, in which the crystal charge density is estimated by overlapping ionic charge densities. No experimental data are used, except for the values of universal constants, and in this sense the results are from first principles. In contrast to earlier models which include some form of charge relaxation, we explicitly include the breathing effects on the self-energy and pair potentials in the model Hamiltonian. PIB is a many-body effect that couples the long-and short-range forces in a way that is not present in any other first principles or empirical models. It leads to the observed violations of the Cauchy relations for the elastic constants whereas central force rigid ion models cannot violate the Cauchy relations. PIB also reduces the predicted LO-TO splitting because the breathing effect introduces dynamical effective charges that are lower in magnitude than the ionic charges. Some results are shown and discussed for MgO (periclase), BeO (bromellite), Al₂O₃ (corundum), TiO₂ (rutile) and SiO₂ (quartz and stishovite).     

Diffusion and sorption of Cs, I and HTO in samples of the argillaceous Wakkanai Formation from the Horonobe URL, Japan: Clay-based modeling approach

Diffusion and sorption behaviors of cationic Cs⁺, anionic I⁻ and neutral HTO in samples of the Wakkanai Formation from the Horonobe underground research laboratory (URL), Japan, were investigated as a function of ionic strength (I) of synthetic groundwater by through-diffusion and batch sorption experiments and mechanistic modeling. The effective diffusivities (D_e) measured by through-diffusion experiments showed cation excess and anion exclusion effects, which were strongly dependent on I; D_e for Cs⁺ decreased as I increased, D_e for I⁻ showed the opposite dependency and D_e for HTO showed no dependence. The sorption of Cs⁺ measured by through-diffusion and batch sorption experiments were described by Freundlich isotherms with consistent parameters and decreased with I as a result of competitive ion exchange.Diffusion and sorption behaviors were interpreted by assuming the clay components of illite and smectite control diffusion and sorption mechanisms. The component additive (CA) sorption model, which includes illite and smectite contents and their ion exchange constants, provided a reasonable account of the Cs⁺ sorption trends measured as functions of I and Cs concentration. The diffusion model was developed by coupling the electrical double layer (EDL) model, describing the change of ionic concentrations (cation excess and anion deficit) and viscoelectric effects caused by electrostatic interaction at negatively charged clay surfaces, and a simplified pore model assuming one type of pore shape and includes their size distribution. When averaging the electrostatic effects by using the pore surface area distribution, the model could predict the cation excess and anion exclusion effects, and its dependence on I reasonably well. This result implies the nanoscale pores dominating the pore surface area can strongly impact on ionic diffusion in argillaceous rocks. The clay-based modeling approach described here provides a useful tool to predict ionic diffusion and sorption in argillaceous rocks.     

Computer simulations of the structural and physical properties of the olivine and spinel polymorphs of Mg2SiO4

The aim of the work presented is to develop a computer simulation technique which will predict the structure and physical properties of forsterite and ringwoodite, the major mantle-forming polymorphs of Mg₂SiO₄. The technique is based upon energy minimization, in which all structural parameters are varied until the configuration with the lowest energy is achieved. The lattice energy and physical properties (e.g. elasticity and dielectric constants) are calculated from interatomic potentials, which generally include electrostatic and short-range terms. We investigate several types of traditional potential models, and present a new type of model which includes partial ionic charges and a Morse potential to describe the effect of covalency on the Si-O bond. This new form of potential model is highly successful, and not only reproduces the zero-pressure structural, elastic and dielectric properties of forsterite and ringwoodite, but also accurately describes their pressure dependence.     

Band theoretical studies of the electronic structure of oxides

The O^2? ion is a useful concept in ionic solids. Its electron density and large polarizability is well described by the Watson sphere model. The large variation in the electronic structure of oxides is illustrated by discussing the ionic MgO, the partly ‘covalent’ Cu₂O, and TiO and NbO which have defects with respect to the ideal NaCl structure. The variation in physical properties is shown for the oxides in the rutile structure ranging from the insulating TiO₂, the metallic RuO₂ to the ferromagnetic CrO₂. Electron densities, total energies and densities of states are used to study these materials.     

Phosphates of the Chalotskoe Pegmatite Deposit,Transbaikal Region

A number of rare phosphates have been found in specimens from the Chalotskoe pegmatite deposit, Transbaikal region, Russia: väyrynenite, MnBe[PO₄](OH,F); parascholzite, CaZn₂[PO₄]₂ · 2H₂O; messelite, Ca₂(Fe²⁺,Mn)[PO₄]₂ · 2H₂O; eosphorite, MnAl[PO₄](OH)₂ · H₂O; moraesite, Be₂[PO₄](OH)4H₂O; and fluorapatite. Väyrynenite forms pink grains 2–3 mm in size, less frequent prismatic crystals up to 0.8 × 3.0 cm, and spheres up to 3 mm in diameter. Parascholzite occurs as pockets up to 0.6 × 1.0 cm composed from snow-white small grains. Messelite forms pale yellow honeycomb grains and poorly shaped crystals up to 1 mm. Eosphorite has been seen in the Chalotskoe pegmatites before, but it has not been studied in detail. It occurs as red-brown prismatic crystals up to 8 cm in length, occasionally forming openbook- like aggregates and pink to pale pink grains up to 5 mm in size. Moraesite forms snow-white fibrous aggregates up to 5 × 6 mm, together with white spheres and short prismatic crystals of fluorapatite up to 1 mm. Microcline, albite, quartz, muscovite, beryl, schorl, almandine-spessartine, columbite-(Fe), and bertrandite are associated minerals. Väyrynenite and parascholzite are found for the first time in Russia.

     

A Chemical Equilibrium Model for Natural Waters

This paper reviews the present status of the Pitzer chemical equilibrium model, which can be used to characterize the one-atmosphere activity coefficients of ionic and non-ionic solutes in natural waters as a function of temperature and ionic strength. The model considers the ionic interactions of the major seasalt ions (H, Na, K, Mg, Ca, Sr, Cl, Br, OH, HCO₃, B(OH)₄, HSO₄, SO₄, CO₃, CO₂, B(OH)₃, H₂O) and is based on the 25 °C model of Weare and co-workers. The model has been extended by a number of workers so that reasonable estimates can be made of the activity coefficients of most of the major seasalt ions from 0 to 250 °C. Recently coefficients for a number of solutes that are needed to determine the dissociation constants of the acids from 0 to 50 °C (H₃CO₃, B(OH)₃, H₂O, HF, HSO ₄ ^- , H₃PO₄, H₂S, NH ₄ ⁺ etc.) have been added to the model. These results have been used to examine the carbonate system in natural waters and determine the activity of inorganic anions that can complex trace metals. The activity and osmotic coefficients determined from the model are shown to be in good agreement with measured values in seawater. This model can serve as the foundation for future expansions that can examine the activity coefficient and speciation of trace metals in natural waters. At present this is only possible from 0 to 50 °C over a limited range of ionic strengths (<1.0) due to the limited stability constants for the formation of the metal complexes. The future work needed to extend the Pitzer model to trace metals is discussed.     

Thermodynamic and Kinetic Properties of Natural Brines

The physical chemistry of natural brines made up of mostly NaCl has been studied over the years. In this article, the work on the thermodynamics and kinetics of processes in NaCl brines will be examined. The importance of ionic interactions of the processes will be stressed. This will include the pressure–volume–temperature and physical–chemical properties of NaCl and other brine salts from 0 to 6 m, 0 to 200°C, and 0 to 1,000 bar applied pressures. Acid–base, gas–liquid, solid–liquid, and ion–complex formation processes in NaCl are examined. Equations that can be used to estimate the equilibria in NaCl are given. Pitzer models are discussed that can be used to estimate ionic equilibria in brines. The oxidation of Fe(II) and Cu(I) with O₂ and H₂O₂ and the reduction of Cu(II) with H₂O₂ in NaCl are examined in terms of ionic complexes of metals with OH^? and CO₃ ^2?. The oxidation of H₂S with O₂ and H₂O₂ is also examined in NaCl media. Equations are given that can be used to estimate the effect of ionic interactions on kinetic processes in NaCl.     

Geochemical constraints on the distribution of trace elements and volatiles in fluorapatite from the Panasqueira tin-tungsten deposit (Portugal)

A large, euhedral crystal of fluorapatite (ca. 19.5 × 20.0 mm) from the Panasqueira tin-tungsten deposit (Portugal) was investigated in terms of the distribution of trace elements by using several microanalytical techniques. The studied material represents almost pure fluorapatite with minor amounts of other cations (mainly Sr, Mn, REE and Fe), OH and Cl. Particular interest was given to the distribution of rare earth elements with respect to the crystallographic orientation. A broad range of analytical techniques were used, including optical microscopy coupled with cathodoluminescence imaging, electron probe microanalysis (EPMA), laser ablation – inductively coupled plasma mass spectrometry (LA-ICPMS), Raman microspectroscopy, and simultaneous thermal analysis coupled with mass spectrometry. The investigated crystal consists of the main crystal with a distinct core and rim (Ap_2core and Ap_2rim, respectively), which grew on a previous, euhedral crystal (Ap₁). The fluorapatite demonstrates various types of zoning: regular oscillatory, irregular, and internal sectoring, which is also reflected in trace elements concentrations. The rim Ap_2rim has lower concentrations of Mn, Sr and Fe, and significantly higher concentrations of REE compared to the core Ap_2core and older crystal Ap₁. Furthermore, the rim Ap_2rim is strongly depleted in Th, U and Pb. The entire crystal shows elevated Eu contents, expressed as a strong positive anomaly in chondrite-normalized REE patterns. With regards to the volatiles, F concentrations are constant in Ap₁, Ap_2core and Ap_2rim, whereas Cl is below the EPMA detection limit. The Ap_2rim was the only part of the investigated material containing OH and CO₃, which were observed in the Raman spectra. Furthermore, part of the crystal Ap_2core is extensively altered, likely due to fluid-induced metasomatic processes. LA-ICPMS U-Pb dating yielded highly discordant dates due to common Pb content. A lower intercept age of 297 ± 13 Ma (MSWD = 0.13) indicates the age of the fluorapatite crystallization. The overall analytical data constrain growth and post-growth processes, including crystallization of Ap₁ and Ap_2core, which both have typical hydrothermal Sn-W deposit characteristics, whereas Ap_2rim is related to a carbonate stage of the mineralization in the Panasqueira deposit.     

Lattice dynamics of pyrite FeS2 — polarizable-ion model

Lattice dynamical calculations of the pyrite FeS₂ were performed using the polarizable-ion model (PIM) with different sets of short-range force constants. Not until the mean deviations between the observed and the calculated phonon energies become smaller than 3 cm^-1, the true force field can be established. In the case of only slightly greater deviations, the force fields computed differ strongly being without any physical meaning. The results are discussed with respect to the force constants K _i , F _i , and H _i , the effective dynamic charges and polarizabilities of the atoms involved, and the eigenvectors and potential energy distributions of the phonon modes. The most important short-range force constants are K ₁ (Fe-S stretching): 0.5 N cm^-1, K ₂ (internal stretching of the S₂ units): 1.0 N cm^-1, F ₁ (Fe^....Fe stretching): 0.2 N cm^-1, which indicate repulsive interactions of Fe atoms due to the occupied t _2g orbitals despite the relatively large Fe?Fe distances of 383 pm, and F ₂ and F ₃ (both intermolecular S₂?S₂ interactions): 0.2 N cm^-1. The great TO/LO splittings of some of the IR allowed phonon modes (species F _u) are caused by the large polarizabilities (2.4^.10⁶ and 3.3^.10⁶ pm³) of the atoms involved rather than by their effective charges (Fe: 0.2 e).     

Mechanisms of formation of barium-rich phlogopite and strontium-rich apatite during the final stages of alkaline magma evolution

Carbonatite veinlets in fergusite from the Dunkeldyk potassium-rich basaltoid complex (southeastern Pamirs) are composed of clinopyroxene, phlogopite, and apatite phenocrysts embedded in a crystallized calcite-bearing groundmass. The examination of back-scattered electron images revealed areas of significantly different compositions in fluorapatite and fluorphlogopite. The content of BaO in the phlogopite ranges from 0.68 to 10.9 wt %. There are also variations in MgO and F contents. The maximum BaO content corresponds to high mole fractions of the Ba end member kinoshitalite (up to 0.24) in the phlogopite. The zoned fluorapatite phenocrysts are rich in SrO (0.77–25.4 wt %). An increase in SrO content is accompanied by an increase in Ce₂O₃, La₂O₃, and BaO and a distinct decrease in CaO. Most of the apatite grains are rimmed by elongated colorless crystals showing the maximum SrO contents. Based on the experimentally determined Ba and Sr partition coefficients between these minerals, silicate and carbonate melts, and fluid, a model was proposed for the enrichment of phases in these trace elements. It was shown that the mineral-forming media of the Ba-rich phlogopites was a residual melt enriched in volatiles (including F) and fluid-mobile elements. During that stage, the decomposition reactions of early Ba-bearing feldspars with subsequent incorporation of BaO in Ba-rich phlogopites played an important role. The mechanism of formation of Sr-rich apatites is fundamentally different: early apatite grains with moderate Sr contents recrystallized under the influence of Sr-rich fluids released during the late magmatic stage. Thus, despite their close association in a single rock, the Ba-bearing phlogopite and Sr-rich apatite were formed by significantly different mechanisms. Our previous investigations of melt and fluid inclusions in minerals from the rocks of the Dunkeldyk complex and the results obtained in this study allowed us to suggest that the barium, fluorite-carbonatite, and rare metal mineralization occurring in the region developed owing to the prolonged evolution of primary magmas, resulting in the formation of melt-solutions (brines) and hydrothermal systems.     

Petrography and mineral chemistry of Alkaline-Carbonatite Complex in Singhbhum Crustal Province,Purulia region,Eastern India

The variant rock types of an Alkaline-Carbonatite Complex (ACC) comprising alkali pyroxenite, nepheline syenite, phoscorite, carbonatite, syenitic fenite and glimmerite along with REE and Nb-mineralization are found at different centres along WNW-ESE trending South Purulia Shear Zone (SPSZ) in parts of Singhbhum Crustal Province. The ACC occurs as intrusions within the Mesoproterozoic Singhbhum Group of rocks. Alkali pyroxenite comprises of aegirine augite, magnesiotaramite, magnesiokatophorite as major constituents. Pyrochlore and eucolite are ubiquitous in nepheline syenite. Phoscorite contains fluorapatite, dahllite, collophane, magnetite, hematite, goethite, phlogopite, calcite, sphene, monazite, pyrochlore, chlorite and quartz. Coarse fluorapatite shows overgrowth of secondary apatite (dahllite). Secondary apatite is derived from primary fluorapatite by solution and reprecipitation. The primary fluorapatite released REE to crystallize monazite grains girdling around primary apatite. Carbonatite is composed dominantly of Srcalcite along with dolomite, tetraferriphlogopite, phlogopitic biotite, aegirine augite, richterite, fluorapatite, altered magnetite, sphene and monazite. The minerals comprising of the carbonatite indicate middle stage of carbonatite development. Fenite is mineralogically syenite. Glimmerite contains 50–60% tetraferriphlogopite. An alkali trend in the evolution of amphiboles (magnesiotaramite-magnesiokatophorite-richterite) and chinopyroxenes (aegirine augite, aegirine) during the crystallization of the suite of rocks is noted. Monazite is the source of REE in phoscorite and carbonatite. Fluorapatite has low contents of REE, PbO, ThO₂ and UO₂. Pyrochlore reflects Nb-mineralization in nepheline syenite and it is enriched in Na₂O, CaO, TiO₂, PbO and UO₂. Pyrochlore containing UO₂ (6.605%) and PbO (0.914%) in nepheline syenite has been chemically dated at 948 ± 24 Ma by EPMA.     

A study of factors affecting on the zeta potential of kaolinite and quartz powder

The purpose of this study was to determine the effects of pH, ion type (salt and metal cations), ionic strength, cation valence, hydrated ionic radius, and solid concentration on the zeta potential of kaolinite and quartz powder in the presence of NaCl, KCl, CaCl₂, CuCl₂, BaCl₂, and AlCl₃ solutions. The kaolinite and quartz powder have no isoelectric point (iep) within the entire pH range (3 < pH < 11). In the presence of hydrolysable metal ions, kaolinite and quartz powder have two ieps. As the cationic valence increases, the zeta potential of kaolinite and quartz powder becomes less negative. Monovalent cation, K⁺, yields more negative zeta potential values than the divalent cation Ba²⁺. As concentration of solid increases, the zeta potential of the minerals becomes more positive under acidic conditions; however, under alkaline conditions as solid concentration increases the zeta potential becomes more negative. Hydrated ionic radius also affects the zeta potential; the larger the ion, the thicker the layer and the more negative zeta potential for both kaolinite and quartz powder.     

Application of empirical ionic models to SiO2 liquid: Potential model approximations and integration of SiO2 polymorph data

Structural and thermodynamic properties of crystalline SiO₂ and SiO₂ liquid have been examined with Monte Carlo (MC), molecular dynamics (MD), and energy minimization (EM) calculations using several ionic potential models obtained from the literature. The MC and MD methods calculate the same structural and thermodynamic properties for liquids when the same potential model is used. The Ewald (1921) method of calculating coulomb interactions reproduced most successfully the structure of liquid silica. Approximating the coulomb interaction by eliminating the inverse lattice sum results in predicted bond distances that are too short and an average 〈Si-O-Si〉 angle of approximately 180°. Introduction of a cut-off in the potential energy function produces irregular tetrahedra and inconsistencies in predicted Si-O coordination in silica liquid. The system internal energies show that liquid structures derived from random starting configurations can be metastable relative to structures calculated from crystalline starting configurations.The static lattice properties of the polymorphs alpha-quartz, coesite, and stishovite were used to evaluate further the accuracy of different sets of repulsive parameters for the full Ewald ionic model. Most of the models studied reproduced poorly the measured structures and elastic constants of the polymorphs. The major weakness of the ionic model is the unreasonably large Si-O bond strength (120 × 10⁻¹² ergs/bond) when formal ionic charges are used. Fractional charge models with a small Si-O bond strength (30 × 10⁻¹² ergs/bond) improve the agreement with experimental data. However, further improvement of the ionic model should include reducing the Si-O bond strength to values in better agreement with published estimates (7 × 10^{− 12} to 13 × 10⁻¹² ergs/bond). By using additional information to constrain the parameterization of the ionic model, such as estimated bond strengths and static properties of the silica polymorphs, a model more representative of the interparticle interactions may be obtained.     

Carbonate complexation of yttrium and the rare earth elements in natural waters

Potentiometric measurements of Yttrium and Rare Earth Element (YREE) complexation by carbonate and bicarbonate indicate that the quality of carbonate complexation constants previously obtained via solvent exchange analyses are superior to characterizations obtained using solubility and adsorptive exchange analyses. The results of our analyses at 25°C are combined with the results of previous solvent exchange analyses to obtain YREE carbonate complexation constants over a wide range of ionic strength (0 ≤ I ≤3 molal). YREE carbonate complexation constants are reported for the following equilibria, M³⁺+nHCO₃⁻?M(CO₃)_n³⁻²ⁿ+nH⁺, where n = 1 or 2. Formation constants written in terms of HCO₃⁻ concentrations require only minor corrections for ion pairing relative to the corrections required for constants expressed in terms of CO₃²⁻ concentrations. Formation constants for the above complexation equilibria, _CO3^Hβ₁=[MCO₃⁺][H⁺][M³⁺]⁻¹[HCO₃⁻]⁻¹ and _CO3^Hβ₂=[M(CO₃)₂⁻][H⁺]²[M³⁺]⁻¹[HCO₃⁻]⁻², have very similar dependencies on ionic strength because the reaction MCO₃⁺+HCO₃⁻?M(CO₃)₂⁻+H⁺ is isocoulombic. Potentiometric analyses indicate that the dependence of log_CO3^Hβ₁ and log_CO3^Hβ₂ on ionic strength at 25°C is given as

(A)     

An experimental study of dissolution–reprecipitation in fluorapatite: fluid infiltration and the formation of monazite

In a series of timed experiments, monazite inclusions are induced to form in the Durango fluorapatite using 1 and 2 N HCl and H₂SO₄ solutions at temperatures of 300, 600, and 900°C and pressures of 500 and 1,000 MPa. The monazite inclusions form only in reacted areas, i.e. depleted in (Y+REE)+Si+Na+S+Cl. In the HCl experiments, the reaction front between the reacted and unreacted regions is sharp, whereas in the H₂SO₄ experiments it ranges from sharp to diffuse. In the 1 N HCl experiments, Ostwald ripening of the monazite inclusions took place both as a function of increased reaction time as well as increased temperature and pressure. Monazite growth was more sluggish in the H₂SO₄ experiments. Transmission electron microscopic (TEM) investigation of foils cut across the reaction boundary in a fluorapatite from the 1 N HCl experiment (600°C and 500 MPa) indicate that the reacted region along the reaction front is characterized by numerous, sub-parallel, 10–20 nm diameter nano-channels. TEM investigation of foils cut from a reacted region in a fluorapatite from the 1 N H₂SO₄ experiment at 900°C and 1,000 MPa indicates a pervasive nano-porosity, with the monazite inclusions being in direct contact with the surrounding fluorapatite. For either set of experiments, reacted areas in the fluorapatite are interpreted as replacement reactions, which proceed via a moving interface or reaction front associated with what is essentially a simultaneous dissolution–reprecipitation process. The formation of a micro- and nano-porosity in the metasomatised regions of the fluorapatite allows fluids to permeate the reacted areas. This permits rapid mass transfer in the form of fluid-aided diffusion of cations to and from the growing monazite inclusions. Nano-channels and nano-pores also serve as sites for nucleation and the subsequent growth of the monazite inclusions.