First principles quantum mechanical calculation of the electric dipole polarizabilities of the borate,carbonate, nitrate and related ions

Electric dipole polarizabilities have been calculated from first principles of quantum mechanics for the BO ₃ ^3? , CO ₃ ^2? , NO ₃ ^? series and for NO ₂ ^? and LiNO₃(g). Calculated trends in average polarizability and polarizability anisotropy in the BO ₃ ^3? -NO ₃ ^? series are in agreement with experiment and can be qualitatively interpreted in terms of the varying energies of the a₁′, a₂″ and e′ symmetry unoccupied MO's of the oxyanions. Embedding a CO ₃ ^2? ion in a D_3h symmetry array of divalent cations reduces both the average polarizability and its anisotropy, particularly when diffuse s and p functions are included in the calculation. Calculations on the gas phase LiNO₃ molecule and on the free NO ₃ ^? ion in the distorted geometry found in LiNO₃(g) allow us to separate polarizability contributions internal to the NO ₃ ^? and Li⁺ ions from those which arise from the Li⁺-NO ₃ ^? interaction. The Li⁺-NO ₃ ^? interaction term so obtained is much smaller than the NO ₃ ^? contribution but is in turn larger than the Li⁺ contribution, suggesting that the inclusion of this interaction term is essential for obtaining accurate results for ion pairs. Although static polarizabilities are in reasonable agreement with experiment for NO ₃ ^? the wavelength dispersion of the polarizability is underestimated by about a factor of two, apparently as a result of inadequacies in the quantum mechanical method. Calculated values are also presented for ¹⁴N NMR shieldings in the nitrogen oxyanions but these are in only qualitative agreement with the experimental values. Similarly, calculated values of magnetic susceptibility are in only qualitative agreement with experiment although trends along the BO ₃ ^3? -NO ₃ ^? series are properly reproduced.     

Localized zones of denitrification in a floodplain aquifer in southern Wisconsin, USA

A floodplain aquifer within an agricultural watershed near Madison, Wisconsin (USA), was studied to determine whether denitrification was occurring below the surface organic layer. Groundwater levels and concentrations of O₂, Cl^?, NO ₃ ^? , SO ₄ ^2? , dissolved organic carbon (DOC), and major cations were monitored over a 1-year period along a 230-m transect between an agricultural field and a stream discharge point. Seventeen groundwater samples were analyzed for δ¹⁵N_NO3 and δ¹⁸O_NO3 composition. Samples in which NO ₃ ^? was too low for stable isotope analysis were analyzed for excess dissolved N₂. Groundwater NO ₃ ^? concentrations declined between the agricultural field and the discharge point. Chloride and δ¹⁵N_NO3/δ¹⁸O_NO3 data indicated that the drop in NO ₃ ^? was caused primarily by dilution of shallow NO ₃ ^? -rich water with deeper, NO ₃ ^? -depleted groundwater. Two localized zones of denitrification were identified in the upland-wetland transition by their δ¹⁵N_NO3 and δ¹⁸O_NO3 signatures, and two in the stream hyporheic zone by the presence of excess dissolved N₂. The combined stratigraphic, hydrologic, and geochemical data in these locations correspond to groundwater mixing zones where NO ₃ ^? is delivered to subsurface layers that support denitrification fueled by dissolved (e.g. DOC or dissolved Fe(II)) and/or solid-phase (e.g. particulate organic carbon, solid-associated Fe(II), or pyrite) electron donors.     

Hydrochemical characteristics of groundwater in the plains of Phalgu River in Gaya, Bihar, India

Assessment of groundwater quality is essential to ensure sustainable use of it for drinking, agricultural, and industrial purposes. The chemical quality of groundwater of Gaya region has been studied in detail in this work to delineate the potable groundwater zones. A total of 30 groundwater samples and 2 surface water samples were collected in and around Gaya district of Bihar. The major cations follow the trend: Ca²⁺?>?Mg²⁺?>?Na⁺?>?K⁺. The domination of calcium ions in the groundwater is due to weathering of rocks. The K⁺ ranged between 0.2 and 47.95 ppm, suggesting its abundance the below desired limit; but some samples were found to be above permissible limit. K⁺ weathering of potash silicate and the use of potash fertilizer could be the source. The major anions abundance followed the order HCO ₃ ^? ?>?Cl^??>?SO ₄ ^2? ?>?NO ₃ ^? ?>?PO ₄ ^3? . Dissolution of carbonates and reaction of silicates with carbonic acid accounts for the addition of HCO ₃ ^? to the groundwater and oxidation of sulphite may be the source of SO ₄ ^2? . Principal component analysis was utilized to reflect those chemical data with the greatest correlation and seven major principal components (PCs) representing >80 % of cumulative variance were able to interpret the most information contained in the data. PC1, PC2 and PC3 reflect the hydrogeochemical processes like mineral dissolution, weathering and anthropogenic sources. PC4, PC5, PC6 and PC7 show monotonic, random and independent relationships.     

Assessment of hydrogeochemical processes in a coastal region: Application of multivariate statistical model

Hydrogeochemical studies have been carried out in a coastal region, using multivariate statistical model, for better understanding the controlling processes that influence the aquifer chemistry. Two principal components (PC1 and PC2) are extracted from the data set of chemical variables (pH, TDS, Ca²⁺, Mg²⁺, Na⁺, K⁺, HCO ₃ ^? , Cl^?, SO ₄ ^2? , NO ₃ ^? and F^?), which account for 79% of the total variation in the quality of groundwater. The PC1 (salinity controlled process) includes the concentrations of TDS, Mg²⁺, Na⁺, K⁺, Cl^?, SO ₄ ^2? and NO ₃ ^? , while the PC2 (alkalinity controlled process) comprises the concentrations of pH, HCO ₃ ^? and F^?. The spatial distribution of PC scores identifies the locations of high salinity and alkalinity processes. The first process corresponds to the influences of geogenic, anthropogenic and marine sources, and the second one to the influence of water-soil-rock interaction. Thus, the present study shows the usefulness of multivariate statistical model as an effective means of interpretation of spatial controlling processes of groundwater chemistry.     

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

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

Characterization and origin of two Fe3+ EPR spectra in kaolinite

The electron paramagnetic resonance (EPR) spectra of Fe³⁺ in a well cristallized kaolinite from Decazeville in France are well resolved. It is shown that in this sample there are mainly two slightly different spectra, well separated at low temperature and characterized at -150° C by the constants B ₂ ⁰ = 0.112 cm^?1, B ₂ ² = 0.0688 cm^?1 for one and B ₂ ⁰ = 0.116 cm^?1, B ₂ ² = 0.0766 cm^?1 for the second. These two spectra arise from Fe³⁺ substituted for Al³⁺ at the two octahedral positions in equal amounts. The temperature dependence of EPR spectra was studied and was explained by a modification of the octahedral sites.     

Electron paramagnetic resonance of PO 3 2− and SO 3 − radicals in anhydrite,celestite and barite: the hyperfine structure and dynamics

Electron paramagnetic resonance (EPR) measurements of natural anhydrite CaSO₄, celestite SrSO₄ and barite BaSO₄ have revealed the presence of PO ₃ ^2? and SO ₃ ^? radicals. Hyperfine structure from³³S has been detected and measured for the first time. Low-symmetry effects, which are manifested in noncoincidence of g and hyperfine axes, were observed for SO ₃ ^? . The dynamics of one of the two SO ₃ ^? types in anhydrite has been investigated. Variations of spin Hamiltonian parameters with temperature have been attributed to thermally induced jumps between the two magnetically inequivalent sites of this center.     

A theoretical study of bond distances,X-ray spectra and electron density distributions in borate polyhedra

The borate polyhedra BO ₃ ^3? , B(OH)₃, BO ₄ ^5? , and B(OH) ₄ ^? are studied using the ab initio and multiple scattering Xα quantum mechanical methods. The ab initio self-consistent-field (SCF) molecular orbital (MO) method, at the minimum basis set level, predicts equilibrium B-O distances within 0.04 Å of their average values in solids so long as the polyhedron charge is small. Orbital energies from double zeta basis set ab initio calculations and analogies with isoelectric compounds are used to assign the X-ray spectra of BO ₃ ^3? and to predict the valence region spectra of BO ₄ ^5? . Contour maps of the difference between molecular and superimposed free atom electron densities show charge buildup along the B-O bond which is only slightly smaller than that observed in CO ₃ ^2? .     

Submarine Groundwater Discharge-Derived Nutrient Loads to San Francisco Bay: Implications to Future Ecosystem Changes

Submarine groundwater discharge (SGD) was quantified at select sites in San Francisco Bay (SFB) from radium (²²³Ra and ²²⁴Ra) and radon (²²²Rn) activities measured in groundwater and surface water using simple mass balance box models. Based on these models, discharge rates in South and Central Bays were 0.3?C7.4?m³?day^?1?m^?1. Although SGD fluxes at the two regions (Central and South Bays) of SFB were of the same order of magnitude, the dissolved inorganic nitrogen (DIN) species associated with SGD were different. In the South Bay, ammonium (NH ₄ ⁺ ) concentrations in groundwater were three-fold higher than in open bay waters, and NH ₄ ⁺ was the primary DIN form discharged by SGD. At the Central Bay site, the primary DIN form in groundwater and associated discharge was nitrate (NO ₃ ^? ). The stable isotope signatures (??¹⁵N_NO3 and ??¹⁸O_NO3) of NO ₃ ^? in the South Bay groundwater and surface waters were both consistent with NO ₃ ^? derived from NH ₄ ⁺ that was isotopically enriched in ¹⁵N by NH ₄ ⁺ volatilization. Based on the calculated SGD fluxes and groundwater nutrient concentrations, nutrient fluxes associated with SGD can account for up to 16?% of DIN and 22?% of DIP in South and Central Bays. The form of DIN contributed to surface waters from SGD may impact the ratio of NO ₃ ^? to NH ₄ ⁺ available to phytoplankton with implications to bay productivity, phytoplankton species distribution, and nutrient uptake rates. This assessment of nutrient delivery via groundwater discharge in SFB may provide vital information for future bay ecological wellbeing and sensitivity to future environmental stressors.     

Urban land-use effects on groundwater phosphate distribution in a shallow aquifer, Nanfei River basin, China

Groundwater, surface water, soil and river sediment samples, and information on land use in the Nanfei River basin (NRB) of China have been analyzed to study the geochemistry, distribution, and mobilization of phosphorus. The distribution of phosphate (PO ₄ ^3??/sup> ) and the relationships between PO ₄ ^3??/sup> and several constituents in groundwater were studied. Partial correlation analysis relating PO ₄ ^3??/sup> to types of land use was conducted using the data analyzing tool SPSS 15.0. The processes controlling the transport of PO ₄ ^3??/sup> are discussed. The conclusions from this study are: (1) urban land use has obvious impact on PO ₄ ^3??/sup> in groundwater, the average concentration of PO ₄ ^3??/sup> being 4.37?mg/L, greater than that resulting from farmland and mixed land use, which have average PO ₄ ^3??/sup> concentrations of 0.10 and 0.18?mg/L, respectively; (2) the partial correlation between PO ₄ ^3??/sup> and types of land use is significant with a coefficient of 0.760; (3) the PO ₄ ^3??/sup> concentrations in surface water are generally higher than those in groundwater, and the total phosphorus (TP) concentrations in river sediments are generally higher than those in soil samples; (4) groundwater is a carrier of PO ₄ ^3??/sup> and is likely responsible for the redistribution of PO ₄ ^3??/sup> in different regions of NRB.     

Le gisement de tungstène de Xihuashan (Sud-Jiangxi,Chine): Relations granites,altérations deutériques-hydrothermales,minéralisations

The Xihuashan tungsten deposit (South Jiangxi, China) is located on the border of a granitic stock composed of four intrusive units: γ _inf2a ^sup5 , γ _inf2b ^sup5 , γ _inf2c ^sup5 and γ _inf2e ^sup5 chronologically. The deposit is situated in γ _inf2a ^sup5 and γ _inf2b ^sup5 whose contact zone is marked by a stockscheider and by a sporadic fine-grained granite designated γ _inf2b′ ^sup5 . The feldspathic episyenitic veins or masses located mainly in γ _inf2b ^sup5 resulted from granite alteration. This alteration characterizes the fluid activity which followed previous mechanical action, remnant liquids draining under structural control. The quartz-bearing Xihuashan veins (Dayu mining district) are a typical wolframite-quartz vein deposit and have developed mainly in γ _inf2b ^sup5 . The density of the veins and the huge mineralizations in γ _inf2b ^sup5 can be explained by the carapace role played by γ _inf2a ^sup5 , γ _inf2b′ ^sup5 and the stockscheider. The deposit is formed by 615 economically valuable veins (medium grade: 1,08% in WO₃) characterized by four stages of mineralization; a reverse vertical zoning is generally observed. Thus, the Xihuashan tungsten deposit possesses a metallogenic value contributing to the comprehension of metallogenic and structural phenomena related to the evolution of granitic masses.     

Geochemical characterization of groundwater from shallow aquifer surrounding Fetzara Lake N. E. Algeria

Hydrogeochemical investigations were carried out around Fetzara Lake, Northeast Algeria, to assess the quality of groundwater for its suitability for drinking and irrigation purposes. The groundwater chemistry is mainly controlled by the water?Crock interactions, but also influenced by other processes such as evapotranspiration and ion exchange. Groundwater samples collected, during two periods (1993 and 2007) from wells in the area were analyzed for pH, EC, TDS, Ca²⁺, Mg²⁺, Na⁺, K⁺, CO ₃ ^2? , HCO ₃ ^? , Cl^?, SO ₄ ^2? , and NO ₃ ^? . The chemical relationships in Piper??s diagram and Gibbs??s diagram suggest that groundwaters mainly belong to noncarbonate alkali type and Cl^? group and are controlled by evaporation dominance, respectively, due to the sluggish drainage conditions, greater water?Crock interaction, and anthropogenic activities. A comparison of the groundwater quality in relation to drinking water quality standards proves that most of the water samples are not suitable for drinking. US Salinity Laboratory??s and Wilcox??s diagrams and %Na⁺ used for evaluating the water quality for irrigation suggest that the majority of the groundwater samples are not good for irrigation.     

The effect of medium chemistry on the solubility and morphology of brushite crystals

An experimental study of the particulars of the solubility and crystallization of brushite Ca(HPO₄) · 2H₂O from aqueous solution in conditions of a variable pH (6.0–3.0) and the contents of impurity ions (K⁺, Na⁺, NH ₄ ⁺ , Mg²⁺, SO ₄ ^2? , CO ₃ ^2? ) has been conducted. It is established that brushite solubility markedly rises with a decrease in pH from 6 to 3 and slightly rises with an increase in Mg²⁺ and SO ₄ ^2? concentrations. The enrichment in K⁺, Na⁺, and NH ₄ ⁺ does not affect brushite solubility. The changeable chemistry of the medium results in variation of the synthetic crystal habit, from rhombic tabular to thickened prismatic crystals.     

Characterisation and quality assessment of groundwater with a special emphasis on irrigation utility: Thirumanimuttar sub-basin, Tamil Nadu, India

Thirumanimuttar sub-basin is of particular importance in the study of groundwater quality due to the release of effluents from industries, agricultural, sewage and urban runoff, brining considerable change in water quality. An investigation was carried out by collecting a total of 194 groundwater samples for two seasons to decipher hydrogeochemistry and groundwater quality for determining its suitability for agricultural purposes. The water is neutral to alkaline in nature with pH ranging from 6.78 to 9.22 with an average of 7.37. Higher electrical conductivity (EC) was noted in NW and mid-downstream parts of the study area. Higher NO ₃ ^? was observed during post-monsoon (POM) due to the action of leaching and anthropogenic process. The piper plot reveals the dominance of Na⁺?CCl^? and Na⁺?CHCO ₃ ^? , mixed Ca²⁺?CNa⁺?CHCO ₃ ^? , mixed Ca²⁺?CMg²⁺?CHCO ₃ ^? and Ca²⁺?CSO ₄ ^? types of hydrogeochemical facies. Higher total hardness in the groundwater is due to the effect of dyeing and bleaching industries discharging effluents affects the quality of water. Residual Sodium Carbonate value indicates 56% of the samples are not suitable for irrigation purposes in both seasons. Higher sodium percentage is noted during PRM indicating the dominance of ion exchange and weathering. Higher sodium adsorption ratio was observed during POM indicating the effect of leaching and dissolution of salts into the aquifer matrix. USSL plot indicates 15% of samples record high salinity to medium sodicity. The Permeability Index indicates water is moderate to good for irrigation purposes. In general, groundwater in the study area is influenced by both natural and anthropogenic activities.     

Characterization, classification, and determination of drinkability of some Algerian thermal waters

In this study, the physicochemical parameters (Conductivity, pH, Cl^?, HCO ₃ ^? , PO ₄ ^3? , SO ₄ ^2? , NO ₃ ^? , NO ₂ ^? , F^?, TH, Ca²⁺, K⁺, Mg²⁺, Na⁺, and DS) were determined for 41 samples collected from fourteen places in Algeria. The temperature of the thermal water samples at collection sites varied from 26°C to 86°C. pH values varied from 6.5 to 8.5 (i.e., from slightly acidic to moderately alkaline); 90.24% of the samples exhibited relatively high salinity (DS?=?550–5,500 mg L^?1). Total hardness measurements indicated these waters to be moderately hard. Forty-six percent of the samples are Na–Cl in character. The ratios Na⁺/Ca²⁺, Na⁺/Mg²⁺, and (Na⁺ + K⁺)/(Ca²⁺ + Mg²⁺) were high in 90.24% of the samples. This indicates the ion exchange process is important, which indicates that most of the Algerian thermal waters had developed over a long period at a depth sufficient to react with the rock. Statistical analyses of the physicochemical data gave positive correlation values, thereby enabling good interpretation of the results and revealing the composition of ions present in the thermal waters, as well as some information about their origin. The therapeutic properties associated with thermal waters encourage people at spas to drink the water they bathe in. Therefore, we examined the drinkability of these thermal waters. World Health Organization (WHO 1993) standards were used to evaluate the thermal water quality for drinking. With respect to hardness, the samples were classified as moderately hard (58.54% of the samples), very hard (36.58% of the samples), and soft (4.88% of the samples). The drinkability study shows that only 16 samples of the investigated waters were drinkable and thus could be consumed without special precaution.     

Gold Solubility in SiO2—HCl—H2O System at 200℃：a Preliminary Assessment of the Implications of Silicification with Regard to Gold Mineralization

The complexation between gold and silica was experimentally, confirmed and calibrated at 200 °C: $$\begin{gathered} Au^ + + H_3 SiO_4^ - \rightleftharpoons AuH_3 SiO_4^0 \hfill \\ \log K_{(200^\circ C)} = 19.26 \pm 0.4 \hfill \\ \end{gathered} $$ Thermodynamic calculations show that AuH₃SiO ₄ ⁰ would be far more abundant than AuCl ₂ ^? under physicochemical conditions of geological interest, suggesting that silica is much more important than chloride as ligands for gold transport. In systems containing both sulfur and silica, AuH₃SiO ₄ ⁰ would be increasingly more important than Au (HS) ₂ ^? as the proportion of SiO₂ in the system increases. The dissolution of gold in aqueous SiO₂ solutions can be described by the reaction: $$\begin{gathered} Au + 1/4O_2 + H_4 SiO_4^0 \rightleftharpoons AuH_3 SiO_4^0 + 1/2H_2 O \hfill \\ log K_{(200^\circ C)} = 6.23 \hfill \\ \end{gathered} $$ which indicates that SiO₂ precipitation is an effective mechanism governing gold deposition, and thus explains the close association of silicification and gold mineralization.     

Hydrogen and carbon in solid solution in oxides and silicates

The dissolution of H₂O and CO₂ in structurally dense, nominally anhydrous and non-carbonate oxide matrices such as MgO and CaO is reviewed. H₂O and CO₂ are treated as gaseous oxide components which enter into solid solution with the refractory oxide hosts. They form anion complexes associated with cation vacancy sites. Evidence is presented that OH^? pairs which derive from the dissolution of H₂O are subject to a charge transfer (CT) conversion into peroxy moieties and molecular hydrogen, O ₂ ^2? ... H₂. Because the O ₂ ^2? moiety is small (O^?-O^? distance ≈ 1.5 Å) high pressure probably favors the CT conversion. Mass spectroscopic studies show that molecular H₂ may be lost from the solid which retains excess oxygen in the form of O ₂ ^2? , leading to the release of atomic O. The dissociation of O ₂ ^2? moieties into a vacancy-bound O^? state and an unbound O^? state can be followed by measuring the internal redox reactions involving transition metal impurities, the transient paramagnetism of the O^? and their effect on the d.c. conductivity. Evidence is presented that CO₂ molecules dissolve dissociatively in the structurally dense oxide matrix, as if they were first to dissociate into CO+O and then to form separate solute moieties CO ₂ ^2? and O ₂ ^2? , both associated with cation vacancy sites. In the CO ₂ ^2? moiety (C-O^? distance 1.2–1.3 Å, OCO angle ≈ 130°) the C atom probably sits off center. The transition of the C atom into interstitial sites is accompanied by dissociation of the CO ₂ ^2? moiety into CO^? and O^?. This transition can be followed by infrared spectroscopy, using OH^? as local probes. Further support derives from magnetic susceptibility, thermal expansion, low frequency dielectric loss and low temperature deformation measurements. The recently observed emission of O and Mg atoms besides a variety of molecules such as CO, CO₂, CH₄, HCN and other hydrocarbons during impact fracture of MgO single crystals is presented and discussed in the light of the other experimental data.     

Hydrotalcite-like solid solutions with variable SO 4 2− and CO 3 2− contents at 50° C

Hydrotalcite-like solid solutions have been synthesized by coprecipitation in basic solutions with variable SO ₄ ^2? /CO ₃ ^2? ratios. Chemical determination of CO ₃ ^2? in the interlayer was impossible because of the presence of minor hydromagnesite. SO ₄ ^2? was determined both by chemical analysis and X-ray photoelectron spectroscopy (XPS), the two methods giving similar results. A Raman spectrometry gave additional data on the SO ₄ ^2? /CO ₃ ^2? ratio. Then, the stoichiometry of the anionic interlayers, X _s , X _c , and X _OH were determined, and the influence of X _s on the c′ parameter (increasing from c′=7.97 Å to c′=8.63 Å between X _s =0 and X _s =1) was characterized. In addition, a partitioning curve of SO ₄ ^2? and CO ₃ ^2? between aqueous solutions and hydrotalcite-like compounds was established. Its general shape strongly suggests a miscibility gap between a sulfate-rich end and a carbonate-rich solid solution (maximum SO ₄ ^2? /CO ₃ ^2? about 0.2). This result explains why most of the hydrotalcites synthesized during experimental alteration of basaltic glasses by sea-water (a sulfate-rich solution) are CO ₃ ^2? -rich solid solutions.     

The competing models for the origin and internal evolution of granitic pegmatites in the light of melt and fluid inclusion research

In this paper we discuss the main petrogenetic models for granitic pegmatites and how these models have evolved over time. We suggest that the present state of knowledge requires that some aspects of these models to be modified, or absorbed into newer ones. Pegmatite formation and internal evolution have long supposed the need for highly water- and flux-enriched magmas to explain the differences between pegmatites and other intrusives of similar major element composition. Compositions and textural characteristics of fluid and melt inclusions in pegmatite minerals provide strong evidence for such magmas. Furthermore, we show that melt inclusion research has increased the number of potential flux components, which may include H₂O, OH^?, CO₂, HCO ₃ ^? , CO ₃ ^2? , SO ₄ ^2? , PO ₄ ^3? , H₃BO₃, F , and Cl, as well as the elements Li, Na, K, Rb, Cs, and Be, herein described as melt structure modifiers. In this paper we emphasize that the combined effect which these components have on the properties of pegmatite melts is difficult to deduce from experimental studies using only a limited number of these components. The combination and the amount of the different magmatic species, together with differences in the source region, and variations in pressure and temperature cause the great diversity of the pegmatites observed. Some volatile species, such as CO ₃ ^2? and alkalis, have the capacity to increase the solubility of H₂O in silicate melt to an extraordinary degree, to the extent that melt-melt-fluid immiscibility becomes inevitable. It is our view that the formation of pegmatites is connected with the complex interplay of many factors.     

Zn and Pb solubility and speciation in aqueous chloride fluids at T-P parameters corresponding to granitoid magma degassing and crystallization

The solubility of all possible Zn and Pb species in aqueous chloride fluids was evaluated by means of thermodynamic simulations in systems ZnO(PbO)-aqueous solution of NaCl (KCl, NaCl + HCl) within broad ranges of temperature (600–900°C), pressure (0.7–5 kbar), and chloride concentrations, under parameters corresponding to the crystallization and degassing of granitoid magmas in the Earth’s crust. Our simulation results demonstrate that the addition of Cl to the fluid phase in the form of Na(K)Cl and HCl significantly increases the concentrations of Cl-bearing Zn and Pb complexes and the total concentration of the metals in the solutions in equilibrium with the solid oxides. In Zn-bearing fluids, the Zn(OH) ₂ ⁰ , ZnOH⁺, and Zn(OH) ₃ ^? —hydroxyl complexes and the ZnCl ₂ ⁰ , and ZnCl⁺ chlorocomplexes, which are predominant at low Cl concentrations (C_Cl < 0.05–0.1 m) give way to ZnCl ₄ ^2? with increasing C_Cl, which becomes the predominant Zn species of the fluid at C_Cl > 0.1–0.5 m throughout the whole temperature range in question and pressures higher than 1 kbar. For Pb-bearing fluids, the T-P-X region dominated by the Pb(OH) ₂ ⁰ , and Pb(OH) ₃ ^? hydroxyl complexes is remarkably wider than the analogous region for Zn, particularly at elevated temperatures (≥700°C) in alkaline solutions. An increase in C_Cl is associated with an increase in the concentration and changes in the speciation of Pb chlorocomplexes: PbCl ₂ ⁰ → PbCl ₃ ^? → PbCl ₄ ^2? . The concentrations of Zn and Pb chlorocomplexes increase with increasing pressure, decreasing temperature, and decrease pH with the addition of HCl to the system. It is demonstrated that the solubility of ZnO at any given T-P-X in alkaline solutions with low chloride concentrations are lower than the solubility of PbO. The Zn concentration increases more significantly than with the Pb concentration with increasing C_Cl and decreasing pH, so that the Zn concentration in acidic solutions is higher than the Pb concentration over broad ranges of temperature, pressure, and Cl concentration. Chloride complexes of Zn (ZnCl ₂ ⁰ , and ZnCl ₄ ^2? ) and Pb (PbCl ₂ ⁰ , and PbCl ₃ ^? are proved to be predominant within broad T-P-X-pH ranges corresponding to the parameters under which magmatic fluid are generated. Our simulation results confirm the hypothesis that chlorocomplexes play a leading role in Zn and Pb distribution between aqueous chloride fluids and granitic melts. These simulation results are consistent with experimental data on the Zn and Pb distribution coefficients (D(Zn)^f/m and D(Pb)^f/m, respectively) between aqueous chloride fluids and granitic melts that demonstrated that (1) D(Zn)^f/m and D(Pb)^f/m increase with increasing Na and K chloride concentrations in the aqueous fluid, (2) both D(Zn)^f/m and D(Pb)^f/m drastically increase when HCl is added to the fluid, and (3) (D(Zn)^f/m is higher than D(Pb)^f/m at any given T-P-X parameters. The experimentally established decrease in D(Zn)^f/m and D(Pb)^f/m with increasing pressure (at unchanging temperature and Cl concentration) is likely explained by an increase in the alkalinity of the aqueous chloride fluid in equilibrium with granite melt and, correspondingly, a decrease in the Zn and Pb solubility in this fluid.