Solubility of rhodochrosite (MnCO3) in water and seawater

The solubility product of rhodochrosite (MnCO₃) was measured in seawater, deionized water and dilute NaCl solutions. The solubility product extrapolated to infinite dilution at 25.0 C was (2.60 ± 0.07)× 10^?11. The stoichiometric solubility product measured in seawater of 34.27%. salinity was (3.24 ± 0.23) × 10^?9 at 25.0 C and (2.28 ± 0.24) × 10^?9 at 3.3 C. The stoichiometric solubility product is in good agreement with the value calculated from an ion association model. The enthalpy of the reaction is in fair agreement with the estimated value.     

Solubility of ettringite (Ca6[Al(OH)6]2(SO4)3 · 26H2O) at 5–75°C

The solubility of ettringite (Ca₆[Al(OH)₆]₂(SO₄)₃ · 26H₂O) was measured in a series of dissolution and precipitation experiments at 5–75°C and at pH between 10.5 and 13.0 using synthesized material. Equilibrium was established within 4 to 6 days, with samples collected between 10 and 36 days. The log K_SP for the reaction Ca₆[Al(OH)₆]₂(SO₄)₃ · 26H₂O ⇌ 6Ca²⁺ + 2Al(OH)₄⁻ + 3SO₄²⁻ + 4OH⁻ + 26H₂O at 25°C calculated for dissolution experiments (−45.0 ± 0.2) is not significantly different from the log K_SP calculated for precipitation experiments (−44.8 ± 0.4) at the 95% confidence level. There is no apparent trend in log K_SP with pH and the mean log K_SP,298 is −44.9 ± 0.3. The solubility product decreased linearly with the inverse of temperature indicating a constant enthalpy of reaction from 5 to 75°C. The enthalpy and entropy of reaction ΔH^°_r and ΔS^°_r, were determined from the linear regression to be 204.6 ± 0.6 kJ mol⁻¹ and 170 ± 38 J mol⁻¹ K⁻¹. Using our values for log K_SP, ΔH^°_r, and ΔS^°_r and published partial molal quantities for the constituent ions, we calculated the free energy of formation ΔG^°_f,298, the enthalpy of formation ΔH^°_f,298, and the entropy of formation ΔS^°_f,298 to be −15211 ± 20, −17550 ± 16 kJ mol⁻¹, and 1867 ± 59 J mol⁻¹ K⁻¹. Assuming ΔC_P,r is zero, the heat capacity of ettringite is 590 ± 140 J mol⁻¹ K⁻¹.     

Preparation and solubility of northupite from brine and its adsorption properties for Cu(II) and Cd(II) in seawater

The mineral northupite Na₃Mg(CO₃)₂Cl was synthesized from a solar Adriatic seawater brine pond to which Na₂CO₃ was added at 373°K. The precipitated northupite had a surface area (P) of 6.0 ± 0.4 m²g⁻¹, and the thermodynamic solubility product was estimated to be log K Na₃Mg(CO₃)₂Cl = −4.8 ± 0.3 at 25°C. This value was used to calculate the interfacial energy (σ = 50 erg cm⁻²) for the homogeneous nucleation of northupite. The solubility constant determined in this study has been used to examine the saturation state of Mahega Lake and Lake Katwe (Uganda). The waters from Lake Katwe were found to be supersaturated with respect to northupite.The adsorption of Cu and Cd onto northupite particles was studied in seawater. Both metals are strongly adsorbed. Adsorption constants and the specific area of northupite occupied by Cd and Cu using Langmuir adsorption isotherms and equilibrium constants for surface complex formation have been determined.     

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

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

Model for the formation of arsenic contamination in groundwater. 1. Datong Basin,China

Mineral equilibria were analyzed in the system As-bearing rock-meteoric water. It was shown that carbonate rocks are the most probable source of As and Sr in the waters of the Datong Basin (People's Republic of China). The reason for groundwater enrichment in As is the shift of the equilibrium FeCO₃ (siderite) + H₂O = FeOOH(goethite) + CO₂(g) + H₂(g) to the left (toward siderite formation) owing to organic matter oxidation by atmospheric oxygen and an increase in the equilibrium partial pressure of CO₂, while the Eh of the system remains below ?0.30 ± 0.06 V.     

Role of molecular oxygen in the dissolution of siderite and rhodochrosite

The dissolution of siderite (FeCO₃) and rhodochrosite (MnCO₃) under oxic and anoxic conditions is investigated at 298 K. The anoxic dissolution rate of siderite is 10^−8.65 mol m⁻² s⁻¹ for 5.5 < pH < 12 and increases as [H⁺]^0.75 for pH < 5.5. The pH dependence is consistent with parallel proton-promoted and water hydrolysis dissolution pathways. Atomic force microscopy (AFM) reveals a change in pit morphology from rhombohedral pits for pH > 4 to pits elongated at one vertex for pH < 4. Under oxic conditions the dissolution rate decreases to below the detection limit of 10⁻¹⁰ mol m⁻² s⁻¹ for 6.0 < pH < 10.3, and hillock precipitation preferential to steps is observed in concurrent AFM micrographs. X-ray photoelectron spectroscopy (XPS) and thermodynamic analysis identify the precipitate as ferrihydrite. At pH > 10.3, the oxic dissolution rate is as high as 10^−7.5 mol m⁻² s⁻¹, which is greater than under the corresponding anoxic conditions. A fast electron transfer reaction between solution O₂ or [Fe³⁺(OH)₄]⁻ species and surficial >Fe^II hydroxyl groups is hypothesized to explain the dissolution kinetics. AFM micrographs do not show precipitation under these conditions. Anoxic dissolution of rhodochrosite is physically observed as rhombohedral pit expansion for 3.7 < pH < 10.3 and is chemically explained by parallel proton- and water-promoted pathways. The dissolution rate law is 10^−4.93[H⁺] + 10^−8.45 mol m⁻² s⁻¹. For 5.8 < pH < 7.7 under oxic conditions, the AFM micrographs show a tabular precipitate growing by preferential expansion along the a-axis, though the macroscopic dissolution rate is apparently unaffected. For pH > 7.7 under oxic conditions, the dissolution rate decreases from 10^−8.45 to 10^−9.0 mol m⁻² s⁻¹. Flattened hillock precipitates grow across the entire surface without apparent morphological influence by the underlying rhodochrosite surface. XPS spectra and thermodynamic calculations implicate the precipitate as bixbyite for 5.8 < pH < 7.7 and MnOOH (possibly feitnkechtite) for pH >7.7.     

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

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

Experimental studies of oxygen isotope fractionation between rhodochrosite (MnCO3) and water at low temperatures

Rhodochrosite crystals were precipitated from Na-Mn-Cl-HCO₃ parent solutions following passive, forced and combined passive-to-forced CO₂ degassing methods. Forced and combined passive-to-forced CO₂ degassing produced rhodochrosite crystals with a small non-equilibrium oxygen isotope effect whereas passive CO₂ degassing protocols yielded rhodochrosite in apparent isotopic equilibrium with water. On the basis of the apparent equilibrium isotopic data, a new temperature-dependent relation is proposed for the oxygen isotope fractionation between rhodochrosite and water between 10 and 40 °C:

1000lnα_{rhodochrosite-water}=17.84±0.18(10³/T)-30.24±0.62     

Solubility of Fe2(OH)3Cl (pure-iron end-member of hibbingite) in NaCl and Na2SO4 brines

Pure-iron end-member hibbingite, Fe₂(OH)₃Cl(s), may be important to geological repositories in salt formations, as it may be a dominant corrosion product of steel waste canisters in an anoxic environment in Na–Cl- and Na–Mg–Cl-dominated brines. In this study, the solubility of Fe₂(OH)₃Cl(s), the pure-iron end-member of hibbingite (Fe^II, Mg)₂(OH)₃Cl(s), and Fe(OH)₂(s) in 0.04 m to 6 m NaCl brines has been determined. For the reactionFe₂(OH)₃Cl(s) + 3H⁺ ? 3 H₂O + 2 Fe²⁺ + Cl^?,the solubility constant of Fe₂(OH)₃Cl(s) at infinite dilution and 25 °C has been found to be log₁₀ K = 17.12 ± 0.15 (95% confidence interval using F statistics for 36 data points and 3 parameters). For the reactionFe(OH)₂(s) + 2H⁺ ? 2 H₂O + Fe²⁺,the solubility constant of Fe(OH)₂ at infinite dilution and 25 °C has been found to be log₁₀ K = 12.95 ± 0.13 (95 % confidence interval using F statistics for 36 data points and 3 parameters). For the combined set of solubility data for Fe₂(OH)₃Cl(s) and Fe(OH)₂(s), the Na⁺–Fe²⁺ pair Pitzer interaction parameter θ_{Na⁺/Fe²⁺} has been found to be 0.08 ± 0.03 (95% confidence interval using F statistics for 36 data points and 3 parameters). In nearly saturated NaCl brine we observed evidence for the conversion of Fe(OH)₂(s) to Fe₂(OH)₃Cl(s). Additionally, when Fe₂(OH)₃Cl(s) was added to sodium sulfate brines, the formation of green rust(II) sulfate was observed, along with the generation of hydrogen gas. The results presented here provide insight into understanding and modeling the geochemistry and performance assessment of nuclear waste repositories in salt formations.     

Ferromanganese carbonate metasediments of the Sob area,Polar Urals: Bedding conditions,composition, and genesis

The paper presents the results of study of ferromanganese carbonate rocks in the Sob area (Polar Urals), which is located between the Rai-Iz massif and the Seida–Labytnangi Railway branch. These rocks represent low-metamorphosed sedimentary rocks confined to the Devonian carbonaceous siliceous and clayey–siliceous shales. In terms of ratio of the major minerals, ferromanganese rocks can be divided into three varieties composed of the following minerals: (1) siderite, rhodochrosite, chamosite, quartz, ± kutnahorite, ± calcite, ± magnetite, ± pyrite, ± clinochlore, ± stilpnomelane; (2) spessartite, rhodochrosite, and quartz, ± hematite, ± chamosite; (3) rhodochrosite, spessartite, pyroxmanite, quartz ± tephroite, ± fridelite, ± clinochlore, ± pyrophanite, ± pyrite. In all varieties, the major concentrators of Mn and Fe are carbonates (rhodochrosite, siderite, kutnahorite, Mn-calcite) and chlorite group minerals (clinochlore, chamosite). The chemical composition of rocks is dominated by Si, Fe, Mn, carbon dioxide, and water (L.O.I.): total SiO₂ + Fe₂O ₃ ^tot + MnO + L.O.I. = 85.6?98.4 wt %. The content of Fe and Mn varies from 9.3 to 55.6 wt % (Fe₂O ₃ ^tot + MnO). The Mn/Fe ratio varies from 0.2 to 55.3. In terms of the aluminum module Al_M = Al/(Al + Mn + Fe), the major portion of studied samples corresponds to metalliferous sediments. The δ¹³C_carb range (–30.4 to–11.9‰ PDB) corresponds to authigenic carbonates formed with carbon dioxide released during the microbial oxidation of organic matter in sediments at the dia- and/or catagenetic stage. Ferromanganese sediments were likely deposited in relatively closed seafloor zones (basin-traps) characterized by periodic stagnation. Fe and Mn could be delivered from various sources: input by diverse hydrothermal solutions, silt waters in the course of diagenesis, river discharges, and others. The diagenetic delivery of metals seems to be most plausible. Mn was concentrated during the stagnation of bottom water in basin-traps. Interruption of stagnation promoted the precipitation of Mn. The presence of organic matter fostered a reductive pattern of postsedimentary transformations of metalliferous sediments. Fe and Mn were accumulated initially in the oxide form. During the diagenesis, manganese and iron oxides reacted with organic matter to make up carbonates. Relative to manganese carbonates, iron carbonates were formed under more reductive settings and higher concentrations of carbon dioxide in the interstitial solution. Crystallization of manganese and iron silicates began already at early stages of lithogenesis and ended during the regional metamorphism of metalliferous sediments.     

Determination of the first ionization constant of silicic acid from quartz solubility in borate buffer solutions to 350°C

The solubility of quartz has been determined in borax buffer solutions having total boron concentrations of 0.10, 0.20, 0.40 and 0.60 mol kg^?1 and over the temperature range 130–350°C at the saturated vapour pressure of the system. The first ionization constant of silicic acid was calculated from the solubility data and varied from ?logK₁ = 8.88 (± 0.15) at 130°C to ?logK₁ = 10.06 (± 0.20) at 350°C. The solubility of quartz in these solutions was due to the presence of the three species, H₄SiO₄, H₃SiO₄^? and NaH₃SiO₄°. The equilibrium constant for the reaction, Na⁺ + H₃SiO₄^? = NaH₃SiO₄° extended from log K_as = 1.18?1.40 (± 0.20) over the temperature interval 135–301°C. The formation of NaH₃SiO₄° ion pairs was concluded to contribute significantly to the solubility of quartz in alkaline hydrothermal solutions when pH > 8 and sodium concentration exceeds 0.10 mol kg^?1.     

Solubility and complexing of Ni in the system NiO-H2O-HCl

The solubility of bunsenite (NiO) in Cl-bearing fluids in the range of 450°–700°C, 1–2 kbar was determined using the Ag + AgCl acid buffer technique. Based on the results of the experiments, it is concluded that the associated NiCl⁰₂ complex is the dominant Ni species in the fluid over the entire temperature-pressure range investigated. The temperature dependence of the equilibrium constant for the reaction NiO(s) + 2HCl⁰(aq) = NiCl⁰₂(aq) + H₂O is given by logK = ?4.17(±0.55) + 4629(±464)/T(K) at 1 kbar, and logK = ?4.75(±0.91) + 5933(±756)/T(K) at 2 kbar. The calculated difference in standard state Gibbs free energy of formation between NiCl⁰₂ and 2HCl⁰ in kcal is G⁰(NiCl⁰₂) ? 2G⁰(HCl⁰) = ?20.77(±2.22) + 0.03264(±0.0026)T(K), at 1 kbar and G⁰(NiCl⁰₂) ? 2G⁰(HCl⁰) = ?25.01(±1.35) + 0.03264(±0.0016)T(K) at 2 kbar. Comparison of the solubilities of Ni end-member minerals with those of Ca, Mn, Fe, and Mg indicates that nickel minerals generally are the least soluble at a given temperature and pressure. The relatively low solubility of Ni end-member minerals, combined with the relatively low concentration of Ni in most rocks, should result in a quite low mobility of Ni in hydrothermal fluids.     

碳酸盐矿物的阴极发光性与微量元素的关系

碳酸盐矿物的阴极发光特征与其成分有关。笔者用阴极发光与电子探针微区分析法对砂岩中碳酸盐矿物进行测试分析 ,其结果表明碳酸盐矿物的阴极发光与微量元素含量有如下规律 :①碳酸盐矿物在铁含量高于猝灭下限或锰含量低于激活下限时 ,不具有阴极发光性 ;②铁的猝灭下限约为 0 0 4mol,锰的激活下限为小于 7× 10 - 5mol;③铁 /锰比值越高 ,越不利于碳酸盐矿物阴极发光 ;但是铁 /锰比值小于 1的碳酸盐一定具有阴极发光 ;④铁或锰二者之一含量很少时不利于碳酸盐矿物的阴极发光。     

Celestite (SrSO4(s)) solubility in water,seawater and NaCl solution

Celestite solubility measurements have been conducted in pure water at temperatures from 10 to 90°C. Equilibrium was achieved with respect to a crystalline solid phase from both undersaturated and supersaturated solutions. The measurements show that the solubility undergoes a maximum near 20°C. LogK values for the solubility reaction are adequately described by the following expression over the temperature range 283.15 to 363.15 K: −logK= −35.3106+0.00422837T+318312/T²+14.99586 logT.The following thennodynamic values for the dissolution reaction of SrSO_4(s), at 25°C have been derived: ΔG_R⁰ = 37852 ± 30 Jmol⁻¹ΔH_R⁰ = −1668±920Jmol⁻¹ΔS_R⁰= −132.6±3.2JK^−1mol−1Celestite solubility measurements were also determined in NaCl solutions up to 5 m concentration and from 10 to 40°C. These data are in good agreement with the work of StrÜbel (1966), who reports solubility measurements to temperatures of 100°C.The application of the Pitzer relations and the solubility constants determined in this study to calculate celestite solubility in NaCl solutions yields excellent agreement between predicted values and experimental measurements over the entire range of temperature and NaCl concentration conditions. For the limited number of solubility measurements in seawater-type solutions and mixed-salt brines, the agreement using the Pitzer relations is within three percent of the measured solubility.     

Experimental study of Pd hydrolysis in aqueous solutions at 25–70°C

The hydrolysis of the Pd²⁺ ion in HClO₄ solutions was examined at 25–70°C, and the thermodynamic constants of equilibrium K ₍₁₎⁰ and K ₍₂₎⁰were determined for the reactions Pd²⁺ + H₂O = PdOH⁺ + H⁺ and Pd²⁺ + 2H₂O = Pd(OH)₂⁰ + 2H⁺, respectively. The values of log K ₍₁₎⁰ = −1.66 ± 0.5 (25°C) and −0.65 ± 0.25 (50°C) and log K ₍₂₎⁰ = −4.34 ± 0.3 (25°C) and −3.80 ± 0.3 (50°C) were derived using the solubility technique at 0.95 confidence level. The values of log K ₍₁₎⁰ = −1.9 ± 0.6 (25°C), −1.0 ± 0.4 (50°C), and −0.5 ± 0.3 (70°C) were obtained by spectrophotometric techniques. The palladium ion is significantly hydrolyzed at elevated temperatures (50–70°C) even in strongly acidic solutions (pH 1–1.5), and its hydrolysis is enhanced with increasing temperature.     

Eh ph Diagrams for mn, fe, co, ni, cu and as Under Seawater Conditions: Application of two new Types of eh ph Diagrams to the Study of Specific Problems in Marine Geochemistry

E_H pH diagrams have been calculated using the PHREEQC programme in order to establish the predominance fields of Mn, Fe, Co, Ni, Cu and As in bottom waters from the Angola Basin. Predominance fields are presented separately for both aquatic species and solid mineral phases in order to simplify interpretation of the data. The diagrams show significant differences from standard E_H pH diagrams for these elements calculated for freshwater at 25 °C and 1 bar which assume an element concentration of 10^-6 M. In particular, our diagrams show that Mn²⁺ and NiCO ₃ ⁰ are the predominant aquatic species for Mn and Ni in bottom seawater and FeOOH, Fe₂O₃, Fe₃O₄, CoFe₂O₄, CuFe₂O₄, CuFeO₂, and Ba₃ (AsO₄)₂ the predominant solid phases for Fe, Co, Cu and As, respectively. Mn and Ni are therefore undersaturated and Fe, Co, Cu and As supersaturated in bottom seawater from the Angola Basin. Neither rhodochrosite (MnCO₃) nor siderite (FeCO₃) can form in this marine environment in equilibrium with seawater. A mixed Mn-Ca carbonate is therefore formed within the pore waters of reducing sediments. The high Ni/Cu ratios in cobalt-rich manganese crusts formed adjacent to the oxygen minimum zone may be explained by the change from Cu²⁺ to CuCl ₃ ^2- as the dominant aquatic species of Cu in seawater at an E_H of +0.48 V.     

Experimental modeling of Au and Pt coupled transport by chloride hydrothermal fluids at 350–450°C and 500–1000 bar

The coupled solubility of Au_(cr) and Pt_(cr) has been measured in acidic chloride solutions at 350–450°С and 0.5 and 1 kb using the autoclave technique with determination of dissolved metal contents after quenching. The constants of the reaction combining the dominant species of Au and Pt in high-temperature hydrothermal fluids (K_(Au–Pt)) have been determined: 2 Au_(cr) + PtCl₄^2- = Pt_(cr) + 2AuCl₂^-; log K_(Au–Pt) =–1.02 ± 0.25 (450°С, 1 kb), 0.09 ± 0.15 (450°С, 0.5 kb), and –1.31 ± 0.20 (350°С, 1 kb). It has been established that the factors affecting the Au/Pt concentration ratio in hydrothermal fluids and precipitated ores are temperature, pressure, redox potential, and sulfur fugacity. An increase in temperature results in an increase in the Au/Pt concentration ratio (up to ~550°С at P = 1 kb). A decrease in pressure and redox potential leads to enrichment of fluid in Au. An increase in sulfur fugacity in the stability field of Pt sulfides results in increase in the Au/Pt concentration ratio. Native platinum is replaced by sulfide mineral in low-temperature systems enriched in Pt (relative to Au).     

Crystallographic studies of some olivine-related (Ni,Mg)3(PO4)2 solid solutions

Olivine-related (Ni, Mg)₃(PO₄)₂ solid solutions were prepared and equilibrated at 1070 K. Accurate monoclinic unit cell dimensions were determined from Guinier-Hägg photographic data. Structural refinements based on the X-ray profile-fitting technique after Rietveld were carried out for pure nickel (II) orthophosphate and for three Ni/Mg solid solutions. (Ni_1-x Mg _x )₃(PO₄)₂ phases with 0.40≦x≦0.60 are probably isostructural with Ni₃(PO₄)₂ (P2₁/a) while phases with low magnesium contents (<27 atom % Mg) deviate structurally from Ni₃(PO₄)₂. The results also show that Ni²⁺ is partially ordered at the octahedralM(1) sites, withK _D (Ni, Mg)=4.0±0.2     

Hydrostatic compression of ilmenite phase of ZnSiO3 and MgGeO3

Accurate measurements of cell parameters were performed on the ilmenite phases of ZnSiO₃ and MgGeO₃ using an X-ray diffraction method under hydrostatic conditions. The linear changes in cell parameter are represented by 1?a/a ₀=(1.06±0.04)×10^?4 P(kbar) and 1?c/c ₀=(2.11±0.04)×10^?4 P for ZnSiO₃, and 1?a/a ₀=(1.37±0.03)×10^?4 P and 1?c/c ₀=(2.05±0.04)×10^?4 P for MgGeO₃. A least-squares calculation using the first-order Birch-Murnaghan equation gives K _T =2.16±0.02 Mbar and K _T =1.87±0.02 Mbar for ZnSiO₃ and MgGeO₃, respectively. Elastic systematics assuming K _T V _m =constant give a predicted value K _T =2.14 Mbar for the ilmenite phase of MgSiO₃.     

Experimental study of fluorite solubility in acidic solutions as a method for boron fluoride complexes studying

Fluorite solubility in HCl and HF solutions with varied concentrations of boric acid was studied at 81, 155, and 208°C and saturated vapor pressure. Our experimental results demonstrate that fluorite solubility increases with increasing B(OH)₃ concentration, and this was interpreted as the formation of the BF3OH–complex (Ryss, 1956). The experimental data were used to determine, using the OptimA software, the free energies of formation of HF°(aq) and, which were then used to calculate the constants of the reactions HF = H⁺ + F^– (1) and B(OH)₃(aq) + 2H⁺ + 3 F^– (2). The pK ₁ values are 3.71 ± 0.013, 4.28 ± 0.015, and 4.89 ± 0.017 and pK ₂ 13.60 ± 0.02, 13.99 ± 0.02, and 14.95 ± 0.03 at saturated vapor pressure and 81, 155, and 208°C, respectively.