Isotopic fractionation of Mg(aq), Ca(aq), and Fe(aq) with carbonate minerals

Density-functional electronic structure calculations are used to compute the equilibrium constants for ²⁶Mg/²⁴Mg and ⁴⁴Ca/⁴⁰Ca isotope exchange between carbonate minerals and uncomplexed divalent aquo ions. The most reliable calculations at the B3LYP/6-311++G(2d,2p) level predict equilibrium constants K, reported as 10³ln (K) at 25 °C, of −5.3, −1.1, and +1.2 for ²⁶Mg/²⁴Mg exchange between calcite (CaCO₃), magnesite (MgCO₃), and dolomite (Ca_0.5Mg_0.5CO₃), respectively, and Mg²⁺(aq), with positive values indicating enrichment of the heavy isotope in the mineral phase. For ⁴⁴Ca/⁴⁰Ca exchange between calcite and Ca²⁺(aq) at 25 °C, the calculations predict values of +1.5 for Ca²⁺(aq) in 6-fold coordination and +4.1 for Ca²⁺(aq) in 7-fold coordination. We find that the reduced partition function ratios can be reliably computed from systems as small as and embedded in a set of fixed atoms representing the second-shell (and greater) coordination environment. We find that the aqueous cluster representing the aquo ion is much more sensitive to improvements in the basis set than the calculations on the mineral systems, and that fractionation factors should be computed using the best possible basis set for the aquo complex, even if the reduced partition function ratio calculated with the same basis set is not available for the mineral system. The new calculations show that the previous discrepancies between theory and experiment for Fe³⁺-hematite and Fe²⁺-siderite fractionations arise from an insufficiently accurate reduced partition function ratio for the Fe³⁺(aq) and Fe²⁺(aq) species.     

Thermodynamic properties of the Sb(III) hydroxide complex Sb(OH)3(aq) at hydrothermal conditions

The standard thermodynamic properties and Helgeson-Kirkham-Flowers (HKF) parameters for Sb(OH)_3(aq) have been estimated. For this purpose, the available solubility data for senarmontite, valentinite, stibnite, and native Sb in a wide range of temperatures (15 to 450°C) and pressures (1 to 1000 bar), and thermodynamic properties of Sb oxides (senarmontite and valentinite) have been critically analyzed. Published data were complimented by results from new experiments performed by solubility and solid-state galvanic cell methods. Both experimental data and thermodynamic calculations show that the hydroxide complex Sb(OH)_3(aq) is primarily responsible for hydrothermal transport of antimony, especially at temperatures above 250°C.     

Thermodynamics of feldspathoid solutions

We have developed models for the thermody-namic properties of nephelines, kalsilites, and leucites in the simple system NaAlSiO₄?KAlSiO₄?Ca_0.5AlSiO₄?SiO₂?H₂O that are consistent with all known constraints on subsolidus equilibria and thermodynamic properties, and have integrated them into the existing MELTS software package. The model for nepheline is formulated for the simplifying assumptions that (1) a molecular mixing-type approximation describes changes in the configurational entropy associated with the coupled exchange substitutions □Si?NaAl and □Ca? Na₂ and that (2) Na⁺ and K⁺ display long–range non-convergent ordering between a large cation and the three small cation sites in the Na₄Al₄Si₄O₁₆ formula unit. Notable features of the model include the prediction that the mineral tetrakalsilite (“panunzite”, sensu stricto) results from anti-ordering of Na and K between the large cation and the three small cation sites in the nepheline structure at high temperatures, an average dT/dP slope of about 55^°/kbar for the reaction over the temperature and pressure ranges 800–1050 ^°C and 500–5000 bars, roughly symmetric (i.e. quadratic) solution behavior of the K–Na substitution along joins between fully ordered components in nepheline, and large positive Gibbs energies for the nepheline reciprocal reactions and and for the leucite reciprocal reaction      

Uranophane dissolution and growth in CaCl2-SiO2(aq) test solutions

The dissolution and growth of uranophane [Ca(UO₂)₂(SiO₃OH)₂·5H₂O] have been examined in Ca- and Si-rich test solutions at low temperatures (20.5 ± 2.0 °C) and near-neutral pH (∼6.0). Uranium-bearing experimental solutions undersaturated and supersaturated with uranophane were prepared in matrices of ∼10⁻² M CaCl₂ and ∼10⁻³ M SiO₂(aq). The experimental solutions were reacted with synthetic uranophane and analyzed periodically over 10 weeks. Interpretation of the aqueous solution data permitted extraction of a solubility constant for the uranophane dissolution reaction and standard state Gibbs free energy of formation for uranophane ( kJ mol⁻¹).     

Thermodynamic properties of Pd chloride complexes and the Pd2+(aq) ion in aqueous solutions

Based on the expert review of literature data on the thermodynamic properties of species in the Cl-Pd system, stepwise and overall stability constants are recommended for species of the composition [PdCl _n ]^{2 ? n} , and the standard electrode potential of the half-cell PdCl ₄ ^2? /Pd(c) is evaluated at E _298,15° = 0.646 ± 0.007 V, which corresponds to Δ _f G _298.15° = ?400.4 ± 1.4 kJ/mol for the ion PdCl ₄ ^2? (aq). Derived from calorimetric data, Δ _f H _298.15° PdCl ₄ ^2? (aq) = ?524.6 ± 1.6 kJ/mol and Δ _f H _298.15° Pd²⁺(aq) = 189.7 ± 2.6 kJ/mol. The assumed values of the overall stability constant of the PdCl ₄ ^2? ion and the standard electrode potential of the PdCl ₄ ^2? /Pd(c) half-cell correspond to Δ _f G _298.15° = 190.1 ± 1.4 kJ/mol and S _298.15° = ?94.2 ± 10 J/(mol K) for the Pd²⁺(aq) ion.     

Solubility of gold in hydrothermal chloride solutions

The solubility of gold has been determined in chloride solutions in the temperature range 300–500°C corresponding to the inferred range for the formation of “hypothermal” gold deposits. The solutions were buffered with respect to HC1 by a K-feldspar-muscovite-quartz assemblage, and to oxygen by the assemblage haematite-magnetite. Solubilities increased rapidly with temperature from about 10 p.p.m. at 300°C, to 500 and 1000 p.p.m. at 500°C at 1000 and 2000 bar, respectively.These results are discussed in terms of possible solution species in this high-temperature region where molecular behaviour predominates in the solution equilibria. It is suggested that gold and other metals may be transported to the site of ore-deposition in undersaturated high-temperature solutions. Ore deposition may take place at lower temperatures where ionic gold chloride or sulfide species dominate the chemistry of the ore solutions.     

Marcasite precipitation from hydrothermal solutions

Pyrite and marcasite were precipitated by both slow addition of aqueous Fe²⁺ and SiO₃²⁻ to an H₂S solution and by mixing aqueous Fe²⁺ and Na₂S₄ solutions at 75°C. H₂S₂ or HS₂⁻ and H₂S₄ or HS₄⁻ were formed in the S₂O₃²⁻ and Na₂S₄ experiments, respectively. Marcasite formed at pH < pK₁ of the polysulfide species present (for H₂S₂, pK₁ = 5.0; for H₂S₄, pK₁ = 3.8 at 25°C). Marcasite forms when the neutral sulfane is the dominant polysulfide, whereas pyrite forms when mono-or divalent polysulfides are dominant. In natural solutions where H₂S₂ and HS₂ are likely to be the dominant polysulfides, marcasite will form only below pH 5 at all temperatures.

The pH-dependent precipitation of pyrite and marcasite may be caused by electrostatic interactions between polysulfide species and pyrite or marcasite growth surfaces: the protonated ends of H₂S₂ and HS₂ are repelled from pyrite growth sites but not from marcasite growth sites. The negative ions HS₂ and S₂²⁻ are strongly attracted to the positive pyrite growth sites. Masking of 1π_g^* electrons in the S₂ group by the protons makes HS₂ and H₂S₂ isoelectronic with AsS²⁻ and As₂²⁻, respectively ( et al., 1981). Thus, the loellingitederivative structure (marcasite) results when both ends of the polysulfide are protonated.

Marcasite occurs abundantly only for conditions below pH 5 and where H₂S₂ was formed near the site of deposition by either partial oxidation of aqueous H₂S by O₂ or by the reaction of higher oxidation state sulfur species that are reactive with H₂S at the conditions of formation e.g., S₂O₃²⁻ but not SO₄²⁻. The temperature of formation of natural marcasite may be as high as 240°C ( and , 1985), but preservation on a multimillion-year scale seems to require post-depositional temperatures of below about 160°C ( , 1973; and , 1985).     

Sulfoarsenide complexes of gold in hydrothermal solutions in the formation of gold ore deposits (thermodynamic modeling)

The composition and conditions of the formation of gold sulfoarsenide complexes were studied by means of the solution of inverse problems of physicochemical modeling on the basis of the SELEKTOR software, with the computer analysis of the experimental data on Au dissolution in the presence of orpiment at 200–300°C and 1 kbar pressure. It was shown that sulfoarsenide complexes of gold formed in sulfurous-arsenious metalliferous hydrothermal solutions quantitatively prevailed in acidic and near-neutral solutions in the presence of As. The stability of the H₂AuAsS ₃ ⁰ sulfoarsenide complex and of its AuAsS ₂ ⁰ deprotonated analogue depends on the concentration of arsenic in the system, just as the ratio of sulfoarsenide and hydrosulfide complexes of gold. The productive metalliferous generations of sulfides in pyrite-arsenopyrite parageneses are deposited involving gold sulfoarsenides.     

Experimental determination of zirconium speciation in hydrothermal solutions

The solubility of ZrO₂(baddeleyite) in HCl, HF, H₂SO₄, NaOH, and Na₂CO₃ solutions was determined by the capsule method at 500°C and 1000 bar. Baddeleyite is the only solid phase detected in the experimental products. Based on the ZrO₂(baddeleyite) solubility measurements, the values of equilibrium constants at 500°C and 1000 bar (consistent with the Gibbs free energies of all the reactants) were obtained for the following reactions: ZrO₂(cr) + H₂SO ₄ ⁰ = Zr(OH)₂OH ₄ ⁰ (pK^o = 4.95), ZrO₂(cr) + 2H₂SO ₄ ⁰ = Zr(SO₄) ₂ ⁰ ) + 2H₂O (pK^o = 3.74), ZrO₂(cr) + H₂O + HF⁰ = Zr(OH)₃F⁰ (pK^o = 3.35), ZrO₂(cr) + 2HF⁰ = Zr(OH)₂F ₂ ⁰ (pK^o = 2.37), and ZrO₂(cr) + 2H₂O + OH^? = Zr(OH) ₅ ^? (pK^o = 4.39). Ionization constants were estimated for the chloride, fluoride, sulfate, and hydroxo complexes of zirconium. Using the experimental data and thermodynamic information derived from experiments and the electrostatic model of the ionization of electrolytes, it was shown that no more than n mg zirconium per one kilogram H₂O can be accumulated in high-temperature fluids at 500°C and 1000 bar.     

Empirical prediction of the Pitzer’s interaction parameters for cationic Al species with both SiO2(aq) and CO2(aq): Implications for the geochemical modelling of very saline solutions

The Pitzer’s interaction parameters, λ_N–M, involving the Mth cationic Al species Al³⁺ or AlOH²⁺ or AlO⁺ and the Nth neutral species SiO_2(aq) (at temperatures of 25–300 °C) or CO_2(aq) (at temperatures of 25–150 °C), have been evaluated through empirical linear relationships between λ_N–M and the surface electrostatic field of the ionic species of interest. These relationships have been obtained starting from the known λ_N–M for both SiO_2(aq) and CO_2(aq) with the main dissolved cations. The Pitzer’s interaction parameter thus estimated for the pair CO_2(aq)–Al³⁺ at 25 °C, 0.327, is 20–40% higher than the corresponding values obtained from CO₂ solubilities in concentrated solutions of AlCl₃, 0.272 ± 0.010 (2σ), and Al₂(SO₄)₃, 0.232 ± 0.002 (2σ), partly corroborating the empirical approach adopted in this study. To test the Pitzer’s interaction coefficients for cationic Al species with aqueous SiO₂, the log K values of the kaolinite dissolution reaction have been computed starting from available experimental data at 23–25 °C and ionic strengths of 0.0001–0.12 mol/kg adopting, alternatively, the Pitzer’s equations and the Debye–Hückel equation. A satisfactory agreement has been found between the log K values obtained through these two approaches, with maximum deviations of 0.11–0.12 log units. This good convergence of results is encouraging as it represents a necessary condition to prove the reliability of the Pitzer’s interaction coefficients estimated in this work. These results are a first step to take into account specific interactions among solutes in concentrated electrolyte solutions, such as those hosted in sedimentary basins or geothermal waters, for instance through the Pitzer’s equations. However, experimental or field data at higher ionic strengths are absolutely necessary to validate the reliability of the Pitzer’s interaction coefficients determined in this study.     

An experimental study of Cobalt (II) complexation in Cl and H2S-bearing hydrothermal solutions

The speciation of cobalt (II) in Cl⁻ and H₂S-bearing solutions was investigated spectrophotometrically at temperatures of 200, 250, and 300 °C and a pressure of 100 bars, and by measuring the solubility of cobaltpentlandite at temperatures of 120-300 °C and variable pressures of H₂S. From the results of these experiments, it is evident that CoHS⁺ and predominate in the solutions except at 150 °C, for which the dominant chloride complex is CoCl₃⁻. The logarithms of the stability constant for CoHS⁺ show moderate variation with temperature, decreasing from 6.24 at 120 °C to 5.84 at 200 °C, and increasing to 6.52 at 300 °C. Formation constants for chloride species increase smoothly with temperature and at 300°C their logarithms reach 8.33 for , 6.44 for CoCl₃⁻, 4.94 to 5.36 for , and 2.42 for CoCl⁺. Calculations based on the composition of a model hydrothermal fluid (Ksp-Mu-Qz, KCl = 0.25 m, NaCl = 0.75 m, ΣS = 0.3 m) suggest that at temperatures ?200 °C, cobalt occurs dominantly as CoHS⁺, whereas at higher temperatures the dominant species is .     

The Benedikt hydrothermal system (north-eastern Slovenia)

Records from magnetic signals in Buenos Aires loess–paleosol sequences have been published, but their relationship with environmental changes has been difficult to establish. Studies on the superparamagnetic (SP) population in present soils can help to understand these processes. Samples from present soils (Argiudolls) and a paleosol from the Buenos Aires Province (Argentina) were analyzed using low and room temperature magnetic measurements. They show that it is possible to support the hypothesis of a lognormal distribution of superparamagnetic particles, the median diameter near 15 nm. The presence of detritic pseudosingledomain (PSD) titanomagnetite, with low titanium content ranging between TM 28 and TM40, has been established. The linear correlation of SP content with ferrimagnetic susceptibility and with magnetization suggests that ferrimagnetic minerals drive the SP generation. Finally, it can be concluded that SP magnetic grains, in the Pampean plain, are generated by an inorganic process in adequate environmental pH and Eh of the soils.     

The solubility of fluorite in hydrothermal solutions,an experimental study

The solubility of fluorite in NaCl solutions increases with increasing temperature at all ionic strengths up to about 100°C. Above this temperature, the solubility passes through a maximum and possibly a minimum with increasing temperature at NaCl concentrations of 1.0M or less, and increases continuously with increasing temperature at NaCl concentrations above 1.0M. At any given temperature, the solubility of fluorite increases with increasing salt concentration in NaCl, KCl and CaCl₂ solutions. The solubility follows Debye-Hückel theory for KCl solutions. In NaCl and CaCl₂ solutions, the solubility of fluorite increases more rapidly than predicted by Debye-Hückel theory: the excess solubility is due to the presence of NaF^c, CaF⁺, and possibly of Na₂F⁺. The solubility of fluorite in NaCl-CaCl₂ and in NaCl-CaCl₂-MgCl₂ solutions is controlled by the common ion effect and by the presence of NaF^c, CaF⁺, and MgF⁺. The solubility of fluorite in NaCl-HCl solutions increases rapidly with increasing initial HCl concentration; the large solubility increase is due to the presence of HF^c. It seems likely that complexes other than those identified in this study rarely play a major role in fluoride transport and fluorite deposition at temperatures below 300°C.     

南秦岭造山带安康石梯—旬阳神河早古生代热水沉积盆地构造-沉积相与热水聚矿特征

南秦岭造山带安康石梯—旬阳神河一带早古生代为裂陷沉积盆地区,发育一套深水相“硅、灰、泥”沉积建造,伸展构造体制下形成的裂陷型盆地中,具有典型的深水沉积、火山喷流沉积与热水沉积同盆共存,形成规模巨大独具特色的以重晶石、磁铁矿为主的多金属成矿带.热水沉积成矿盆地构造-沉积相反映了不同的构造变形-岩石组合-地球化学-沉积岩相的多维组合.南秦岭带三级热水沉积盆地发育的构造-沉积相,可初步划分为3种类型:(火山)热水沉积成矿盆地构造-沉积相、深水缺氧环境中裂陷沉积成矿盆地的构造-沉积相、碳酸盐岩台地浅水沉积盆地的构造-沉积相.(火山)热水沉积成矿盆地构造沉积相主要表现为火山沉积、热水沉积、深水化学沉积、热水沉积成矿四位一体.裂陷沉积成矿盆地的构造沉积相主要表现为热水沉积、深水化学沉积、热水沉积成矿三位一体.碳酸盐岩浅水沉积盆地的构造沉积相主要表现为正常浅水沉积、热水沉积、热水沉积成矿三位一体.通过对区内沉积成矿盆地的识别分析,三级构造热水沉积成矿盆地受控于盆地中的同生断裂和火山活动,具有沉积岩相、热水沉积岩组合、火山喷流沉积组合、显著成矿作用及物化探异常分布.三级构造热水沉积成矿盆地是矿床定位的构造空间,四级热水沉积洼地为矿体(矿层)的容纳空间.区内热水沉积岩主要为重晶石(毒重石)岩、硅质岩、磁铁钠长石岩和铁碳酸盐岩类,重晶石、磁铁矿等矿产多产于热水沉积岩中或火山(喷流)沉积岩的上盘.     

Experimental determination of the stability and stoichiometry of sulphide complexes of silver(I) in hydrothermal solutions to 400°C

The solubility of silver sulphide (acanthite/argentite) has been measured in aqueous sulphide solutions between 25 and 400°C at saturated water vapour pressure and 500 bar to determine the stability and stoichiometry of sulphide complexes of silver(I) in hydrothermal solutions. The experiments were carried out in a flow-through autoclave, connected to a high-performance liquid chromatographic pump, titanium sampling loop, and a back-pressure regulator on line. Samples for silver determination were collected via the titanium sampling loop at experimental temperatures and pressures. The solubilities, measured as total dissolved silver, were in the range 1.0 × 10⁻⁷ to 1.30 × 10⁻⁴ mol kg⁻¹ (0.01 to 14.0 ppm), in solutions of total reduced sulphur between 0.007 and 0.176 mol kg⁻¹ and pH_T,p of 3.7 to 12.7. A nonlinear least squares treatment of the data demonstrates that the solubility of silver sulphide in aqueous sulphide solutions of acidic to alkaline pH is accurately described by the reactions0.5Ag₂S(s) + 0.5H₂S(aq) = AgHS(aq) K_s,1110.5Ag₂S(s) + 0.5H₂S(aq) + HS⁻ = Ag(HS)2− K_s,122Ag₂S(s) + 2HS⁻ = Ag₂S(HS)22− K_s,232where AgHS(aq) is the dominant species in acidic solutions, Ag(HS)2− under neutral pH conditions and Ag₂S(HS)22− in alkaline solutions. With increasing temperature the stability field of Ag(HS)2− increases and shifts to more alkaline pH in accordance with the change in the first ionisation constant of H₂S(aq). Consequently, Ag₂S(HS)22− is not an important species above 200°C. The solubility constant for the first reaction is independent of temperature to 300°C, with values in the range logK_s,111 = −5.79 (±0.07) to −5.59 (±0.09), and decreases to −5.92 (±0.16) at 400°C. The solubility constant for the second reaction increases almost linearly with inverse temperature from logK_s,122 = −3.97 (±0.04) at 25°C to −1.89 (±0.03) at 400°C. The solubility constant for the third reaction increases with temperature from logK_s,232 = −4.78 (±0.04) at 25°C to −4.57 (±0.18) at 200°C. All solubility constants were found to be independent of pressure within experimental uncertainties. The interaction between Ag⁺ and HS⁻ at 25°C and 1 bar to form AgHS(aq) has appreciable covalent character, as reflected in the exothermic enthalpy and small entropy of formation. With increasing temperature, the stepwise formation reactions become progressively more endothermic and are accompanied by large positive entropies, indicating greater electrostatic interaction. The aqueous speciation of silver is very sensitive to fluid composition and temperature. Below 100°C silver(I) sulphide complexes predominate in reduced sulphide solutions, whereas Ag⁺ and AgClOH⁻ are the dominant species in oxidised waters. In high-temperature hydrothermal solutions of seawater salinity, chloride complexes of silver(I) are most important, whereas in dilute hydrothermal fluids of meteoric origin typically found in active geothermal systems, sulphide complexes predominate. Adiabatic boiling of dilute and saline geothermal waters leads to precipitation of silver sulphide and removal of silver from solution. Conductive cooling has insignificant effects on silver mobility in dilute fluids, whereas it leads to quantitative loss of silver for geothermal fluids of seawater salinity.     

A geochemical model for removal of iron(II)(aq) from mine water discharges

A steady state geochemical model has been developed to assist in understanding surface-catalysed oxidation of aqueous Fe(II) by O₂(aq), which occurs rapidly at circumneutral pH. The model has been applied to assess the possible abiotic removal of Fe(II)(aq) from alkaline ferruginous mine water discharges using engineered reactors with high specific-surface area filter media. The model includes solution and surface speciation equilibrium, oxidation kinetics of dissolved and adsorbed Fe(II) species and mass transfer of O₂(g). Limited field data for such treatment of a mine water discharge were available for model development and assessment of possible parameter values. Model results indicate that an adsorption capacity between 10⁻⁶ and 10⁻⁵ mol l⁻¹ is sufficient for complete removal, by oxidation, of the Fe(II)(aq) load at the discharge. This capacity corresponds approximately to that afforded by surface precipitation of Fe(III) oxide onto plastic trickling filter media typically used for biological treatment of wastewater. Extrapolated literature values for microbial oxidation of Fe(II)(aq) by neutrophilic microbial populations to the simulated reactor conditions suggested that the microbially-mediated rate may be several orders-of-magnitude slower than the surface-catalysed oxidation. Application of the model across a range of mine water discharge qualities shows that high Fe(II)(aq) loadings can be removed if the discharge is sufficiently alkaline. Additional reactor simulations indicate that reactor efficiency decreases dramatically with pH in the near acid region, coinciding with the adsorption edge for Fe²⁺ on Fe oxyhydroxide. Alkaline discharges thus buffer pH within the range where Fe(II)(aq) adsorbs onto the accreting Fe hydroxide mineral surface, and undergoes rapid catalytic oxidation. The results suggest that the proposed treatment technology may be appropriate for highly ferruginous alkaline discharges, typically associated with abandoned deep coal mines.     

Existing forms of tungsten in hydrothermal solutions and forming conditions of scheelite

Experiments on the solubility of WO₃ in HCl and KCl solutions at 200°C show that tungsten cannot migrate in the form of chloride in solutions. In Cl-rich hydrothermal solutions of moderate salinity, tungsten migrates mainly as alkali salts of HWO ₄ ^? and WO ₄ ^2? . Determination of the solubility of WO₃ in HF and KHF₂ solutions at 100–300°C shows that tungsten migrates steadily as WO₃F^? and WO₂F ₃ ^? in F-rich hydrothermal solutions. Experiments and thermodynamic calculations also indicate that silico-wolframic acid, polymeric wolframic acid and sulfoxy wolframic acid cannot extensively occur in hydrothermal solutions. In addition, the physicochemical conditions of formation of scheelite are also discussed in the present paper.     

Experimental study of the alteration process of labradorite in acid hydrothermal solutions

Labradorite was altered artificially by HC1 solution ranging from M = 1 to M = 0.003 at 245 and 230°C. The products of alteration were examined by X-ray diffraction, electron microscopy, electron diffraction, infrared spectroscopy and the electron microprobe and the solution was analyzed chemically.Amorphous silica only was formed in solutions with M_HCl = 1 and M_HCl = 0.3. In a solution with M_HCl = 0.2, amorphous silica was initially formed, later dissolved and replaced by kaolinite. A mixture of microcrystalline boehmite and amorphous aluminosilicate was formed, altering to kaolinite in solutions with M_HCl = 0.1 and 0.3. Small amounts of kaolinite were initially formed but the alteration soon stopped in solution with M_HCl = 0.003. Relationships between the alteration processes and pH of the solutions can be roughly explained by using solubility diagrams assuming the congruent dissolution of labradorite and precipitation of the products in partial equilibrium. However, these assumptions are not valid with strongly acid solutions.The rate of dissolution of labradorite is controlled not only by its surface area, but also by the diffusion of matter through the layer of alteration products.