Decomposition of alkyl dixanthogens in aqueous solutions

Alkyl dixanthogens, (ROCSS)₂, decompose in aqueous solution in the presence of nucleophiles in many ways.It is proposed here that in alkaline solution the principal methods of decomposition of ethyl dixanthogen are by simultaneous attack of OH^? ions on the sulphur-sulphur bond to give products which include xanthate ion (ROCSS^?) and peroxide (H₂O₂) and on the carbon-sulphur bond to give products which include monothiocarbonate ion (ROSCO^?), sulphide ion (S^2?), and sulphur (S⁰). Above pH 12 reaction is complete in a few minutes, and more monothiocarbonate than xanthate is formed. At pH 9 the reaction takes over 20 h and more xanthate than monothiocarbonate is formed.The primary products react further to give various ions which depend in part on the pH of the system. In alkaline solution some of the xanthate and peroxide react to give perxanthate (ROCSSO^?). In acid solution both xanthate and monothiocarbonate decompose rapidly; CS₂ is formed from xanthate and OCS from monothiocarbonate.In the presence of other nucleophiles at pH 9.2, dissolved dixanthogen decomposes much more quickly than with OH^? alone, and other reactions occur. With thiosulphate a higher proportion of xanthate is formed together with some xanthyl thiosulphate and monothiocarbonate but no perxanthate. With sulphite (in the absence of oxygen) or cyanide the products include xanthate and monothiocarbonate but no perxanthate. With sulphite in the presence of oxygen, perxanthate is also formed.Suspensions of dixanthogens react slowly but in a similar fashion to dissolved dixanthogens.Longer-chain dixanthogens are much less soluble than ethyl dixanthogen but, in general, react in a similar way. Higher temperatures increase the rate of decomposition by OH^?.This work has various implications in operating plants.     

Perxanthates — A new factor in the theory and practice of flotation

A compound with a UV absorption maximum at 348 nm was observed in Mount Isa copper flotation plant solution. This spectrum was similar to that of the product of reaction of xanthate and peroxide in dilute, alkaline aqueous solution. The compound was termed perxanthate (more correctly “O”-alkyl dithiomonoperoxycarbonate).A new compound, ammonium sec-butyl perxanthate (C₄H₉ OCSSO·NH₄), was prepared by reacting potassium sec-butyl xanthate and hydrogen peroxide in dilute alkaline solution, acidifying, extracting into iso-octane, and precipitating with anhydrous ammonia. Solutions of this compound were compared with solutions containing the Mount Isa compound. Each compound was found to have the same UV absorption spectrum in a given solvent (alkaline aqueous, acid aqueous, chloroform, iso-octane, iso-amyl alcohol, and n-butyl acetate), but the spectra were different in different solvents (especially in alkaline and acid aqueous solutions). Both compounds could be extracted from acid, but not alkaline, aqueous solutions by organic solvents, and both had similar IR and mass spectra.It was concluded that the perxanthate in plant solution resulted from reaction of xanthate with peroxide derived from reduction of oxygen during flotation. This lends credence to the electrochemical theory of flotation and has some important theoretical and practical implications.     

Flotation of germanium n and p with potassium ethyl xanthate

Steady-state potential of the germanium electrode, flotation tests and results of spectrophotometric ATR (Attenuated Total Reflection) measurements of germanium surface in the presence and absence of potassium ethyl xanthate are presented.On the basis of the steady-state potentials of the germanium electrode in ethyl xanthate solutions, the standard potential E⁰ of the reaction: Ge + 2EtX^? = Ge (EtX)₂ + 2e is estimated. The pH ranges of the dixanthogen (EtX)₂ and the germanium xanthate Ge(EtX)₂ species predominance in the bulk solution are calculated. It has been established that flotation of germanium is greater than the natural floatability (in the absence of collectors) in the pH ranges where (EtX)₂ or Ge (EtX)₂ are present in the bulk solution. Spectrophotometric results reveal the presence of (EtX)₂ and Ge (EtX)₂ on the surface in the same pH ranges as is calculated for the bulk reactions.No significant differences in the surface properties and the flotation behaviour between germanium n and p have been found.     

Thermodynamic calculations on sulfide flotation systems, II. Comparisons with electrochemical experiments on the galena-ethyl xanthate system

Extensive thermodynamic calculations have been carried out to study the oxidation of galena and its flotation by potassium ethyl xanthate (KEX). In order to include some kinetic effects, three cases have been considered by assuming that sulfur oxidation can proceed as far as the formation of sulfate, thiosulfate and elemental sulfur.The results of these calculations have been compared with those of linear sweep voltammetry and intermittent galvanostatic polarization experiments conducted on galena at pH 9.2. Analysis of the experimental data indicates that in xanthate-free solutions, galena is oxidized primarily to PbO and S⁰ and, to a lesser extent, S₂O₃²⁻, rather than to the thermodynamically most favored PbOH⁺ and SO₄²⁻. Tests carried out in the presence of collector confirm the previous finding (Woods, 1971) that the interaction of xanthate with the mineral begins with chemisorption by a one-electron reaction. This cannot, of course, be predicted by calculations based on bulk thermodynamics.     

Flotation of galena. Influence of grain size,of pulp deoxygenation,and of cleaning with ammonium acetate

The flotation of < 10, 10–20, and 20–40 μm galena fractions was studied. For uncleaned galena a given collector coverage produced better floatability with increasing grain size. Nitrogen had a detrimental effect only for the < 10 μm fraction, producing at a given collector coverage a recovery smaller than that obtained with air.Galena cleaned with 400 g/l ammonium acetate had very poor floatability, although xanthate abstraction was fairly high; this confirms that strong xanthate adsorption is necessary for flotation. Formation of monothiocarbonate was small in all cases, which points to a very minor influence, if any, of this compound in the flotation process.In blank flotation tests, or for very low residual xanthate concentrations, a peak at 208 nm and a shoulder at 255 nm were observed. The former was assigned to the uncomplexed Pb²⁺ ion, and the latter was tentatively attributed to the PbOH⁺ ion.Lead in solution results from dissolution of the oxidation products of galena, as galena itself has an exceedingly low solubility. The curve for total lead in solution vs. initial xanthate concentration, had a minimum for an initial xanthate concentration of 10^?5M, the further increase in dissolved lead is attributed to formation of complexes such as PbX⁺ (X = xanthate). Dissolved lead concentrations were nearly as high for cleaned as for uncleaned galena, which indicates a high oxidation rate of the mineral.     

Effect of surface oxidation and iron contents on xanthate ions adsorption of synthetic sphalerites

Abstraction of xanthate ions from solution by copper-activated synthetic sphalerites with various iron contents was measured. It was found that raising the iron content in the samples caused a decrease in xanthate ions abstraction. For sphalerite samples characterized by the same percentage of iron deoxidized surfaces took up more xanthate ions than oxidized surfaces.Surface coverage by EtX^? ions was determined to be less than one monolayer. These values were compared with copper ion coverage at sphalerite surface. The amount of EtX^? ions abstracted from solution was noted to be independent of the pH of the solution in the range from 6 to 10 pH units, above $\text{pH}\text{=10}$ a decrease in the abstraction was observed.     

The solubility of gold and silver in the system AuAgSO2H2O at 25°C and 1 atm.

During supergene alteration of auriferous carbonate ore, the weathering fluids formed are likely to be alkaline and therefore unsuitable as a medium for gold transport as a chloride complex. Secondary gold remobilization in such deposits can often be attributed instead to gold complexing by sulphur-bearing ligands. Gold and silver solubility in the systems AuSO₂H₂O and AgSO₂H₂O respectively, from the thermodynamic data available, is due to complex formation with thiosulphate and bisulphide ligands. The most stable gold complexes, Au(S₂O₃)₂³⁻ (at φO₂ > 10⁻⁶⁰) and Au(HS)⁻₂ (atφO₂ < 10⁻⁶⁰), exist in neutral or alkaline solutions. Like gold, silver forms a stable thiosulphate complex, Ag(S₂O₃)³⁻₂ in moderately oxidizing, and bisulphide complexes, AgHS⁰ and Ag(HS)⁻₂ in reducing, alkaline media. Silver solubility in highly oxidized, neutral or acid solutions is increased by formation of AgS₂O₃⁻, Ag⁺ and AgSO₄⁻ complexes.Colloidal, crystalline and alloyed gold and silver reacted with 0.1 M Na₂S₂O₃ do not, however, demonstrate independent solubility. The rate of gold solubility in 0.1 M Na₂S₂O₃, for example, is increased both by the presence of silver-thiosulphate complexes and alloyed silver. It is possible that such behaviour is due to the formation a mixed metal complex of the type (Au, Ag)(S₂O₃)₂³⁻.The nature and mineral association of secondary gold in the oxidized zone of carbonate ore at Wau. in Papua New Guinea, is consistent with prior remobilization as a thiosulphate complex. Here the secondary gold is coarsely crystalline, alloyed with 50–75 at% Ag and enriched at the watertable and with manganese dioxide in the oxidized zone.     

Kinetic study of abstraction of ethyl xanthate ions by oxidized chalcocite surface

The results of potassium ethyl xanthate (EtXK) consumption measurements by synthetic chalcocite (Cu₂S grain-sized classes of 60–75 μm and 120–200 μm, respectively) are presented. These measurements were done in a modified circulation apparatus in an argon or oxygen atmosphere at pH=9.5. The experimental results were compared with theoretical results predicted by a mathematical model based on the assumption that the EtX^? ions were immobilized as a result of a chemical reaction (or reactions), e.g., of an ion-exchange type, taking place within the oxidation product layer (OPL), formed on the Cu₂S surface.The experimental and theoretical results are in good agreement especially for small values of Q₀ (Q₀ is the initial mass of the EtX^? in solution per mass unit of Cu₂S). In this case both the theoretical and experimental results show that the EtX^? concentration in solution decays exponentially with time.     

Flotation and depression control of arsenopyrite through pH and pulp redox potential using xanthate as the collector

The role of pH and pulp redox potential (E_H) to control the flotation and depression of arsenopyrite has been investigated through studies on microflotation of arsenopyrite crystals and batch flotation of an arsenopyritic ore using isopropyl xanthate as collector. The transition between flotation and depression of arsenopyrite is established by the reversible potential of the xanthate/dixanthogen couple. Adsorption of arsenate ions on ferric hydroxide has been studied through electrokinetics to delineate mechanisms involved in the depression of arsenopyrite using oxidants. Chemical binding between arsenate species and ferric hydroxide sites on arsenopyrite is suggested as the mechanism responsible for depression of arsenopyrite. E_H conditions are given for the flotation and depression of arsenopyite at various pH values for the arsenopyritic ore.     

Stability of ethylxanthate ion in neutral and weakly acidic media. Part 1: Influence of pH

Xanthates are used in the flotation of sulfide ores although their aqueous solutions are not stable under certain conditions. Their stability in acidic and weakly acidic aqueous solutions was therefore investigated, as these media are required for some processes.The peak absorbances of ethylxanthate ion and carbon disulfide were first determined in aqueous solution. The decomposition of ethylxanthate ion was analyzed by measuring variations in absorbance (at 301 nm) and pH with respect to time. A pH regulation system was then used while measuring variations in absorbance and productions of protons caused by xanthate decomposition.The results concerning xanthate half-lives show good agreement with the literature, but the kinetic results deviate substantially. The following relation was obtained for half-life:

$\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{=9.67×10}{}^{\mathrm{?6}}\text{(pH)}{}^{11}\text{;4?7;T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{in seconds}$

We established that ethylxanthate decomposition at pH 4 is a first order reaction with respect to ethylxanthate concentration, and postulating this order to the other pH values, the following kinetic relation was found:

$\text{v= ?(1.22×10}{}^4\text{[H}{}^{\mathrm{+}}\text{]?1.36×10}{}^{\mathrm{?2}}\text{)([}\text{EtX}{}^{\mathrm{?}}\text{]}{}_{\mathrm{\infty }}\text{) (4?pH?7)}$

where v is the rate of decomposition (mol l^?1 min^?1), and [EtX^?]_∞ is the ethylxanthate concentration when the decomposition equilibria are reached (mol l^?1). The better concentration was found to obey the law:

$\text{[}\text{EtX}{}^{\mathrm{?}}\text{]}{}_{\mathrm{\infty }}\text{=3.142×10}{}^{\mathrm{?5}}\text{pH ? 1.255 × 10}{}^{\mathrm{?4}}\text{(4?pH?6)}$

     

The stabilizing effect of Na2S on the collector coating of chrysocolla

The heterogeneous reaction between natural and sulphidized chrysocolla and potassium amyl xanthate in solution is investigated.It is found that, together with the consumption of large amounts of xanthate, particles composed of the reaction products are spontaneously released from the surface of chrysocolla giving rise to a colloidal dispersion. In principle the mechamism is related to the peptizing effect of the excess xanthate ions that are found when Cu²⁺ and X^? ions are made to react in a homogeneous phase.In sulphidized chrysocolla this phenomenon is markedly reduced, showing that Na₂S has a stabilizing effect on the collector coating.Flotation experiments performed with aqueous amyl dixanthogen emulsions show that stable adhesion occurs only on properly sulphidized chrysocolla. It is believed that the mechanism controlling adhesion in this case is similar to that which determines the stability of the collector layer.     

Interaction of finely ground galena and potassium amylxanthate in relation to flotation, 3. Influence of acid and neutral grinding

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), Hallimond tube flotation and microelectrophoresis have been utilized to investigate the reactions in the adsorption-abstraction of K-amylxanthate on finely ground galena. The mineral was ground in a laboratory stainless steel rod mill under controlled conditions (pH 4.0 and 7.0) using HCl as a pH regulator. X-ray photoelectron spectroscopic (XPS) studies have been carried out in order to characterize the surface oxidation products after grinding (weak amounts of S_n and PbS₂O₃). The two-stage adsorption process discovered in previous studies was confirmed. For low concentrations or submonolayer capacity, the layer is formed with 1:1 monocoordinated lead xanthate and dixanthogen. For higher values of surface coverage, it is composed of lead xanthate (stoichiometric at pH 7 and non-stoichiometric at pH 4), amyldixanthogen and amylcarbonate disulphide. In the second stage mainly dixanthogen is formed. This stage corresponds to complete flotation and to a sharp decrease in zeta potentials.     

Electrochemical studies of sulphides,I. The electrocatalytic activity of galena,pyrite and cobalt sulphide for oxygen reduction in relation to xanthate adsorption and flotation

The electrocatalytic activity of galena, pyrite and Co₃S₄ for oxygen reduction has been studied by potentiostatic methods. Open circuit potentials of the sulphide electrodes have also been measured as a function of pH in nitrogen, air and oxygen atmospheres and also in the presence of H₂O₂ and ethyl xanthate. The adsorption of xanthate on sulphides was followed by observing bubble attachment to the electrodes.The catalytic activity for oxygen (or H₂O₂) reduction (the cathodic currents), the electrode potentials and the xanthate adsorption as shown by bubble attachment within certain pH limits, all varied as $\text{Co}{}_3\text{S}{}_4\text{> pyrite (≈ PbS in H}{}_2\text{O}{}_2\text{) ? PbS}$ indicating considerable dependence of the redox processes in flotation on the d - electron character of the sulphides.In the absence of oxygen, xanthate is probably bonded to the water structure of the surface through hydrogen-bonding, thus keeping the surface hydrophilic. Such adsorption reduces the electrode potential and inhibits oxygen reduction.     

Fluoride contamination of groundwater in parts of eastern India and a preliminary experimental study of fluoride adsorption by natural haematite iron ore and synthetic magnetite

Incidence of high fluoride (F^?) in groundwater (>1.5 mg/L) in two tribal belts of eastern India, one around Chukru in the Palamau district of Jharkhand and the other around Karlakot in the Nuapada district of Odisha, has been studied. The maximum concentration of F^? in groundwater from dug wells and tube wells is 10.30 mg/L in Chukru and 4.62 mg/L in Karlakot. The groundwaters are mildly alkaline with pH ranges of 7.52–8.22 and 7.33–8.20 in Chukru and Karlakot, respectively. The F^? concentration is positively correlated with pH, electrical conductivity and SO₄ ^2? in both areas. The high F^? in groundwater resulted mainly from dissolution of biotite and fluorapatite in quartzofeldspathic gneiss. The ionic dominance pattern (in meq/L) is mostly in the order Ca²⁺ > Na⁺ > Mg²⁺ > K⁺ among cations and HCO₃ ^? > SO₄ ^2? > < Cl^? > F^? among anions in the Karlakot groundwater. Preliminary adsorption experiments were conducted on natural haematite iron ore and synthetic magnetite to evaluate their potential for F^? removal from water. Effects of different parameters such as contact time, pH, adsorbent dose and initial F^? concentration on the adsorption capacity of these materials were investigated. Strong dependence of F^? removal on pH was observed for both the adsorbents. With natural haematite iron ore, the maximum F^? removal of 66 % occurred at an initial pH of 3.2 for a solution with F^? concentration of 3 mg/L, adsorbent dose of 7 g/L and overnight contact time. The haematite iron ore was observed to increase the pH of the F^? solution. Adsorption equilibrium was not achieved with this adsorbent even after a contact time of 45.2 h. In the case of synthetic magnetite, 84 % F^? removal was achieved after 2 min of contact time for a solution with F^? concentration of 6 mg/L, adsorbent dose of 10 g/L and initial pH of 7. The results indicate high potential of both natural haematite iron ore and synthetic magnetite as adsorbents of F^? in water.     

Isopropylxanthate ions uptake by modified natural zeolite and removal by dissolved air flotation

This work describes a laboratory study concerning the adsorption of isopropylxanthate ions onto modified zeolites particles. The separation of the loaded carrier and their removal, from aqueous solutions, was conducted by flocculation followed by dissolved air flotation, DAF. The zeolite employed was a natural sample (approximately 48% clinoptilolite and 30% mordenite) which was previously treated with sodium ions (activation) and modified with copper ions (Cu–Z) before the xanthate ions uptake. Adsorption capacities (q_m) for Cu–Z were 0.34 meq g^− 1 for the powdered form, and 1.12 meq g^− 1 for the floc form. The adsorption capacity for the floc form appears to involve an enhanced electrostatic adsorption due to the positive sites on the floc surface. In all cases, the isopropylxanthate concentration in the treated water was found to be negligible (< 0.04 mg L^{− 1}). The flotation technique showed to be a fast process, requires a low recycle ratio (20%) in air saturated water, and the treated water ended up with a very low residual turbidity (6.8 NTU). It is believed that this adsorption–flotation technique, here named adsorptive particulate flotation, using activated and modified natural zeolite has a high potential as an alternative for pollutants removal (copper and isopropylxanthate ions) from waste mining effluents.     

Adsorption of barium ions by β-manganese dioxide and its activation in oleate flotation

The Ba²⁺ ion adsorption isotherms on β-MnO₂ were of the Langmuir type. The endothermic heat of adsorption (40 kJ mol^?1) is ascribed to entropy contributions associated with the $\text{Na}{}^{\mathrm{+}}\text{Ba}{}^{\mathrm{2+}}$ ion-exchange mechanism. The Ba²⁺ ion adsorption density was higher at pH 10 than that at pH 7, due to the more negative surface charge at the higher pH. Ba²⁺ ions were found to reverse the sign of the ζ potential of the MnO₂ particles.More oleate was adsorbed by β-MnO₂ in the presence of Ba²⁺ ions than in their absence. The oleate adsorption isotherms on Ba²⁺-activated MnO₂ were of the Freundlich type and indicated an exothermic process. Hallimond flotation recovery of Ba²⁺-activated MnO₂ was higher at pH 10 than at pH 7, although less oleate was adsorbed at the higher pH. At pH 7, Mn²⁺-activation led to higher recoveries than Ba²⁺-activation. It seems that the attraction between the surface and the activator plays an important rôle in determining the flotation recovery.     

Effect of compacton on chemistry of solutions expelled from montmorillonite clay saturated in sea water

The total mineralization of solutions squeezed out of montmorillonite clay saturated in sea water was determined at different overburden pressures. The subsequent fractions of expelled solutions were also analysed for various anions (Cl^?, SO^2-₄, HCO^?₃, F^?) and cations (Na⁺, K⁺, Mg²⁺, Ca²⁺, B³⁺). The results indicate that the concentrations of squeezed-out solutions during the initial stages of compaction (at pressures up to 35 kg/cm²) are slightly higher than that of interstitial solution present initially. The concentration of squeezed-out solution goes through a maximum, or at least remains constant, before starting to decrease with increasing overburden pressure.     

Adsorption of uranyl onto ferric oxyhydroxides: Application of the surface complexation site-binding model

Uranyl adsorption was measured from aqueous electrolyte solutions onto well-characterized goethite, amorphous ferric oxyhydroxide, and hematite sols at 25°C. Adsorption was studied at a total uranyl concentration of 10^?5 M, (dissolved uranyl 10^?5 to 10^?8 M) as a function of solution pH, ionic strength and electrolyte concentrations, and of competing cations and carbonate complexing. Solution pHs ranged from 3 to 10 in 0.1 M NaNO₃ solutions containing up to 0.01 M NaHCO₃. All the iron oxide materials strongly adsorbed dissolved uranyl species at pHs above 5 to 6 with adsorption greatest onto amorphous ferric oxyhydroxide and least onto well crystallized specular hematite. The presence of Ca or Mg at the 10^?3 M level did not significantly affect uranyl adsorption. However, uranyl carbonate and hydroxy-carbonate complexing severely inhibited adsorption. The uranyl adsorption data measured in carbonate-free solutions was accurately modeled with the surface complexation-site binding model of Davis et al. (1978), assuming adsorption was chiefly of the UO₂OH⁺ and (UO₂)₃(OH)⁺₅, aqueous complexes. In modeling it was assumed that these complexes formed a monodentate UO₂OH⁺ surface complex, and a monodentate, bidentate or tridentate (UO₂)₃(OH)⁺₅surface complex. Of the latter, the bidentate surface complex is the most likely, based on crystallographic arguments. Modeling was less successful predicting uranyl adsorption in the presence of significant uranyl carbonate and hydroxy-carbonate complexing. It was necessary to slightly vary the intrinsic constants for adsorption of the di- and tricarbonate complexes in order to fit the uranyl adsorption data at total carbonate concentrations of 10^?2 and 10^?3 M.     

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.     

Thermodynamic and kinetic constraints on reaction rates among minerals and aqueous solutions. II. Rate constants,effective surface area,and the hydrolysis of feldspar

Analysis of experimental data reported by Lagache (1965, 1976), Evans (1965), Busenberg (1975), Busenberg and Clemency (1976), Holdren and Berner (1979), Siegel and Pfannkuch (1984), and Chou and Wollast (1984) with the aid of irreversible thermodynamics and transition state theory (Aagaard and Helgeson, 1977, 1982) suggests that at temperatures at least up to 650°C, the rate of both congruent and incongruent feldspar hydrolysis in aqueous solutions far from equilibrium at pH ? 10.6 ? (2300/T), where T stands for temperature in kelvins, is a function solely of effective surface area and pH at constant pressure and temperature. At higher pH, the rate is apparently pH-independent up to ~pH 8 at 25°C, where it again becomes pH-dependent at higher pH. Observations of scanning electron micrographs indicate that the cross-sectional area of etch pits on hydrolyzed feldspar grains is of the order of 10^?9 to 10^?8 cm² and that the ratio of the effective to total surface area (which may or may not change with reaction progress) ranges from <0.01 to 1, depending on the grain size, dislocation density, and the extent of comminution damage on the surfaces of the grains. Apparent rate constants retrieved from experimental data reported in the literature for feldspar hydrolysis in the lower pH-dependent range extend from ~10^?13 to ~10^?7 moles cm^?2 sec^?1 at temperatures from 25° to 200°C, which is consistent with activation enthalpies for albite and adularia of the order of 20 kcal mole^?1. In contrast, the apparent rate constants for the pH-independent rate law range from ~10^?16 to ~10^?11 moles cm^?2 sec^?1 at temperatures from 25° to 650°C, which requires an activation enthalpy for adularia of ~ 9 kcal mole^?1. These observations are consistent with surface control of reaction rates among minerals and aqueous solutions. The rate-limiting step in the pH-dependent case apparently corresponds at the lower end of the pH scale to breakdown of a protonated configuration of atoms on the surface of the reactant feldspar, but at higher pH the rate is limited by decomposition of an activated surface complex corresponding in stoichiometry to hydrous feldspar. In highly alkaline solutions, an activated complex containing hydroxyl ions apparently controls the rate of feldspar hydrolysis. Nevertheless, near equilibrium, regardless of pH the rate is proportional to the chemical affinity of the overall hydrolysis reaction.