Stephen U. Aja

The aqueous geochemistry of Zr and the solubility of some Zr-bearing minerals

Abstract Literature data on the thermodynamics of complexation of Zr with inorganic species, at 25°C, have been critically reviewed. The preponderance of published complexation constants deal with F− and OH− ions. Stability constants for the complexation reactions are relatively independent of ionic strength and thus recomended values for each ligand type are averages of the most reliable data. Complexation constants under elevated conditions (T ⪕ 250°C andPv = PH2O) have been predicted for various Zr complexes (F−, Cl−, SO42− and OH−) using Helgesons electrostatic approach. Predominance diagrams (calculated for simple systems with these constants) suggest that, over a wide range of pH conditions, Zr(OH)4(aq) will dominate the aqueous geochemistry of Zr except under very high activities of competing ligands (e.g., F−, SO42−). The solubilities of vlasovite [Na2ZrSi4O11] and weloganite [Sr3Na2Zr(CO3)6·3H2O have been measured in KCI solutions (0.5–1.0 M) at 50°C. Weloganite dissolution is complicated by the predictable precipitation of strontianite (SrCO3) whereas vlasovite dissolves incongruently. Solubility products for the dissolution of welonganite and vlasovite are determined to be −28.96±0.14 and −20.40±1.18, respectively. Concentrations of Zr up to 10−3 m were present in the experimental solutions; the presence of large amounts of Zr in aqueous solutions support the possibility of extensive remobilization of Zr during hydrothermal mineralization.

Acid–base surface chemistry and sorption of some lanthanides on K+-saturated Marblehead illite: I. results of an experimental investigation

Abstract The surface reactivity and sorption of Nd and Eu onto K+-saturated, Marblehead illite has been investigated in 0.01, 0.1, and 1.0 M KCl solutions at 25°C; the potentiometric titrations were conducted using back-titration techniques. Batch experimental protocols were used in both series of studies. The ionic strength-dependent, proton surface charge density (σH) varies from −1500 to −1700 mC/m2 in 1.0 M KCl solutions and from −1800 to −2200 mC/m2 in 0.01 and 0.1 M KCl solutions. An isoelectric point was not defined by the σH vs. pH curves, which reflects the multi-phase nature of natural illitic materials. The functional dependence of REE binding constants (log Ke) on surface coverage (log ΓREE) indicates the existence of a multiplicity of energetically distinct surface types. These surface site types include amphoteric silanol and aluminol sites, basal planar surfaces, and “frayed edges”; the frayed edges are observed only in low ionic strength solutions (I ≤ 0.1 M KCl).

Acid–base surface chemistry and sorption of some lanthanides on K+-saturated Marblehead illite: II. a multisite–surface complexation modeling

Abstract The surface reactivity and sorption of Nd and Eu onto K+-saturated Marblehead illite at 25°C, measured in aqueous 0.01, 0.1, and 1.0 M KCl solutions, were interpreted with a multi-site-surface complexation model. Model potentiometric titration and sorption curves (computed using the Gibbs free energy minimization code, Selektor-A) resolve into reactions on variable-charge amphoteric sites on edge surfaces and on permanent-charge siloxane surfaces (φx). Standard partial molal Gibbs free energy of formation from elements (g2980) for surface complexes were derived from oxide (SiO2,am and γ-Al2O3) surface deprotonation KA10, KA20 and electrolyte adsorption constants KCl0, KNa0. Because surface complexation reactions on siloxane basal surfaces are negligible in 1 M KCl, models of surface charge and adsorption edges of Nd and Eu presumed that C1 is equal to 1.6 Fm−2 for amphoteric site types, and a maximum site density of 1.2 ± 0.2 sites nm−2 for the outer-sphere species, (Al>OH2+Cl−). To obtain values of g2980 for exchangeable cations and charged X∼REE complexes, ion exchange sites were assumed to be fully deprotonated in 1.0 M KCl solutions (pH > 2.7). Proton release and REE3+ uptake on ion exchange sites were then simulated (pH 4.0) from initial values of 20 to 48%. The application of Gibbs free energy minimization to sorption processes is innovative in that simultaneous treatment of surface complexation reactions and minerals stability is feasible in any system without introducing mass-balance constraints particular to surface species.

The sorption of the rare earth element, Nd, onto kaolinite at 25 degrees C

The question of the fractionation of rare earth metals (REE) between clay minerals and coexisting aqueous solutions is one of interest to the geochemical community. Research interest in the marine geochemistry of the REE, for instance, is partly driven by the need to better understand REE metal budgets in seawater and/or the geochemical cycle of REE. This, in turn, is linked to the proposition that models of hydrothermal fluxes associated with spreading ridges must consider the roles of secondary minerals as sinks for the REE, especially in view of the fact that smectite and chlorites are important products of low-temperature alteration of mid-oceanic ridge basalts (Chamley 1989). Clay minerals are also common products of hydrothermal alteration, weathering and diagenesis, and petrographic evidence indicates that some REE redistribution occurs under these low-temperature conditions (McLennan 1989 and references therein). Indeed, some petrographic evidence shows that REE remohilization may occur during diagenesis and that adsorption by clay minerals plays important roles in such fractionation processes (Awwiner and Mack 1991; Zhao et al. 1992). A limited number of experimental studies have reported coefficients for the partition of REE between various minerals and coexisting aqueous solutions. Koeppenkastrop and De Carlo (1992) studied the sorption of REE, in the presence of seawater (pH = 7.8), onto the following synthetic phases: vernadite (~MnO~), hydroxylapatite [CaI0(PO4)(OH)2], amorphous goethite and crystalline goethite. The fractionation trends observed by them included the preferential uptake of light rare earth element (LREE) relative to heavy rare earth element (HREE), the existence of a positive Ce anomaly for sorption onto vernadite and a relatively enhanced uptake of Nd by hydroxylapatite. According to Koeppenkastrop and De Carlo (1992), this enhanced uptake of Nd by hydroxylapatite and the positive anomaly for Ce sorption on vernadite suggest that the mechanism of sorption in the former was due to substitution for Ca 2§ and to oxidative scavenging of Ce 3+ in the latter. Nonetheless, mechanistic models of REE sorption have not yet evolved and the question of the relevant adsorption model (isotherm) on these phases is not resolved. Beall et al. (1979) measured distribution coefficients for the sorption of La, Sm, Yb, Am, Cm and Cf on kaolinite, montmorillonite and attapulgite at (room temperature); the experiments were conducted in NaC1 solutions (0.25-4 M) buffered to a pH of 5 with acetate solutions. Their study suggested that metal uptake decreased with ionic strength, although the extent to which this might be due to the effects of competing metal complexation reactions was not addressed. At each ionic strength and for a given clay mineral, the values of the distribution ratios measured for both the lanthanides and actinides were only marginally different. Amongst the minerals, however, the actinides demonstrated a strong preference for attapulgite. Prel iminary experimental investigations (Mecherri et al. 1990) into the sorption of Nd onto orthoclase and calcite at 50 ~ have also been reported. According to them, Nd sorption onto orthoclase is best modeled by a Langmuir-type isotherm whereas a Freundlich-type isotherm characterized the sorption onto calcite. In addition, Mecherri et al. (1990) observed that only a small fraction of the sorbed metals could be desorbed and that increasing pH tends to decrease the partition of Nd onto the solid phases. The focus of the studies discussed above has understandably been the determination of the distribution coefficients for various lanthanides. Besides the obvious petrologic utility of partition coefficients, they also have important practical applications. Such data are

Illite equilibria in solutions: I. Phase relationships in the system K2OAl2O3SiO2H2O between 25 and 250°C

Natural illite from Marblehead, Wisconsin (MH), USA, has been equilibrated with 0.2 and 2.0M KCl/KOH and KCl/HCl solutions in the presence of excess kaolinite or microcline and quartz or amorphous silica at temperatures between 25 and 250°C and Pv = PH2O- Reversibility of univariant equilibria was demonstrated by approach from high and low aK+aH+ and from silica under- and super-saturation. Solutions were separated after experiments using immiscible displacement techniques. Isothermal, isobaric log ak+/aH+ vs. log aSiO2,aq diagrams have been constructed denning possible stability fields for kaolinite, microcline, gibbsite (or boehmite or diaspore), muscovite, and four illitic phases. Assuming an R+2-free stoichiometry, K-content per half cell, estimated from the slopes of univariant lines, are 0.29, 0.50, 0.69, and 0.85 K; these phases are compositional analogs of smectite (S), mixed-layer illite I/S (i.e., IS, ISII) and illite (I), respectively. Illitization reactions are strongly affected by temperature and porewater chemistry. At quartz saturation, direct conversion of smectite or kaolinite to endmember illite can occur at high pH; at low pH, these reactions are unlikely inasmuch as K+ requirements exceed concentrations observed in most natural pore waters. In silica-supersaturated solutions, illitization reactions proceed through crystallization of intermediate phases with compositions between smectite and endmember illite (I).

Illite equilibria in solutions: II. Phase relationships in the system K2OMgOAl2O3SiO2H2O

The stability of the Marblehead illite has been investigated in the five-component system K2O-MgO-Al2O3-SiO2-H2O in the presence of kaolinite and microcline. The mica-like solubility-controlling phases [Kx(MgyAl2−y)(Alx−ySi4−(x−y)) O10(OH)2]identified include (i) smectite x = 0.29 ± 0.04, y = 0.26 ± 0.02; (ii) illite x = 0.50 ± 0.05, y = 0.22 ± 0.14; (iii) illite x = 0.69 ± 0.08, y = 0.16 ± 0.03; (iv) illite x = 0.85 ± 0.05, y = 0.12 ± 0.04; and (v) muscovite. Possible stability regions have been defined for these solubility-controlling phases using isothermal isobaric log aMg12+2/2aH+ vs. log aH+ diagrams. When illite stability is referred to the quaternary system K2O-Al2O3-SiO2-H2O, the effect of (R+2)VI substitution is neglected. The error inherent in this simplification has been estimated. Inasmuch as a four-fold increase in aMg2+ shifts the illite-smectite-kaolinite-solution invariant point by less than 0.3 log units, stability relationships in the quaternary system provide an adequate representation of illite solution equilibria, to a first approximation. However, the error caused by neglecting Mg+2 is close to or within experimental error. Thus, a more precise determination of the effect of Mg+2 on illite solution equilibria may not be possible using the solution equilibration method.

The stability of Fe–Mg chlorites in hydrothermal solutions—I. Results of experimental investigations

Abstract Three natural chlorites have been equilibrated with kaolinite (±quartz±gibbsite±hematite) in aqueous MgCl2 or NaCl solutions at saturated vapor conditions (PV=PH2O). The compositions of the chlorites, (Fe0.603+Fe5.432+Mg2.30Al2.98Mn0.05Ca0.03Zn0.01□0.60)(Si5.63Al2.37)O20(OH)16, (Al2.33Fe1.002+Fe0.143+Ca0.02Mn0.01Ni0.02Cr0.01Mg8.40□0.07)(Si5.66Al2.34)O20(OH)16 and (Si5.26Al2.74)(Al1.94Ti0.28Fe6.162+Fe0.563+Mn0.05Mg2.05Ca0.31P0.19V0.01□0.46)O20(OH)16 were determined from X-ray fluorescence (XRF) and Mossbauer spectroscopy. The experiments were conducted in sealed bottles (LDPE or PTFE) or Teflon-lined (PTFE) reaction vessels, and the starting solution compositions were constructed such that equilibrium boundaries were approached from high and low values of log a Mg 2+ 0.5 a H + , log a Na + a H + and log a SiO aq . The attainment of stable equilibrium is indicated by the reversibility of fluid-mineral equilibria, agreement of results obtained in the different aqueous media, and the variance of the 6-component system, MgO–Al2O3–SiO2-Fe2O3–FeO–H2O. To fully reflect the complexity of the equilibria being investigated, a number of reactions (with corresponding equilibrium constants) have been used to model the behavior of each chlorite under isothermal, isobaric conditions. These solution equilibration data also validate the applicability of solubility product conventions to chlorite solubility data and thus contradict the general presumption that the behaviors of complex layer silicates in aqueous solutions are not amenable to the law of mass action.

The Solubility of Some Alkali-Bearing Zr Minerals in Hydrothermal Solutions

The hydrolytic dissolution of some Zr-bearing minerals has been measured in aqueous KCl and KF solutions. Depending on the mineral being investigated, Zr concentrations varied from about 9 ppb to nearly 40 ppm in equilibrated solutions; this validates the notion of Zr mobility in some crustal fluids. At 50 C, {Delta}{sub f}G were determined (using the measured solubility products) for Na catapleiite, elpidite, vlasovite and weloganite to be {minus}4,654.2 {+-} 5.6, {minus}7,391.0 {+-} 11.6, {minus}5,181.0 {+-} 7.3 and {minus}7,130.5 {+-} 1.0 kJ/mol, respectively. Calculated phase relationships among the zirconosilicates found at the Strange Lake peralkaline complex indicate that elpidite and gittinsite cannot coexist stably implying a two-stage hydrothermal mineralization process. The relative stability of Zr-bearing minerals and the aqueous geochemistry of Zr has been attracting some recent interest owing to their relevance to the problem of radioactive waste management, hydrothermal Zr mineralization, the geochemical cycle of zirconium and the role of zirconium as a strategic metal.

Illite equilibria in solutions: III. A re-interpretation of the data of Sass et al. (1987)

Abstract In a recent solubility study of Goose Lake and Beavers Bend illite, Sass et al. (1987) inferred the existence of three components of natural illites [ K 0.24 O 10 (OH) 2 ], [ K 0.67 O 10 (OH) 2 ], and [ K 0.90 O 10 (OH) 2 ] which were interpreted to be smectite, illite, and K-mica, respectively. They also speculated that illite-smectite equilibrium is metastable under diagenetic conditions except between 90 and 110°C where it is stabilized by an ordering transition. A re-interpretation of the data of Sass et al. (1987) indicates that the solubility-controlling phases have the following K atoms per half cell: 0.29, 0.52, 0.69, 0.84, and 1.0. Furthermore, solution equilibration investigations of kaolinite-microcline mixtures have shown that these two minerals do not coexist stably. Thus, the question of an ordering transition whose main effect is to stabilize illite-smectite equilibria relative to kaolinite-microcline assemblage does not arise.

THE STABILITY OF Fe-Mg CHLORITES IN HYDROTHERMAL SOLUTIONS: II. THERMODYNAMIC PROPERTIES

The hydrothermal stabilities of a low-Fe clinochlore and a high-Mg chamosite, in the presence of kaolinite, were investigated recently at T ⩽ 200°C and