Kinetics of gypsum crystal growth from high ionic strength solutions: A case study of Dead Sea - seawater mixtures

Gypsum precipitation kinetics were examined from a wide range of chemical compositions , ionic strengths (4.75-10 m) and saturation state with respect to gypsum (1.16-1.74) in seeded batch experiments of mixtures of Ca²⁺-rich Dead Sea brine and -rich seawater. Despite the variability in the experimental solutions, a single general rate law was formulated to describe the heterogeneous precipitation rate of gypsum from these mixtures:

     

The ionic strength dependence of lead (II) carbonate complexation in perchlorate media

Lead speciation in many aqueous geochemical systems is dominated by carbonate complexation. However, direct observations of Pb²⁺ complexation by carbonate ions are few in number. This work represents the first investigation of the equilibrium over a range of ionic strength. Through spectrophotometric observations of formation at 25 °C in NaHCO₃-NaClO₄ solutions, formation constants of the form were determined between 0.001 and 5.0 molal ionic strength. Formation constant results were well represented by the equation:

     

Sorption of yttrium and rare earth elements by amorphous ferric hydroxide: Influence of solution complexation with carbonate

The influence of solution complexation on the sorption of yttrium and the rare earth elements (YREEs) by amorphous ferric hydroxide was investigated at 25 °C over a range of pH (4.0-7.1) and carbonate concentrations . Distribution coefficients, defined as , where [MS_i]_T is the total concentration of sorbed YREE, M_T is the total YREE concentration in solution, and [S_i] is the concentration of amorphous ferric hydroxide, initially increased in magnitude with increasing carbonate concentration, and then decreased. The initial increase of is due to sorption of YREE carbonate complexes , in addition to sorption of free YREE ions (M³⁺). The subsequent decrease of , which is more extensive for the heavy REEs, is due to the increasing intensity of YREE solution complexation by carbonate ions. The competition for YREEs between solution complexation and surface complexation was modeled via the equation:

     

The kinetics of chlorite dissolution

A model for the dissolution of chlorite has been developed based on fast ligand assisted proton attack of the alumina tetrahedra within the alumina-silica lattice followed by slower dissolution of the remnant silica lattice. While the rate determining step is within the silica dissolution regime, the rate is a function of the H⁺ and Al³⁺ concentrations and the dominant aqueous Al species. Individual rates may be described by a generic rate equation applicable across the spectrum of Al species:

     

Toward a general viscosity equation for natural anhydrous and hydrous silicate melts

A new and empirical viscosity equation for anhydrous and hydrous natural silicate melts has been developed using the following formulation:

     

The dissociation of carbonic acid in NaCl solutions as a function of concentration and temperature

Potentiometric measurements of the stoichiometric constants for the dissociation of carbonic acid in NaCl solutions ( and ) have been made as a function of molality (0-6 m) and temperature (0-50 °C). The results have been fitted to the equations

     

Solubility and dissolution rate of silica in acid fluoride solutions

We performed 57 batch reactor experiments in acidic fluoride solutions to measure the dissolution rate of quartz. These rate data along with rate data from published studies were fit using multiple linear regression to produce the following non-unique rate law for quartz

where 10^−5.13 < a_HF < 10^1.60, −0.28 < pH < 7.18, and 298 < T < 373 K. Similarly, 97 amorphous silica dissolution rate data from published studies were fit by multiple linear regression to produce the following non-unique rate law for amorphous silica

where 10^−2.37 < a_HF < 10^1.61, −0.32 < pH < 4.76 and 296 < T < 343 K. Regression of the rates versus other combinations of solution species, e.g.  + H⁺, F⁻ + H⁺, HF + , HF + F⁻, or  + F⁻, produced equally good fits. Any of these rate laws can be interpreted to mean that the rate-determining step for silica dissolution in fluoride solutions involves a coordinated attack of a Lewis acid, on the bridging O atom and a Lewis base on the Si atom. This allows a redistribution of electrons from the Si–O bond to form a O–H group and a Si–FH group.     

Diffusion of helium in zircon and apatite

Diffusion of helium has been characterized in natural zircon and apatite. Polished slabs of zircon and apatite, oriented either normal or parallel to c were implanted with 100 keV ³He at a dose of 5 × 10^{15 3} He/cm². Diffusion experiments on implanted zircon and apatite were run in Pt capsules in 1-atm furnaces. ³He distributions following experiments were measured with Nuclear Reaction Analysis using the reaction ³He(d,p)⁴He. For diffusion in zircon we obtain the following Arrhenius relations:

Although activation energies for diffusion normal and parallel to c are comparable, there is marked diffusional anisotropy, with diffusion parallel to c nearly 2 orders of magnitude faster than transport normal to c. These diffusivities bracket the range of values determined for He diffusion in zircon in bulk-release experiments, although the role of anisotropy could not be directly evaluated in those measurements.In apatite, the following Arrhenius relation was obtained over the temperature range of 148–449 °C for diffusion normal to c:

In contrast to zircon, apatite shows little evidence of anisotropy. He diffusivities obtained in this study fall about an order of magnitude lower than diffusivities measured through bulk release of He through step-heating, and within an order of magnitude of determinations where ion implantation was used to introduce helium and He distributions measured with elastic recoil detection.Since the diffusion of He in zircon exhibits such pronounced anisotropy, helium diffusional loss and closure cannot be modeled with simple spherical geometries and the assumption of isotropic diffusion. A finite-element code (CYLMOD) has recently been created to simulate diffusion in cylindrical geometry with differing radial and axial diffusion coefficients. We present some applications of the code in evaluating helium lost from zircon grains as a function of grain size and length to diameter ratios, and consider the effects of “shape anisotropy”, where diffusion is isotropic (as in the case of apatite) but shapes of crystal grains or fragments may depart significantly from spherical geometry.     

A spectrophotometric study of neodymium(III) complexation in sulfate solutions at elevated temperatures

The formation constants of neodymium complexes in sulfate solutions have been determined spectrophotometrically at temperatures of 30-250 °C and a pressure of 100 bars. The dominant species in the solution are NdSO₄⁺ and Nd(SO₄)₂⁻, with the latter complex being more important at higher temperature. Equilibrium constants were calculated for the following reactions:

     

A spectrophotometric study of samarium (III) speciation in chloride solutions at elevated temperatures

The speciation of samarium (III) in chloride-bearing solutions was investigated spectrophotometrically at temperatures of 100-250 °C and a pressure of 100 bars. The simple hydrated ion, Sm³⁺, is predominant at ambient temperature, but chloride complexes are the dominant species at elevated temperatures. Cumulative formation constants for samarium chloride species were calculated for the following reactions:

     

A potentiometric study of the stability of aqueous yttrium-acetate complexes from 25 to 175 °C and 1-1000 bar

The stability of yttrium-acetate (Y-Ac) complexes in aqueous solution was determined potentiometrically at temperatures 25-175 °C (at P_s) and pressures 1-1000 bar (at 25 and 75 °C). Measurements were performed using glass H⁺-selective electrodes in potentiometric cells with a liquid junction. The species YAc²⁺ and were found to dominate yttrium aqueous speciation in experimental solutions at 25-100 °C (log [Ac⁻] < −1.5, pH < 5.2), whereas at 125, 150 and 175 °C introduction of into the Y-Ac speciation model was necessary. The overall stability constants β_n were determined for the reaction

     

Prediction of the vapor-liquid distribution constants for volatile nonelectrolytes in water up to its critical temperature

The distribution of solutes between coexisting liquid and vapor phases of water can be expressed by the distribution constant, K_D, defined as

     

The influence of temperature and seawater composition on calcite crystal growth mechanisms and kinetics: Implications for Mg incorporation in calcite lattice

The composition of carbonate minerals formed in past and present oceans is assumed to be significantly controlled by temperature and seawater composition. To determine if and how temperature is kinetically responsible for the amount of Mg incorporated in calcite, we quantified the influence of temperature and specific dissolved components on the complex mechanism of calcite precipitation in seawater. A kinetic study was carried out in artificial seawater and NaCl-CaCl₂ solutions, each having a total ionic strength of 0.7 M. The constant addition technique was used to maintain [Ca²⁺] at 10.5 mmol kg⁻¹ while [] was varied to isolate the role of this variable on the precipitation rate of calcite.Our results show that the overall reaction of calcite precipitation in both seawater and NaCl-CaCl₂ solutions is dominated by the following reaction:

     

Comparative carbonate and hydroxide complexation of cations in seawater

Comparative concentrations of carbonate and hydroxide complexes in natural solutions can be expressed in terms of reactions with bicarbonate that have no explicit pH dependence (). Stability constants for this reaction with n = 1 were determined using conventional formation constant data expressed in terms of hydroxide and carbonate. Available data indicate that stability constants appropriate to seawater at 25 °C expressed in the form are on the order of 10^4.2 for a wide range of cations (M^z+) with z = +1, +2 and +3. Φ₁ is sufficiently large that species appear to substantially dominate MOH^z−1 species in seawater. Evaluations of comparative stepwise carbonate and hydroxide stability constant behavior leading to the formation of n = 2 and n = 3 complexes suggest that carbonate complexes generally dominate hydroxide complexes in seawater, even for cations whose inorganic speciation schemes in seawater are currently presumed to be strongly dominated by hydrolyzed forms (). Calculated stability constants, and , indicate that the importance of carbonate complexation is sufficiently large that carbonate and hydroxide complexes would be generally comparable even if calculated Φ₂ and Φ₃ values are overestimated by two or more orders of magnitude. Inclusion of mixed ligand species in carbonate-hydroxide speciation models allows cation complexation intensities (M_T/[M^z+]) to be expressed in the following form:

     

Pb and rare earth element diffusion in xenotime

Diffusion of Pb and the rare earth elements Sm, Dy and Yb have been characterized in synthetic xenotime under dry conditions. The synthetic xenotime was grown via a Na₂CO₃–MoO₃ flux method. The sources of diffusant for the rare earth diffusion experiments were REE phosphate powders, with experiments run using sources containing a single REE. For Pb, the source consisted a mixture of YPO₄ and PbTiO₃. Experiments were performed by placing source and xenotime in Pt capsules, and annealing capsules in 1 atm furnaces for times ranging from 30 min to several weeks, at temperatures from 1000 to 1500 °C. The REE and Pb distributions in the xenotime were profiled by Rutherford Backscattering Spectrometry (RBS).The following Arrhenius relations are obtained for diffusion in xenotime, normal to (101):

Diffusivities among the REE do not differ greatly in xenotime over the investigated temperature range, in contrast to findings for the REE in zircon [Cherniak, D.J., Hanchar, J.M., Watson, E.B., 1997. Rare earth diffusion in zircon. Chem. Geol. 134, 289–301.], where the LREE diffuse more slowly, and with higher activation energies for diffusion, than the heavier rare earths. In zircon, these differences among diffusion of the rare earths are attributed to the relatively large size of the REE with respect to Zr, for which they likely substitute in the zircon lattice. With the systematic increase in ionic radius from the heavy to lighter REE, this size mismatch becomes more pronounced and diffusivities of the LREE are as consequence slower. Although xenotime is isostructural with zircon, the REE are more closely matched in size to Y, so in xenotime this effect appears much smaller and the REE diffuse at similar rates. In addition, the process of diffusion in xenotime likely involves simple REE^+ 3 → Y^+ 3 exchange, without charge compensation as needed for REE^+ 3 → Zr^+ 4 exchange in zircon. This latter factor may also contribute to the large activation energies for diffusion of the REE in zircon (i.e., 691–841 kJ mol^− 1, [Cherniak, D.J., Hanchar, J.M., Watson, E.B., 1997. Rare earth diffusion in zircon. Chem. Geol. 134, 289–301.]), in comparison with those for xenotime.For Pb, the following Arrhenius relation is obtained (also normal to (101)):

These measurements suggest that Pb diffusion in xenotime is quite slow, even slower than Pb diffusion in monazite and zircon, and considerably slower than diffusion of the REE in xenotime. Xenotime may therefore be even more retentive of Pb isotope signatures than either monazite or zircon in cases where Pb isotopes are altered solely by volume diffusion. However, because the activation energy for Pb diffusion in xenotime is lower than those for monazite and zircon, Pb diffusion may be somewhat faster at many temperatures of geologic interest in xenotime than in monazite or zircon.     

The solubility of platinum and gold in NaCl brines at 1.5 kbar, 600 to 800°C: A laser ablation ICP-MS pilot study of synthetic fluid inclusions

The concentration and distribution of Pt and Au in a fluid-melt system has been investigated by reacting the metals with S-free, single-phase aqueous brines (20, 50, 70 wt% eq. NaCl) ± peraluminous melt at a confining pressure of 1.5 kbar and temperatures of 600 to 800 °C, trapping the fluid in synthetic fluid inclusions (quartz-hosted) and vesicles (silicate melt-hosted), and quantifying the metal content of the trapped fluid and glass by laser ablation ICP-MS. HCl concentration was buffered using the assemblage albite-andalusite-quartz and fO₂ was buffered using the assemblage Ni-NiO. Over the range of experimental conditions, measured concentrations of Pt and Au in the brines (, ) are on on the order of 1-10³ ppm. Concentrations of Pt and Au in the melt (, ) are ∼35-100 ppb and ∼400-1200 ppb, respectively. Nernst partition coefficients (, ) are on the order of 10²-10³ and vary as a function of (non-Henry’s Law behavior). Trapped fluids show a significant range of metal concentrations within populations of inclusions from single experiments (∼ 1 log unit variability for Au; ∼2-3 log unit variability for Pt). Variability in metal concentration within single inclusion groups is attributed to premature brine entrapment (prior to metal-fluid-melt equilibrium being reached); this allows us to make only minimum estimates of metal solubility using metal concentrations from primary inclusions. The data show two trends: (i) maximum and average values of and in inclusions decrease ∼2 orders of magnitude as fluid salinity () increases from ∼4 to 40 molal (20 to 70 wt % eq. NaCl) at a constant temperature; (ii) maximum and average values of increase approximately 1 order of magnitude for every 100°C increase temperature at a fixed . The observed behavior may be described by the general expression: