Pb diffusion in monazite: a combined RBS/SIMS study

We report measurements of Pb diffusion in both synthetic (CePO₄) and natural monazites run under dry, 1-atm conditions. Powdered mixtures of prereacted CePO₄ and PbZrO₃ were used as the source of Pb diffusant for “in-diffusion” experiments conducted in sealed Pt capsules for durations ranging from a few hours to several weeks. Following the diffusion anneals, Pb concentration profiles were measured with Rutherford Backscattering Spectroscopy (RBS) and supplemented by measurements with secondary ion mass spectrometry (SIMS). In order to evaluate potential compositional effects upon Pb diffusivity and simulate diffusional Pb loss that might occur in natural systems, we also conducted “out-diffusion” experiments on Pb-bearing natural monazites. In these experiments, monazite grains were surrounded by a synthetic zircon powder to act as a “sink.” Monazites from these experiments were analyzed with SIMS. Over the temperature range 1100 to 1350°C, the Arrhenius relation determined for in-diffusion experiments on synthetic monazite is given by:

     

The coarsening kinetics associated with exsolution in an iron-free clinopyroxene

The coarsening kinetics of exsolution lamellae have been determined for an initial composition of 0.541 CaMgSi₂O₆–0.459 Mg₂Si₂O₆ (Di_54.1) at 1300 °, 1200 ° and 1100 ° C. Portions of this material were annealed for varying lengths of time, and the average wavelength () was obtained from measurements made by utilizing transmission electron microscopy (TEM). All experiments were conducted within the (001) coherent spinodal for this system (McCallister and Yund, 1977), and the lamellae are coherent up to and including the largest wavelength observed, 1054 Å.At all three temperatures mentioned previously the data are consistent with the following relationship: The activation energy for the process, as determined from the respective kinetic constants (k), is 99±2 Kcal/mol. This value is similar to that found for Ca self diffusion in pseudo-wollastonite and wollastonite, and suggests the possibility that the diffusion of Ca may be controlling the rate of coarsening.A comparison of exsolution microstructures in the synthetic samples with those observed in clinopyroxenes found in kimberlites and with similar Ca/Ca + Mg indicates that the latter cooled rapidly to the effective quench temperature.     

Surface properties, solubility and dissolution kinetics of bamboo phytoliths

Although phytoliths, constituted mainly by micrometric opal, exhibit an important control on silicon cycle in superficial continental environments, their thermodynamic properties and reactivity in aqueous solution are still poorly known. In this work, we determined the solubility and dissolution rates of bamboo phytoliths collected in the Réunion Island and characterized their surface properties via electrophoretic measurements and potentiometric titrations in a wide range of pH. The solubility product of “soil” phytoliths ( at 25 °C) is equal to that of vitreous silica and is 17 times higher than that of quartz. Similarly, the enthalpy of phytoliths dissolution reaction is close to that of amorphous silica but is significantly lower than the enthalpy of quartz dissolution. Electrophoretic measurements yield isoelectric point pH_IEP = 1.2 ± 0.1 and 2.5 ± 0.2 for “soil” (native) and “heated” (450 °C heating to remove organic matter) phytoliths, respectively. Surface acid-base titrations allowed generation of a 2-pK surface complexation model. Phytoliths dissolution rates, measured in mixed-flow reactors at far from equilibrium conditions at 2 ? pH ? 12, were found to be intermediate between those of quartz and vitreous silica. The dissolution rate dependence on pH was modeled within the concept of surface coordination theory using the equation:

     

Lamellar exsolution systems in clinopyroxene

Exsolution systems in synthetic pyroxenes were studied by transmission electron microscopy. An iron free sample En₈₀Wo₂₀ was prepared by devitrifying glass at 1300°C. Samples with bulk composition En₅₀Fs₃₀Wo₂₀ and En₃₅Fs₃₈Wo₂₇ were given various but well-defined heat treatments. The exsolution systems observed cannot unambiguously be related to the heat treatment. Periodic lamellar exsolution was observed parallel to (001) and (100) with sharp satellite reflections in the diffraction diagram. In more complex exsolution systems coarse (100) lamellae were found together with fine lamellae parallel to (001) and (100). An unusual phenomenon occurs at a (100) twin boundary where both individuals display exsolution lamellae parallel to (001). Pigeonite lamellae in one twin meet augite lamellae of the other individual at the twin boundary and vice-versa. The precise matching is achieved by a change in width near the boundary. Smoothly curved phase boundaries are developed in the obtuse angle of crosshatched (100) and (001) pigeonite lamellae in augite, whereas the boundaries in the acute angle are straight with sharp edges. This is consistent with elastic energy constraints.     

TEM study of the transformation of augite to sodic pyroxene in eclogitized ferrogabbro

Transmission and analytical electron microscopy has been used to study relicts of augite that occur in various stages of transformation to sodic pyroxene. The augite relicts are characterized by a hatching produced by two sets of former 001 exsolution lamellae that possess high dislocation densities and were altered completely to sodic pyroxene, even where the augite matrix is still fresh. With further alteration, the sodic pyroxene in these 001 lamellae recrystallized and grew into the augite matrix, resulting in irregular lamellae that consist of subgrains having low dislocation densities. Needles and thin (100) lamellae of sodic pyroxene developed on the 001 lamellae. Alteration of the augite matrix proceeded by growth of areas with defects (dislocations, stacking faults). All sodic pyroxenes in these microstructures have the same orientation as the precursor augite, indicating a topotactic reaction mechanism. The reactions occurred at roughly constant Si and mainly involved replacement of Ca and Mg by Na and Al. Dislocations may have played a prominent role in the transformation by acting as diffusion pathways and by migrating into untransformed augite, leaving sodic pyroxene in their wake. At the grain boundaries of the augite, discrete grains of sodic pyroxene formed without any fixed orientation relation with the augite, consistent with a non-topotactic reaction. The predominance of the topotactic reaction inside the augite over the non-topotactic grain-boundary reaction is attributed to the scarcity of fluids during eclogite metamorphism.     

Subcalcic diopsides from kimberlites: Chemistry,exsolution microstructures,and thermal history

Twenty-six subcalcic diopside megacrysts (Ca/(Ca+ Mg)) = 0.280–0.349, containing approximately 10 mol% jadeite, from 15 kimberlite bodies in South Africa, Botswana, Tanzania, and Lesotho, have been characterized by electron microprobe analysis, X-ray-precession photography, and transmission electron microscopy. Significant exsolution of pigeonite was observed only in those samples for which Ca/(Ca+Mg)0.320. The exsolution microstructure consists of coherent (001) lamellae with wavelengths ranging from 20 to 31 nm and compositional differences between the hosts and lamellae ranging from 10 to 30 mol% wollastonite. These observations suggest that the exsolution reaction mechanism was spinodal decomposition and that the megacrysts have been quenched at various stages of completion of the decomposition process.Annealing experiments in evacuated SiO₂ glass tubes at 1,150° C for 128 hours failed to homogenize microstructure, whereas, at 5 kbar and 1,150° C for only 7.25 hours, the two lattices were homogenized. This pressure effect suggests that spinodal decomposition in the kimberlitic subcalcic diopside megacrysts can only occur at depths less than 15 km; the cause of the effect may be the jadeite component in the pyroxene. Apparent quench temperatures for the exsolution process in the megacrysts range from 1,250° C to 990° C, suggesting that decomposition must have commenced at temperatures of more than 1,000° C.These P–T limits lead to the conclusion that, in those kimberlites where spinodal decomposition has occurred in subcalcic diopside megacrysts, such decomposition occurred at shallow levels (<15 km) and, at the present erosion level, temperatures must have been greater than 1,000° C.     

Exsolution of Ca-amphibole from glaucophane and the miscibility gap between sodic and calcic amphiboles

Transmission and analytical electron microscopy (TEM/AEM) of glaucophane from glaucophane + Ca-amphibole-bearing blueschist and eclogite from two Vermont localities (Ecologite Brook and Tillotson Peak) and one California locality (Cazadero) has revealed the first evidence from exsolution for the miscibility gap between sodie and calcic amphiboles. The Tillotson Peak samples and the Cazadero samples contain coarsegrained glaucophane—actinolite pairs, while the Eclogite Brook samples contain coexisting glaucophane and actinolitic hornblende. Ca-rich glaucophanes contain abundant fine-scale lamellae of Ca-rich amphibole. These lamellae are usually oriented near (100) and . High-resolution TEM (HRTEM) images show them to be coherent. The exsolution lamellae are so narrow, beyond the resolution of AEM, that their true Ca contents are obscured by analytical contributions from the surrounding host. The AEM data suggest that the lamellae are either winchite or actinolite, depending on the true Ca concentration. In most cases, the exsolution lamellae have very curved interfaces and show variable orientation. This is attributed to the close similarity of unit-cell parameters for the two amphiboles. Three-dimensional optimal phase-boundary calculations using EPLAG (Fleet 1982) show that the observed 100 and orientations are consistent with the minimization of area strain between the two lattices along the interface. Some samples show evidence for incipient exsolution in the form of homogeneously distributed, fine-scale precipitates. These results suggest that Ca-rich glaucophanes from other glaucophane—actinolite assemblages may be exsolved at the TEM scale. The coexisting amphiboles from Eclogite Brook also have been studied using the electron microprobe (EMP). The compositional gap defined by the Eclogite Brook pairs is consistent with previously reported results, but shows a wider break along the glaucophane—actinolite pseudobinary join, suggesting very limited solid solution up to temper-atures of about 500–550°C. The glaucophanes are relatively poor in Ca, except for one anomalous grain containing Ca-amphibole lamellae. Ferric iron, estimated by normalization to fixed cation numbers, is strongly partitioned into the actinolitic hornblende and the glaucophanes are very poor in the riebeckite component.     

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:

     

Exsolution and coarsening mechanisms and kinetics in an ordered cryptoperthite series

Cryptoperthites from the Klokken layered syenite intrusion were examined by TEM to determine the role of exsolution, ordering and twinning in the development of the coherent microtextures during slow cooling, the stratigraphic position of the samples in the layered series giving an independent variable in determining their evolution. Both periodicity (primary and secondary) and morphology change with distance from the top of the series. Most samples contain low microcline in the diagonal association.Partial ordering occurred before exsolution, which was followed by Albite-twin formation in the albite lamellae. The twin periodicity depends on the average lamellar thickness (or on the primary lamellar periodicity, ₁) and no longer changes during subsequent morphological evolution. In the Or-rich lamellae long-period Albite twins develop before waves form in the lamellar interface. The interfaces rotate with increasing order to give parallel-sided zig-zag lamellae of low microcline with Albite twinned lamellae of low albite, which may pinch and swell. Where the albite lamellae are discontinuous, adjacent microcline lamellae coalesce giving oblique lamellae and Pericline or M-type twins. Thickening of some oblique lamellae gives a distinct secondary periodicity, ₂, which outlines lozenge-shaped areas with relics of the primary periodicity and, if coarse enough, is responsible for optically-visible braid microperthite. Coherency, demonstrated by high resolution images, is maintained through all stages of the coarsening.A time-temperature-transformation diagram for continuous cooling is presented and can be used to interpret the kinetics and morphological evolution of cryptoperthites from rocks with very different cooling rates (dykes and lavas to very large plutons), which have, however, similar primary lamellar periodicities. The finest periodicities are only slightly larger than the supposed initial periodicities ( _o) for spinodal decomposition and little coarsening can have occurred. Coarsening at cooling rates slow enough to produce significant ordering may be much slower than coarsening in disordered feldspars. Primary coarsening may be stopped by the development of Albite twins in the Abrich phase, which will require reversal of the order-antiorder sense of parts of the framework. Coarsening may also be slowed if the phases at intermediate temperatures order at different rates or have different equilibrium degrees of Al-Si order. Secondary coarsening can develop at much lower temperatures (<400° C) on the formation of low microcline, when both phases have the same framework order.     

Sigmoidal exsolution by internal shear stress in pyroxenes from chondritic meteorites

Pyroxenes of pigeonitic and augitic bulk compositions in H3–4 chondritic meteorites commonly exhibit sigmoidal precipitates, rather than the elsewhere common lamellar associations. Most often, submicrometric sigmoids with calcic clinopyroxene composition occur within clinoenstatite; more rarely, clinoenstatite sigmoids occur within calcic clinopyroxene. The sigmoids appear as 001 terminated lamellae, with terminations rotated in opposite directions towards the 100 orientation. Pre-exsolution pigeonite and augite formed at temperatures higher than 980 °C, whereas sigmoidal exsolution occurred between 990 and 830 °C. Local anomalous lattice parameters determined by electron diffraction suggest that lattice parameters are most strained where the exsolution texture is most poorly defined. Shear strain occurs during exsolution due to mismatching lattice parameters and variable angles. In response to shear stress, the lamellae relax and assume sigmoidal strained morphologies. Sigmoidal exsolution is strongly controlled by (100) orthoenstatite stacking faults that possibly trigger exsolution.     

Exsolution of Ca-clinopyroxene from orthopyroxene aided by deformation

Monoclinic calcium-poor shear-transformation lamellae and calcium-rich exsolution lamellae occur parallel to (100) in orthopyroxene. The formation of both structures from an orthopyroxene host involves a shear on (100) parallel to [001], with additional cation exchange in the exsolution case. The shear transformation involves a macroscopic simple shear angle of 13.3° (shear strain of 0.236) and produces a specific a-axis orientation with respect to the sense of shear; we have found that this orientation dominates in exsolution lamellae in kinked orthopyroxene, where the sense of shear is known. In undeformed orthopyroxene, there is generally no preferred sense of orientation of the monoclinic a axes. We advance a specific model for exsolution involving nucleation and growth by shear transformation combined with cation exchange, thus circumventing the classical nucleation barrier and permitting exsolution at lower solute supersaturations.     

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

     

Solid solutions of trace Eu(III) in calcite: Thermodynamic evaluation of experimental data over a wide range of pH and pCO2

The thermodynamics of dilute Eu-calcite solid solutions formed under widely different pH-pCO₂ conditions at T = 25°C and p = 1 bar were investigated using three sets of Eu(III) uptake experiments, two of which were taken from the literature: (a) recrystallization in synthetic cement pore water at pH ∼ 13 and pCO₂ ∼ 10⁻¹³ bar (this work); (b) coprecipitation in 0.1 M NaClO₄ at pH ∼ 6 and pCO₂ ∼ 1 bar; (c) coprecipitation in synthetic seawater at pH ∼ 8 and pCO₂ ranging from 3 × 10⁻⁴ to 0.3 bar.Solid solution formation was modeled using the Gibbs energy minimization (GEM) method. In a first step (“forward” modeling), we tested ideal binary solid solution models between calcite and the Eu end-members Eu₂(CO₃)_3, EuNa(CO₃)₂, Eu(OH)CO₃ or Eu(OH)₃, for which solids with independently measured solubility products exist. None of these four binary solid solutions was capable of reproducing all three experimental datasets simultaneously. In a second step (“inverse” modeling), ideal binary solid solutions were constructed between calcite and the candidate Eu end-members EuO(OH), EuH(CO₃)₂ and EuO(CO₃)_0.5, for which no independent solubility products are available. For each single data point and each of these end-members, a free energy of formation with inherent activity coefficient term ( = G_α^o + RT lnγ_α) was estimated from “dual thermodynamic” GEM calculations. The statistical mean of was then calculated for each of the three datasets. A specific end-member was considered to be acceptable if a standard deviation of ± 2 kJ mol⁻¹ or less resulted for each single dataset, and if the mean -values calculated for the three datasets coincided. No binary solid solution with any of the seven above mentioned end-members proved to satisfy these criteria.The third step in our analysis involved consideration of ternary solid solutions with CaCO₃ as the major end-member and any two of the seven considered Eu trace end-members. It was found that the three datasets can only be reproduced simultaneously with the ternary ideal solid solution EuH(CO₃)₂ - EuO(OH) - CaCO₃, setting = −1773 kJ mol⁻¹ and = −955 kJ mol⁻¹, whereas all other end-member combinations failed. Our results are consistent with time-resolved laser fluorescence data for Cm(III) and Eu(III) indicating that two distinct species are incorporated in calcite: one partially hydrated, the other completely dehydrated. In conclusion, our study shows that substitution of trivalent for divalent cations in carbonate crystal structures is a more complex process than the classical isomorphic divalent-divalent substitution and may need consideration of multicomponent solid solution models.     

Electron microscopical study of garnet exsolution in orthopyroxene

By means of electron-optical methods the structure and orientation of lamella-like garnet exsolutions in an orthopyroxene from the eclogite of Gilsberg, Saxony Granulite Complex, are investigated. It can be shown that besides the macroscopic and microscopic submicroscopic exsolution lamellae also exist; they have widths up to nearly 5 nm. The intergrowth conditions are such shat (100)_op is parallel to {100}_gr and [001]_op parallel to 〈110〉_gr. The lamellae are coherent, but sometimes extended dislocation arrangements are found in microscopic lamellae which are caused by a deformation. These investigations point to formation of the rock as a result of a solid state reaction under high pressure conditions with subsequent diffusive-dissociative exsolution of garnet from primary pyroxene.     

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:

     

Effects of magnesium ions on near-equilibrium calcite dissolution: Step kinetics and morphology

Dissolution kinetics at the aqueous solution-calcite interface at 50 °C were investigated using in situ atomic force microscopy (AFM) to reveal the influence of magnesium concentration and solution saturation state on calcite dissolution kinetics and surface morphology. Under near-equilibrium conditions, dissolved Mg²⁺ displayed negligible inhibitory effects on calcite dissolution even at concentrations of . Upon the introduction of , the solution saturation state with respect to calcite, , acted as a “switch” for magnesium inhibition whereby no significant changes in step kinetics were observed at Ω_calcite<0.2, whereas a sudden inhibition from Mg²⁺ was activated at Ω_calcite?0.2. The presence of the Ω-switch in dissolution kinetics indicates the presence of critical undersaturation in accordance with thermodynamic principles. The etch pits formed in solutions with exhibited a unique distorted rhombic profile, different from those formed in Mg-free solutions and in de-ionized water. Such unique etch pit morphology may be associated with the anisotropy in net detachment rates of counter-propagating kink sites upon the addition of Mg²⁺.     

Oxygen self-diffusion “fast-paths” in titanite single crystals and a general method for deconvolving self-diffusion profiles with “tails”

Like most other minerals, titanite rarely if ever forms perfect crystals. In addition to the point defects that might affect lattice diffusion, there may be extended line- or planar defects along which fast diffusion could occur. During the course of an experimental study of oxygen lattice diffusion in titanite, we found that almost all of the ¹⁸O uptake profiles produced in natural titanite crystals departed from the complementary error function solution expected for simple lattice diffusion with a constant surface concentration. Instead, they exhibited “tails” extending deeper into the samples than expected for simple lattice diffusion. The purpose of this contribution is to report on these features—described as “fast-paths” for oxygen diffusion—and outline a method for coping with them in extracting information from diffusion profiles.For both dry and hydrothermal experiments in which the “fast paths” are observed, ¹⁸O was used as the diffusant. In dry experiments, the source material was ¹⁸O-enriched SiO₂ powder, while ¹⁸O-enriched water was used for the hydrothermal experiments. Diffusive uptake profiles of ¹⁸O were measured in all cases by nuclear reaction analysis (NRA) using the ¹⁸O (p,α)¹⁵N reaction [see Zhang X. Y., Cherniak D. J., and Watson E. B. (2006) Oxygen diffusion in titanite: lattice and fast-path diffusion in single crystals. Chem. Geol.235 105-123].In our experiments, different sizes of “tails” (with varying ¹⁸O concentrations) were observed. Theoretically, under the same temperature and pressure conditions, the sizes of tails should be affected by two factors: the diffusion duration and the defect density. For the same experiment duration, the higher the defect density, the larger the “tail”; for the same defect densities, the longer the diffusion duration, the larger the “tail.”The diffusion “tails” could be a result of either planar defects or one-dimensional “pipe” diffusion. AFM imaging of HF etched titanite surfaces confirmed that the etched features might be caused by either parallel planar defects or parallel pipe defects, but could not differentiate between these possibilities. Through theoretical calculations simulating the tailed diffusion profiles using reasonable assumptions of lattice diffusivities and fast-path diffusivities, and comparing these with tail features measured in our samples, it can be concluded that the “tails” observed in our experiments are caused by planar defects rather than pipe defects.A new method was developed for separating the “fast-path” contribution from the overall composite diffusion profile consisting of both “fast-path” and lattice diffusion. Through this process, the lattice diffusion coefficient could be determined, which is required to analyze the tail. The oxygen diffusion rates in the fast-paths were obtained by traditional graphical analysis methods, using the Whipple-Le Claire equation (for 2-D defects) assuming that the width of the fast-path is 1 nm. Two Arrhenius relations were obtained for the fast-path diffusion phenomenon, one for experiments under dry conditions, and the other for hydrothermal conditions: