Roland Hellmann

Dissolution at dislocation etch pits in quartz

Abstract Several samples of quartz were etched hydrothermally at 300°C in etchams of controlled dissolved silica concentration in order to measure the critical concentration, Ccrit, above which dislocation etch pits would not nucleate on the quartz surface. Ccrit for 300°C was theoretically predicted to be 0.6C0 and the measured Ccrit, was 0.75 ± 0.15C0 (C0 is the equilibrium concentration). Above this value, some dislocation etch pits formed, but the rate of formation significantly decreased. These results are the first experimental validation of etch pit formation theory under hydrothermal conditions. Dune sands showed a generally angular and pitted surface when etched in dilute solutions, while sands etched at C ~ Ccrit showed less angular pitting. Analysis of a soil profile developed in situ on the Parguaza granite, Venezuela, revealed a gradual change from angular, pitted grain surfaces at the top of the profile to rounded surfaces on grains sampled just above bedrock. Since quartz dissolution without surface pitting continues deep in the profile, the Si concentration must exceed Ccrit, at depth. These results indicate that for C >Ccrit, dissolution occurs at edges and kinks on the surface of quartz and very few pits form; in contrast, at C ⪡ Ccrit, dislocation etch pits grow rapidly, contributing to the overall dissolution rate.

The formation of leached layers on albite surfaces during dissolution under hydrothermal conditions

Hydrothermally altered (225°C) albite was compositionally depth-profiled using X-ray photoelectron spectroscopy (XPS)coupled with calibrated Ar ion sputtering. Solution data were collected during dissolution runs for the same crystals which were spectroscopically analyzed. We found that leached zones depleted in Na, Al, and O develop during the initial, incongruent phase of dissolution. Angle resolved XPS (ARXPS) demonstrated that Na and Al are significantly depleted from the upper few monolayers. Depths of leaching, which range from 10 to 900 A, decrease with increasing pH in theacid region and increase with pH in the basic region. Based on calculated dissolution rates the depth of leaching can be roughly correlated with the release rate of Si. From the observation that the equivalents of H+ consumed always exceed the equivalents of Na+ and Al3+ released, hydrolysis cannot be considered to be a simple ion exchange process. The XPS spectra also revealed the presence of Cl− over the entire leaching depth for samples run at pH pHzpc, suggesting electrostatic adsorption of aqueous species at charged sites within the leached layer. The presence of Cl− and Ba2+ also show that preferential leaching creates a porous and open structure which allows for the large-scale influx of solvent molecules. Preliminary evaluations of diffusion transport rates through leached layers suggest that dissolution is not rate limited by diffusion. Instead, the kinetics of dissolution seem to be related to the intrinsic rate of structural hydrolysis. Using the XPS and solution data in conjunction with theoretical and experimental studies in the literature, we propose a dissolution mechanism based on initial ion exchange followed by the hydrolysis of Al and Si, which is modeled as the breakdown of activated complexes formed at bridging oxygen (Obr) sites. Elemental mass balances based on comparisons between the XPS and solution data suggest that dissolution occurs non-uniformly and is probably preferentially constrained to dislocations and macroscopic defects within the structure.

The albite-water system: Part III. Characterization of leached and hydrogen-enriched layers formed at 300°C using MeV ion beam techniques

Abstract Samples of albite feldspar were dissolved at 300C and 170 bars for periods up to 24 h in flow-through reactors at acid, neutral, and basic pH conditions. Three MeV ion beam techniques, Resonant Nuclear Reaction Analysis (RNRA), Rutherford Backscattering Spectrometry (RBS), and Elastic Recoil Detection Analysis (ERDA) were employed to obtain elemental depth profiles and information on the composition of the near-surface region after dissolution. Based on the anti-correlative trends of the H and Na profiles obtained by RNRA, Na loss and H permeation are coupled by an ion exchange process in acidic and neutral pH solutions. At basic pH conditions, the evidence is ambiguous as to whether there is a limited degree of ion exchange between aqueous cations and Na, as based on RBS spectra and Na RNRA profiles. The recorded depths of H permeation and Na leaching range from a maximum at acid pH (H permeation exceeding ∼10,000A, Na leaching ∼20,000A) to a minimum at basic pH (no H enrichment, Na leaching depths of several hundredA). The composition of the leached/H-enriched region is a function of pH. This is postulated to be primarily a function of two factors: the H ion concentration gradient between the solution and the solid, which directly controls the pH-dependence of the ion exchange couple H+ (or H3O+) ← Na+ and secondly, the speciation of Al -OH and t.sbnd; Si -OH groups created by hydrolysis reactions and the subsequent preferential release of Al within the leached/H-enriched zone. Based on the ratios of H uptake to Na loss at acid and neutral pH, which range between 0.7 and 2.5, it is not possible to distinguish between H+, H2O, and H3O+ species permeating into the structure. Free water may be created within the leached/ H-enriched structure via recondensation (repolymerization) reactions of adjacent Si&-OH groups. Excess H concentration profiles potentially provide indirect evidence for recondensation reactions at depths

The albite-water system: Part IV. Diffusion modeling of leached and hydrogen-enriched layers

Measured H and Na concentration depth profiles in albite samples dissolved at 300°C at various pH conditions (Hellmann et al., 1997-Part III) are indicative of the complex nature of diffusion within leached/H-enriched layers. A qualitative comparison between the measured profiles and profiles based on various diffusion models reveals that the inward diffusion of H species and the outward diffusion of Na are not independent, but rather are interrelated by- an interdiffusion process that can be modeled with a single interdiffusion coefficient ~D. The coefficient ~D varies as a function of the concentration of either H or Na and is thus dependent on depth. The proposed interdiffusion model is based on rates of Na diffusion that are up to several orders of magnitude faster than H diffusion (DNa/DH ≫ 1), this being in accord with direct diffusion measurements from the glass dissolution literature. Modeling results reveal that the rate of H diffusion is one of the most important parameters in determining the depths of leached/ H-enriched layers. Based on a qualitative comparison between the measured profiles and the interdiffusion model, a lower limit of DH ≥ 10−13cm2s−1 can be estimated for leached/H-enriched layers created at acid pH (3.3-3.4) and 300°C. Depending on the estimated value of DNa/DH, this corresponds to DNa ≫ 10−13cm2s−1. The use of a structural factor with ~D imparts an even greater concentration dependence on the interdiffusion coefficient. Increasing the value of the structural factor has the effect of greatly increasing the depth of leaching/H enrichment for any given set of constant DH and DNa/DH values. Irreversible chemical reactions which result in the uptake of H species, such as framework bond hydrolysis reactions, are also potentially important in correctly modeling diffusion of leached/ H-enriched layers. Increasing the rate of reaction acts as a damping factor on steady-state diffusion profiles. Chemical reactions within leached/ H-enriched layers potentially necessitate the addition of a chemical reaction term to the applied diffusion model in order to avoid an underestimation of diffusion rates.

Albite feldspar hydrolysis to 300°C

Abstract The dissolution of albite in aqueous solutions at temperatures ranging from 25 to 300°C and pH s from 2.4 to 10.1 was investigated by determining the stoichiometric ratios of Al/Si, Na/Si and Al/Na in the effluent solution. The experiments were carried out in a tubular plug flow reactor. The solution data collected at temperatures above 100°C were representative of steady state dissolution behavior. Under these conditions, both the Al/Si and the Na/Si ratios showed negative incongruency. The deficiency of Al was attributable to the observed precipitation of a crystalline boehmite (AlO(OH)) reaction rim. The high porosity of the boehmite phase prevented it from behaving as a diffusional barrier to the outward flux of Si from the structure. The deficiency of Na was much less than that of Al, and was probably due to the sorption of Na ions either at the feldspar-boehmite interface and/ or within the boehmite. If these factors are taken into account, then it can be assumed that steady state dissolution is congruent. At temperatures of 25 and 100°C, the Al/Si and Na/Si ratios generally showed positive incongruency, indicating the preferential release of Al and Na with respect to Si. This reveals that the initial, non-steady state phase of dissolution leads to a leached near-surface zone which is depleted in Na and Al. This altered layer is probably not uniform in depth or lateral extent since there was evidence for preferential attack of the feldspar at specific sites on the surface, leading to etch pit formation. When the low temperature, non-steady state results are taken together with the high temperature, steady state results, then the hydrolysis of feldspars can be considered to be a two-step process: first, there is the initial development of a non-uniform leached zone due to incongruent dissolution; the subsequent steady state, stoichiometric dissolution process occurs when the rate of diffusion of product ions through the leached zone equals the rate of structure breakdown.

Atomic-scale imaging of albite feldspar, calcium carbonate, rectorite, and bentonite using atomic-force microscopy

Atomic force microscopy (AFM) was used to investigate the (010) surface of Amelia albite, the basal and (001) planes of CaCO3 (calcite), and the basal planes of rectorite and bentonite. Atomic scale images of the albite surface show six sided, interconnected en-echelon rings. Fourier transforms of the surface scans reveal two primary nearest neighbor distances of 4.7 and 4.9 +/- 0.5 angstroms. Analysis of the images using a 6 angstroms thick projection of the bulk structure was performed. Close agreement between the projection and the images suggests the surface is very close to an ideal termination of the bulk structure. Images of the calcite basal plane show a hexagonal array of Ca atoms measured to within +/- 0.3 angstroms of the 4.99 angstroms predicted by x-ray diffraction data. Putative images of the (001) plane of carbonate ions, with hexagonal 5 angstroms spacing, are also presented and discussed. Basal plane images of rectorite show hexagonal symmetry with 9.1 +/- 2.5 angstroms spacing, while bentonite results reveal a 4.9 +/- 0.5 angstroms nearest neighbor spacing.