David A. Crerar

Silica diagenesis; II, General mechanisms

ABSTRACT Amorphous silica phases (opal-A) precipitate in nature due to the formation of dense colloids in supersaturated alkaline aqueous solutions with low relative concentrations of other ions. Opal-A dissolves and yields solutions of still relatively high silica content. In pore waters containing abundant cations, open framework polymers form which flocculate to yield opal-CT. Opal-CT becomes increasingly ordered, primarily due to preferential growth of cristobalite relative to tridymite and crystal size increase. Opal-CT dissolves to yield pore waters of low silica concentration, which allows slow growth of quartz crystals from monomeric solution. The quartz crystals then slowly increase in size and crystallinity. Carbonates appear to enhance opal-CT formation, possibly due to the activity of positively charged hydroxyl complexes. Hence, polymerization in relatively pure systems is involved in opal-A formation and polymerization in impure systems is involved in opal-CT formation, and slow growth from monomeric (low silica concentration) solutions is involved in precipitation of quartz in sedimentary realms.

Migration of Radioactive Wastes: Radionuclide Mobilization by Complexing Agents

Ion exchange, gel filtration chromatography, and gas chromatographymass spectrometry analyses have demonstrated that ethylenediaminetetraacetic acid (EDTA), an extremely strong complexing agent commonly used in decontamination operations at nuclear facilities, is causing the low-level migration of cobalt-60 from intermediate-level liquid waste disposal pits and trenches in the Oak Ridge National Laboratory burial grounds. Because it forms extremely strong complexes with rare earths and actinides, EDTA or similar chelates may also be contributing to the mobilization of these radionuclides from various terrestrial radioactive waste burial sites around the country.

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.

Healing of microcracks in quartz: Implications for fluid flow

Microcracks in quartz {approximately} 100{mu}m in length and < {approximately}10 {mu}m in width heal in 4 h at 600 C and water pressure of 200 MPa (fluid pressure (P{sub f}) = confining pressure (P{sub c})). Healing is thermally activated; the activation energy is estimated to be between 80 and 35 kJ/mol, depending on the model assumed. Rates also show dependence on fluid pressure, chemistry, and crack dimensions. Faster healing rates are observed in smaller cracks. Thus, when new cracks are not being produced in rocks at elevated temperatures and pressures, fractures will have a vast range of lifetimes: macrofractures transport most of the fluid volume and seal relatively slowly, whereas microcracks allow pervasive penetration of fluid into the rock mass but heal quickly.

Dissolution kinetics of strained calcite

Abstract Interaface-limited dissolution of minerals occurs non-uniformly with preferential attack at sites of excess surface energy such as dislocations, edges, point defects, microfractures, etc. Strained crystals are predicted to show higher dissolution rates due to the increased internal energy associated with dislocations and due to enhanced nucleation of dissolution pits at dislocation outcrops on the surface. Using calcite strained to different degrees, we have observed a measurable rate enhancement of two to three times relative to unstrained crystals at temperatures from 3 to 80°C. This rate enhancement is large compared to that predicted from the calculated increase in crystal activity due to strain energy, but small compared to the three orders of magnitude difference in dislocation densities for the crystals tested (106–109 cm−2). Measurements over a range of pH (4.5–8.3) and temperature (3–80°C) showed that the rate enhancement increased with increasing pH and decreasing temperature. Calculations based on the excess free energy of screw dislocations suggest that dissolution rate enhancement should become significant above a critical defect density of roughly 107 cm−2, in apparent agreement with our observations. Crystal dissolution comprises several parallel processes operating in parallel at active sites. The small relative enhancement of dissolution rate with defect density reflects the greater quantity of dissolved material delivered to solution from receding edges and ledges relative to material coming from point defects and dislocations. Our data, coupled with existing information on other minerals, suggest that generally applicable kinetic measurements can be made on low-strain, macroscopic mineral specimens. However, kinetic data on highly strained minerals should include measurement of defect density because of the rate vs. strain correlation. Selective dissolution can be expected to occur in naturally-deformed rocks, where heterogeneity in dislocation distribution could cause solution transfer and deformation.

Relative degradation rates of NTA, EDTA and DTPA and environmental implications

Abstract Multidentate chelating agents such as NTA, EDTA and DTPA are receiving widespread use in a variety of industrial applications and are entering natural water systems. The presence of these chelates in the environment can be undesirable because they solubilise toxic heavy metals. We have analysed the relative biodegradabilities of NTA, EDTA and DTPA in several different chemical environments. The objective was to determine whether any particular chelate is significantly more biodegradable than the others and therefore more desirable from an environmental point of view. Our results suggest that total degradation (including biological and non-biological) rates decrease in the order: DTPA > EDTA > NTA over the short term and NTA ∼ DTPA > EDTA over the long term. However, photolysis appeared to account for a significant proportion of DTPA degradation. Therefore, considering only biodegradation: NTA > EDTA ∼ DTPA. Degradation rates of all three chelates are not rapid enough, even under ideal laboratory conditions, to preclude concern about their release to the environment.

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.

Adsorption of Co and selected actinides by Mn and Fe oxides in soils and sediments

Abstract Iron and Mn oxides and associated radionuclides in soils and sediments from the radioactive waste burial grounds at Oak Ridge National Laboratory have been selectively extracted using wet chemical techniques. Product-moment-correlation analyses have demonstrated that 60 Co and various actinides, principally 244 Cm, 241 Am and 238 Pu are dominantly associated with Mn oxides. Correlation coefficients between these radionuclides and Fe oxides and organic C are generally very low. The important role of Mn oxides in radionuclide adsorption is attributed to their unique surface and colloidal properties. The data illustrate the importance of the Mn oxide component of soils and sediments in controlling transition metal and actinide solubility. These results suggest two major implications for the disposal of radioactive waste. First, in order to minimize future 60 Co and actinide mobilization from disposal sites, a chemical environment in which Mn oxides are least soluble should be maintained. Second, the liberal use of Mn oxides in waste management operations might improve long-term retention of these radionuclides. Deep-sea Mn modules, which may in the future be mined for their trace metal contents, could serve as a ready supply of Mn oxide for waste disposal applications.

Solubility of the buffer assemblage pyrite + pyrrhotite + magnetite in NaCl solutions from 200 to 350°C

Abstract In the design of hydrothermal solubility studies it is important that the system be completely defined chemically. If the solubilities of minerals containing m metallic elements are to be determined in hydrothermal NaCl solutions, the phase rule requires that a total of m + 6 independent intensive parameters be controlled or measured in order to determine completely the system. In this study the solubility of the univariant assemblage pyrite + pyrrhotite + magnetite has been determined in vapor saturated hydrothermal solutions from 200 to 350°C for NaCl concentrations ranging from 0.0 to 5.0 molal. At any temperature, oxygen and sulfur fugacities were buffered by the chosen assemblage. System pH was determined from excess CO 2 partial pressures and computed ionic equilibria. Equilibrium constants were calculated by regression analysis of solubility data. The results show that more than 10 ppm of each mineral can dissolve in typical hydrothermal solutions under geologically realistic conditions. Solubilities were best represented by the species Fe 2+ and FeCl + at 200 and 250°C; Fe 2+ , FeCl + and FeCl 2 0 at 300°C; and Fe 2+ and FeCl 2 0 at 350°C. Ore deposition would occur by lowering temperature, diluting chloride concentration, or by raising pH through wall rock alteration reactions.

Biogeochemistry of bog iron in the New Jersey Pine Barrens

Abstract Rivers and swamps of the southern New Jersey Coastal Plain contain sporadic but quantitatively important deposits of bog iron. This consists of unconsolidated to massive limonite impregnating sands and silts, the only X-ray identifiable Fe mineral being goethite. The chemistry and hydrology of river, swamp and ground waters suggest that Fe is supplied by lateral and vertical migration of corrosive, acidic ground waters up through Fe-rich sediments toward aerated surfaces. Here oxidation of Fe is catalyzed by Fe-fixing bacteria including Thiobacillus ferrooxidans, Leptothrix ochracea, Crenothrix polyspora, Siderocapsa geminata, and Metallogenium sp. These bacteria are essential to the precipitation of Fe, which would not otherwise oxidize at significant rates given the acid pH and chemical composition of surface waters.