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At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

A Geo‐Structurally Based Correction Factor for Apparent Dissolution Rates in Fractured Media

Abstract Field measurements of apparent geochemical weathering reaction rates in subsurface fractured porous media are known to deviate from laboratory measurements by multiple orders of magnitude. To date, there is no geologically based explanation for this discrepancy that can be used to predict reaction rates in field systems. Proposed correction factors are typically based on ad hoc characterizations related to geochemical kinetic models. Through a series of high‐fidelity reactive transport simulations of mineral dissolution within explicit 3D discrete fracture networks, we are able to link the geo‐structural attributes with reactive transport observations. We develop a correction factor to linear transition state theory for the prediction of the apparent dissolution rate based on measurable geological properties. The modified rate law shows excellent agreement with numerical simulations, indicating that geological structure could be a primary reason for the discrepancy between laboratory and field observations of apparent dissolution rates in fractured media.

58 GEOSCIENCES↗
DOE OSTI2016

Tandem dissolution of UO 3 in amide-based acidic ionic liquid and in situ electrodeposition of UO 2 with regeneration of the ionic liquid: a closed cycle

A closed cycle is demonstrated for the tandem dissolution and electroreduction of UO 3 to UO 2 with regeneration of the acidic ionic liquid. The dissolution is achieved by use of the acidic ionic liquid N,N-dimethylacetimidium bis(trifluoromethanesulfonimide) in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonimide) serving as the diluent. Bulk electrolysis performed at 1.0 V vs. Ag reference yields a dark brown-black uranium deposit (UO 2 ) on the cathode. Anodic oxidation of water in the presence of dimethylacetamide regenerates the acidic ionic liquid. We have demonstrated the individual steps in the cycle together with a sequential dissolution, electroreduction, and regeneration cycle.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

A reductive dissolution study of magnetite

Magnetite dissolution tests using a hydrazine base solution were performed at a temperature range of 90 to 150 deg. C. The dissolution rate of magnetite increased with [N{sub 2}H{sub 4}], time, and temperature. The optimum solution pH in the experimental range was 3. The addition of copper ion to the hydrazine base solution greatly increased the magnetite dissolution rate. This was explained by the complex formation between N{sub 2}H{sub 4} and Cu ions, and the reducing power of the hydrazine-Cu complex to the ferric ions of magnetite. The reductive decontamination solution can be applied below 100 deg. C by the addition of copper ions. The chemical decontamination of a Type 304 stainless steel specimen using a hydrazine base reductive decontamination solution was also performed. The contact dose rate was greatly decreased by the repetitive application of NP and the hydrazine base solution. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2011

Progress toward bridging from atomistic to continuum modeling to predict nuclear waste glass dissolution.

This report summarizes research performed for the Nuclear Energy Advanced Modeling and Simulation (NEAMS) Subcontinuum and Upscaling Task. The work conducted focused on developing a roadmap to include molecular scale, mechanistic information in continuum-scale models of nuclear waste glass dissolution. This information is derived from molecular-scale modeling efforts that are validated through comparison with experimental data. In addition to developing a master plan to incorporate a subcontinuum mechanistic understanding of glass dissolution into continuum models, methods were developed to generate constitutive dissolution rate expressions from quantum calculations, force field models were selected to generate multicomponent glass structures and gel layers, classical molecular modeling was used to study diffusion through nanopores analogous to those in the interfacial gel layer, and a micro-continuum model (K{mu}C) was developed to study coupled diffusion and reaction at the glass-gel-solution interface.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2011

EVALUATION OF ARG-1 SAMPLES PREPARED BY CESIUM CARBONATE DISSOLUTION DURING THE ISOLOK SME ACCEPTABILITY TESTING

Evaluation of Defense Waste Processing Facility (DWPF) Chemical Process Cell (CPC) cycle time identified several opportunities to improve the CPC processing time. The Mechanical Systems & Custom Equipment Development (MS&CED) Section of the Savannah River National Laboratory (SRNL) recently completed the evaluation of one of these opportunities - the possibility of using an Isolok sampling valve as an alternative to the Hydragard valve for taking DWPF process samples at the Slurry Mix Evaporator (SME). The use of an Isolok for SME sampling has the potential to improve operability, reduce maintenance time, and decrease CPC cycle time. The SME acceptability testing for the Isolok was requested in Task Technical Request (TTR) HLW-DWPF-TTR-2010-0036 and was conducted as outlined in Task Technical and Quality Assurance Plan (TTQAP) SRNLRP-2011-00145. RW-0333P QA requirements applied to the task, and the results from the investigation were documented in SRNL-STI-2011-00693. Measurement of the chemical composition of study samples was a critical component of the SME acceptability testing of the Isolok. A sampling and analytical plan supported the investigation with the analytical plan directing that the study samples be prepared by a cesium carbonate (Cs{sub 2}CO{sub 3}) fusion dissolution method and analyzed by Inductively Coupled Plasma - Optical Emission Spectroscopy (ICP-OES). The use of the cesium carbonate preparation method for the Isolok testing provided an opportunity for an additional assessment of this dissolution method, which is being investigated as a potential replacement for the two methods (i.e., sodium peroxide fusion and mixed acid dissolution) that have been used at the DWPF for the analysis of SME samples. Earlier testing of the Cs{sub 2}CO{sub 3} method yielded promising results which led to a TTR from Savannah River Remediation, LLC (SRR) to SRNL for additional support and an associated TTQAP to direct the SRNL efforts. A technical report resulting from this work was issued that recommended that the mixed acid method be replaced by the Cs{sub 2}CO{sub 3} method for the measurement of magnesium (Mg), sodium (Na), and zirconium (Zr) with additional testing of the method by DWPF Laboratory being needed before further implementation of the Cs{sub 2}CO{sub 3} method at that laboratory. While the SME acceptability testing of the Isolok does not address any of the open issues remaining after the publication of the recommendation for the replacement of the mixed acid method by the Cs{sub 2}CO{sub 3} method (since those issues are to be addressed by the DWPF Laboratory), the Cs{sub 2}CO{sub 3} testing associated with the Isolok testing does provide additional insight into the performance of the method as conducted by SRNL. The performance is to be investigated by looking to the composition measurement data generated by the samples of a standard glass, the Analytical Reference Glass - 1 (ARG-1), that were prepared by the Cs{sub 2}CO{sub 3} method and included in the SME acceptability testing of the Isolok. The measurements of these samples were presented as part of the study results, but no statistical analysis of these measurements was conducted as part of those results. It is the purpose of this report to provide that analysis, which was supported using JMP Version 7.0.2.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2022

Dissolution Flowsheet for Skull Oxide Generated during U-Mo Alloy Casting for High Performance Research Reactor Fuel

The Savannah River National Laboratory was requested to develop dissolution flowsheets for high assay low enriched U scrap generated during the fabrication of high performance research reactor fuel. The scrap streams include U-10Mo and U-10Mo-Zr foils, rejected Al-clad fuel plates, and skull oxide from casting molds. Flowsheets for the dissolution of the U-10Mo-Zr foils and Al-clad U-10Mo-Zr mini-plates received from BWX Technologies, Inc were developed and demonstrated in the first phase of this project. In the second phase (this work), the skull oxide from a U-10Mo casting mold was obtained from the Y-12 National Security Complex (Y-12) and small-scale experiments were performed to demonstrate dissolution flowsheets.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2015

A Study of autunite dissolution in the presence of Shewanella oneidensis MR1 and different bicarbonate concentrations - 15736

Remediation of uranium at Hanford Site 300 area via tripolyphosphate injections could sequester uranium (VI) in an insoluble form, creating different types of uranium phosphate minerals like autunite. The factors that influence the dissolution of such minerals come into question. Bacteria can play a significant role in the dissolution of minerals and the formation of secondary minerals. Formed autunite, as a phosphorus-containing mineral, can attract bacteria to liberate phosphorus, meeting their nutrient requirements and causing uranium release back into the environment. Previous studies have been conducted under aerobic conditions. This research was extended to investigate the stability of the autunite mineral under anaerobic conditions pertaining to the Hanford Site, and to study the effect of facultative anaerobic bacteria on the uranium (VI) release from autunite. Objectives: - Evaluate bacterial interactions with uranium (VI) by focusing on facultative anaerobic bacteria, Shewanella Oneidensis MR1. - Study the effect of bacteria on the dissolution of the uranyl phosphate solid phases created as a result of sodium tripolyphosphate injections into the subsurface at the Hanford 300 Area. - Evaluate the role of anaerobic bacteria as one of the factors affecting the outcome of environmental remediation. Experimental Approach: - 16 crimp-sealed bottles were prepared with 50 ml of media solution and 90 mg (4.4 mM of uranium VI) Ca-autunite mineral obtained from Hanford site. Bottles were inoculated with Shewanella Oneidensis MR1 strains when autunite was equilibrated with the media solutions. Bacteria was obtained from Pacific Northwestern National Laboratory (PNNL). - Solution media preparation: 20 mM NaHepes buffer: pH=7, 10 mM Lactate plus bicarbonate in four concentrations (0, 3, 5 and 10 of KHCO{sub 3}) and a bacteria-free control. Samples were taken from the supernatant solutions of the experimental and control bottles. Wet and dry digestion was performed until a white solid residue was acquired. Dilution was made on solid samples for KPA analysis to determine concentration of U (VI) as function of time. The results suggests that in the absence of HCO{sub 3}{sup -} there was a decrease in the of uranium (VI) concentration due to metal reducing bacteria Shewanella Oneidensis MR1, but for higher concentrations of HCO{sub 3}{sup -}, there is no significant difference in uranium (VI) reduction before and after inoculation with bacteria. - Although bottles were sealed, there is an hypothesis that it was oxygen restricted but not strictly performed under anaerobic conditions. For that matter there is a reason to repeat the experiment using the glove box to understand the uranium (VI) reduction mechanism and behavior of bacteria with oxic/anoxic interfaces. Conclusions: - Higher bicarbonate concentrations might interfere with uranium (VI) reduction by anaerobic bacteria. - It is necessary to reduce time of sampling before the inoculation and prolong sampling time after inoculation. - It might be necessary to increase concentration of lactate. Future Work: - Conduct experiment in strictly anaerobic conditions by using the glove box and evaluate if further reduction of uranium (VI) is obtained in the presence of carbonates (HCO{sub 3}{sup -}). - Finalize analysis of calcium and phosphorous via ICP instrument.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2015

Experimental study on effects of geologic heterogeneity in enhancing dissolution trapping of supercritical CO 2

Abstract Dissolution trapping is one of the primary mechanisms that enhance the storage security of supercritical carbon dioxide (scCO 2 ) in saline geologic formations. When scCO 2 dissolves in formation brine produces an aqueous solution that is denser than formation brine, which leads to convective mixing driven by gravitational instabilities. Convective mixing can enhance the dissolution of CO 2 and thus it can contribute to stable trapping of dissolved CO 2 . However, in the presence of geologic heterogeneities, diffusive mixing may also contribute to dissolution trapping. The effects of heterogeneity on mixing and its contribution to stable trapping are not well understood. The goal of this experimental study is to investigate the effects of geologic heterogeneity on mixing and stable trapping of dissolved CO 2 . Homogeneous and heterogeneous media experiments were conducted in a two‐dimensional test tank with various packing configurations using surrogates for scCO 2 (water) and brine (propylene glycol) under ambient pressure and temperature conditions. The results show that the density‐driven flow in heterogeneous formations may not always cause significant convective mixing especially in layered systems containing low‐permeability zones. In homogeneous formations, density‐driven fingering enhances both storage in the deeper parts of the formation and contact between the host rock and dissolved CO 2 for the potential mineralization. On the other hand, for layered systems, dissolved CO 2 becomes immobilized in low‐permeability zones with low‐diffusion rates, which reduces the risk of leakage through any fault or fracture. Both cases contribute to the permanence of the dissolved plume in the formation.

Agartan, Elif↗
DOE OSTI2019

Enhanced Drug Delivery by Dissolution of Amorphous Drug Encapsulated in a Water Unstable Metal–Organic Framework (MOF)

Abstract Encapsulating a drug molecule into a water‐reactive metal–organic framework (MOF) leads to amorphous drug confined within the nanoscale pores. Rapid release of drug occurs upon hydrolytic decomposition of MOF in dissolution media. Application to improve dissolution and solubility for the hydrophobic small drug molecules curcumin, sulindac, and triamterene is demonstrated. The drug@MOF composites exhibit significantly enhanced dissolution and achieves high supersaturation in simulated gastric and/or phosphate buffer saline media. This combination strategy where MOF inhibits crystallization of the amorphous phase and then releases drug upon MOF irreversible structural collapse represents a novel and generalizable approach for drug delivery of poorly soluble compounds while overcoming the traditional weakness of amorphous drug delivery: physical instability of the amorphous form.

Suresh, Kuthuru↗
DOE OSTI2019

Enhanced Drug Delivery by Dissolution of Amorphous Drug Encapsulated in a Water Unstable Metal–Organic Framework (MOF)

Encapsulating a drug molecule into a water-reactive metal–organic framework (MOF) leads to amorphous drug confined within the nanoscale pores. Rapid release of drug occurs upon hydrolytic decomposition of MOF in dissolution media. Application to improve dissolution and solubility for the hydrophobic small drug molecules curcumin, sulindac, and triamterene is demonstrated. The drug@MOF composites exhibit significantly enhanced dissolution and achieves high supersaturation in simulated gastric and/or phosphate buffer saline media. This combination strategy where MOF inhibits crystallization of the amorphous phase and then releases drug upon MOF irreversible structural collapse represents a novel and generalizable approach for drug delivery of poorly soluble compounds while overcoming the traditional weakness of amorphous drug delivery: physical instability of the amorphous form.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Incorporation of trace metals in Hanford waste boehmite mineral phases and dissolution rate impacts

Abstract Data from radioactive Hanford tank waste samples was analyzed to assess whether trace metals present were incorporated into the bulk boehmite matrix or were simply intermingled with the bulk material and subsequent impacts to the boehmite dissolution rate. Results suggest that chromium is primarily blended into the bulk boehmite, with a small fraction present on the surface of the solids. However, increasing the level of chromium incorporation 30‐fold decreased the dissolution rate of boehmite by only 8%, suggesting incorporation of chromium into these samples had a minimal impact on the dissolution rate. Iron was also found to be incorporated in the boehmite solids. Silicon appears to be simply intermingled and not blended into the boehmite crystals.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2013

Buoyant currents arrested by convective dissolution

[1] When carbon dioxide (CO 2 ) dissolves into water, the density of water increases. This seemingly insubstantial phenomenon has profound implications for geologic carbon sequestration. In this work we show, by means of laboratory experiments with analog fluids, that the up–slope migration of a buoyant current of CO 2 is arrested by the convective dissolution that ensues from a fingering instability at the moving CO 2 –groundwater interface. We consider the effectiveness of convective dissolution as a large–scale trapping mechanism in sloping aquifers, and we show that a small amount of slope is beneficial compared to the horizontal case. We review the development and coarsening of the fingering instability along the migrating current and predict the maximum migration distance of the current with a simple sharp–interface model. We show that convective dissolution exerts a powerful control on CO 2 plume dynamics and, as a conclusion, on the potential of geologic carbon sequestration.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2022

The solubility of titanite in silicate melt determined from growth and dissolution experiments

The solubility of titanite (CaTiSiO 5 ) in Si-rich melts was measured experimentally through growth experiments at 800–1000 °C, 0.5–1.0 GPa, log fO 2 ~ CCO–0.8, t = 72–168 h, and H 2 O = 0 to 4 wt.%, and in dissolution experiments at 925–1300 °C, 0.8 GPa, t = 18–118 h, and H 2 O = 1–10 wt.% in a piston-cylinder apparatus. Run product glasses in growth experiments were homogeneous, and iron loss suppressed ilmenite crystallization. Saturation concentrations in dissolution experiments were estimated by fitting measured diffusion profiles. Titanite solubility increases with increasing temperature and melt composition parameter \(M = {\text{ molar }}\left( {{\text{Na }} + {\text{ K }} + {\text{ 2Ca}}} \right)/\left( {{\text{Al }} \times {\text{ Si}}} \right)\) . Multiple linear regression of glass composition data from growth and dissolution experiments ( n = 29) plus 39 experiments from the LEPR database (Hirschmann et al. 2008 ) yielded the titanite solubility equation (adj. r 2 = 0.95): \(\left( {TiO_{2} } \right)^{melt} \left( {wt.\% } \right) = 0.978 \times M + 0.0048 \times T\left( K \right){-}5.90\) . This model correctly predicted undersaturation in 95% of 2344 experiments from the LEPR database that did not have titanite or rutile. Application to natural rocks yields saturation temperatures that are similar to independent temperature estimates. Finally, this equation should be useful for constraining the temperatures of titanite-saturated melts, for determining whether titanite saturation in magmatic source regions is likely, and for determining when titanite can crystallize and begin to exert an influence on melt geochemistry.

58 GEOSCIENCES↗
DOE OSTI2025

Dissolution of Nb(C,N) and (Ti,V)N in the Coarse-Grained Heat-Affected Zone of Electric Resistance Welded X65 Line Pipe

Nb-rich and Ti/V-rich precipitates were introduced into the X65 linepipe via thermomechanical controlled processing. Their dissolution kinetics were investigated in the coarse-grained heat-affected zone after electric resistance welding. Nb-rich precipitates were found to dissolve completely, whereas Ti/V-rich precipitates were found to dissolve partially, because of slower dissolution kinetics. In conclusion, a complete dissolution of Nb-rich precipitates was associated with significant austenite grain coarsening, which promoted bainite as a quench product, which might have reduced the impact toughness.

microstructure↗
DOE OSTI2019

The Dissolution of Fe in HCl from the Ilmenite Concentrate; Evaluating the Effect of Operating Parameters and Mutual Interactions

Selective HCl dissolution of ilmenite components for obtaining Ti or titanium dioxide (TiO{sub 2}) has been highly recognized due to its advantages, greater environmental friendliness, and simplicity, compared to H{sub 2}SO{sub 4} and Cl{sub 2} methods. The effect of numerous parameters has been studied with the one-factor-at-a-time method. The present study aimed to evaluate the effect of key operation parameters, such as acid-to-solid ratio (A/S: 5 to 20 mL/g), reaction temperature (T: 70 °C to 100 °C), and acid concentration (A pct: 15 to 30 wt pct), on the dissolution of Fe in HCl solution with the minimum Ti losses to the leachate from its abundant, domestic, and low-cost mineral source (Kahnooj ilmenite concentrate) using central composite design–response surface methodology. After 90 minutes of leaching, the Ti/Fe (pct) in terms of dissolved amounts was selected as the process assessment response function. Based on the conducted experimental and statistical analysis, increasing the levels of parameters in the studied domain leads to an increase in Ti/Fe (pct), in the order of A pct > T > A/S. Two statistically significant mutual interactions between A/S-T and T-A pct, with 95 pct confidence level, were revealed for the first time in this study. The optimization strategy was set to the minimization of Ti/Fe (pct) by considering the objective of study and the selected response function. The A/S, T, and A pct were determined to be 5 mL/g, 70 °C, and 15 pct, respectively, for maximum impurity dissolution and minimum Ti loss to the leachate.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Fayalite Dissolution and Siderite Formation in Water-Saturated Supercritical CO2

Olivines, a significant constituent of basaltic rocks, have the potential to immobilize permanently CO2 after it is injected in the deep subsurface, due to carbonation reactions occurring between CO2 and the host rock. To investigate the reactions of fayalitic olivine with supercritical CO2 (scCO2) and formation of mineral carbonates, experiments were conducted at temperatures of 35 °C to 80 °C, 90 atm pressure and anoxic conditions. For every temperature, the dissolution of fayalite was examined both in the presence of liquid water and H2O-saturated scCO2. The experiments were conducted in a high pressure batch reactor at reaction time extending up to 85 days. The newly formed products were characterized using a comprehensive suite of bulk and surface characterization techniques X-ray diffraction, Transmission/Emission Mössbauer Spectroscopy, Scanning Electron Microscopy coupled with Focused Ion Beam, and High Resolution Transmission Electron Microscopy. Siderite with rhombohedral morphology was formed at 35 °C, 50 °C, and 80 °C in the presence of liquid water and scCO2. In H2O-saturated scCO2, the formation of siderite was confirmed only at high temperature (80 °C). Characterization of reacted samples in H2O-saturated scCO2 with high resolution TEM indicated that siderite formation initiated inside voids created during the initial steps of fayalite dissolution. Later stages of fayalite dissolution result in the formation of siderite in layered vertical structures, columns or pyramids with a rhombus base morphology.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2023

Dissolution and reprecipitation of amorphous silica in silica Rich shales induces Non-Monotonic evolution of porosity in acidic reactive environments

Advances in sustainable subsurface energy technologies are crucial for meeting our energy and resource needs for a climate-resilient future. Novel strategies to harness subsurface shale reservoirs for recovering valuable metals and for enabling CO 2 storage are influenced by the morphological and mineralogical heterogeneities of these materials. In this context, delineating the interactions of highly acidic solutions such as wet supercritical CO 2 on shales with varying mineralogy is crucial to inform the stability of caprock seal for CO 2 storage and enhancements in permeability for fluid transport, reactivity, and storage. The feedback chemical effects associated with the interactions of acidic solutions on the morphologies and mineralogies of shales have not been extensively investigated. These insights are crucial for assessing temporal changes in the reactivity and the fate of the fluids in subsurface environments. In this study, we investigate the effect of 1M HCl solution on the chemistry and morphology of three different shale samples with varying carbonate, clay and silica contents. An increase in the amorphous content, from 37% to 41.3%, of silica-rich and carbonate/clay lean shale is noted due to reactions with an acidic solution which is attributed to the dissolution of Si-bearing phases such as clays, accompanied by SiO 2 precipitation. In shales bearing high content of clays and carbonates, significant increase in the pore volumes and surface areas are noted. Non-monotonic changes in the micron-scale porosity of silica rich–carbonate/clay lean (e.g., Mowry shale) are noted using in-situ X-ray microtomography experiments. Due to the initial mobilization of silica and dissolution of carbonate/clay phases, the total porosity slightly increases from 6.7% to 10.7% followed by a decrease to similar to 4% caused by SiO 2 reprecipitation. In conclusion, these findings suggest that even though silica is less reactive in acidic environments, the changes in the amorphous and crystalline content due to dissolution and reprecipitation alter the porosity and fluid flow paths.

04 OIL SHALES AND TAR SANDS↗
DOE OSTI2024

Concurrent measurement of strain and chemical reaction rates in a calcite grain pack undergoing pressure solution: Evidence for surface-reaction controlled dissolution

Pressure solution is inferred to be a significant contributor to sediment compaction and lithification, especially in carbonate sediments. For a sediment deforming primarily by pressure solution, the compaction rate should be directly related to the rate of calcite dissolution, transport along grain contacts, and calcite reprecipitation. Previous experimental work has shown that there is evidence that deformation in wet calcite grain packs is consistent with control by pressure solution, but considerable ambiguity remains regarding the rate limiting mechanism. We present the results of laboratory compaction experiments designed to directly measure calcite dissolution and precipitation rates (recrystallization rates) concurrently with strain rate to test whether measured rates are consistent with predicted rates both in absolute magnitude and time evolution. Recrystallization rates are measured using trace element chemistry (Sr/Ca, Mg/Ca) and isotopes (87Sr/86Sr) of fluids flowing slowly through a compacting grain pack as it is being triaxially compressed. Imaging techniques are used to characterize the grain contacts and strain effects in the post-experiment grain pack. Our data show that calcite recrystallization rates calculated from all three geochemical parameters are in approximate agreement and that the rates closely track strain rate. The geochemically inferred rates are close to predicted rates in absolute magnitude. Uncertainty in grain contact dimensions makes distinguishing between surface reaction control and diffusion control difficult. Measured reaction rates decrease faster than predicted from standard pressure solution creep flow laws. This inconsistency may indicate that calcite dissolution rates at grain contacts are more complex, and more time-dependent, than suggested by geometric models designed to predict grain contact stresses.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗