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Ø. Hammer D.K. Dysthe H. Lund P. Meakin B. Jamtveit 《Geochimica et cosmochimica acta》2008,72(20):5009-5021
We present a 2D numerical model for the growth of calcite from supersaturated aqueous solutions under laminar, open-channel flow conditions. The model couples solution chemistry, precipitation at solution/calcite interfaces, hydrodynamics, diffusion and degassing. The model output is compared with experimental results obtained using an oversaturated calcite solution produced by mixing CaCl2 and Na2CO3. The precipitation rate is observed to increase when the supersaturated solution flows over an obstruction, leading to a growth instability that causes the formation of terraces. At relatively high flow rates, the most important mechanism for this behaviour seems to be hydrodynamic advection of dissolved species either towards or away from the calcite surface, depending on location relative to the obstruction, which deforms the concentration gradients. At lower flow rates, steepening of diffusion gradients around protrusions becomes important. Enhanced degassing over the obstruction due to shallowing and pressure drop is not important on small scales. Diffusion controlled transport close to the calcite surface can lead to a fingering-type growth instability, which generates porous textures. Our results are consistent with existing diffusive boundary layer theory, but for flow over non-smooth surfaces, simple calcite precipitation models that include empirical correlations between fluid flow rate and calcite precipitation rate are inaccurate. 相似文献
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Sergey Zubtsov Franois Renard Jean-Pierre Gratier Robert Guiguet Dag K Dysthe Vladimir Traskine 《Tectonophysics》2004,385(1-4):45-57
Experimental observations are reported of weakening of sediment-like aggregates by addition of hard particles. Sieved mixtures of calcite and halite grains are experimentally compacted in drained pressure cells in the presence of a saturated aqueous solution. The individual halite grains deform easily by pressure solution creep whereas calcite grains act as hard objects and resist compaction. The fastest rate of compaction of the mixed aggregate is not obtained for a 100% halite aggregate but for a content of halite grains between 45% and 75%. We propose that this unusual compaction behavior reflects the competition between two mechanisms at the grain scale: intergranular pressure solution at grain contacts and grain boundary healing between halite grains that prevent further compaction. 相似文献
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We investigated the dissolution behaviour of polished calcite surfaces in situ using a fluid-cell atomic force microscope. Polished calcite surfaces enabled us to study the effects of applied surface stress and crystallographic orientation on calcite dissolution pattern formation. Thin-sections of Iceland spar single-crystals polished either parallel or with a 5° miscut angle to cleavage planes were studied. Compressive surface stresses of up to 50 MPa were applied to some of the thin-section samples by means of elastic concave bending. Experiments were carried out in semi-stagnant deionized water under mainly transport limited dissolution conditions. Samples polished parallel to cleavage planes dissolved by the formation of etch-pits originating from polishing defects. The dissolution behaviour of 5° miscut surfaces was relatively unaffected by polishing defects, since no etch-pits developed in these samples. Dissolution of the miscut samples led to stepped or rippled surface patterns on the nanometer scale that coarsened during the first 30-40 min of the experiments. Possible reasons for the pattern-coarsening were: (i) progressive bunching of retreating dissolution steps and (ii) surface energy driven recrystallization (Ostwald ripening) under transport limited dissolution conditions. A flat polished miscut surface in calcite may recrystallize into a hill-and-valley structure in a (near-)saturated solution so as to lower its total surface free energy in spite of a larger surface area. No clear effect of applied stress on dissolution pattern formation has been observed. 相似文献
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We present an experimental investigation on the dissolution of uniaxially stressed crystals of NaClO3 in contact with brine. The crystals are immersed in a saturated fluid, stressed vertically by a piston and monitored constantly in situ with a CCD camera. The experiments are temperature-controlled and uniaxial shortening of the sample is measured with a high-resolution capacitance analyzer. Once the crystal is stressed it develops dissolution grooves on its free surface. The grooves are oriented with their long axis perpendicular to the direction of compressive stress and the initial distance between the parallel grooves is in accordance with the Asaro-Tiller-Grinfeld instability. We observe a novel, transient evolution of this roughness: The grooves on the crystal surface migrate upwards (against gravity), grow in size and the inter-groove distance increases linearly with time. During the coarsening of the pattern this switches from a one-dimensional geometry of parallel grooves to a two-dimensional geometry with horizontal and vertical grooves. At the end of the experiment one large groove travels across the crystal and the surface becomes smooth again. Uniaxial shortening of the crystal by pressure solution creep decays exponentially with time and shows no long term creep within the range of the resolution of the capacitance analyzer (accuracy of 100nm over a period of 14 days). This indicates that, while active, the fast transient processes on the free surface increase the solution concentration and thereby significantly slow down or stop pressure solution at the top of the crystal. This novel feedback mechanism can explain earlier results of cyclic pressure solution creep and demands development of a more complex theory of pressure-solution creep including processes that act on free surfaces. 相似文献
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In situ measurements of mineral surface evolution during the process of pressure solution are possible with the high brightness of synchrotron X-ray sources. This capability has been explored through the use of newly developed reaction vessels that allow transmission of the incident and scattered X-ray beam through a low atomic weight piston. Several new vessels are described, along with details of computational algorithms that are used to simulate X-ray scattering in this unconventional geometry. Results using calcite (CaCO3) and halite (NaCl) as reactant crystals are presented and compared to other atomic-scale measurements of surface dissolution processes. Calcite was reacted with an unsaturated fluid at 30 bars of pressure for approximately 24 h. During reaction the root mean square surface roughness (σ) evolved from 13.7 Å (± 0.5 Å) to 19.5 Å (± 1.0 Å), giving a roughening rate of: dσ/dt = +6.3 × 10− 5 Å s− 1. This is consistent with other measurements made with free calcite surfaces and is driven almost entirely by chemical disequilibrium. Analysis of the surface ex situ post-reaction gives an identical σ value, showing that the in situ measurements are well-constrained. Experiments also at 30 bars but in a saturated solution indicate that the calcite surface does not significantly roughen, giving the result that pressure solution of calcite at this pressure cannot be monitored in experiments of several days duration. Experiments with halite, a much more reactive phase, in saturated solutions showed the reflectivity profile to be dynamic on a time scale of hours. This experiment was left to reach equilibrium over 108 days and then re-analyzed, showing that σ had increased from 34 Å (± 2 Å) to 41 Å (± 2 Å), giving a roughening rate of: dσ/dt ≤ +6.4 × 10− 7 Å s− 1. This is two orders of magnitude smaller than the calcite roughening rate caused by chemical disequilibrium and provides the first direct in situ atomic-scale measurement of the rate of surface roughening due to pressure solution. 相似文献
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