Calcite kinetics

Since all calcite is dissolved after 100 days at the latest, the following line for the calcite kinetics is inserted at the beginning of the BASIC program to save calculation time ... 

Carbonate Decomposition. The carbonate content of Green River oil shale is high (see Table 4). In addition, the northern portion of the Piceance Creek basin contains significant quantities of the carbonate minerals nahcoUte and dawsonite. The decomposition of these minerals is endothermic and occurs at ca 600—750°C for dolomite, 600—900°C for calcite, 350—400°C for dawsonite, and 100—120°C for nahcohte. Kinetics of these reactions have been studied (19). Carbon dioxide, a product of decomposition, dilutes the off-gases produced from retorting processes at the above decomposition temperatures. 

The decomposition of dolomite shows many points of similarity with the reactions of calcite and of other single carbonates of Group IIA metals (Sects. 3.1.1 and 3.1.2) the reaction is reversible, occurs at an interface, and both apparent kinetic parameters and reactivity are influenced by the prevailing C02 pressure. 

In a final application of kinetic reaction modeling, we consider how sodium feldspar (albite, NaAlSisOs) might dissolve into a subsurface fluid at 70 °C. We consider a Na-Ca-Cl fluid initially in equilibrium with kaolinite [Al2Si20s (OF )/ ], quartz, muscovite [KAl3Si30io(OH)2, a proxy for illite], and calcite (CaC03), and in contact with a small amount of albite. Feldspar cannot be in equilibrium with quartz and kaolinite, since the minerals will react to form a mica or a mica-like... 

The initial methodology used assumed that all excess calcite from the solution would be deposited in a predetermined volume of the geothermal pipe. This approach is conservative considering that calcite deposition is an instantaneous process. However, the literature frequently states that calcite deposition is kinetically-controlled where the process of deposition could either be slower or faster (Sjoberg ... 

The utilization periods for AP01D given by the direct deposition of the excess calcite method has a minimum of 1 month and maximum of 6 months as shown in Fig. 2. It should be noted that the calcite precipitation rate equation at a saturation ratio above 1.72 provided shorter utilization time than the direct deposition method, indicating that the rate law overestimated the amount of calcite deposited. At a saturation ratio below 1.72, however, the rate law indicated a longer utilization period, which was expected since the calcite deposition is kinetically controlled rather than instantaneous deposition of the excess calcite. 

Berner, R.A. Morse, J.M. 1974. Dissolution kinetics of calcium carbonate in seawater. IV. Theory of calcite dissolution, American Journal of Science, 274,108-134. 

Busenberg, E. Plummer, L.N. 1986. A comparative study of the dissolution and crystal growth kinetics of calcite and aragonite. U S. Geological Survey Bulletin, 1578, 139-168. 

Lebron, I. Suarez, D.L. 1996. Calcite nucleation and precipitation kinetics as affected by dissolved organic matter at 25°C and pH > 7.5. Geochimica et Cosmochimica Acta, 60(15), 2765-2776. 

Shiraki, R. Brantley, S.L. 1995. Kinetics of near-equilibrium calcite precipitation at 100°C An evaluation of elementary reaction-based and affinity-based rate laws. Geochemica et Cosmochimica Acta, 59, 1457-1471. 

Sjoberg, E.L. Rickard, D.T. 1984. Calcite dissolution kinetics surface speciation and the origin of the variable pH dependence, Chemical Geology, 42, 119-136. 

The most likely mineral phases to oxidize under the kinetic testing conditions are the sulfides. Acidity generated from their oxidation is likely to react with calcite and to a lesser extent magnesite. Talc minerals are unlikely to react with leachate. Expected reactions are therefore of the forms ... 

The simulated C02 fugacity matches the initial reservoir C02 content and indicates that the pH is buffered by C02-calcite equilibrium. Further modelling was carried out using the Geochemists Workbench React and Tact modules with the thermodynamic database modified to reflect the elevated P conditions and kinetic rate parameters consistent with the Waarre C mineralogy. The Waarre C shows low reactivity and short-term predictive modelling of the system under elevated C02 content changes little with time (Fig. 1). 

Pronounced discrepancies between observed composition and the calculated equilibrium composition illustrate that the formation of the solid phase, for example, the nucleation of dolomite and calcite in seawater, is often kinetically inhibited, and the formation of phosphates, hydrated clay and pyrite is kinetically controlled. 

The processes described and their kinetics is of importance in the accumulation of trace metals by calcite in sediments and lakes (Delaney and Boyle, 1987) but also of relevance in the transport and retention of trace metals in calcareous aquifers. Fuller and Davis (1987) investigated the sorption by calcareous aquifer sand they found that after 24 hours the rate of Cd2+ sorption was constant and controlled by the rate of surface precipitation. Clean grains of primary minerals, e.g., quartz and alumino silicates, sorbed less Cd2+ than grains which had surface patches of secondary minerals, e.g., carbonates, iron and manganese oxides. Fig. 6.11 gives data (time sequence) on electron spin resonance spectra of Mn2+ on FeC03(s). 

Busenberg, E., and L. N. Plummer (1986b), "A Comparative Study of the Dissolution and Crystal Growth Kinetics of Calcite and Aragonite", in F. A. Mumpton, Ed., Studies in Diagenesis, U.S. Geol. Surv. Bull. 1578, pp. 139-168. 

Figure 7. A Mg vs. 5 Mg plot of calcite speleothems and their drip waters from the Soreq cave site, Israel (data from Galy et al. 2002) compared with seawater. The horizontal trend of the data suggests that Mg in carbonates is related to aqueous Mg by equilibrium fractionation processes. Results of a three-isotope regression, shown on the figure and in Table 3, confirm that the (3 value defined by the data is similar to the predicted equilibrium value of 0.521 and distinct from kinetic values. The positive A Mg characteristic of the speleothem carbonates is apparently inherited from the waters. The positive A Mg values of the waters appear to be produced by kinetic fractionation relative to primitive terrestrial Mg reservoirs (the origin).

Although surfece waters are supersaturated with respect to calcium carbonate, abiogenic precipitation is imcommon, probably because of unfevorable kinetics. (The relatively rare formation of abiogenic calcite is discussed further in Chapter 18.) Marine organisms are able to overcome this kinetic barrier because they have enzymes that catalyze the precipitation reaction. Because fl declines with depth, organisms that deposit calcareous shells in deep waters, such as benthic foraminiferans, must expend more energy to create their hard parts as compared to surfece dwellers. 

Inskeep WP, Bloom PR. 1986. Kinetics of calcite precipitation in the presence of water-soluble organic ligands. Soil Science Society of America Journal 50 1167-1172. 

Romanek C. S., Grossman E. L. and Morse J. W. (1989). Carbon isotope fractionation in aragonite and calcite Experimental study of temperature and kinetic effects. Geol Soc. Amer. Abstr. Prog., 2LA76. 

Sjoberg E. L. (1976). A fundamental equation for calcite dissolution kinetics. Geochim. Cosmochim. Acta, 40 441-447. 

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