Calcite precipitation controls

The first alternative is unlikely but not impossible. Past experience suggests that KdSr probably depends on the degree of supersaturation of the solutions with respect to calcite during calcite precipitation (Katz et al., 1972). In the experiments by Shikazono and Holland (1983), the solutions from which anhydrite was deposited were considerably supersaturated. It is therefore possible that the values of K sr extracted from their experimental data are higher than those which controlled the incorporation of Sr in anhydrite during the formation of Kuroko deposits. Experiments at very low degrees of anhydrite supersaturation are needed to determine whether this is a possible explanation for the low Sr content of some of the Kuroko anhydrites. 

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. 

Figure 7.15 A simple ocean-atmosphere-continent system. Pressure of C02 enhances Ca release from the continental crust (which is assumed to be made of CaSi03) and controls the depth of calcite saturation. Calcite precipitation is therefore controlled by the hypsometric curve, equation (7.4.8), and Pco2-...

Heavy metal cations precipitate readily as hydroxides or carbonates in alkaline media. Dissolved carbonate content will be limited by calcite precipitation or by conversion of hydroxyl AFm to carbonate AFm. Hydroxide ions, on the other hand, are abundant. Here only the solids that may be present under oxic conditions will be discussed. Figure 5a shows the total dissolved heavy metal cation concentrations that would prevail if hydroxide precipitation were to be the dominant solubility-controlling process. Figure 5b shows the solubility of Ca metallate species, as these are likely to act as solubility-controlling phases for oxyanionic species. 

Let us now consider the problem from the standpoint of calcite precipitation kinetics. At saturation states encountered in most natural waters, the calcite reaction rate is controlled by surface reaction kinetics, not diffusion. In a relatively chemically pure system the rate of precipitation can be approximated by a third order reaction with respect to disequilibrium [( 2-l)3, see Chapter 2]. This high order means that the change in reaction rate is not simply proportional to the extent of disequilibrium. For example, if a water is initially in equilibrium with aragonite ( 2c=1.5) when it enters a rock body, and is close to equilibrium with respect to calcite ( 2C = 1.01), when it exits, the difference in precipitation rates between the two points will be over a factor of 100,000 The extent of cement or porosity formation across the length of the carbonate rock body will directly reflect these... 

Rate expressions of this form were derived for calcite precipitation with = 1 (Nancollas and Reddy, 1971 Reddy and Nancollas, 1971), and with mj = 0 and = 0.5 (Sjoberg, 1976 Kazmierczak et al, 1982 Rickard and Sjoberg, 1983 Sjoberg and Rickard, 1983). Rate equations such as (57) wherein rates are linearly proportional to AG close to equilibrium have been attributed to adsorption-controlled growth (Nielsen, 1983 Shiraki and Brantley, 1995). Such rate models have been used by some researchers to model dissolution and precipitation of quartz over a wide range in temperature and pressure (Rimstidt and Barnes, 1980) however, it has been pointed out that this has only been confirmed with experiments at high temperature (Dove, 1995). 

Sanyal A., Nugent M., Reeder R. J., and Bijma J. (2000) Seawater pH control on the boron isotopic composition of calcite evidence from inorganic calcite precipitation experiments. Geochim. Cosmochim. Acta 64, 1551-1555. 

Schmittner, K.-E. Giresse, P. (1999) Micro-environmental controls on biomineralization superficial processes of apatite and calcite precipitation in Quaternary soils, Roussillon, France. Sedimentology 46, 463-476. 

Dandurand, J.L., Gout, R., Hoefs, J., Menschel, G., Schott, J. Usdowski, E. (1982) Kinetically controlled variations of major components and carbon and oxygen isotopes in a calcite-precipitating spring. Chemical Geology 36, 299-315. 

This means that soils formed from primary minerals rich in Ca " and Mg " are less likely to become alkaline than soils formed from minerals rich in and Na". The alkalinity in Ca -rich soils is precipitated in the form of CaCOa (calcite), which controls the solution pH at a value near 8.2 (assuming a CO2 gas pressure of 0.3 millibar). These soils are termed calcareous, and are not considered to be alkaline despite their pH because they do not possess the negative attributes of the more strongly alkaline sods. If, on the other hand, alkalinity is present in the form of sodium and potassium bicarbonates and carbonates, the moist soil has a very high pH, and poses an alkalinity hazard to plants. 

Two hypotheses have been put forward to explain the origin of calcite concretions in the Namorado Sandstone (i) growth was essentially controlled by local sources of carbonate and early bacterial processes, with the initial calcite precipitating either at... 

A luetic simulation of the microbial-facilitated calcite precipitation was conducted using geochemical conditions consistent with the SRPA, realistic concentrations of urea, and rates of microbial urea hydrolysis that are consistent with those measured on conqilex microbial communities. In the simulation, urea hydrolysis and then calcite precipitation proceeded at maximal rates. We expect that in a real field test of our process in which an electron donor (e.g., molasses) is added to the system to stimulate urea hydrolysis, the process would proceed even more rapidly. The modeling suggested that over tiie long periods that die process would occur in an aquifer, the prec itation rate would be controlled by the urea hydrolysis rates and would be independent of tiie concentration of calcium in the aquifer. 

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