Calcite dissolution influences

A number of factors have been investigated that influence calcite dissolution in relatively simple systems. These include the important variables of temperature, pH, and the partial pressure of CO2. The equations describing the net dissolution rate of carbonate minerals in simple experimental solutions differ somewhat, depending on experimental conditions and the interpretations placed on the experimental results by various investigators (cf., for example, Plummer et al., 1978 Chou et al., 1989). 

Mg2+ influences calcite dissolution rates the same way, but not to the same extent as Ca2+. The inhibition effects of Mg2+ can be described in terms of a Langmuir adsorption isotherm. Sjoberg (1978) found he could model results for the combined influences of Ca2+ and Mg2+ in terms of site competition consistent with ion exchange equilibrium. The inhibition effects of Mg2+ in calcite powder runs increase with increasing Mg2+ concentration and as equilibrium is approached. 

Phosphate has been found to be an extremely strong inhibitor of carbonate reaction kinetics, even at micromolar concentrations. This constituent has been of considerable interest in seawater because of its variability in concentration. It has been observed that phosphate changes the critical undersaturation necessary for the onset of rapid calcite dissolution (e.g., Berner and Morse, 1974), and alters the empirical reaction order by approximately a factor of 6 in going from 0 to 10 mM orthophosphate solutions. Less influence was found on the log of the rate constant. Walter and Burton (1986) observed a smaller influence of phosphate on calcite... 

The influence of heavy metals on calcium carbonate reaction rates has not been extensively studied. Experiments have shown that many metals exhibit inhibitory effects on calcite dissolution. Ions tested by Terjesen et al. (1961), in decreasing order of effectiveness, were Pb2+, La3+, Y3+, Sc3+, Cd2+, Cu2+, Au3+, Zn2+, Ge4+, and Mn2+, and those found to be about equal were Ni2+, Ba2+, Mg2+, and Co2+. The general trend follows the solubility of the metal carbonate minerals, with the exception of Zn2+ and the "about equal" group whose solubilities are all greater than calcite. 

FIG. 3.3. Influence of pH on the rates of calcite dissolution and precipitation (left, adapted from Chou et al.7 with the permission of Elsevier Science Publishers), and an initial-rate plot for calcite precipitation indicating the applicability of Eq. 3.9 (right, data from W. P. Inskeep and P. R. Bloom,8 adapted with permission. Copyright by Pergamon Press Ltd., Headington Hill Hall, Oxford 0X3 OBW, England.)... 

Figure 8 Results of a model study of the influence of the depth distribution of organic matter oxidation below the sediment-water interface on sedimentary calcite dissolution. The e-folding depth for organic C oxidation is approximately equal to l/ji. Results are shown for 3 degrees of saturation with respect to calcite - -20 p,molkg (supersaturated bottom water), 0 p,mol kg (bottom water at saturation), and —10 p,mol kg (undersaturated).

Taylor, T.R. (1990) The influence of calcite dissolution on reservoir porosity in Miocene sandstones. Picaroon Field, offshore Texas Gulf Coast. J. sediment. Petrol., 60, 322-334. 

Influence of transport and reaction on wormhole formation in porous media. AICHE Journal. September 1933-1949. C. N. Fredd and H. S. Fogler. 1998. The influence of chelating agents on the kinetics of calcite dissolution. JWrwi / (f Colloid and Interface Science. 204 187-197. Fredd,... 

The influence of Na+, K+, calcite, and EDTA on complete hydroxyapatite dissolution and hydroxypyromorphite formation was also examined (Ma 1996). Only high EDTA concentrations (EDTA/Pb molar ratios up to 5) significantly affected the ability of dissolving hydroxyapatite from sequestering hydroxypyromorphite the strong bond between the EDTA ligand and the Pb2+ was fairly competitive with the tendency to precipitate out as hydroxypyromorphite. 

Different factors have an influence on the calcite solubility. First of all, like for gypsum (chapter 4.1.1.4) the formation of the CaC03° complex is endothermic (AH(CaC03°) = +3.5), while the mineral dissolution is exothermic (AH(CaC03(s)) = -2.3). Thus, the maximum solubility occurs at some medium temperature, at which the formation of the CaC03° complex is already increased and the decreasing solubility of CaC03(s) does not predominate yet. 

Because aragonite is more susceptible to dissolution than calcite, especially under the influence of meteoric waters, and because most fossil corals are recovered from uplifted terrestrial deposits, diagenesis is an especially important limiting factor in recovering older coral records. This problem can be circumvented by drilling into submerged fossil deposits, but because of logistical difficulties, so far this has been accomplished in only a few key spots such as Barbados and Tahiti (Eairbanks, 1989 Bard et al., 1996). 

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