Calcite-water equilibrium

In 1947 Harold Urey, the 1934 Nobel Laureate, recognized that the temperature dependence of the isotope exchange equilibrium between water and calcite (the principal mineral in marine limestones) could be employed as a paleo-thermometer. At 298.15 K the fractionation factor for calcite-water is 1.0286,... 

Both authors calculations also indicated that it is possible for solutions of reasonable compositions for natural waters to produce mixtures of freshwater and seawater that were undersaturated with respect to calcite but supersaturated with respect to dolomite. This observation is a cornerstone for some dolomitization models that are discussed later in this chapter. It is also important to note that the extent of undersaturation which results from mixing is strongly dependent on the initial Pco2 °f the dilute water when it is in equilibrium with calcite. Waters high in CO2 can cause more extensive dissolution. If these waters enter a vadose zone where CO2 can be degassed, they will become supersaturated and calcium carbonate can precipitate. This process provides an excellent mechanism for cementation near the water table. Because the water table can oscillate vertically, a considerable zone of cementation can result. 

It is planned to reactivate the well B4 to support peak times of water consumption and to mix the extracted water with that of the current drinking water well B3. Check with the help of PHREEQC modeling if and in which shares this can be done with regard to general requirements of drinking water standards and to the technical requirements in terms of the calcite-carbondioxide equilibrium (chapter 3.1.5.2). [key word for mixing of two waters see the exercise in chapter 3.1.3.3.J... 

The temperature dependence of equilibrium isotope exchange in the calcite-water system has been intensively studied since Urey (1947) first suggested that the paleotemperature of the ancient oceans could be estimated by the 0-isotope distribution between seawater and the calcium carbonate precipitated from it. Urey et al. (1951) argued that O-isotope equilibrium between seawater and CaC03 was likely and support for this idea has come from the close agreement between the CaC03-H20 isotopic fractionation observed in natural systems and those derived from both thermodynamic calculations and laboratory experiments (e.g. Epstein et al., 1951, 1953 Emiliani, 1955 O Neil et al., 1969 O Neil et al., 1975). 

Another way in which the sense of speleothem S O change can be estimated is by reference to modem speleothem calcite that is actively forming in the same cave at the present time, once equilibrium deposition of the modern calcite has been established through analysis of ratios for modern speleothem calcite-water pairs (Table 1). 

How soluble is calcite in equilibrium with the atmosphere (pco2 = 10 atm) and what is the composition of the water at equilibrium The following species are in equilibrium Ca, H, H2CO, HCOf, C03, OH, and C02(g). In order to calculate the equilibrium concentrations of the six species in solution, we need to consider, in addition to the equilibria already used in the previous calculations,... 

From the preceding equations, we can see that when calcite approaches equilibrium with water at high pH, an excess of negative HCOs and C03 will exist, whereas at low pH an excess of positive Ca and CaHCOs and CaOH" will occur. These ionic species may be produced at the solid/solution interface or may form in solution and subsequently adsorb on the mineral in amounts proportional to their concentration in solution. In either case, the net result will be a positive charge on the surface at low pH and a negative charge at high pH. 

Fig. 6. Isotopic composition of calcite cements from North Coles Levee, South Coles Levee and Canal fields. North Coles Levee dolomite cements shown for comparison. North Coles Levee data from Schultz et al. (1989) South Coles Levee and Canal data from Table 2. Increasingly negative 5 OpDB and lower Sr ratios are correlated with higher temperatures of crystallization. Strontium ratio decrease is attributed to Sr from plagioclase alteration. Shaded box shows calculated composition of calcite in equilibrium with present pore water at Coles Levee fields based on fluid temperature and composition in Fisher Boles (1990) and Sr isotopic data in Feldman et al.( 993).

The results are shown in Figures 8.3 and 8.4. It will take dissolution of 0.125 and 0.04 moles of calcite to neutralize 1.0 liter of TS-3 and MW-86 water to a pH of 6.08 and 6.39, respectively, at which the solutions are saturated with calcite. At these points, no more calcite is dissolved into the solutions - titration of additional calcite into the system merely increases the mass of solid calcite. The equilibrium conditions concerning other constituents did not change from these points on. The amounts of precipitated gypsum do not increase. 

We determined the shell C and O isotopic composition of the cultured foraminifera, and compared these isotopic values with the water chemistry of the culture chambers, and also with the shell chemistry of field specimens collected from sites on the North Carolina and South Carolina (USA) continental margin. The culmred foraminifera showed substantial offsets from the 8 C of system water dissolved inorganic carbon (—0.5 to —2.5%c, depending on species) and smaller offsets (0 to — 0.5%o) from the predicted 8 0 of calcite in equilibrium with the culture system water at the growth temperature. These offsets reflect at least three factors species-dependent vital effects ontogenetic variations in shell chemistry and the aqueous carbonate chemistry ([COJ] or pH) of the experimental system. 

Fig. 9. Continued) and Ab O values (this study, and McCorkle et al 1997). The carbon isotopic offsets are calculated relative to bottom water DIC, and the oxygen isotopic offsets are calculated relative to calcite in equilibrium with bottom water using the expressions of McCorkle et al (1997) (solid diamonds) and Shackleton (1974) (open circles). The vertical line shows the trend in isotopic offsets predicted by a pore water b C effect only. The observed b C depletions of pore water DIC in the 0-0.5 cm depth interval range from — 1 to — 2.2%o at the nearby sites discussed by McCorkle et al (1997) (white hexagons, dotted line). Because pore water carbonate ion concentration is likely to be lower than bottom water values at these relatively shallow continental margin sites, a carbonate ion influence on shell composition would tend to cause Ab O values greater than zero (ellipse).

In seawater, the differences between activities and concentrations must always be considered (cf. Sect. 15.1.1). The activity coefficients for monovalent ions in seawater assume a value around 0.75, for divalent ions this value usually lies around 0.2. In most cases of practical importance, the activity coefficients can be regarded with sufficient exactness as constants, since they are, over the whole range of ionic strengths in solution, predominately bound to the concentrations of sodium, chloride, and sulfate which are not directly involved in the calcite-carbonate-equilibrium. The proportion of ionic complexes in the overall calcium or carbonate content can mostly be considered with sufficient exactness as constant in the free water column of the ocean. Yet, this cannot be applied to pore water which frequently contains totally different concentrations and distributions of complex species due to diage-netic reactions. 

Examples for Calculation of the Calcite-Carbonate-Equilibrium in Ocean Waters... 

Fig. 1.17 Relation between the and Cl concentrations of geothermal waters and inclusion fluids. The solid line indicates (i) albite-K-feldspar-muscovite-quartz-calcite-solution equilibrium at aHjCOj = 10 (2) albite-K-feldspar-muscovite-quartz-calcite-solution equilibrium at aHjCO, = 10 (3) anhydrite-solution at XSo (total dissolved sulfur concentration) = 10 (4) anhydrite-solution equilibrium at = 10 (Shikazono 1978a, b). For symbols used, see caption to Fig. 1.15...

Two uncertainties permit such a range of opinion (l) the solubility of calcite in equilibrium with sea water cannot be... 

Hence, it is assumed that the minerals in equilibrium with geothermal waters are albite, K-feldspar, muscovite, quartz, calcite, anhydrite, chlorite and wairakite. 

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