Carbonate equilibrium geochemistry

Microbial reactions also change the chemical composition of the groundwater e.g. due to production of carbonate and sulfide. If the inorganic composition of groundwater is to be used to estimate microbial degradation, chemical reactions have to be considered, too. In this example a simplified geochemistry is simulated, consisting of a calcium-carbonate equilibrium system and precipitation of iron sulfide. 

Calcium carbonate is insoluble in pure water but dissolves in weakly acidic water. The role of this solubility phenomenon in the geochemistry of caverns is described in Box. We can understand this dependence on pH by examining the acid-base properties of the species involved in the solubility equilibrium. 

Despite the vast amount of work on 14C dating which has already been accomplished and despite the fact that it is the best developed method available today, numerous difficulties still exist with its application. First, carbonate geochemistry which helped control 14C concentrations in the past is not simple to reconstruct. Carbonate minerals are commonly in a state of near equilibrium with groundwater, and only slight changes in water temperature or chemistry will promote either dissolution or precipitation of carbonate ions. In this way, the proportion of modern carbon in the water can be changed and some isotope... 

In theory, it should be possible to deal with all carbonate geochemistry in seawater simply by knowing what the appropriate activity coefficients are and how salinity, temperature, and pressure affect them. In practice, we are only now beginning to approach the treatment of activity coefficients under this varying set of conditions with sufficient accuracy to be useful for most problems of interest. That is why "apparent" and stoichiometric equilibrium constants, which do not involve the use of activity coefficients, have been in widespread use in the study of marine carbonate chemistry for over 20 years. The stoichiometric constants, usually designated as K. involve only the use of concentrations, whereas expressions for apparent equilibrium constants contain both concentrations and aH+ derived from "apparent pH". These constants are usually designated as K Examples of these different types of constants are ... 

Chacko, T., Cole, D.R., and Horita, J. (2001) Equilibrium oxygen, hydrogen and carbon isotope fractionation factors applicable to geologic systems. In Reviews in Mineralogy and Geochemistry Stable Isotope Geochemistry (Valley, J.W., and Cole, D.R., eds.), Vol. 43, pp. 1-81, Mineralogical Society of America, Chantilly, VA. 

One of the primary aims in the study of the geochemistry of carbonates in marine waters is the calculation of the saturation state of the seawater with respect to carbonate minerals. The saturation state of a solution with respect to a given mineral is simply the ratio of the ion activity or concentration product to the thermodynamic or stoichiometric solubility product (Equation (3)). In seawater the latter is generally used and Hmingjai is the symbol used to represent the ratio. If H = 1, the solid and solution are in equilibrium if H < 1, the solution is undersaturated and mineral dissolution can occur, and if H > 1, the solution is supersaturated and precipitation should... 

Taylor BE, O Neil JR (1977) Stable isotope studies of metasomatic Ca-Fe-Al-Si skams and associated metamorphic and igneous rocks, Osgood Mts, Nevada. Contrib Mineral Petrol 63 1-49 Taylor BE, Bucher-Nurminen K (1986) Oxygen and carbon isotope and cation geochemistry of metasomatic carbonates and fluids-Bergell aureole, N. Italy. Geochim Cosmochim Acta 50 1267-1279 Thompson JB Jr (1959) Local equilibrium in metasomatic process. In Abelson PH (ed) Res Geochem 1 427-457... 

One of the more useful diagrams in geochemistry is the predominance diagram. In this, species activities are plotted versus pH, or often as pH versus log/o, and one of the more informative of these is the predominance diagram for aqueous CO2. There are three carbonate species, HjCOj, HCO3, and CO which would result from dissolving carbon dioxide gas in water. We have already calculated the equilibrium constant for one of the relevant ionic equilibria ( 9.3.1) ... 

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