Calcium calcite dissolution

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. 

Carbonate minerals are among the most chemically reactive common minerals under Earth surface conditions. Many important features of carbonate mineral behavior in sediments and during diagenesis are a result of their unique kinetics of dissolution and precipitation. Although the reaction kinetics of several carbonate minerals have been investigated, the vast majority of studies have focused on calcite and aragonite. Before examining data and models for calcium carbonate dissolution and precipitation reactions in aqueous solutions, a brief summary of the major concepts involved will be presented. Here we will not deal with the details of proposed reaction mechanisms and the associated complex rate equations. These have been examined in extensive review articles (e.g., Plummer et al., 1979 Morse, 1983) and where appropriate will be developed in later chapters. 

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. 

Reddy M.M., Plummer L.N. and Busenberg E. (1981) Crystal growth of calcite from calcium bicarbonate solutions at constant Pc02 and 25°C A test of the calcite dissolution model. Geochim. Cosmochim. Acta 45,1281-1291. 

Open Ocean Dissolution Experiments. The first direct studies of calcium carbonate dissolution in deep seawater were made by Peterson (41) and Berger (42). Peterson suspended spheres of Iceland spar calcite, held in pronged plastic containers, at various depths in the Central Pacific Ocean for four months. The amount of dissolution was determined by weight loss, which was small relative to the total weight of the spheres. On the same mooring Berger suspended sample chambers, which consisted of... 

As soon as the calcite saturation level is reached, biogeochemical carbonate crystals accumulate, mixed with remains of ostracods sediments prograding towards the lake centre form foresets. Foresets can include turbidite beds a few centimetres thick due to turbid underflow. Calcium carbonate precipitates only when Characeae are present. Along the talus, and with depth, the water cools down increasing calcite dissolution. 

Although the mass transfer portion of the model overpredicts calcite dissolution, the calculations of pre- and post-C02 calcite saturation indices are still useful. Subtracting one from the other yields a modelled, or predicted, change in the Sl aic (Fig- 2), and it correlates with the observed increases in the calcium concentration. Before CO2 injection the calcium concentrations in the five fields were very similar. However, the maximum calcium concentration observed after the CO2 injection, which suggests calcite dissolution, varied considerably. When these numbers are compared with the original con-... 

The potential for well-bore scale during these CO2 treatments is determined by first predicting how much carbonate mineral dissolution will take place. After a calcite saturation index has been calculated for a water analysis for both pre- and post-C02 treatment, then Fig. 7 can be used to determine how much calcium (and therefore bicarbonate) will have been added to treated waters from calcite dissolution. These calcium and bicarbonate values can be added to the original formation water, which is then used to calculate a new calcite saturation index for the new water. 

This was done for Crooks Gap using nine sets of calculations at different pressures to represent the declining pressure conditions up the production tubing to surface lines. The results are shown in Fig. 11, which is a plot of Sl ajc vs depth. Even with extra calcium in the water from calcite dissolution, the water in the subsurface production tubing remains undersaturated with respect to calcium carbonate, and therefore scale would not be expected to precipitate. After 230 days of production the downhole pump at Crooks Gap was retrieved in preparation for another CO2 treatment, and indeed no scale was observed on the pump. 

Four flux categories are given in Table 9.7 The COj produced due to oxic respiration, calcite dissolution, the alkalinity as a sum parameter for calcium carbonate dissolution and CO from oxic respiration, and, hitherto neglected in the discussion, the dissolved organic carbon (DOC). 

The solubility of calcite and aragonite increases with increasing pressure and decreasing temperature in such a way that deep waters are undersaturated with respect to calcium carbonate, while surface waters are supersaturated. The level at which the effects of dissolution are first seen on carbonate shells in the sediments is termed the lysocline and coincides fairly well with the depth of the carbonate saturation horizon. The lysocline commonly lies between 3 and 4 km depth in today s oceans. Below the lysocline is the level where no carbonate remains in the sediment this level is termed the carbonate compensation depth. 

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