CaCO3 calcite

Calcite. A common crystalline form of natural calcium carbonate, caco3, that is the basic constituent of limestone, marble, and chalk. Also called calcspar. 

B. Calcite (CaCO3) + Pyrite (FeS2) 1 2 750 Anhydrite (CaS04) + Calcite (CaC03) + Hematite (Fe203) Oxidation Pyritic sulfur fixation... 

Secondary minerals include (a) calcite — CaCO3 (b) siderite — FeCO3,... 

One mechanism for the removal of Ca is the precipitation of calcite, already noted as one of the principal cementing minerals, in the bedrock. The fact that groundwaters in the drift and bedrock aquifers are generally oversaturated with respect to calcite (see Fig. 5) supports the existence of this mechanism. The saturation indices indicate that calcite either precipitates from solution or simply does not dissolve. Fig. 17 is a plot of the log of the activity ratio of [Ca ] /[Mg ] vs. log of the equilibrium partial pressure of CO2, using computations made with SOLMNEQ. Apparently, the [Ca ]/ [Mg ] ratio of groundwater from the drift aquifers is controlled by calcite and dolomite in as much as similar concentrations of Ca and Mg are in solution. In the bedrock aquifers, precipitation of CaCO3 likely accounts for the decrease in the [Ca ] /[Mg ] ratio and the drop in partial pressure of CO2. ... 

In an example of a sliding fugacity path, we calculate how CO2 fugacity affects the solubility of calcite (CaCO3). We begin by defining a dilute solution in equilibrium with calcite and the CO2 fugacity of the atmosphere... 

In a first example of how minerals can buffer a fluid s chemistry, we consider how a hypothetical groundwater that is initially in equilibrium with calcite (CaCO3) at 25°C might respond to the addition of an acid. In REACT, we enter the commands... 

By this reaction, we can expect the modeled fluid to be rather acidic, since it is rich in potassium. We could have chosen to fix pH by equilibrium with the siderite, which also occurs in the veins. It is not clear, however, that the siderite was deposited during the same paragenetic stages as the fluorite. It is difficult on chemical grounds, furthermore, to reconcile coexistence of the calcium-rich ore fluid and siderite with the absence of calcite (CaCO3) in the district. In any event, assuming equilibrium with kaolinite leads to a fluid rich in fluorine and, hence, to an attractive mechanism for forming fluorite ore. 

In the calculation results (Fig. 18.1), amorphous silica, calcite (CaCO3), and sepiolite precipitate as water is removed from the system. The fluid s pH and ionic strength increase with evaporation as the water evolves toward a Na-CO3 brine (Fig. 18.2). The concentrations of the components Na+, K+, Cl-, and SO4-rise monotonically (Fig. 18.2), since they are not consumed by mineral precipitation. The HCO3 and SiO2(aq) concentrations increase sharply but less regularly, since they are taken up in forming the minerals. The components Ca++ and Mg++ are largely consumed by the precipitation of calcite and sepiolite. Their concentrations, after a small initial rise, decrease with evaporation. 

In a final application of kinetic reaction modeling, we consider how sodium feldspar (albite, NaAlSi3Ox) might dissolve into a subsurface fluid at 70°C. We consider a Na-Ca-Cl fluid initially in equilibrium with kaolinite [Al2Si2O5(OH)4], quartz, muscovite [KAl3Si3O10(OH)2, a proxy for illite], and calcite (CaCO3), and in contact with a small amount of albite. Feldspar cannot be in equilibrium with quartz and kaolinite, since the minerals will react to form... 

NaAl3Si3Oio(OH)2 +. 6 H2O -> CaCO3+NaAlSi2O6-H2O paragonite calcite analcime... 

To model the effect of carbonate minerals on drainage chemistry, we continue our calculations from the previous section (in which we reacted pyrite with a hypothetical groundwater in contact with atmospheric oxygen). This time, we include calcite (CaCO3) in the initial system... 

The weathering of carbonate rocks and the precipitation and resolution of CaCO3 in natural waters are problems of intense interest. It is estimated that over 99 % of the earth s carbon is present in CaCO3 minerals limestone, calcite, aragonite, vaterite, marble, etc. To the equilibria of the previous problem, we now add... 

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