Calcite precipitation from water

One of the most important uses of oxygen isotopes in carbonates is as geothermometers. Isotope fractionation is pressure independent (Clayton et al., 1975), giving it an advantage over other geothermometers which are generally both temperature and pressure dependent. The equation for calculating the temperature at which calcite precipitates from water is ... 

According to Equation 48 calcite should precipitate from waters having a Mg/Ca ratio below a certain value, while dolomite should precipitate from waters having a Mg/Ca ratio above that critical value. This rule is obeyed under conditions of precipitation from very slightly supersaturated aqueous solutions like those occurring in certain areas of the ocean. Ocean water is close to equilibrium with both calcite and dolomite (53). 

Urbane, J., Pezdic, J., Bronc, I.K., and Scdoc, D., 1987, Comparison of isotopic composition of different forms of calcite precipitated from fresh water. Proceedings International Symposium on the Use of Isotope Techniques in Water Resources Development, International Atomic Energy Agency, Vienna, lAEA-SM-131P, pp. 783-787. 

There are at least three genetic types of calcite present in the studied sandstones, but not all of them are differentiated by texture or stable isotopic composition. The presence of deep-burial, late calcites precipitated from evolved pore waters is likely, but these cements are restricted to specific intervals. 

The difference in the isotopic composition of oxygen indicates that the silica precipitated from water that may have gained 0 by isotope exchange with silicate minerals of the bedrock underlying the ice sheet, whereas the calcite precipitated from glacial meltwater at the base of the ice sheet. In other words. 

Fig. 17.14 Range of isotopic compositions of oxygen, carbon, and strontium of the clasts of black calcite and opaline silica from the Elephant Moraine. If the calcite precipitated from glacial meltwater at 0°C, then the water must have had a value between -47.3%o and -54.9%o (SMOW) in good agreement with the value of ice in the Elephant Moraine. The carbon in the black calcite is strongly depleted in which is characteristic of photosynthetic plants and of bituminous coal in the Beacon Supergroup. Fragments of coal occur in the Elephant Moraine which is composed of clasts derived from the base of the ice sheet. The strontium in the black calcite is strongly enriched in radiogenic Sr compared to seawater (SW) of Phanerozoic age...

In the calculation results (Fig. 24.1), amorphous silica, calcite (CaCCF), 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 an Na-C03 brine (Fig. 24.2). The concentrations of the components Na+, K+, Cl-, and SO4- rise monotonically (Fig. 24.2), since they are not consumed by mineral precipitation. The HCO3 and Si02(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. 

Figure 29.2 shows the mineralogic results of the calculation. Dolomite dissolves, since it is quite undersaturated in the waste fluid. The dissolution adds calcium, magnesium, and carbonate to solution. Calcite and brucite precipitate from these components, as observations from the wells indicated. The fluid reaches equilibrium with dolomite after about 11.6 cm3 of dolomite have dissolved per kg water. About 11 cm3 of calcite and brucite form during the reaction. Since calculation... 

The isotope composition of biogenic and authigenic mineral precipitates from lake sediments can be used to infer changes in either temperature or the isotope composition of lake water. Knowledge of the factors that may have influenced the isotope composition of the lake water is essential for the interpretation of the precipitated phases (Leng and Marshall 2004). In many lakes the combined analysis of different types of authigenic components (precipitated calcite, ostracodes, bivalves, diatoms, etc.) may offer the possibility of obtaining seasonally specific information. 

In this case study, the selected phases are pyrite, amorphous FeS, calcite (present in limestones in the roof strata Fig. 5), dolomite (possibly also present in the limestones), siderite (which occurs as nodules in roof-strata mudstones), ankerite (present on coal cleats in the Shilbottle Seam), melanterite and potassium-jarosite (representing the hydroxysulphate minerals see Table 3), amorphous ferric hydroxide (i.e., the ochre commonly observed in these workings, forming by precipitation from ferruginous mine waters), and gypsum (a mineral known to precipitate subaqueously from mine waters with SO4 contents in excess of about 2500 mg/L at ambient groundwater temperatures in this region, and with which most of the mine waters in the district are known to be in equilibrium). In addition, sorption reactions were included in some of the simulations, to contribute to the mole transfer balances for Ca, Na, and Fe. 

Cave deposits consist of minerat calcite fCaCOgj that precipitate from ground-water that enters a cavern and evaporates into the air. An abundance of caicite deposits form the rock travertine (a variety of limestone). 

Let us now consider the problem from the standpoint of calcite precipitation kinetics. At saturation states encountered in most natural waters, the calcite reaction rate is controlled by surface reaction kinetics, not diffusion. In a relatively chemically pure system the rate of precipitation can be approximated by a third order reaction with respect to disequilibrium [( 2-l)3, see Chapter 2]. This high order means that the change in reaction rate is not simply proportional to the extent of disequilibrium. For example, if a water is initially in equilibrium with aragonite ( 2c=1.5) when it enters a rock body, and is close to equilibrium with respect to calcite ( 2C = 1.01), when it exits, the difference in precipitation rates between the two points will be over a factor of 100,000 The extent of cement or porosity formation across the length of the carbonate rock body will directly reflect these... 

---