Dolomites equilibrium

Lafon, G. M., G. A. Otten and A. M. Bishop, 1992, Experimental determination of the calcite-dolomite equilibrium below 200 °C revised stabilities for dolomite and magnesite support near-equilibrium dolomitization models. Geological Society of America Abstracts with Programs 24, A210-A211. 

Dolomite Equilibrium. Figure 2 is a plot of the ion product, (Ca2+) (Mg2+) (C032 )2, as a function of temperature. Saturation of lake waters with respect to dolomite is similar to the results for calcium carbonate. The same general conclusions are apparent. Dolomite tends to be saturated with respect to lake water at 14°C. 

Powell and Searcy [1288], in a study of CaMg(C03)2 decomposition at 750—900 K by the torsion—effusion and torsion—Langmuir techniques, conclude that dolomite and C02 are in equilibrium with a glassy phase having a free energy of formation of (73 600 — 36.8T)J from 0.5 CaO + 0.5 MgO. The apparent Arrhenius parameters for the decomposition are calculated as E = 194 kJ mole-1 and activation entropy = 93 JK-1 (mole C02)-1. 

If the above argument is correct and the /CO2 of hydrothermal solutions from back-arc basins is in equilibrium with alteration mineral assemblage including dolomite and calcite, t[ g2+ lmQ + of fluids can be estimated to be 0.03-0.055 from dolomite-calcite-... 

In our example, we test the consequences of reacting an isotopically light (i.e., nonmarine) limestone at 60 °C with an isotopically heavier groundwater that is relatively rich in magnesium. We start by defining the composition of a hypothetical groundwater that is of known CO2 fugacity (we initially set /co2 to 1) and in equilibrium with dolomite ... 

Fig. 19.2. Isotopic composition (bold lines) of dolomite formed by reaction between a limestone and migrating groundwater, assuming that minerals maintain isotopic equilibrium over the simulation. Fine lines show results of simulation holding minerals segregated from isotopic exchange, as already presented (Fig. 19.1).

To prepare the initial system, we use the analysis in Table 25.1 for the saline water, which we assume to be in equilibrium with potassium feldspar, quartz, muscovite, and dolomite ( dolomite-ord is the most stable variety in the database). The commands... 

In fact, the choice of CO2 fugacity has little effect on the mineralogical results of the mixing calculation. In the model, the critical property of the Fountain fluid is that it is undersaturated with respect to calcite, so that calcite dissolves when the fluid mixes into the Lyons. Because we assume equilibrium with dolomite and magnesite, the saturation index (log Q/K) of calcite is fixed by the reaction... 

Fig. 25.4. Oxygen and carbon stable isotopic compositions of calcite ( ) and dolomite ( ) cements from Lyons sandstone (Levandowski et al., 1973), and isotopic trends (bold arrows) predicted for dolomite cements produced by the mixing reaction shown in Figure 25.3, assuming differing CO2 fugacities (25, 50, and 100) for the Fountain brine. Fine arrows, for comparison, show isotopic trends predicted in calculations which assume (improperly) that fluid and minerals maintain isotopic equilibrium over the course of the simulation. Figure after Lee and Bethke (1996).

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... 

Pronounced discrepancies between observed composition and the calculated equilibrium composition illustrate that the formation of the solid phase, for example, the nucleation of dolomite and calcite in seawater, is often kinetically inhibited, and the formation of phosphates, hydrated clay and pyrite is kinetically controlled. 

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). 

For WGS, commercial catalysts are only operated up to 550 °C and no catalysts are available for higher temperatures, because adverse equilibrium conversion makes the process impractical in the absence of a CO2 sorbent. Han and Harrison [38] have shown that, at 550 °C, dolomite and limestone have a sufficiently high WGS activity. For SMR a conventional Ni SMR catalyst is used in a 1 1 ratio with CaO [30]. Meyer et al. [32] have also used a Ni-based catalyst in combination with limestone and dolomite, and achieved CH4 conversions of 95% at 675 °C while the CH4 conversion at equilibrium was 75%. 

Carbon dioxide has a dominant effect on the dissolution of carbonate minerals, such as calcite and dolomite (Table 2.1). If a carbonate mineral dissolves in water that is equilibrated with a constant source of CO, then the concentration of dissolved carbonic acid remains constant and high. However, when calcite dissolution is accompanied by consumption of carbonic acid and a continuous source of CO is not maintained, the reaction proceeds further to achieve equilibrium. 

As a specific example of the problem, let us calculate the equilibria for an actual case study a deep water from the Sarcidano region (Sardinia, Italy) in equilibrium with Mesozoic dolomites (Bertorino et al., 1981). The compositions in mEq/1 of water sampled in a drilled well are listed in table 8.8. The in situ temperature is 21 °C we assume here that the in situ T is 25 °C at 1 bar, to simplify calculations. We also assume for the sake of simplicity that the main ion species in solution are HCO3, Mg, Ca, CO3, OH, and H, and that all Ca and Mg are in the ionic forms Ca and Mg. ... 

Our approximation in the calculation is 150 cal/mole, which is quite satisfactory. The fact that dolomite is the mineral phase nearest to complete equilibrium is in agreement with the geology of the area and was in fact already evident at first sight from the chemical analyses, indicating similar molalities for Ca and Mg in solution. The resulting /co2(j) 10 - , which is much higher than the atmospheric value. The previous recommendation about pH measurements is particularly obvious in this case. 

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. 

Table X gives data for two elements whose equilibrium chemistry in sea water has been investigated by many workers. Although some important questions, such as the solubility of dolomite, are still open, the numbers given are probably more reliable than those in Tables VIII and IX.

To a good first approximation, the Great Lakes fit a model involving the equilibrium of calcite, dolomite, apatite, kao-Unite, gibbsite, Na- and K-feldspars at 5°C., 1 atm. total pressure with air of PCo2 = 3.5 X 10" atm. and water. Dynamic models, considering carbon dioxide pressure and temperature as variables (but gross concentrations fixed), show that cold waters contain excess carbon dioxide and are unsaturated with respect to calcite, dolomite, and apatite, whereas warm waters are nearly at equilibrium with the atmosphere but somewhat supersaturated with respect to calcite, dolomite, and apatite. 

For the calculations, averages of the results of the two 5. -equilibrium models of Ca2+ = 35 p.p.m., Mg2+ = 7 p.p.m., and alkalinity = 1.55 X 10 3 equiv./liter are used. Solubility data of Larson and Buswell (11), carbon dioxide solubility data of Hamed and Davies (2), and the carbonate ionization data of Hamed and Hammer (3) and Hamed and Scholes (4) are used. Linear interpolations are made for dolomite between pK(soly) = 16.3(5°C.) and 17.0(25°C.). Equations outlining the calcite and dolomite calculations are ... 

Figures 5 and 6 show the results of the calculations, and they are compared with the actual data distribution as shown by a dashed bounding line. With both calcite and dolomite, colder waters match conditions of carbon dioxide pressure greater than atmospheric, and high temperature conditions match carbon dioxide pressure nearly the same as the atmosphere. It appears the degree of mixing and the rate of carbon dioxide diffusion is of prime importance when considering approach to liquid, gas, and solid equilibrium.

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