Calcite-siderite equilibrium

Calcite, Siderite, Magnetite - Equilibrium precipitation and dissolution (if present) - Precipitation causes passivation of iron... 

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 (CaCOs ) 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 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. 

Estimates of storage capacity based on simple flow and equilibrium geochemical models indicate that the Rose Run Sandstone, by itself, potentially can store 30 years of emissions from the five largest coal-burning power plants in eastern Ohio. Ultimately the injected C02 can dissolve into the brine and be converted to the stable, immobile, carbonate mineral phases, primarily siderite, dawsonite, and calcite. 

Fe, Mn, Zn, Cd, Pb, and UO2 carbonates are over 100 times less soluble. In most cases (except in some ore deposits), the Zn, Cd, Co, Sr, Pb, and UO2 carbonates come to equilibrium in systems that are first saturated with respect to rock-forming carbonates (e.g., calcite, dolomite, rhodocrosite, or siderite). For these conditions, maximum concentrations of the trace divalent metals are limited by... 

Siderite and rhodocrosite may approach saturation when acid mine waters are neutralized by contact with carbonate rocks. Figure 6.18 shows the trend toward saturation with respect to carbonate minerals of some surface waters and groundwaters affected by bituminous coal mining (Gang and Langmuir 1974). Plotted are the saturation indices of various metal carbonates versus the index for calcite. The plots show that siderite and rhodocrosite reach equilibrium in some streams (probably because of CO2 loss), whereas calcite and the other carbonates remain undersaturated in all samples. 

The maximum concentrations of trace metals may be limited in groundwaters at equilibrium with minerals such as calcite, dolomite, siderite, and rhodocrosite. Explain. 

A further constraint of pH buffering and of the availability of TIC, Ca, Fe and Mn is caused by the solubility equilibrium with rock forming solid carbonates, as calcite, rhodochrosite, and siderite. The solubility products Lcakite. and Ls.dente... 

The saturation states of these waters with respect to a range of minerals were calculated using the computer code PHREEQE. This approach indicated that the groundwaters are approximately in equilibrium with respect to quartz, calcite, kaolinite, fluorite, ferrosilite, ferric hydroxide and siderite (Table 2). The water/rock interactions deduced from the SEM-EDS analyses and the theoretical calculations using PHREEQE, are summarized in Table 3. 

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