Carbonate minerals calcite solubility

Those minerals whose solubilities are greater than those of most oxides and silicates, but lower than those of soluble salt minerals, are grouped as semisoluble salt minerals (also called salt-type minerals). The list comprising this particular minerals group is calcite, dolomite, magnesite, barite, gypsum, scheelite, carbonate, phosphate, sulfate and some others. These minerals are characterized mainly by their tonic bonding and as has already been pointed out, by their moderate solubility. 

Calcium carbonate solubility is also temperature and pressure dependent. Pressure is a 6r more important fector than temperature in influencing solubility. As illustrated in Table 15.1, a 20°C drop in temperature boosts solubility 4%, whereas the pressure increase associated with a 4-km increase in water depth increases solubility 200-fold. The large pressure effect arises from the susceptibility of the fully hydrated divalent Ca and CO ions to electrostriction. Calcite and aragonite are examples of minerals whose solubility increases with decreasing temperature. This unusual behavior is referred to as retrograde solubility. Because of the pressure and temperature effects, calcium carbonate is fer more soluble in the deep sea than in the surfece waters (See the online appendix on the companion website). 

A classic example of metastability is surface-seawater supersaturation with respect to calcite and other carbonate minerals (Morse and Mackenzie 1990 Millero and Sohl 1992). The degree of calcite supersaturation in surface seawater varies from 2.8- to 6.5-fold between 0 and 25 °C (Morse and Mackenzie 1990). In Fig. 3.18, experimental calcite solubility (metastable state) is approaching model calcite solubility (stable state) at subzero temperatures. In Table 5.1, the difference in seawater pH, assuring saturation or allowing supersaturation with respect to calcite, is 0.38 units. Moreover, in running these calculations, it was necessary to remove magnesite and dolomite from the minerals database (Table 3.1) because the latter minerals are more stable than calcite in seawater. But calcite is clearly the form that precipitates... 

Temperature and pressure variations in natural systems exert major influences on carbonate mineral solubility and the distribution of carbonic acid chemical species. For example, the solubility of calcite decreases with increasing temperature, as does the solubility of CO2 gas in water. These two effects on solubilities can lead to precipitation of calcite as a cement in a marine sediment-pore water system that undergoes moderate burial. 

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. 

Natural carbonate minerals do not form from pure solutions where the only components are water, calcium, and the carbonic acid system species. Because of the general phenomenon known as coprecipitation, at least trace amounts of all components present in the solution from which a carbonate mineral forms can be incorporated into the solid. Natural carbonates contain such coprecipitates in concentrations ranging from trace (e.g., heavy metals), to minor (e.g., Sr), to major (e.g., Mg). When the concentration of the coprecipitate reaches major (>1%) concentrations, it can significantly alter the chemical properties of the carbonate mineral, such as its solubility. The most important example of this mineral property in marine sediments is the magnesian calcites, which commonly contain in excess of 12 mole % Mg. The fact that natural carbonate minerals contain coprecipitates whose concentrations reflect the composition of the solution and conditions, such as temperature, under which their formation took place, means that there is potentially a large amount of information which can be obtained from the study of carbonate mineral composition. This type of information allied with stable isotope ratio data, which are influenced by many of the same environmental factors, has become a major area of study in carbonate geochemistry. 

Another phyllosilicate mineral, namely, biotite, has larger sorption capability as expected. It is likely related to the iron content of biotite. Similarly, the carbonates containing iron and magnesium (ankerite and dolomite Table 3.8) show more significant cesium sorption as calcite (calcium carbonate), which practically does not adsorb cesium. The low sorption ability of calcite can be explained by the Hahn adsorption rule (Chapter 1, Section 1.2.4) that is, the sorption is low when the sorbate (cesium carbonate) has great solubility. 

The most common cation in reservoir water is sodium (Na+). Both sodium sulfide and sodium carbonate are soluble in water and thus do not provide a mechanism for sequestering the acid gas. Perhaps the next most common cation in the reservoir is calcium (Ca2+). Carbon dioxide can react with the calcium ion and form one of many calcium carbonate minerals including calcite (CaC03). Calcium sulfide is not a very stable compound and readily decomposes and thus is not common on the earth. However, H2S can react with other cations in the reservoir water and produce several sulfide minerals including pyrite. 

Marine carbonate minerals have both biotic (dominant) and abiotic (minor) sources. Their formation is often controlled by kinetic factors or biomediated processes in organisms. Surface seawater is most highly supersaturated (the ion activity product (lAP) is much greater than the solubility product) with respect to dolomite ( 50X), followed by pure calcite ( 6 X), then by aragonite ( 4x). It may be close to... 

The minerals nesquehonite through huntite in Table 6.1 are comparatively rare. Among these only hydromagnesite can precipitate in water at 25°C and atmospheric CO2 pressures (Fig. 6.2). Because of their high solubilities, the Mg-carbonate and hydroxy-carbonate minerals, are generally uncommon except in some evaporative cave pools, evaporating ocean embayments, arid soils, and closed-basin lakes (cf. Hostetler 1964). Nesquehonite is about 24 times as soluble as calcite at 25°C and a CO2 pressure of I bar. Similarly, at atmospheric CO2 pressure (10 bar) the solubility of hydromagnesite is 5.5 mmol/kg, whereas that of calcite is only 0.52 mmol/kg. 

There are several ways one can write the solubility of carbonate minerals in water. Most useful are reactions written in terms of CO2 or H+. As noted earlier, carbon dioxide is the chief source of acidity in most natural waters. The reaction between calcite and CO2 is... 

It is useful to construct a graph relating carbonate mineral solubilities to CO2 pressure. This can be done for calcite starting with equilibrium constant expression (6.2) above. If done rigorously, the derivation accounts for the effects of ion activity coefficients and the presence of CaHCOI and CaCOf ion pairs and of CaOH. Considering all complexation, the exact charge-balance equation for a pure water in which calcite is dissolving is... 

This explains why the solubility of calcite (and carbonate minerals generally) varies roughly as the cube root of the CO2 pressure. The accurate, empirical equation for calcite solubility at 25°C that has been plotted in Fig. 6.1 is, in fact,... 

Although we have discussed separately the natural processes affecting carbonate mineral solubilities, in fact such processes often act together. For example, the calcite supersaturation observed in lakes and streams and shallow ocean embayments may result from the simultaneous operation of three processes. These include evaporation, CO2 removal by photosynthesis, and increases in temperature that reduce the solubilities of both CO2 and the carbonate minerals. 

Dolomite is the second most abundant carbonate mineral after calcite. In their occurrences, dolomitic rocks are usually associated with limestones. In the crystal structure of ideal (ordered) dolomite, which is the thermodynamically most stable phase, layers of carbonate groups are separated by and coordinated with alternating layers of calcium and then magnesium ions. In disordered dolomite, which is less stable than the ordered form, a significant number of calcium and magnesium ions are mixed throughout the cation layers. Recently formed dolomite tends to be disordered, whereas the dolomite found in older rocks, such as those of Paleozoic age, is usually well-ordered. The molar Ca /Mg ratio in ordered dolomites tends to be close to unity, whereas that ratio in disordered dolomites is usually several percent enriched in calcium. In Table 6.1, solubility products are given for both ordered and disordered dolomite. As expected, ordered dolomite is less soluble than its disordered form. 

Table 6.4 lists formulas and solubility products of the pure carbonate minerals. The 1 1 carbonates of divalent Fe, Mn, Zn, Cd, Co, Sr, Pb, UO, are all less soluble than calcite and, in fact, the... 

There are a variety of potential explanations for carbonate mineral supersalurations that exceed uncertainties in the saturation indices of the pure, well-crystallized carbonates (about 0.1 SI units for calcite). As discussed earlier in this chapter, calcite supersaturation in surface-waters may result from a temperature increase, evaporation, and/or a loss in CO2 to photosynthesis or by exsolution to the atmosphere. Mixing of surface-waters or groundwaters can also produce a supersaturated mixture. The dissolution of more soluble gypsum causes calcite supersaturation in some groundwaters of the Floridan aquifer. 

Hypothetical reaction pathways chosen to model the L2 leachate-Uinta Sandstone system are illustrated in Figure 5. As a first approximation, dissolution/precipitation reactions affecting the mass balance of Na, K, Mo, SO4, and Cl were not considered. Instead, based upon the solubility controls discussed in the previous sections of this paper, the working hypothesis for the simulations is that the recarbonation of L2 leachate drives the reactions toward equilibrium. Along the path toward equilibrium, recarbonation is accompanied by the precipitation and dissolution of sepiolite, calcite, and an inferred hydrated magnesium carbonate mineral such as hydromagnesite. 

Adsorption plays important role in the formation of groimd water composition. It conduces coprecipitation of some ions in the process of mineral formation. At that, according to the Fajam-Paneth rule adsorption way are coprecipitated those ions, which form poorly soluble salt with the oppositely charged ion of the precipitating mineral. The lower the solubility of a given salt, the more active is coprecipitation. That is why, for instance, along with the formation of the calcite, Zn and Pb well coprecipitate (their carbonates are poorly soluble) and MoO ", WO , PO " (their compounds with Ca are poorly soluble). 

The principal carbonate minerals are calcite (CaC03), magnesite (MgC03), dolomite [MgCa(C03)2], and siderite (FeC03). Calcite is the principal mineral in limestone and the main constituent of marble, chalk, pearls, coral reefs, and the shells of marine animals such as clams and oysters. Although CaC03 has low solubility in pure water, it dissolves readily in acidic solutions with evolution of CO2 ... 

---