The Solubility of Dolomite

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. 

By algebraic manipulation of the calcite and dolomite expressions, we obtain 

With K p values for ordered and then disordered dolomite from Table 6.1, we find mCa )/(mMg ) = 1.35 and 0.38, respectively, at equilibrium between dolomite and calcite. 

Equilibrium reactions among all the calcium-magnesium carbonates can be written in terms of the CO2 pressure and mCa VmMg ratio in water ([H2O] = 1). A diagram in which the phase bound- 

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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.

The action of carbonic acid on limestone produces a calcium bicarbonate solution that is exceedingly soluble in water. (For comparison, at 20°C the solubility of calcium carbonate in water is only 0.0145 g per liter while the solubility of calcium bicarbonate is 166 g per literJ ) Magnesium ions from dolomite are also released into aqueous solution according to the same mechanism. The weathering of gypsum, calcium sulfate, also releases calcium ions into natural water supplies. 

Magnesium is produced in several ways. An important source is dolomite from which, after calcination, the calcium is removed by ion exchange using seawater. The equilibrium is favorable because the solubility of Mg(OH)2 is lower than that... 

Langmuir and Melchior (1985) examined the solubility of Ca, Sr, and Ba sulfates in saline groundwaters of the Wolfcamp Formation of north Texas. The Wolfcamp is composed of limestone and dolomite. The groundwater, which is at a depth of about 970 m. was at 32"C and 67 bars pressure and had a pH of 6.1. Species total concentrations are listed here. 

Unlike the aluminosilicates and most other minerals, the carbonates have an exothermic heat of dissolution, which means that their solubilities decrease with increasing temperature. For example, for calcite decreases from 10 at 0°C to 10 at 30°C. The effect of temperature on the solubility products of aragonite, calcite, and ordered dolomite is plotted in Fig. 6.8. The figure shows that between 0 and 90°C solubilities of the carbonates decrease by about 6-fold for aragonite and calcite and 14-fold for dolomite. This decreasing solubility with temperature is magnified by the fact that the solubility of CO2 gas also declines with temperature. Kqq, decreases from 10 " to 10 between 0 and 30°C. 

Based on a knowledge of the mineralogy of the Uinta Sandstone, the mineral phases most likely to be controlling the solubility of Ca in the sandstone - L2 leachate system are calcite, dolomite, gypsum, and fluorite. Stability lines and saturation indices calculated for these minerals are present in Figure 1 and Thble III, respectively. The observed data point (black circle) plotted in Figure 1 represents the measured pH and log Ca + activity in the L2 leachate after reaction with the Uinta Sandstone. The log CO2 gas partial pressure of -2.95 atmosphere is based on the measured pH and alkalinity of the reacted solution. The open circle represents the log Ca + activity and pH calculated by MINTEQ for the raw leachate recarbonated to a log CO2 partial pressure of -2.95 atmosphere. The leachate apparently developed a CO2 gas overpressure because equilibrium with calcite was attained in sealed containers at the relatively low pH of 7.91 (18). The calcite-dolomite line shown in the figure represents the pH-dependent activity of Ca + in equilibrium with both calcite and dolomite. 

As shown by the raw L2 leachate data point (empty circle) in Figure 3, the leachate is initially supersaturated with respect to (at least) calcite, dolomite, talc, magnesite, sepiolite, hydromagnesite, nesquehonite, and brucite. After reaction with the Uinta Sandstone (indicated by the solid circle, point L2-USS in Figure 3), Mg + activity appears to be most nearly controlled by the solubility of magnesite. 

Solubility in water of magnesium hydroxide and calcined dolomite. Magnesium hydroxide is only sparingly soluble in water (about 0.01 g/1) [19.5]. The magnesium hydroxide present in calcined dolomite is reported not to affect the solubility of calcium hydroxide (but see rate of solution below). 

Dolomite can be thought of as a double salt of MgCOs and CaCOs. Double salts usually have a solubility similar to the solubility of the least soluble simple salt constituent. Because MgCOs is less soluble than CaCOs, dolomite is less soluble than calcite. 

There are various hypotheses explaining this aggregate expansion mechanism. The most wide spread refer to the swelling of clay minerals and of osmotic pressure formation [141,142]. All these hypotheses agree that this phenomenon relates to the reaction of soluble alkalis from cement with the aggregate leading to the decomposition of dolomite ... 

PRACTICE EXAMPLE A Write the solubility product constant expression for (a) MgCOs (one of the components of dolomite, a form of limestone) and (b) Ag3P04 (used in photographic emulsions). 

Data on the solubihty of magnesium hydroxide in water are not all in agreement, but the solubihty is extremely low. The extent of Mg(OH)2 solubihty is 10 mg/L, which is about 1/100 the solubihty of Ca(OH)2. In concentrated solutions of NH Cl and NH CO, the solubihty of Mg(OH)2 is markedly increased, but in no instance does its solubihty equal that of MgCO in water heavily permeated with CO2. Dolomitic hydrates are slightly less soluble than high calcium hydrates, but much nearer the latter in value than Mg(OH)2, because the presence of MgO and Mg(OH)2 does not impede the dissolution of its Ca(OH)2 constituent. 

Compare the solubilities in water of calcium carbonate, calcium sulfite, calcium sulfate, magnesium sulfate, and dolomite. 

The amount of hardness present in natural surface and groundwaters depends to a large extent on the action of dissolved carbon dioxide in rainwater on the watershed s geological formations (such as limestone, dolomite, gypsum, or magnesite). The dissolved hardness levels remain relatively low because of the sparingly soluble nature of the salts formed. Typically, MU water sources initially contain anywhere from 5... 

In arid and semi-arid soils, calcite, dolomite, leonhardite (Ca2Al4Si8024.7H20) and lawsonite (CaAl2Si208.2H20) can be possible minerals. Calcium carbonate strongly influences soil properties in arid and semi-arid soils. Most calcareous soils have soil a pH in the range of 7.3-8 5. When sodium is predominant in soils, soil pH is above 8.5. In most arid and semi-arid soils, calcium carbonates (calcite and dolomite) generally accumulate and are most likely to control the Ca2+ and Mg2+ solubility in these soils (Lindsay, 1979). 

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