Dolomites solubilities

The relationship between calcite (or aragonite) and dolomite solubilities has been extensively discussed in terms of reactions for the dolomitization of limestones. Although this relationship is relatively simple, it has been a source of controversy (e.g., Machel and Mountjoy, 1986 Stoessel, 1987). If both calcite and dolomite are in equilibrium with the same solution, they must be in equilibrium with each other. Under these conditions the following relations hold (e.g., Carpenter, 1980, see Chapter 6). 

Barnes, Ivan, Back, William, Dolomite Solubility in Ground Water, U.S. 

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. 

Magnesium (eighth most abundant element) is found principally as Mg+2 ion in salt deposits, particularly as the slightly soluble carbonate, MgC03, and also in sea water. The natural deposits of MgCOj with CaC02 are called dolomite. Magnesium is present as a cation in the asbestos silicates. 

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... 

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. 

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. 

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). 

The impact of water hardness due to calcium or magnesium ions on detergents was explained in Section 7.3.1 The source of most Ca2+ and Mg2+ in hard water is the dissolution of limestone (CaCOs) or dolomite [CaMg(COs)2]. Magnesium carbonate is fairly soluble (1.26 mmol L 1 at ambient temperature), but CaCOs is much less so (0.153 mmol L 1). However, if the water contains dissolved CO2 (as indeed it will if it is exposed to the air see Exercise 14.9), the relatively freely soluble Ca(HCOs)2 forms, and the limestone slowly dissolves away ... 

Sulphate of magnesia is also obtained by the action of sulphuric acid upon calcareous rocks rich in carbonate of magnesia, such as dolomite. The compound is calcined, and reduced to powder by being aspersed with water it is then diffused through water, and sulphuric acid is added sulphate of lime and sulphate of magnesia are formed, the one barely soluble in water, and the other very much so consequently, they are readily separated. 

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.

Figure 6. Ion product for dolomite as a function of temperature and Pco2. Areas outlined by dashed line bound lake water analyses, and double line represents solubility constants...

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 ... 

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