Muscovite dissolution rates

The rates of dissolution of carbonates and aluminosilicates as a function of pH are generalized in Fig. 2.11. Calcite and dolomite dissolution rates are generally 10 to 1 O -fold faster than rates for the silicates and decrease with pH up to saturation with the carbonates, usually between pH 8 and 10. Dissolution rates among the silicates range widely and are greatest for rapidly weathered minerals such as nepheline and olivine and slowest for quartz, muscovite (illite) and kaolinite, important products of chemical weathering in soils, discussed in more detail in Chap. 7. 

The results of preliminary dissolution experiments with muscovite are din-played in Figure 12. They illustrate that oxalate increases the dissolution rate of muscovite at pH 3. However, the catalytic effect of oxalate is hardly mcasurablo and, hence, an evaluation of data leading to a more mechanistic understanding of muscovite dissolution appeared to be difficult. 

Generally, the protonation of Al sites promotes the dissolution process with increasing H+ activity in acid solution (A1203, kaolinite, muscovite), whereas the rate of silica dissolution even decreases or remains constant (pH < 3). Obviously (lie more Al centers are exposed per unit surface area, the higher the proton-promoted dissolution rate and the more effective are surface chelates in catalyzing the weathering process. 

Figure 15. The pH dependence of the proton-promoted dissolution rates ofkaolinite, muscovite, and their constitutent oxides of A1203 and amorphous Si02 or quartz, respectively. With increasing H4 activity, the rate of A1 detachment is promoted whereas the rate of Si detachment is slowed down.

Kinetic considerations. Studies of phosphate solubility reveal kinetic limitations to dissolution rates. Harrison and Watson (1984) and Rapp and Watson (1986) measured the dissolution rates of apatite and monazite, respectively, and found that the dissolution rate is limited by diffusion of P or LREEs away from the dissolving apatite or monazite. Furthermore, the diffusivity, and hence dissolution rate, is strongly dependent on the H2O content of the melt. In dry melts, dissolution is so slow that complete dissolution of even small crystals of apatite or monazite is unlikely. In melts produced by dehydration melting of muscovite or biotite, where the H2O content is in the range of 4-8 wt % H2O, apatite crystals on the order of 500 pm diameter will dissolve in 100-1000 years. 

The dissolution rates for quartz and albite were taken from Knauss Wolery (1986, 1988), and that for muscovite from Knauss Wolery (1989). The rate for calcite dissolution was based on the review work in Rochelle et al. (1998). Precipitation rates for all secondary phases were set to be at least an order of magnitude greater than that of the mineral dissolution rate, for all conditions likely to be found in the experiments. Thermodynamic data for the potential secondary phases were taken from Sarkar et al. (1982)... 

The overall rate of a fluid-rock reaction can also be modeled, rather than computing the dissolution and precipitation of each solid separately. Eor example, one could write an overall reaction between solids and fluids such as Muscovite -b Quartz = Sillimanite -b K-feldspar -b H2O. The model for overall reactions in metamorphic rocks advanced by Lasaga and Rye (1993)... 

The program then has enough information to use Equation (11.29). However, if some other rate equation is preferred, we can enter it in another part of the kinetic statement (see below). For example, to use Equation (11.29) to examine the kinetic dissolution of albite in water in equilibrium with kaolinite, muscovite, and quartz at 25°C and a pH of 6.0,2 we would prepare a script similar to that shown in Table 11.1. 

The predicted reaction of the muscovite/ quartz mixture with the evolved fluid was much simpler than the above. Dissolution was predicted to take place along the entire length of the column but at about half the rate found with the young fluid, and with more variation between the inlet and outlet ends. The only products expected to be found in this experiment were hillebrandite, which was predicted to form over the first 240 mm of the column, and foshagite, predicted to form from then on. It was predicted that there would be small net reductions in the porosity throughout the column. 

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