Muscovite, dissolution

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. 

In agreement with the results presented, a mechanistic interpretation of kaolinite and muscovite dissolution is suggested ... 

The predictions for the muscovite/quartz mixture reacting with the young fluid showed quartz dissolving quickly. The quartz dissolution front was predicted to be sharp and appeared to be associated with a brief period of zeolite precipitation. The muscovite dissolution front was also predicted to be sharp. However, the muscovite took longer than the quartz to dissolve. A region of muscovite precipitation in front of the dissolution front was predicted. 

Tobermorite was predicted to form at the dissolution front together with the zeolite phase, mesolite. Foshagite was predicted to precipitate behind the dissolution front. There was a predicted increase in porosity for the entire column. At short timescales, a sharp decrease in porosity was predicted, associated with the zeolite precipitation at the quartz dissolution front. This disappeared over longer timescales following removal of quartz from the column. In the first 100 mm of the column, there was predicted to be a large increase in porosity corresponding to the muscovite dissolution front. For the remainder of the column, there was also a predicted increase in porosity but this was minor by comparison. 

Fig. 25.1. Mineralogical consequences of mixing the two fluids shown in Table 25.1 at 60 °C in the presence of microcline, muscovite, quartz, and dolomite. Results shown as the volume change for each mineral (precipitation is positive, dissolution negative), expressed per kg of pore water.

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)... 

Kalinowski B. E. and Schweda P. (1996) Kinetics of muscovite, phiogopite and biotite dissolution and alteration at pH 1 -4, room temperature. Geochim. Cosmochim. Acta 60, 367-385. 

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. 

I-inure 12. The proton- and oxalate-promoted dissolution of muscovite. The slow weathering kinetics is a characteristic of micas. Oxalate affects the stoichiometry of A1 and Si release, but has not i significant catalytic effect. Figure 12c displays a schematic representation of the muscovite structure. It reveals the 2 1 structure. For example, an A1 layers (black) exists in an octahedral sheet between two tetrahedral sheets (white) whose cations are composed or25% A1 and 75% Si. Siioxane and edge surfaces are exposed lo solution. 

The dissolution is controlled by the detachment of Al. Since the dissolution of silica is not promoted in presence of oxalate and salicylate (Bennett et al., 1988 Wieland, 1988), we may conclude that Si centers do not form stable surface complexes with these ligands. Hence, the siloxane layer of kaolinite and muscovite is not reactive with respect to dissolution reactions. Therefore, the detachment of both Al and Si is a consequence of the formation of surface complexes with Al sites. 

Reconstitution of a secondary Al phase. The pH dependence of the Al detachment (Fig. 13) may not be explained simply by surface protonation reactions. Aluminum(III) may adsorb on the siloxane layer (inset in Fig. 14) and reconstitute a secondary precipitate. Hence, the pH dependence of Al detachment reflects the release of Al from this Al-rich precipitate rather than the dissolution process at the kaolinite surface. In the presence of oxalate, the Al phase is dissolved and the dissolution process occurs stoichiometrically at low pH. The accumulation of Al on mica surfaces has already been postulated (t Serstevens et al., 1978). Figure 15 reveals that the Al center occurring at the surface mainly affects the dissolution characteristic of kaolinite and muscovite. 

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.

Organic and mineral O, A, E, Bh Quartz/ muscovite Organic acids (Primary/secondary Al-silicate) + (organic acids) (Al-organo-complex) + (Mg, Ca, Na, K) + (organic anions) -1- H4Si04 Very fast congruent dissolution... 

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. 

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. 

Because the ratio (aK+/ajj+) is fixed, but SiOj continues to increase as K-feldspar dissolves, kaolinite reacts to form muscovite, and the solution follows path D E, at which point the solution becomes saturated with quartz, and if equilibrium is maintained, quartz will begin to precipitate. With four components (KjO, AljOj, Si02, H2O), a maximum of four phases can coexist at our arbitrarily chosen T and P (25 °C, 1 atm) according to the phase rule. With quartz, muscovite, kaolinite, and water, this number has now been reached and cannot be exceeded (K-feldspar doesn t count it is being used as a source of solutes, and has not yet equilibrated with the solution). Therefore, if we continue to add KjO, AI2O3, and SiOj from the K-feldspar to the solution, the solution will stay at point E while kaolinite reacts with the solution to form muscovite, and quartz continues to precipitate. When kaolinite is all used up, additional dissolution of K-feldspar will drive the solution composition along E F, with the Si02 content of the solution buffered by the presence of quartz. At point F, K-feldspar finally becomes stable. 

Sylvestre granite at 285 m.y. and, at the same time, mica-epi-syenitization created further sites of concentration of uranium and led to the dissolution of quartz and muscovitization of other minerals. Another type of episyenitization, more strictly bound to diaclases and fissures, consists in the dissolution of... 

---