K-feldspar dissolution

Gautier, J. M., Oelkers, E. H. Schott, J. (1994). Experimental study of K-feldspar dissolution rates as a function of chemical affinity at 150 °C and pH-9. Geochimica et Cosmochimica Acta, 58, 4549-60. 

Lagache and others ( 7) found that for K-feldspar dissolution at 200 C, the rate of release of silica and alumina was... 

A useful introduction to modeling reaction paths is Steinmann et al. (1994). They show that the reaction path for K-feldspar dissolution as projected in logaK+/ H+ versus logasi02(aj) space is highly dependent on the starting solution composition, and that with certain assumptions, the path can be calculated using a spreadsheet. 

Assuming that plagioclase is more unstable than K-feldspar and therefore more likely to be dissolved, any carboxylic acids remaining after plagioclase dissolution may be used for K-feldspar dissolution. The carboxylic acids produced are insufficient to account for all of the porosity generated by plagioclase dissolution. Therefore, no calculations for K-feldspar were made. 

It is important to note that the layer thicknesses reported above were based strictly on solution chemistry analyses. Several reports have appeared on the thicknesses of leached layers using surface chemistry techniques. Petrovic et al. (1976) used XPS and analyzed K, Al, and Si content of altered K-feldspar grains and found the leached layer was <1.7 nm. Layer thicknesses for dissolution of enstatite, diopside, and tremolite based on XPS data are reported in Table 7.3. 

Model the dissolution of quartz and K-feldspar (adularia) over time. Are the parameters temperature and C02 partial pressure of any importance Within the key word RATES use the BASIC program from the data set PHREEQC.dat. The calculation is done with distilled water (pH = 7, pE = 12) as a batch reaction over a time span of 10 years in 100 time steps at a temperatures of 5 °C and of 25 °C and at C02 partial pressures of 0.035 Vol% (atmosphere) and of 0.7 Vol% (soil). Calculate also the kinetics of the dissolution with 0.035 Vol% C02 and 25 °C for a period of 10 minutes. 

Fig. 64 Kinetics of the dissolution of K-feldspar for four models with different temperatures and partial pressures...

Fig. 63 and Fig. 64 show, that both quartz and K-feldspar show different dissolution kinetics depending on temperature and C02 partial pressure. The difference between quartz and K-feldspar is significant while quartz reaches dissolution equilibrium after 150 to 550 days, K-feldspar does not show any equilibration even after 10 years for all of the four possible scenarios. To reach saturation for K-feldspar in all four models, the simulation time would have to be about 1000 years. 

Reactions M-O represent incongruent dissolution of Ca(OH)2s because the solubility of CaCOjS is much smaller than the solubility of Ca(OH)2s (Table 2.7). Therefore, introduction of Ca(OH)2s to water in equilibrium with atmospheric C02 leads to spontaneous formation of CaC03s. The well-known incongruent dissolution phenomena are those representing the dissolution of aluminosilicate minerals. For example, K-feldspars (orthoclase) undergo incongruent dissolution when exposed to water and carbonic acid to form kaolinite ... 

Changes in pH also affect the magnitude of individual rate expressions. Wollast (28) found for K-feldspar a progressive decrease in the parabolic rate constant for silica with increases in pH over the range 4-10 at 25 C. In investigating magnesium silicate dissolution at 25 C, Luce and others ( ) found a slight... 

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)... 

Figure 3 Log (dissolution rate) versus pH for (a) albite, and (b) K-feldspar. Data for (a) from published sources Busenberg and Clemency (1976), Holdren and Berner (1979), Chou and Wollast (1984), Knauss and Wolery (1986), Manley and Evans (1986), Casey et al (1991), Rose (1991), and Stillings et al (1996), and for (b) McClelland (1950), Wollast (1967), Tan (1980), Manley and Evans (1986), Schweda (1989), Stillings and Brantley (1995) (after Blum...

The calculation of mineral weathering rates based on solute concentrations is complicated by the fact that individual solute species are commonly produced by more than one weathering reaction. In granite weathering, for example, aqueous silicon is produced not only from plagio-clase weathering (Equation (2)), but also by K-feldspar, biotite, hornblende and quartz dissolution. In addition, secondary minerals such as kaolinite take up aqueous silicon (Equation (2)). 

Detrital feldspars in modern sediments have a compositional range that reflects their composition in the source rocks (Trevena and Nash, 1981). During diagenesis, sodium-rich K-feldspars and more calcium-rich detrital plagioclase grains are more subject to dissolution (and replacement) than potassium-rich K-feldspar or sodium-rich plagioclases (Maynard, 1984 Milliken, 1988, 1992 Milliken et al, 1989). 

In a general sense, dissolution of feldspars can be viewed as a kind of weathering and can be described by hydrolysis reactions. The exact formulation of these reactions is far from certain, however. In the simplest construction, dissolution of K-feldspar and calcium plagioclase consumes H, the plagioclase consuming slightly more ... 

Unfortunately, the speciation of aluminum in subsurface pore fluids is poorly known. Formulations of feldspar dissolution that utilize other aluminum-bearing ions require less H, e.g., for a K-feldspar ... 

Thus, dissolution of 0.25 mol of K-feldspar buffers sufficient acid to allow (potentially) precipitation of I mol of calcite. Buffering by silicates has been postulated as the primary control on subsurface pH (Hutcheon and Abercrombie, 1989,1990 Smith and Ehrenberg, 1989 Hutcheon etal, 1993), consistent with the observation that dissolution affecting feldspars typically leaves adjacent calcite cements and skeletal debris unaffected (Siebert et al, 1984). 

At equilibrium Q = K and R = 0.) Eq. (2.107) perhaps applies to the rate of feldspar dissolution in a fresh, unconfined groundwater, whereas Eq. (2.108) is more consistent with the near-equilibrium conditions we might expect in older confined groundwaters. Far from equilibrium at pH values above 7 to 8, feldspar dissolution rates again increase, and the rate equation has the general form... 

---