Potassium feldspar dissolution

Thyne G., Boudreau B. P., Ramm M., and Midtbp R. E. (2001) Simulation of potassium feldspar dissolution and ilhtization in the Statljord Formation, North Sea. Am. Assoc. Petrol. Geologists Bull. 85, 621-635. 

Much attention has been given to feldspar dissolution kinetics over the past 20 years or so. This is largely attributable to feldspars being the most abundant minerals in igneous and metamorphic rocks. Their ubiquity in soils is also well known where they affect the resultant clay mineralogy and potassium status of soils. Another reason feldspar dissolution rates have been studied profusely has been the controversy over dissolution mecha-... 

Diffusional transfers of potassium and silicon between sandstones and shales may be sufficient to accomplish feldspar dissolution, illitization, and quartz cementation (Thyne, 2001 Thyne et al, 2001). Losses of the magnitude observed for detrital carbonates in shales exceed the capacity of diffusion-mediated transfer. Large-scale advection seems required, although our understanding of shale permeabilities seems to preclude this (Bjprlykke, 1989, 1993 and Lynch, 1997). The possibility of convection driven by salinity heterogeneity within thick shale sequences has been demonstrated by Sharp et al (2001), who note that more information for rock properties and fluid compositions within deep basinal shales is needed before the generality of their results can be assessed. 

Fig. 10. SEM pictures of a Melke Formation siltstone sample (well 6506/12-4 depth 3979.3 mKB). (A) Detrital potassium feldspar undergoing dissolution. (B) Authigenic kaolinite.

Figure 2.26 Suggested schematics of potassium feldspar in water dissolution processes (Aagaard and Helgeson, 1982).

H. S., K. Lund, and C. C. McCune. 1975. Acidization—part 3 The kinetics of the dissolution of sodium and potassium feldspar in HF/HCl acid mixtures. Chemical Engineering Science. 30 1325-1332. Fogler, H. 

There is some evidence for cement/aggregate reaction. Some quartz, potassium feldspar and calcareous material has undergone dissolution... 

Manning DAC, Rae EIC, Small JS (1991) An exploratory study of acetate decomposition and dissolution of quartz and Pb-rich potassium feldspar at 150°C, 50mPa (500 bars). Mineral Mag 55 183-195... 

Total elemental analysis of K and Na has been used extensively to estimate the amount of mica present in a sample free of potassium-feldspars (see Sections BIIIl and BIII2). Micas are dissolved preferentially by Na2S207 fusion (Kiely and Jackson [1965]). In their study, this procedure was applicable to fractions greater than 50 nm in diameter, but in the 2 to 0.2 ixm fraction of a sample, only 68 % of the feldspar was recovered because of dissolution of this mineral. 

Dissolution of feldspars is a logical source of dissolved silica, calcium, sodium, and potassium in groundwater. Similarly, the reaction of carbon dioxide-charged water with silicate minerals is a logical source of bicarbonate. Rogers (1987) examined these and other hypotheses using a mass-balance approach. In these calculations, chloride and sulfate were not considered, and the beginning concentrations were considered to be... 

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). 

Continued dissolution of K-feldspar as the water moves downward through the soil tends to raise the pH and concentrations of both dissolved potassium and silica in soil waters. At point B saturation with kaolinite is reached, and the weathering reaction along line B —> C is... 

Beyond the positions of these atoms, we also need to specify their occupation factors. Feldspars are widely reported to exhibit incongruent dissolution, whereby the alkali and aluminum atoms are dissolved preferentially. Thus, we allow for removal of individual species at the surface by varying the occupation of the near-surface potassium ions. This is an additional multiplicative factor in the expression for the structure factor (Eqn. 14) for each atom in the crystal, and varies between 0 and 1. 

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