Feldspar dissolution

Blum, A. E. and Stillings, L. L. (1995). Feldspar dissolution kinetics. In "Chemical Weathering Rates of Silicate Minerals" (A. F. White and S. L. 

Blum, A.E. and L.L. Stillings, 1995, Feldspar dissolution kinetics. Reviews in Mineralogy 31,291-351. 

Current best estimates for natural plagioclase weathering rates are one to three orders of magnitude lower than laboratory rates. Surface characteristics which may play a role in determining rates and mechanisms of feldspar dissolution (including non-stoichiometric dissolution and parabolic kinetics) in the laboratory include adhered particles, strained surfaces, defect and dislocation outcrops, and surface layers. The narrow range of rates from experiments with and without pretreatments indicates that these surface characteristics alone cannot account for the disparity between artificial and natural rates. 

Figure 2 Effect of sample pretreatment on feldspar dissolution kinetics (modified from Ref. 11). See text for discussion.

The morphology of weathered feldspar surfaces, and the nature of the clay products, contradicts the protective-surface-layer hypothesis. The presence of etch pits implies a surface-controlled reaction, rather than a diffusion (transport) controlled reaction. Furthermore, the clay coating could not be "protective" in the sense of limiting diffusion. Finally, Holdren and Berner (11) demonstrated that so-called "parabolic kinetics" of feldspar dissolution were largely due to enhanced dissolution of fine particles. None of these findings, however, addressed the question of the apparent non-stoichiometric release of alkalis, alkaline earths, silica, and aluminum. This question has been approached both directly (e.g., XPS) and indirectly (e.g., material balance from solution data). 

Numerous surface characteristics have been postulated to play a role determining mechanisms, rate-limiting steps, and rates of feldspar dissolution during weathering (as discussed above). These include ... 

Table I shows the results of several such studies. The pH s of the specific experimental runs used in Table I were chosen to facilitate comparison with natural (fresh) surface waters. Rates were taken directly from reported results, or were determined from the linear portions of tabulated or graphed data. Rates were normalized using either reported surface areas, or estimates of surface area based on reported particle size data. Results in Table 1 compare favorably with a similar compilation by Lasaga (50). Table I shows that feldspar dissolution rates in slightly acidic solutions vary by less than one order of magnitude, despite different experimental conditions and sample pretreatments.

More recent studies of alkali feldspar dissolution have found variations on the hypotheses listed above. For example, Nugent et al. [97] have proposed that naturally weathered albite feldspar surfaces are sodium and aluminum depleted, as found for... 

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-... 

A number of workers (Wollast, 1967 Huang and Kiang, 1972 Luce et al., 1972) have observed that feldspar weathering conforms to the parabolic diffusion law (Chapter 2). An example of this is shown in Fig. 7.2. Much research effort has gone into explaining why parabolic kinetics could be operational for feldspar dissolution. Several explanations have been given, and these are discussed below. 

Holdren, G. R., Jr., and Speyer, P. M. (1986). Stoichiometry of alkali feldspar dissolution at room temperature and various pH values. In Rates of Chemical Weathering of Rocks... 

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... 

Petrovic, R. Rate control in feldspar dissolution-II. The protective effect of precipitate, Geochim. Cosmochim. Acta 40, 1509-1521 (1976). 

The model of Oelkers (2001b) was originally developed and applied to alkali feldspar dissolution at 150 °C and pH 9 (Gautier et al, 1994 Oelkers et al, 1994) ... 

The most commonly used rate equation for feldspar dissolution under ambient conditions can be written as... 

In contrast, the presence of these cations has been observed to decrease the rate of dissolution of several feldspars (Sjoberg, 1989 Nesbitt et al, 1991 Stillings and Brantley, 1995) and to decrease the thickness of the leached layers formed on the feldspar surface. Schweda (1990), Nesbitt et al. (1991), and Brantley and Stillings (1996) proposed that feldspar dissolution in acidic solution far from equilibrium is related to competitive adsorption of and other cations on the surface of feldspar. Brantley and Stillings derived a rate equation based on a leached layer and Langmuir adsorption model ... 

Dove (1995) further summarizes evidence suggesting that adsorption of both Al " " and Fe3+ onto quartz surfaces inhibits reactivity of that phase. Inhibition of feldspar dissolution also occurs when Al is present in solution (Chou and Wollast, 1985 Nesbitt et al, 1991 Chen and Brantley, 1997). For example, Nesbitt et al. (1991) argued that adsorption of Al " " retarded the rate of dissolution of labradorite more than other cations. Furthermore, the effect of aqueous Al " " on dissolution of albite may increase with increasing temperature due to the enhanced adsorption of cations with temperature (Machesky, 1989 Chen and Brantley, 1997 note however that Oelkers (2001b) disputes this trend). In contrast, the addition of aqueous aluminum was not observed to affect the rate of forsterite dissolution at pH 3 and 65 °C (Chen and Brantley, 2000). It may be that aqueous aluminum becomes incorporated into surfaces and affects dissolution wherever the connectedness of surface silicon atoms is >0. Brantley and Stillings (1996, 1997) and Chen and Brantley (1997) suggest that Equation (51) can be used to model aluminum inhibition on feldspars. Sverdrup (1990) has reviewed the effects of aqueous Al on many minerals and incorporated these effects into rate equations. 

Hellmann R., Penisson J.-M., Hervig R., Thomassin J. H., and Abrioux M.-F. (2002) Solution-reprecipitation responsible for altered near-surface zones during feldspar dissolution do leached layers really exist Goldschmidt Conference Abstracts, A320. 

Oelkers E. H. and Schott J. (1998) Does organic acid adsorption affect alkali-feldspar dissolution rates Chem. Geol. 151(1-4), 235-245. 

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