Dissolution feldspar weathering

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 ... 

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. 

Equation (2.101) corresponds to transport-controlled kinetics (cf. Stumm 1990). White and Claassen conclude that after long times in natural water/rock systems parabolic rates tend to become linear. Helgeson et al. (1984) show that feldspar dissolution rates are linear if the feldspar is pretreated to remove ultrafine reactive particles. In other words initial parabolic rates are probably an artifact of sample preparation. It seems likely that, in general, the dissolution or weathering of most silicates in natural water/rock systems obeys zero-order kinetics. 

MECHANISMS OF PRIMARY MINERAL DISSOLUTION 6.2a. Feldspar Weathering... 

Not much is known for certain about the rate of weathering of secondary minerals. Gibbsite dissolves in acid solutions (pH 2-3) at a rate per unit of surface area that is not greatly different from that of feldspars (A 10 " moles/mVsec). Naturally, because gibbsite crystals tend to be much smaller (micron-sized) than particles of feldspars and other primary minerals, actual dissolution rates of feldspars and gibbsite on a mass basis are very different. Consequently, when H" ions enter acid soils, they are more likely to react with clays and thereby dissolve Al , rather than be consumed by the process of primary mineral (e.g, feldspar) weathering. 

Dissolution of most aluminosilicate minerals also consinnes H ions and contributes base cations (Ca, Mg, Fe(II)), alkali elements (Na, K) and dissolved Si and Al to the tailings pore water (Blowes and Ptacek, 1994). Though, dissolution of almninosilicate minerals is slower than of metal hydroxides and much slower than that of carbonates. Feldspar weathering is mainly controlled by pH, silica, Na, K and Ca concentrations. One possible reactions path is ... 

Berner RA, Holdren GR Jr (1979) Mechanism of feldspar weathering. II. Observations of feldspars from soils. Geochim Cosmochim Acta 43 1173-1186 Bevan J, Savage D (1989) The effect of organic acids on the dissolution of K-feldspar under conditions relevant to burial diagenesis. Mineral Mag 53 415-425 Blum AE, Lasaga AC (1988) The role of surface speciation in the low-temperature dissolution of minerals. Nature 4 431-433... 

Search of structure. Wiley, New York, 764 pp Holdren GR Jr, Berner RA (1979) Mechanism of feldspar weathering. I. Experimental studies. Geochim Cosmochim Acta 43 1161-1171 Huang WH, Keller WD (1970) Dissolution of rock-forming silicate minerals in organic acids simulated first-stage weathering of fresh mineral surfaces. Am Mineral 55 2076-2094... 

As in dissolution, a chemical and structural change can occur from hydrolysis as the ions replaced by or OH may be of a different size so that the crystal structure is stressed and weakened. An example of this is the weathering of feldspar or goethite by H ... 

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

The continental pattern for Na matches the pattern for total feldspar percentages, as Na values are primarily correlated with plagioclase (Eberl Smith 2009). Feldspars are much more susceptible to chemical dissolution than quartz and, with sufficient time and precipitation, will weather mainly to clay minerals. As a result, total feldspar contents and Na contents decrease with increasing precipitation from west to east (Fig. 3). 

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. 

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). 

Ca (aq), Mg (aq), and HCOjCaq). Silicate weathering is an incongruent process. The most important of these reactions involves the weathering of the feldspar minerals, ortho-clase, albite, and anorthite. The dissolved products are K (aq), Na (aq), and Ca (aq), and the solid products are the clay minerals, illite, kaolinite, and montmorillonite. The weathering of kaolinite to gibbsite and the partial dissolution of quartz and chert also produces some DSi,... 

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... 

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... 

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