Feldspars in Weathering

Abstract

Feldspar weathering occurs via dissolution of all components into solution, with the subsequent precipitation of secondary minerals from solution, and it is the feldspars dissolution rate which controls the overall rate of feldspar weathering. The rate of feldspar dissolution is controlled by the kinetics of surface reactions at the mineral-water interface, not by mass transfer processes, either in solution or through a protective surface layer. At neutral to basic pH conditions, the entire range of feldspars compositions appears to dissolve nearly stoichiometrically, although a thin Al enriched surface layer (<20Å) may form, and cations, particularly Na⁺, may be exchanged with H⁺ to depths of several 100 Å. The exchange of cations with H⁺ appears to be reversible, with cation occupancy favored in the basic pH region. It is not clear whether the observed Al enrichment on the surface is a consequence of slightly non-stoichiometric dissolution, or readsorption of Al from solution at charged surface sites. At acidic solution pH’s, a silica-enriched surface layer 100’s to 1000’s of Å thick may form. This layer is highly hydrated and disordered, and analogous to an amorphous SiO₂ gel. The silica-enriched surface layer does not provide a diffusional barrier to the transport of Al and cations to solution, and does not appear to effect the destruction rate of the feldspar tetrahedral lattice.

Feldspars of all compositions have an experimental dissolution rate which increases with increasing H+ activity at pH <6, and increasing OH^- activity above pH 8.5, a pattern typical of many silicate minerals. In the acidic pH region <pH 6) albite and K-feldspar have nearly identical experimental dissolution rates, with the dissolution rate (R) proportional to [H⁺]^~0.5. This is in contrast to the observation in natural soils that albite weathers much more rapidly than K-feldspar. In the basic pH region, albite dissolution rates have a pH dependence of Roc[OH^-]^~0.3, with K-feldspar dissolution rates having a stronger dependence on [OH'] than albite, and absolute dissolution rates approximately a factor of 10 slower than albite. The plagioclase series has a pH dependence of the dissolution rate of Roc[H⁺]^~0.5 in the acidic region in the compositional range An_0–70However, at compositions of An_>70 there is a transition to increasing values in the exponent until Roc [H⁺]^~1.0for An_>90. At a single solution pH, the dissolution rate of the plagioclase series increases gradually with increasing Ca content until~An_>75, with a rapid increase in the dissolution rate of plagioclase between the compositions An_>75. and An_>90.

There is general agreement that feldspar dissolution in the acid region occurs by selective attack on the Al sites in the tetrahedral framework, and the rate limiting process must involve the hydrolysis of the AI-O-Si bond. It is therefore the increasing Al/Si ratio with increasing An content, and not the Na/Ca ratio, which causes the increasing dissolution rate with increasing An content. In the basic pH region, the relative importance of the AI/Si ratio and cation occupancy are more uncertain.

The most comprehensive models for the dissolution of feldspars employ surface speciation models, in which the dissolution rate is proportional to the equilibrium concentration of surface chemical complexes, which are preferential sites for dissolution reactions, Most commonly, the surface species involve the protonation and deprotonation of dangling tetrahedral oxygens to form positively and negatively charged surface sites in the acid and basic regions, respectively. The concentration of charged surface sites is controlled by adsorption isotherms of species in solution. The large effect of solution pH on feldspar dissolution rates is indirect, controlling the equilibrium concentration of surface species through an

adsorption process. The surface speciation model has major implications for the effects of temperature and adsorption of impurities on dissolution kinetics, which differ significantly from models based upon homogenous kinetic theory.

Field measurements of plagioclase dissolution rates from watershed mass-balance studies and from changes in the abundance of plagioclase in soils of different ages indicate that feldspar weathering is one to three orders of magnitude slower than predicted from laboratory studies. The leading explanation for this effect is the isolation of a large proportion of the feldspar surface area in isolated micropores. Alternative explanations include; (i) adsorption of inhibitors, of which Al and Fe are the most likely, (ii) the high saturation states of soil solutions, and (iii) experimental artifacts in the dissolution experiments. The reconciling of natural and experimental feldspar dissolution rates is a critical step before feldspar dissolution kinetics can be used reliably in predictive models.