Albite, weathering

Many minerals have been found to dissolve and precipitate in nature at dramatically different rates than they do in laboratory experiments. As first pointed out by Paces (1983) and confirmed by subsequent studies, for example, albite weathers in the field much more slowly than predicted on the basis of reaction rates measured in the laboratory. The discrepancy can be as large as four orders of magnitude (Brantley, 1992, and references therein). As we calculate in Chapter 26, furthermore, the measured reaction kinetics of quartz (SiC>2) suggest that water should quickly reach equilibrium with this mineral, even at low temperatures. Equilibrium between groundwater and quartz, however, is seldom observed, even in aquifers composed largely of quartz sand. 

Chou and Wollast (1984) used a fluidized bed reactor to study albite weathering. An illustration of their device is shown in Fig. 3.5. The flow needed to maintain the feldspar particles in suspension is provided by the pumping rate Plt while P2 is the rate of addition of fresh solution P2 is also the rate of output of the reacted solution. By changing the rate of renewal of P2 one can vary the residence time of the fluid in the reactor. To maintain a small difference in concentration between the input at the bottom of the fluidized bed and the output at the top of the bed, P2 must be small in comparison to Pi. Chou and Wollast (1984) maintained the renewal rate P2 between 3 and 6% of the mixing rate Pi. 

An example of aluminosilicate weathering is the reaction of the feldspar albite to a montmor-illonite-type mineral... 

The weathering of silicates has been investigated extensively in recent decades. It is more difficult to characterize the surface chemistry of crystalline mixed oxides. Furthermore, in many instances the dissolution of a silicate mineral is incipiently incongruent. This initial incongruent dissolution step is often followed by a congruent dissolution controlled surface reaction. The rate dependence of albite and olivine illustrates the typical enhancement of the dissolution rate by surface protonation and surface deprotonation. A zero order dependence on [H+] has often been reported near the pHpzc this is generally interpreted in terms of a hydration reaction of the surface (last term in Eq. 5.16). 

Chou, L., and R. Wollast (1984), "Study of the Weathering of Albite at Room Temperature and Pressure with a Fluidized Bed Reactor", Geochim. Cosmochim. Acta 48, 2205-2218. 

Nickel (8) calculated the thickness of the proposed "residual layer" on albite from the mass of dissolved alkalis and alkaline earths released during laboratory weathering and the measured surface area, and determined that the thickness ranges from 0.8 to 8.0 nm in the pH range of natural surface waters. Although he interpreted his results differently, they anticipate later findings on the pH dependence of residual layer compositions (see below). 

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... 

The Hostrock and Backfill Material. Most crystalline igneous rocks, including granite and gneiss, are composed of a comparatively small number of rock forming silicate minerals like quartz, feldspars (albite, microcline, anorthite etc.) micas (biotite, muscovite) and sometimes pyroxenes, amphiboles, olivine and others. Besides, there is a rather limited number of common accessory minerals like magnetite, hematite, pyrite, fluorite, apatite, cal cite and others. Moreover, the weathering and alteration products (clay minerals etc.) from these major constituents of the rock would be present, especially on water exposed surfaces in cracks and fissures. 

Wollast, R., and Chou, L. (1985). Kinetic study of the dissolution of albite with a continuous flow through fluidized bed reactor. In Chemistry of Weathering (J. I. Drever, ed.), pp. 75-96, Reidel Publ., Dordrecht, The Netherlands. 

Berner, R. A., Holdren, G. R., Jr., and Schott, J. (1985). Surface layers on dissolving silicates. (Comments on Study of the weathering of albite at room temperature and pressure with a fluidized bed rector by L. Chou and R. Wollast.) Geochim. Cosmochim. Acta 49, 1657-1658. 

W. Stumm and R. Wollast. Coordination chemistry of weathering Kinetics of the surface-controlled dissolution of oxide minerals, Rev. Geophys. 28 53 (1990). See also A. E. Blum and A. C. Lasaga, The role of surface speciation in the dissolution of albite, Geochim. Cosmochim. Acta 55 2193 (1990). 

In this chapter, the focus is on weathering of feldspars, aluminosilicate minerals, which are the most abundant mineral species in the earth crust (Banfield Hamers, 1997). Feldspars contain aluminium and silicon, which are arranged in a tetrahedral structure, with other cations in the voids of this structure. The common feldspars have compositions ranging between albite (NaAlSisOg) and K-feldspar (KAlSigOg) (alkali feldspars) and between albite and anorthite (CaAl2Si20g) (plagioclase feldspars). 

Chou L. and Wollast R. (1984) Study of the weathering of albite at room temperature and pressure with a fluidized bed reactor. Geochim. Cosmochim. Acta 48, 2205-2217. 

Wollast R. and Chou L. (1992) Surface reactions during the early stages of weathering of albite. Geochim. Cosmochim. Acta 56, 3113-3121. 

Figure 7.1 Goldich s sequence of increasing weatherability of common minerals (cf. Loughnan 1969 Faure 1991). In parentheses are the lifetimes in years from Table 7.1, assuming olivine = forsterite, augite = diopside, hornblende = tremolite, Ca-plagioclase = anorthite, Na-plagioclase = albite, K-feldspar = microcline, and the stability of muscovite is comparable to that of the related clay, illite.

Irreversible reaction paths (dotted and dashed lines are shown in the diagram for the hydrolysis of albite (ABCDEF) and coexisting K-feldspar and albite with relative reaction rates of 1 1 (A b c d e f g h i ) and 0.1 1 (A b"c"d"e"f, weathering of Sierra Nevada rocks (GH), and reaction of clay minerals with Bermuda seawater (IJ. The area labeled M designates the composition of surface seawater and point N represents the average composition of world streams. 

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