Silicate rocks, dissolution kinetics

Dissolution Kinetics of Silicate Rocks—Application to Solute Modeling... 

WHITE AND CLAASSEN Dissolution Kinetics of Silicate Rocks... 

Experimentally determined dissolution kinetics are applicable to natural weathering processes of silicate rocks. Mass transfer from the mineral to the aqueous phase was determined to be incon-gruent under a range of experimental conditions. Transfer rates of individual species (Q) at times (t) can usually be described by one of two rate expressions ... 

White, A. F., and Claassen, H. C. Dissolution kinetics of silicate rocks, application to solute modeling, JjL Jenne, E.A., ed., Chemical Modeling—Speciation, Sorption, Solubility, and Kinetics in Aqueous Systems, Am. Chem. Soc., 1978 (this volume). 

Figure 2,5 (a) An Arrhenius plot of log k versus I/TXK) for the dissolution rates of various silicate rocks and minerals. The data points and curves for rhyolite, basalt glass, and diabase are from Apps (1983), as is the curve labeled silicates, which Apps computed from the results of Wood and Walther (1983). Curves for the S1O2 polymorphs are based on Rimstidt and Barnes (1980). Modified from Langmuir and Mahoney (1985). Reprinted from the National Well Water Assoc. Used by permission, (b) An Arrhenius plot of log k versus 1 /T(K) for the precipitation of quartz and amorphous silica based on Rimstidt and Barnes (1980). Reprinted from Geochim. Cosmochim. Acta, 44, J.D. Rimstidt and H.L. Barnes, The kinetics of silica water reactions, 1683-99, 1980, with permission from Elsevier Science Ltd, The Boulevard. Langford Lane. Kidlington OXS 1GB, U.K. 

Empirical studies of silicate rock or mineral solution rates at low temperatures, under conditions where the water is far from equilibrium with the solid, obey zero-order kinetics (cf. Apps 1983 Paces 1983, Bodek et al. 1988), also called linear kinetics (White and Claassen 1979). The best example of such behavior is the dissolution of S1O2 polymorphs (see Rimstidt and Barnes 1980 and Section 2.7.8). Linear or zero-order kinetics is observed when the area of reacting mineral exposed to a volume of solution or volume of the water-rock system (also called the specific wetted surface, A, in cm or m /m ) may be considered constant with time. The general form of the empirical rate law is... 

The two main compositional rock families discussed above, i.e. silicate and carbonate rocks, give also rise to two different water characteristics. The kinetics of silicate weathering reactions observed in the held are slow, compared to carbonate or sulfate rock weathering (Swoboda-Colberg and Drever, 1993 Drever and Clow, 1995). The solubility of most secondary minerals formed during silicate weathering (such as clay minerals and oxy-hydroxides) is small (Sposito, 1989). Therefore, in temperate regions of Europe, such as the Alps, dissolution of silicate rocks produces weakly... 

In summary, a significant amount of secondary porosity appears to be due to dissolution of feldspar and silicate rock fragments this apparently requires transport of aluminum in solution at relatively high concentrations. Carboxylic acids, which are known to form strong complexes with metal cations (Lind and Hem 1975) and are abundant in oil-field waters, may enhance the solubility and dissolution kinetics of silicates, mobilize aluminum and other cations, and modify the porosity of sandstones undergoing diagenesis. 

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. 

Petrovich, R. 1981. Kinetics of dissolution of mechanically comminuted rock-forming oxides and silicates - I. Deformation and dissolution of quartz under laboratory conditions. Geochim. Cosmochim. Aclu 45 1665-1674. 

Lasaga, A. C. (1983), Kinetics of Silicate Dissolution, in Fourth International Symposium on Water-Rock Interaction, Proceedings, Misasa, Japan, pp. 269-274. Lasaga, A. C. (1984), Chemical Kinetics of Water-Rock Interactions, J. Geophys. Res. 89, (B6), 4009 4025. 

Petrovich, R. (1981). Kinetics of Dissolution of Mechanically Comminuted Rock-Forming Oxides and Silicates —I Deformation and Dissolution of Quartz under Laboratory Conditions . Geochimica et Cosmochimica Acta 45 1665—1674. 

---