Silicate rocks, dissolution rates

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 ... 

The oxidation of sulfides preferentially dissolves carbonates, rather than silicates, because the rate of carbonate dissolution is orders of magnitude faster. For example, Haut Glacier d Arolla has a bedrock which is composed of metamorphic silicate rocks. Carbonates and sulfides are present in trace quantities in bedrock samples (0.00-0.58% and <0.005-0.71%, respectively). There are also occasional carbonate veins present in the schistose granite. Despite the bedrock being... 

Note that this relationship is similar to the van t Hoff equation. If the Arrhenius expression is obeyed, then a plot of log k versus l/T is a straight line, with a slope of-EJ2303 R. Such plots are shown in Figs. 2.5(a) and (b), which describe the effect of increasing temperature on the rates of dissolution of some silicate rocks and minerals. 

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... 

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. 

If lake water reacts with lake bottom carbonate (limestone, dolomite), pH increases rapidly because of fast dissolution rates of carbonates (Sect. 6.3.1.1). In contrast dissolutions of silicates (granite, metamorphic rocks etc.) do not proceed considerably because of slow dissolution rate of silicates, and pH of lake water does not change quickly. Figure 6.15 shows that pH of lake water depends on lithology of lake bottom. Examples of ion exchange reactions controlling pH are given by... 

A combination of uncertainty in thermodynamic data accuracy at elevated temperatures and overall lack of completeness with respect to organic compound class and interaction types (complex formation, adsorption, solid precipitate) poses some real limitations in the use of chemical and reaction path models. Thus, a cautionary note is that to compare results of different numerical studies it is necessary to know whether similar thermodynamic data bases were used. As we will show below, apparently discrepant conclusions drawn from modeling studies probably result from the use of more or less complete thermodynamic data. Of concern in the investigation of organic acid interactions in silicate rocks is the complete lack of any equilibrium measurements of silica-organic acid anion interactions, although dissolution rate measurements have been made (Bennett et al. 1988 Bennett 1991). 

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