Silicate rocks, dissolution

H2CO3 in recharge water was made from six Rainier Mesa water samples for which reliable pH and HCO3 values were available. In simple silicate rock dissolution, all carbonate species result from reaction of H2CO3 with the rock. Thus, HCO3 + C03 + H2CO3 at any point in the flow path is assumed to be a constant. Calculations of concentrations of these species in the six samples previously mentioned allowed calculation of H2CQ3 initially available for reaction. These data are presented in Table I. The arithmetic mean was used in subsequent calculations. 

Priyono, J. and Gilkes, R.J. 2004. Dissolution of milled-silicate rock fertilisers in the soil. Australian Journal of Soil Research 42 441-448. 

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). 

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... 

In the dissolved load, the preferential dissolution of the different types of lithology or mineral can cause large variations in trace-element abundances in rivers. A classical tracer of rock dissolution is strontium. The isotopic ratio of strontium in river waters is, to a first approximation, explained by mixing between different sources. Rain, evaporite dissolution, and carbonate and silicate weathering are the predominant sources of strontium in rivers (see Chapter 5.12). Examples of the use of Ca/Sr, Na/Sr, and Sr isotopic ratios to quantify the proportion of strontium derived from carbonate weathering in large basins can be found in Negrel et al. (1993)... 

The dynamics of the process of dissolution and sorption of biogenic silica from siliceous rocks, diatomite and opoka, has been studied. 

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... 

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. 

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). 

Dissolution of CaCOs is a congruent reaction the entire mineral is weathered and results completely in soluble products. The above reaction is driven to the right by an increase of CO2 partial pressure and by the removal of the Ca and/or bicarbonate. Any impurities present in the calcareous rock, such as silicates, oxides, organic compounds, and others, are left as residue. As the calcium and bicarbonate leach... 

There are no unequivocal weathering reactions for the silicate minerals. Depending on the nature of parent rocks and hydraulic regimes, various secondary minerals like gibbsite, kaolinite, smectites, and illites are formed as reaction products. Some important dissolution processes of silicates are given, for example, by the following reactions ... 

As the rock cycle continues, the calcium silicate minerals are eventually uplifted onto land where they imdergo chemical weathering. This reaction involves acid hydrolysis driven by carbonic acid. The latter is derived from the dissolution of the magmatic CO2 in rainwater ... 

The concentrations of dissolved species in natnral waters depend ultimately on the dissolution of basic rocks-carbonates, silicates and aluminosilicates-induced by the action of weak acids in the water derived from dissolved gases-e.g. H2CO3 derived from CO2. Anions produced in acid-base reactions balance cations produced in dissolution reactions. The charge balance is ... 

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