Feldspar reaction

The reaction path begins at log Uk/uh = -1.0 and log an arM = 5-2. As the amount of K and Si in the system is incremented by the addition of potassium feldspar, reaction A occurs followed by reactions B, C, and D. The stepwise increase in the extent of reaction, is reflected by changes in the concentration (activity) of the aqueous species and by changes in the number of moles of each mineral. 

The that is supphed by reactions in Eqs. 2 and 3 can react with sihcate minerals such as the sodium-bearing form of feldspar ... 

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

If K" is added to the rock accompanied by the destruction of feldspar, the following reactions occur. 

Albite and K-feldspar are commonly observed to coexist. If the following reaction is in equilibrium,... 

As in the case of K" ", if the eoncentration of alkali elements (Cs" ", Rb, Li ) are controlled by feldspars, it is convenient to take into aecount of the following exchange reaction,... 

If muscovite, K-feldspar and quartz are saturated with geothermal waters, the following reaction can be written ... 

Fig. 2.11. The temperature dependence of cation/proton activity ratios of geothermal well discharges in Japan. The lines in the figure are recalculated temperature dependences of cation/proton ratios in Icelandic geothermal waters. The dashed curve in B represents the reaction 1.5 K-feldspar + H+ = 0.5 K-mica + 3 quartz (or chalcedony) + K+ (Chiba, 1991). Open circle Takigami, open triangle Kakkonda, open square Okuaizu, solid circle Kirishima, solid triangle Sumikawa, solid square Nigoiikawa.

Once the initial equilibrium state of the system is known, the model can trace a reaction path. The reaction path is the course followed by the equilibrium system as it responds to changes in composition and temperature (Fig. 2.1). The measure of reaction progress is the variable , which varies from zero to one from the beginning to end of the path. The simplest way to specify mass transfer in a reaction model (Chapter 13) is to set the mass of a reactant to be added or removed over the course of the path. In other words, the reaction rate is expressed in reactant mass per unit . To model the dissolution of feldspar into a stream water, for example, the modeler would specify a mass of feldspar sufficient to saturate the water. At the point of saturation, the water is in equilibrium with the feldspar and no further reaction will occur. The results of the calculation are the fluid chemistry and masses of precipitated minerals at each point from zero to one, as indexed by . 

Mass transfer can be described in more sophisticated ways. By taking in the previous example to represent time, the rate at which feldspar dissolves and product minerals precipitate can be set using kinetic rate laws, as discussed in Chapter 16. The model calculates the actual rates of mass transfer at each step of the reaction progress from the rate constants, as measured in laboratory experiments, and the fluid s degree of undersaturation or supersaturation. 

In our second example, we calculate the same ratio for the reaction between muscovite and potassium feldspar (KAlSiaOs maximum microcline in the database) in the presence of quartz ... 

We consider as an example the hydrolysis of potassium feldspar (KAlSisOg), the first reaction path traced using a computer (Helgeson et al., 1969). We specify the composition of a hypothetical water... 

Several chemical geothermometers are in widespread use. The silica geothermometer (Fournier and Rowe, 1966) works because the solubilities of the various silica minerals (e.g., quartz and chalcedony, Si02) increase monotonically with temperature. The concentration of dissolved silica, therefore, defines a unique equilibrium temperature for each silica mineral. The Na-K (White, 1970) and Na-K-Ca (Fournier and Truesdell, 1973) geothermometers take advantage of the fact that the equilibrium points of cation exchange reactions among various minerals (principally, the feldspars) vary with temperature. 

Strangely, Reaction 25.2 proceeds backward in the early part of the calculation (Fig. 25.1), producing a small amount of potassium feldspar at the expense of muscovite and quartz. This result, quite difficult to explain from the perspective of mass transfer, is an activity coefficient effect. As seen in Figure 25.2, the activity coefficient for K+ increases rapidly as the fluid is diluted over the initial segment of the reaction path, whereas that for H+ remains nearly constant. (The activity coefficients differ because the a parameter in the Debyc-IIuckcl model is 3 A for K+ and 9 A for H+.) As a result, aK+ increases more quickly than aH+, temporarily driving Reaction 25.2 from right to left. 

In a final application of kinetic reaction modeling, we consider how sodium feldspar (albite, NaAlSisOs) might dissolve into a subsurface fluid at 70 °C. We consider a Na-Ca-Cl fluid initially in equilibrium with kaolinite [Al2Si20s (OF )/ ], quartz, muscovite [KAl3Si30io(OH)2, a proxy for illite], and calcite (CaC03), and in contact with a small amount of albite. Feldspar cannot be in equilibrium with quartz and kaolinite, since the minerals will react to form a mica or a mica-like... 

In the simulation, CO2 in the soil gas reacts with the feldspars, leading to the alkali leaching and separation of silica from alumina observed to result from soil weathering. Near the top of the profile, the reaction produces gibbsite and adds Na+, K+, and Si02(aq) to the migrating pore fluid, according to the reactions,... 

Holdren Jr., G. R., and P. M. Speyer (1987), "Reaction Rate-Surface Area Relationships during the Early Stages of Weathering. II. Data on Eight Additional Feldspars", Geochim. Cosmochim. Acta 51/9, 2311-2318. 

Reaction with K-Bearing Minerals. WD experiments with mixtures of Na-Kinney and sparingly soluble K-minerals were undertaken to simulate natural conditions. When K-feldspar was shaken with Na-Kinney at room temperature without WD for as long as 1 year, no illite layers were found in the experimental product. Nor were illite layers formed when muscovite was shaken with Na-Kinney for... 

Similarly to Mn(IV)- and Fe(III)-oxides, some primary minerals were shown to promote polymerization of hydroquinone (19). Olivines, pyroxenes, and amphiboles accelerated the polymerization reaction to a greater extent than micas and feldspars. Microcline and quartz were ineffective- The effect was greatest for tephroite, a manganese-bearing silicate with the ideal chemical formula M SiO. Fayalite, the corresponding Fe(II) analog (Fe2Si0 ), was effective, but to a lesser extent. 

Mass fluxes of alkali elements transported across the solid-solution interfaces were calculated from measured decreases in solution and from known surface areas and mineral-to-solution weight-to-volume ratios. Relative rates of Cs uptake by feldspar and obsidian in the batch experiments are illustrated in Figure 1. After initial uptake due to surface sorption, little additional Cs is removed from solution in contact with the feldspars. In contrast, parabolic uptake of Cs by obsidian continues throughout the reaction period indicating a lack of sorption equilibrium and the possibility of Cs penetration into the glass surface. 

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