Feldspars stoichiometry

One additional aspect of laboratory dissolution experiments is the question of stoichiometric vs. non-stoichiometric dissolution. Many of the studies cited above analyzed only a few of the elements released by feldspar that is, although alkalis, alkaline earths, silica, and aluminum may be released during dissolution of feldspar, few studies report analyses for all elements. Often, only silica was analyzed. Where multiple elements are analyzed, they are often released to the solution in proportions which do not correspond to the bulk stoichiometry of the feldspar ( ] ). 

Nonlinear Precipitation of Secondary Minerals from Solution. Most of the studies on dissolution of feldspars, pyroxenes, and amphiboles have employed batch techniques. In these systems the concentration of reaction products increases during an experiment. This can cause formation of secondary aluminosilicate precipitates and affect the stoichiometry of the reaction. A buildup of reaction products alters the ion activity product (IAP) of the solution vis-a-vis the parent material (Holdren and Speyer, 1986). It is not clear how secondary precipitates affect dissolution rates however, they should depress the rate (Aagaard and Helgeson, 1982) and could cause parabolic kinetics. Holdren and Speyer (1986) used a stirred-flow technique to prevent buildup of reaction products. 

Holdren, G. R., Jr., and Speyer, P. M. (1986). Stoichiometry of alkali feldspar dissolution at room temperature and various pH values. In Rates of Chemical Weathering of Rocks... 

Because it is likely that most of the Na+ and K+ in river waters derived from silicates comes from weathering reactions like those involving feldspars, and most of the Ca2+ and Mg2+ stems from reactions like those above for anorthite and chlorite, we will use the stoichiometry of these reactions to complete the balance. First, we remove the remaining Ca2+ and Mg2+ according to reactions (9.21) and (9.22) ... 

With a known mineral, as determined by electron diffraction or other technique (such as X-ray diffraction), determination of the stoichiometry and structural formula can be a suitable test for analytical precision of thin-film elemental analyses. This simple test follows the practice commonly employed for electron microprobe data in which the accuracy (and completeness) of an analysis is judged by the departure from stoichiometry calculated for a given mineral. Thus, thin-film analyses of olivines, pyroxenes, garnets, feldspars and many other common rock-forming minerals can be examined for internal consistency via a calculation of structural formulae. 

Clayton J. L. (1988) Some observations of the stoichiometry of feldspar hydrolysis in granitic soils. J. Environ. Qual. 17, 153-157. 

The prediction by our speciation model that in multiple oxides (as in quartz) Si sites are not susceptible to adsorption by H+ is confirmed by the results of surface titrations of albite, labradorite, and anorthite (Fig. 6). One can see that the net adsorption of H + at the feldspar solution interface increases markedly with the relative number of Al sites to Si sites, which is reflected by the stoichiometry of Al and Si in the different feldspars. These results help clarify the typical features of the dependence on pH of silicate dissolution (Table 2) ... 

However, this does not tell the whole story. Also increasing during dissolution of the feldspar are all the other species produced (AP", Al(OH), and HaSiO ), though at lower concentrations. These are shown in Figure 19.6, rising from the lower left comer with a slope of 1.0. Initially, the concentrations of all these species are so small that the concentrations of and OH remain constant at 10 . The species all increase from zero (logm = —00) maintaining the overall 1 1 3 stoichiometry, and would stop when the solution became saturated with K-feldspar. However, before that happens, the solution may become saturated with other minerals, which will precipitate as K-feldspar continues to dissolve. This is the essential element of the reaction path calculations. 

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