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Mineral dissolution/precipitation To determine whether or not a water is saturated with an aluminosilicate such as K-feldspar, we could write a dissolution reaction such as: KAlSi 3 O 8 + 4H + + 4H 2 O K + + Al 3+ + 3H 4 SiO 4 0 We could then determine the equilibrium constant: from Gibbs free energies of formation. The IAP could then be determined from a water analysis, and the saturation index calculated.
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INCONGRUENT DISSOLUTION Aluminosilicate minerals usually dissolve incongruently, e.g., 2KAlSi 3 O 8 + 2H + + 9H 2 O Al 2 Si 2 O 5 (OH) 4 + 2K + + 4H 4 SiO 4 0 As a result of these factors, relations among solutions and aluminosilicate minerals are often depicted graphically on a type of mineral stability diagram called an activity diagram.
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ACTIVITY DIAGRAMS: THE K 2 O-Al 2 O 3 -SiO 2 -H 2 O SYSTEM We will now calculate an activity diagram for the following phases: gibbsite {Al(OH) 3 }, kaolinite {Al 2 Si 2 O 5 (OH) 4 }, pyrophyllite {Al 2 Si 4 O 10 (OH) 2 }, muscovite {KAl 3 Si 3 O 10 (OH) 2 }, and K-feldspar {KAlSi 3 O 8 }. The axes will be a K + /a H + vs. a H 4 SiO 4 0. The diagram is divided up into fields where only one of the above phases is stable, separated by straight line boundaries.
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Activity diagram showing the stability relationships among some minerals in the system K 2 O-Al 2 O 3 -SiO 2 -H 2 O at 25°C. The dashed lines represent saturation with respect to quartz and amorphous silica.
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Seeing this, what are the reactions these lines represent?
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Lines Fe(OH) 3(ppd) + H + = H 2 O + Fe(OH) 2 + Log K = -0.7799 (From G R ) K 1 = [H 2 O][Fe(OH) 2 + ] / [Fe(OH) 3(ppd) ][H + ] K 1 = [Fe(OH) 2 + ] / [H + ] But need to plot Fe 3+ vs. H + … Fe 3+ + 2 H 2 O = Fe(OH) 2 + + 2 H+ K 2 = [Fe(OH) 2 + ][H + ] 2 / [Fe 3+ ] = log K 2 =- 5.6689 [Fe(OH) 2 + ] = K 2 [Fe 3+ ] / [H + ] 2 substitute K 1 = K 2 [Fe 3+ ] / [H + ] 3 [Fe 3+ ] = K 1 [H + ] 3 / K 2 log[Fe 3+ ] = 3log[H + ] + (logK 1 - logK 2 )
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