Mass Action & Mass BalanceMineral dissolution/precipitationINCONGRUENT DISSOLUTIONACTIVITY DIAGRAMS: THE K2O-Al2O3-SiO2-H2O SYSTEMSlide 5Slide 6Redox GeochemistryWHY?Mass Action & Mass Balance•mCa2+=mCa2++MCaCl+ + mCaCl20 + CaCL3- + CaHCO3+ + CaCO30 + CaF+ + CaSO40 + CaHSO4+ + CaOH+ +…•Final equation to solve the problem sees the mass action for each complex substituted into the mass balance equationlcnciHLCHCL][][][][nxLmCamCa22Mineral 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:•KAlSi3O8 + 4H+ + 4H2O K+ + Al3+ + 3H4SiO40•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.43443HSiOHAlKaaaaKINCONGRUENT DISSOLUTION•Aluminosilicate minerals usually dissolve incongruently, e.g., 2KAlSi3O8 + 2H+ + 9H2O Al2Si2O5(OH)4 + 2K+ + 4H4SiO40•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.ACTIVITY DIAGRAMS: THE K2O-Al2O3-SiO2-H2O SYSTEMWe will now calculate an activity diagram for the following phases: gibbsite {Al(OH)3}, kaolinite {Al2Si2O5(OH)4}, pyrophyllite {Al2Si4O10(OH)2}, muscovite {KAl3Si3O10(OH)2}, and K-feldspar {KAlSi3O8}.The axes will be a K+/a H+ vs. a H4SiO40. The diagram is divided up into fields where only one of the above phases is stable, separated by straight line boundaries.log aH4SiO40-6 -5 -4 -3 -2 -1log (aK+/aH+)01234567KaoliniteGibbsiteMuscoviteK-feldsparPyrophylliteQuartzAmorphoussilicaActivity diagram showing the stability relationships among some minerals in the system K2O-Al2O3-SiO2-H2O at 25°C. The dashed lines represent saturation with respect to quartz and amorphous silica.2 3 4 5 6 7 8 9 10 11 12-10-8-6-4-202pHlog a Al+++Al+++Al(OH)2+Al(OH)4-AlOH++Gibbsite25oCGreg Mon Nov 01 2004Diagram Al+++, T = 25 C, P = 1.013 bars, a [H2O] = 1Seeing this, what are the reactions these lines represent?Redox GeochemistryWHY?•Redox gradients drive life processes!–The transfer of electrons between oxidants and reactants is harnessed as the battery, the source of metabolic energy for organisms•Metal mobility redox state of metals and ligands that may complex them is the critical factor in the solubility of many metals–Contaminant transport–Ore deposit
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