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THE GEOCHEMISTRY OF NATURAL WATERS

2. LEARNING OBJECTIVES. Learn about incongruent dissolution of silicates.Learn to calculate and use activity diagrams.Learn about the use of mass-balance calculations to infer weathering reactions.Apply the knowledge gained to rationalize compositions of natural waters in igneous and metamorphic

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THE GEOCHEMISTRY OF NATURAL WATERS

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    1. 1 THE GEOCHEMISTRY OF NATURAL WATERS MINERAL WEATHERING AND MINERAL SURFACE PROCESSES - II INCONGRUENT DISSOLUTION AND ACTIVITY DIAGRAMS CHAPTER 4 - Kehew (2001) Activity diagrams - 2

    2. 2 LEARNING OBJECTIVES Learn about incongruent dissolution of silicates. Learn to calculate and use activity diagrams. Learn about the use of mass-balance calculations to infer weathering reactions. Apply the knowledge gained to rationalize compositions of natural waters in igneous and metamorphic rocks. Explore the implications of acid rain in igneous and metamorphic terrains. In Lectures 4 and 5, we dealt with congruent dissolution of oxides and carbonates, and incongruent dissolution of carbonates. In this lecture we concentrate on methods for dealing with incongruent dissolution of silicates. This will involve learning to construct and use a type of phase diagram called an activity diagram. We will also learn how to do mass-balance calculations to infer the weathering reaction that control the composition of waters during weathering of silicate minerals. In the process, we will apply what we learn to understanding how incongruent dissolution affects the compositions of natural waters in igneous and metamorphic rocks, and we will also briefly discuss the effect of acid rain on natural waters occurring in such rocks. In Lectures 4 and 5, we dealt with congruent dissolution of oxides and carbonates, and incongruent dissolution of carbonates. In this lecture we concentrate on methods for dealing with incongruent dissolution of silicates. This will involve learning to construct and use a type of phase diagram called an activity diagram. We will also learn how to do mass-balance calculations to infer the weathering reaction that control the composition of waters during weathering of silicate minerals. In the process, we will apply what we learn to understanding how incongruent dissolution affects the compositions of natural waters in igneous and metamorphic rocks, and we will also briefly discuss the effect of acid rain on natural waters occurring in such rocks.

    3. 3 ANOTHER EXAMPLE: THE Na2O-Al2O3-SiO2-H2O SYSTEM Now we will now calculate an activity diagram for the following phases: gibbsite {Al(OH)3}, kaolinite {Al2Si2O5(OH)4}, pyrophyllite {Al2Si4O10(OH)2}, Na-beidellite {Na1/3Al7/3Si11/3O10(OH)2 }, analcite {NaAlSi2O6·H2O}, and albite {NaAlSi3O8}. The axes will be a Na+/a H+ vs. a H4SiO40. Thermodynamic data are given in the following table.

    4. 4 THERMODYNAMIC DATA The thermodynamic data given here come from Drever, J.I. (1997) The Geochemistry of Natural Waters, 3rd. ed., Prentice-Hall (Table 10-2, p. 208). The thermodynamic data given here come from Drever, J.I. (1997) The Geochemistry of Natural Waters, 3rd. ed., Prentice-Hall (Table 10-2, p. 208).

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