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MINERAL WEATHERING PROCESSES-I

MINERAL WEATHERING PROCESSES-I. Chapter 7. LEARNING OBJECTIVES. Be introduced to the factors influencing the resistance of primary minerals to weathering. Become familiar with the nature of the products of weathering (clays, oxides and hydroxides).

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MINERAL WEATHERING PROCESSES-I

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  1. MINERAL WEATHERING PROCESSES-I Chapter 7

  2. LEARNING OBJECTIVES • Be introduced to the factors influencing the resistance of primary minerals to weathering. • Become familiar with the nature of the products of weathering (clays, oxides and hydroxides). • Understand the principles governing the solubility of quartz. • Understand the principles governing the solubility of Al- and Fe-oxyhydroxides.

  3. WHAT IS WEATHERING? • Weathering refers to the low-temperature reactions between water (containing O2 and CO2) and minerals in soils and rocks. • Weathering is the readjustment of unstable, high-pressure and -temperature minerals to oxidizing, acidic, low-temperature conditions. • There are three main weathering processes: • Congruent dissolution • Incongruent dissolution • Oxidation

  4. GOLDICH’S STABILITY SERIES Weathering rates follow a series roughly the inverse of the order of original crystallization for igneous minerals (Bowen’s Reaction Series).

  5. WHY GOLDICH’S SERIES? • Goldich’s series is in part a reflection of the crystal structures of the silicates and the molar ratio of silica to oxygen. • The higher the Si:O ratio, the more covalent the structure and the fewer other metal cations required for charge balance. • Quartz, the most resistant to weathering, has an Si:O ratio of 0.5, and olivine, the least resistant to weathering, has an Si:O ratio of 0.25.

  6. EXPERIMENTAL RATES OF MINERAL WEATHERING

  7. EVALUATION OF THE SATURATION STATE OF A SILICATE MINERAL 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.

  8. WHAT’S WRONG WITH THIS APPROACH? • Dissolved Al is not routinely determined in natural waters. • When determined, Al is often below detection limit, or so low that accurate analysis is difficult. • Much of the Al present is probably not actually dissolved, but in colloidal form. • The dissolved Al needs to be converted to free Al3+ by calculation.

  9. INCONGRUENT 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.

  10. ACTIVITY DIAGRAMS: THE K2O-Al2O3-SiO2-H2O SYSTEM We 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 aK+/aH+ vs. aH4SiO40. The diagram is divided up into fields where only one of the above phases is stable, separated by straight line boundaries.

  11. Activity 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.

  12. THERMODYNAMIC DATA

  13. A preliminary mapping of the approximate relative locations where we would expect the stability fields of the phases of interest to plot based on their compositions.

  14. GIBBSITE/KAOLINITE BOUNDARY - I The reactions are always written to conserve Al in solid phases: 2Al(OH)3(s) + 2H4SiO40 Al2Si2O5(OH)4 + 5H2O rG° = (-3800) + 5(-237.13) - 2(-1151) - 2(-1316.6) = -50.45 kJ mol-1

  15. GIBBSITE/KAOLINITE BOUNDARY - II But the equilibrium constant is written: So this plots as a vertical boundary, independent of aK+/aH+.

  16. The first boundary is plotted on this diagram. At this point, we do not know where the gibbsite/ kaolinite boundary will terminate, so we draw it along the length of the diagram.

  17. KAOLINITE/PYROPHYLLITE BOUNDARY - I Once again, Al is conserved: Al2Si2O5(OH)4  Al2Si4O10(OH)2

  18. KAOLINITE/PYROPHYLLITE BOUNDARY - I Once again, Al is conserved: Al2Si2O5(OH)4+ 2H4SiO40 Al2Si4O10(OH)2 + 5H2O rG° = (-5275) + 5(-237.13) - (-3800) - 2(-1316.6) = -27.45 kJ mol-1

  19. KAOLINITE/PYROPHYLLITE BOUNDARY - II But the equilibrium constant is written: So this also plots as a vertical boundary, independent of aK+/aH+.

  20. The second vertical boundary is also plotted on the diagram. Like the first, we don’t yet know where it will terminate, so we draw it across the length of the diagram.

  21. GIBBSITE/MUSCOVITE BOUNDARY - I 3Al(OH)3(s)  KAl3Si3O10(OH)2

  22. GIBBSITE/MUSCOVITE BOUNDARY - I 3Al(OH)3(s) + 3H4SiO40 + K+  KAl3Si3O10(OH)2 + H+ + 9H2O rG° = (-5606) + (0) + 9(-237.13) - 3(-1151) - 3(-1316.6) - (-283.27) = -54.10 kJ mol-1

  23. GIBBSITE/MUSCOVITE BOUNDARY - II But the equilibrium constant is written:

  24. GIBBSITE/MUSCOVITE BOUNDARY - II But the equilibrium constant is written: So this plots as a straight line with a slope of -3.

  25. Now the muscovite/ gibbsite boundary has been added.

  26. KAOLINITE/MUSCOVITE BOUNDARY - I 3Al2Si2O5(OH)4 + 2K+  2KAl3Si3O10(OH)2 + 2H+ + 3H2O rG° = 2(-5606) + 2(0) + 3(-237.13) - 3(-3800) - 2(-283.27) = 43.15 kJ mol-1

  27. KAOLINITE/MUSCOVITE BOUNDARY - II This boundary plots as a horizontal line, independent of silicic acid activity.

  28. The horizontal muscovite/ kaolinite boundary has now been added. At this point, we do not know whether the muscovite/kaolinite boundary will first intersect the kaolinite/pyrophyllite boundary, or whether the muscovite/kaolinite boundary will first intersect the muscovite/K-feldspar boundary that we have yet to draw. 3.78

  29. K-FELDSPAR/MUSCOVITE BOUNDARY - I 3KAlSi3O8 +2H+ + 12H2O  KAl3Si3O10(OH)2 + 2K+ + 6H4SiO40 rG° = (-5606) + 2(-283.27) + 6(-1316.6) - 3(-3767) - 2(0) - 12(-237.13) = 74.42 kJ mol-1

  30. K-FELDSPAR/MUSCOVITE BOUNDARY - II So this plots as a straight line with a slope of -3.

  31. When we plot the K-feldspar/ muscovite boundary quantitatively, we see that it intersects the horizontal muscovite/kaolinite boundary before the latter intersects the kaolinite/pyrophyllite boundary.

  32. K-FELDSPAR/KAOLINITE BOUNDARY - I 2KAlSi3O8 +2H+ + 9H2O  Al2Si2O5(OH)4 + 2K+ + 4H4SiO40 rG° = (-3800) + 2(-283.27) + 4(-1316.6) - 2(-3767) - 2(0) - 9(-237.13) = 35.23 kJ mol-1

  33. K-FELDSPAR/KAOLINITE BOUNDARY - II So this plots as a straight line with a slope of -2.

  34. Almost done!

  35. K-FELDSPAR/PYROPHYLLITE BOUNDARY - I 2KAlSi3O8  Al2Si4O10(OH)2

  36. K-FELDSPAR/PYROPHYLLITE BOUNDARY - I 2KAlSi3O8 +2H+ + 4H2O  Al2Si4O10(OH)2 + 2K+ + 2H4SiO40 rG° = (-5275) + 2(-283.27) + 2(-1316.6) - 2(-3767) - 2(0) - 4(-237.13) = 7.78 kJ mol-1

  37. K-FELDSPAR/PYROPHYLLITE BOUNDARY - II

  38. K-FELDSPAR/PYROPHYLLITE BOUNDARY - II So this plots as a straight line with a slope of -1.

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