THE GEOCHEMISTRY OF NATURAL WATERS

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THE GEOCHEMISTRY OF NATURAL WATERS

• MINERAL WEATHERING AND MINERAL SURFACE PROCESSES
  - I
• CHAPTER 4 - Kehew (2001)
• Silica solubility

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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.

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SILICA SOLUBILITY - I

• In the absence of organic ligands or fluoride,
  quartz solubility is relatively low in natural
  waters.
• Below pH 9, the dissolution reaction is
• SiO2(quartz) 2H2O(l) ? H4SiO40
• for which the equilibrium constant at 25C is
• At pH lt 9, quartz solubility is independent of
  pH.
• Quartz is frequently supersaturated in natural
  waters because quartz precipitation kinetics are
  slow.

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SILICA SOLUBILITY - II

• Thus, quartz saturation does not usually control
  the concentration of silica in low-temperature
  natural waters. Amorphous silica can control
  dissolved Si
• SiO2(am) 2H2O(l) ? H4SiO40
• for which the equilibrium constant at 25C is
• Quartz is formed diagenetically through the
  following sequence of reactions opal-A
  (siliceous biogenic ooze) ? opal-A (nonbiogenic
  amorphous silica) ? opal-CT ? chalcedony ?
  microcrystalline quartz

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SILICA SOLUBILITY - III

• At pH gt 9, H4SiO40 dissociates according to
• H4SiO40 ? H3SiO4- H
• H3SiO4- ? H2SiO42- H
• The total solubility of quartz (or amorphous
  silica) is

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SILICA SOLUBILITY - IV

• The equations for the dissociation constants of
  silicic acid can be rearranged (assuming a M )
  to get
• We can now write

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Activities of dissolved silica species in
equilibrium with quartz and amorphous silica at
25C. Note that silica solubility is
pH-independent at pH lt 9, but increases
dramatically with increasing pH at pH gt9.
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SILICA SOLUBILITY - V

• An alternate way to understand quartz solubility
  is to start with SiO2(quartz) 2H2O(l) ?
  H4SiO40
• Now adding the two reactions
• SiO2(quartz) 2H2O(l) ? H4SiO40 Kqtz
• H4SiO40 ? H3SiO4- H K1
• SiO2(quartz) 2H2O(l) ? H3SiO4- H K

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SILICA SOLUBILITY - VI

• Taking the log of both sides and rearranging we
  get
• Finally adding the three reactions
• SiO2(quartz) 2H2O(l) ? H4SiO40 Kqtz
• H4SiO40 ? H3SiO4- H K1
• H3SiO4- ? H2SiO42- H K2
• SiO2(quartz) 2H2O(l) ? H2SiO42- 2H K

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SILICA SOLUBILITY - VII

• SUMMARY
• Silica solubility is relatively low and
  independent of pH at pH lt 9 where H4SiO40 is the
  dominant species.
• Silica solubility increases with increasing pH
  above 9, where H3SiO4- and H2SiO42- are
  dominant.
• Fluoride, and possibly organic compounds, may
  increase the solubility of silica.
• Saturation with quartz does not control silica
  concentrations in low-temperature natural waters
  saturation with amorphous silica may.