THE GEOCHEMISTRY OF NATURAL WATERS

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THE GEOCHEMISTRY OF NATURAL WATERS

• MINERAL WEATHERING AND MINERAL SURFACE PROCESSES
  - II
• INCONGRUENT DISSOLUTION AND ACTIVITY DIAGRAMS
• CHAPTER 4 - Kehew (2001)
• Activity diagrams - 1

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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.

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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.

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WHATS 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.

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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.

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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 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.

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Activity diagram showing the stability
relationships among some minerals in the system
K2O-Al2O3-SiO2-H2O at 25C. The dashed lines
represent saturation with respect to quartz and
amorphous silica.
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THERMODYNAMIC DATA
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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.
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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

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GIBBSITE/KAOLINITE BOUNDARY - II

• But the equilibrium constant is written
• So this plots as a vertical boundary, independent
  of aK/aH.

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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.
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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

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KAOLINITE/PYROPHYLLITE BOUNDARY - II

• But the equilibrium constant is written
• So this also plots as a vertical boundary,
  independent of aK/aH.

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The second vertical boundary is also plotted on
the diagram. Like the first, we dont yet know
where it will terminate, so we draw it across the
length of the diagram.
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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

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GIBBSITE/MUSCOVITE BOUNDARY - II

• But the equilibrium constant is written
• So this plots as a straight line with a slope of
  -3.

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Now the muscovite/ gibbsite boundary has been
added.
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Explanation of the choice of line segments when
phase boundaries intersect. See notes for details.
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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

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KAOLINITE/MUSCOVITE BOUNDARY - II

• This boundary plots as a horizontal line,
  independent of silicic acid activity.

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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
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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

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K-FELDSPAR/MUSCOVITE BOUNDARY - II

• So this plots as a straight line with a slope of
  -3.

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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.
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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

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K-FELDSPAR/KAOLINITE BOUNDARY - II

• So this plots as a straight line with a slope of
  -2.

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Almost done!
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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

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K-FELDSPAR/PYROPHYLLITE BOUNDARY - II

• So this plots as a straight line with a slope of
  -1.

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Activity diagram showing the stability
relationships among some minerals in the system
K2O-Al2O3-SiO2-H2O at 25C. The dashed lines
represent saturation with respect to quartz and
amorphous silica.
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SILICA SATURATION

• Lines representing conditions of silica
  saturation can also be added to the diagram. For
  quartz, we start with the mass-action expression
  for
• SiO2(quartz) 2H2O(l) ? H4SiO40
• or
• Thus, the quartz saturation line is a vertical
  line. Similarly for amorphous silica we have