Chemical Weathering

1
Chemical Weathering

• Most minerals in rocks exposed at surface not
  thermo- dynamically stable all undersaturated in
  rain and soil waters
• Important chemical reactions include hydrolysis,
  dissolution, and oxidation, i.e., minerals are
  attacked by H2O, H, and O2.
• Minerals dissolve congruently if all components
  soluble (e.g., calcite. forsterite)
• Mg2SiO4 4 H 2 Mg2 4 H4SiO4
• Minerals with insoluble components (e.g., Al)
  dissolve incongruently through hydrolysis to form
  clays
• CaAl2Si2O8 H2O 2 H Ca2 Al2Si2O5(OH)4
  (kaolinite)
• Fluids contain dissolved products of weathering
  soluble cations (alkalis, alkaline earths),
  anions (HCO3-), silica

2
What are activity diagrams?

• Phase diagrams that plot stability fields for
  minerals in terms of the chemical composition of
  aqueous solutions in equilibrium with them.
• Usually used to characterize incongruent
  dissolution of aluminosilicate minerals in
  igneous and metamorphic rocks to form clays
• Usually y log ratio of activities of cation
  produced to H consumed, x log activity H4SiO4
• As phase diagrams they obey the phase rule and
  Schreinemakers rules.

3
What questions can activity diagrams
answer?(after Faure, 1991, pg. 256)

• What environmental conditions are required to
  allow a particular mineral to form?
• What minerals are stable in a given geochemical
  environment?
• What ions or molecules are consumed or produced
  when an unstable mineral reacts in a given
  geochemical environment?
• How does the water evolve chemically when
  reactions occur in a closed system with a small
  water/rock ratio?
• What minerals were likely to be in equilibrium
  with fluid of a given composition?

4
Rules for writing weathering reactions and
constructing activity diagrams

• Assuming no changes in valence states, start by
• Al is conserved (insoluble, immobile),
  transferred into solid product of reaction (clay)
• Excess Si forms silicic acid, which does not
  dissociate unless the environment is very basic
  (pH gt 9)
• Alkalis and alkaline earths form ions in solution
• Balance equation with respect to Al, Si, and
  soluble cations, then balance oxygen by adding
  H2O
• Balance hydrogen by adding H (usually reactant,
  from environment)
• Make sure equation balanced in terms of mass and
  charge