Chemical Weathering – Importance in Geomorphology
Dissolution of minerals is a form of erosion with the interesting quality that it can
occur in the subsurface. Despite this difference, chemical weathering should be
considered as a significant factor in the total denudation of a basin.
Chemical weathering alters the physical properties of rocks - decreasing or
increasing material strength.
Chemical weathering of silicate minerals is part of the global (geologic) carbon cycle,
which factors into the CO2 concentration within the atmosphere.
Solutes provide a snapshot of present chemical processes, whereas soil and rock
chemistry provide an integrated history of past chemical processes.
Chemical Equilibrium
Le Châtelier principle: If a system at equilibrium is subjected to a
change of pressure, temperature, or the number of moles of a
component, there will be a tendency for a net reaction in the direction
that reduces the effect of this change.
Rocks, when produced, are usually out of equilibrium with Earth surface
conditions, so reactions occur that strive to attain equilibrium.
Gibbs Free Energy - a measure of work that a system can do at
constant Temperature (T) and Pressure(P), related to the heat content,
or Enthalpy (H), and degree of disorder, or Entropy (S), of a system.
Spontaneous processes release Gibbs Free Energy and therefore have
negative ΔG values.
Chemical Potential – change in Gibbs free energy per change in
number of particles of a component species in a system.
Chemical potential can also be written in terms of activity
(~concentration)
Solubility and Saturation
Solubility Product
Solubility
Saturation Index
Rivers, Continental Crust, and Common Weathering Reactions
The top seven solutes in rivers. Note log scale.
Congruent Dissolution
Simple process of solution of a solid
mineral into dissolved products.
An example of a congruent dissolution
reaction is the dissolution of calcite:
Incongruent Dissolution
Feldspars (abundant) and other aluminosilicates
dissolve incongruently, meaning they release
solutes and form one or more new minerals:
Incongruent Dissolution
Weathering Steps for Major Silicate Groups
Redox Reactions
Oxidation and Reduction reactions involve
transfer of electrons between atoms. The loss of
an electron is known as oxidation, whereas the
gaining of an electron is known as reduction.
Oxidation of Iron and Sulfur bearing compounds
are most common on the Earth's surface: Pyrite
Oxidation Reaction.
Chemical Kinetics
Temperature Dependent Reaction Rates
Svante Arrhenius
Temperature and Reaction Rates in a Soil Profile
pH Dependent Reaction Rates
Chemical Affinity
Mineral Surface Age
Kinetic rate constants, as determined in laboratory studies, tend to be
higher by an order of magnitude than rate constants determined in field
studies. Why?
Hypothesis #1: The mineral surface area in contact with moving soil
solutions may be lower than in the laboratory reactors, which may account
for the discrepancy.
Hypothesis #2: Mineral surfaces alter as they age in ways that may affect
the dissolution rate.
A long-term (6-year) lab experiment investigated this and found a power
law relationship with a negative exponent that indicated a 10-fold decrease
in dissolution rate in 50 years, for K-feldspar, hornblende, and biotite.
Mineral surfaces tend to roughen with age - they develop pits, steps, and
defects which will tend to increase the BET-determined surface area over
time. So, in long-term experiments, solute losses from reactors must be
normalized to increasing surface areas, resulting in a decline of dissolution
rate.