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.