5.2 Chemical Weathering

Physical Geology: Weathering and Soil

5.2 Chemical Weathering

• Definition of Chemical Weathering: Chemical weathering refers to the process through which minerals undergo chemical changes upon exposure to surface conditions, resulting in destabilization and alteration of the minerals.

• Mineral Reactivity:

  • Quartz: Roughly unaffected by chemical weathering.

  • Feldspar: Easily altered by chemical processes.

• Environmental Influence:

  • Chemical weathering is most pronounced in warm, wet climates and minimal in cold, dry climates.

• Key Surface Conditions Affecting Chemical Weathering:

  • Presence of water (both in the air and on the ground).

  • Abundance of oxygen.

  • Presence of carbon dioxide - reacts with water to form weak carbonic acid:

  • Reaction formula: $H<em>2O + CO</em>2 <br>ightarrow H<em>2CO</em>3$

  • Dissociation of carbonic acid: $H<em>2CO</em>3 <br>ightarrow H^+ + HCO_3^-$

  • In soil, the concentration of CO2 can increase the acidity of the percolating water significantly.

Types of Chemical Weathering

• Chemical weathering can be subdivided into two primary types:

  1. Alteration to Other Minerals

  • Hydrolysis of feldspar to form clay minerals.

  • Example:

    • Reaction: $CaAl<em>2Si</em>2O<em>8 + H</em>2CO<em>3 + rac{1}{2} O</em>2 <br>ightarrow Al<em>2Si</em>2O<em>5(OH)</em>4 + Ca^{2+} + CO_3^{2-}$

    • Involved Minerals: Calcium plagioclase feldspar altering into kaolinite, producing dissolved calcium and carbonate ions.

  1. Complete Dissolution

  • Minerals can dissolve entirely, their components entering solution.

  • Example:

    • Calcite (CaCO3) solubility:

    • Reaction: $ext{CaCO}<em>3 + H^+ + HCO</em>3^- <br>ightarrow Ca^{2+} + 2HCO_3^-$

    • Calcite is the primary component of limestone, which dissolves under surface conditions.

Hydrolysis Process

• Illustration: Diagrammatic representation of unweathered vs weathered granitic surfaces (Figure 5.9) showing the transformation of fresh feldspar to clay-like kaolinite.

  • Transformation of Minerals: Other silicate minerals can experience hydrolysis, leading to various clay minerals:

  • Pyroxene → Chlorite or Smectite.

  • Olivine → Serpentine.

Oxidation Process

• Significance of Oxidation: Important mechanism of chemical weathering especially concerning ferromagnesian silicates.

• Oxidation of Iron:

  • Starting with iron dissolution, followed by oxidation:

  • Example with Olivine:

  • Reaction: $Fe<em>2SiO</em>4 + 4H<em>2CO</em>3 <br>ightarrow 2Fe^{2+} + 4HCO<em>3^- + H</em>4SiO_4$

  • The iron ions then oxidize to form Hematite:

  • Reaction: $2Fe^{2+} + 4HCO<em>3^- + rac{1}{2} O</em>2 + 2H<em>2O ightarrow Fe</em>2O<em>3 + 4HCO</em>3^-$

  • Conclusion: Iron oxidation is relevant to various silicates such as pyroxene, amphibole, and biotite, forming several iron oxide minerals.

Special Case: Acid Rock Drainage (ARD)

• Detailed Chemistry of Pyrite Oxidation:

  • Reactions involving pyrite and its transformation into sulfuric acid:

  • Reaction:

    • $2FeS<em>2 + 7O</em>2 + 2H<em>2O ightarrow 2Fe^{2+} + H</em>2SO_4 + 2H^+$

  • Consequences of ARD:

  • Results in acid runoff leading to low pH (below 4), affecting aquatic life and causing toxicity (e.g., dissolved copper being toxic to salmon in rivers impacted by mine runoff).

  • Remediation Efforts: Current activities are ongoing to mitigate the impacts of ARD in mining sites, such as the Mt. Washington Mine, B.C.

Summary of Weathering Reactions

• Chemical weathering processes lead to the creation of softer and weaker rocks, more vulnerable to mechanical weathering.

• Distinct processes highlighted:

  • Hydrolysis of feldspars to clay.

  • Dissolution of calcite in weak acids.

Exercises

Exercise 5.2: Chemical Weathering

• Task: Identify which chemical weathering process is primarily responsible for each change:
  | Chemical Change | Process |
  |-------------------------------------|------------------|
  | Pyrite to Hematite | Oxidation |
  | Calcite to Calcium and Bicarbonate Ions | Dissolution |
  | Feldspar to Clay | Hydrolysis |
  | Olivine to Serpentine | Hydrolysis |
  | Pyroxene to Iron Oxide | Oxidation |