Pedosphere - Weathering and Dissolution of Minerals

Weathering and Dissolution of Minerals

The process of soil formation is dominated by chemical weathering of silicate minerals, aided by acidic products of pioneering plants and organisms as well as carbonic acid inputs from the atmosphere. Carbonic acid is produced in the atmosphere and soil layers through the carbonation reaction.

H₂O + CO₂ → H++ HCO₃- → H₂CO₃

This is the dominant form of chemical weathering and aides in the breakdown of carbonate minerals like calcite and dolomite and silicate minerals like feldspar. The breakdown of the Na-feldspar, albite, by carbonic acid to form kaolinite clay is as follows:

2NaAlSi₃O₈ + 2H₂CO₃ + 9H₂O → 2Na+ + 2HCO₃- + 4H₄SiO₄ + Al₂Si₂O₅(OH)₄

Evidence of this reaction in the field would be elevated levels of bicarbonate (HCO₃-), sodium and silica ions in the water runoff. The breakdown of carbonate minerals:

CaCO₃ + H₂CO₃ → Ca2+ + 2HCO₃- or CaCO₃ → Ca2+ + CO₃2-

The further dissolution of carbonic acid (H₂CO₃) and bicarbonate (HCO₃) produces CO₂ gas. Oxidization is also a major contributor to the breakdown of many silicate minerals and formation of secondary minerals (diagenesis) in the early soil profile. Oxidation of Olivine (FeMgSiO₂) releases Fe, Mg and Si ions. The Mg is soluble in water and is carried in the runoff but the Fe often reacts with oxygen to precipitate Fe₂O₃ (hematite), the oxidized state of iron oxide. Sulfur, a byproduct of decaying organic material will also react to Fe to form pyrite (FeS₂) but often in reducing environments. Pyrite dissolution leads to high pH levels due to elevated H+ ions and further precipitation of Fe₂O₃ ultimately changing the redox conditions of the environment.