Physical Geography 102
Chemical Weathering

Chemical Weathering

The process of weathering by which chemical reactions transform rocks and minerals into new chemical combinations that are stable under prevailing conditions at or near the Earth's surface
5 processes: Hydrolysis, Hydration, Oxidation, Solution/Dissolution, Ion Exchange


Hydrolysis is a decomposition reaction involving water and a silicate mineral
H+ ions or OH- ions are taken in by a mineral, altering the mineral's chemistry
Example: Orthoclase + H+ reacts to form Muscovite + Quartz + K+
Not only does the chemistry change - the crystal structure changes - this can weaken the rock structure


Hydration is also a reaction involving water and a silicate mineral
H2O is taken in by certain minerals
Hydration will also add water to a clay mineral which will cause the mineral to expand
Grus is formed when this reaction takes place in granite
Hydration reactions also produce clay minerals: Orthoclase + H2O + HCl reacts to form Illite + Quartz + K+ + Cl-


Oxidation occurs when an element combines with an oxygen ion, O-2
Combustion is an oxidation reaction
Fe+2 combines with O-2 to form Fe2O3 hematite (rust)
Many rocks have a reddish color due to the presence of hematite

Reduction Reactions

The opposite of oxidation is reduction
In this case a mineral looses an oxygen
Fe2O3 + C + heat reacts to form 2Fe + CO2 + O-2
This reaction is used in blast furnaces to produce iron from iron ore
Both oxidizing and reducing reactions alter the original mineral, causing chemical and crystalline changes

Solution - Dissolution

This process involves the transformation of minerals containing Ca, Mg, K, Na, and Fe into carbonate minerals (calcite) by reactions with CO2 and water
CO2 + H2O reacts to form H2CO3     Carbonic Acid
Both water and carbon dioxide are readily available in the atmosphere, surface water, and groundwater
Carbonic acid is a weak acid but over long periods of time can dissolve large quantities of calcite/limestone/marble
Calcite is dissolved by the acid, then is carried in solution
Eventually it is deposited in oceans as new limestone beds, in fractures in the rock, in caves as stalactites and stalagmites (travertine)

Ion Exchange

Ion exchange is the substitution of ions in solution for ions in minerals
Water contains ions of H+, OH-, Ca+, Na+, K+, Cl-
Most minerals have the capability to exchange ions - most common in the clay minerals which attract ions
Which ions are exchanged will depend on the chemistry and environment