Weathering geochemistry

Three reactions

Chemical weathering proceeds by biological, chemical and physical routes together, but its chemistry is dominated by water made slightly acidic by dissolved carbon dioxide, reacting with the minerals it meets.^[1,2]

Hydrolysis attacks silicates: feldspar breaks down, the soluble cations go into solution, and aluminium and silicon stay behind as clay. Kaolinite is the residue of that reaction taken far enough.^[1]

Oxidation attacks iron. Ferrous iron released as a mineral breaks down is converted to ferric, which is far less soluble, so the iron is fixed in place as oxide rather than carried away.^[1,2]

Carbonation attacks carbonates. Acidified rainwater dissolves calcite, which is the reaction that shapes limestone landscapes and that links atmospheric carbon to the rock record.^[1]

Sorting by solubility

The net effect is separation. Soluble elements leave in solution; insoluble ones remain. The residue is enriched in whatever the water could not move — a concentration mechanism that needs no magma and no hydrothermal system.^[1]

This is where Phase 2 stops. That residual enrichment happens is process understanding; which residues become ore is a question about mineral systems, and belongs to a later phase.^[1,2]

Fluids without melting

Hot aqueous fluids do the same job at depth and at speed: dissolving elements where conditions favour solution, precipitating them where they do not, and altering the rock they pass through on the way.^[1]

Commonly misunderstood