At the surface, materials from inside Earth are exposed to the atmosphere and the hydrosphere - where they are chemically modified.
The chemical modification is of two types:

• Physical
• Chemical

Physical breakdown processes -

• rock expansion due to unloading
• thermal expansion and contraction
• frost action due to water freezing - wedging or heaving
• organic activities - plant roots, animal burrows.

Chemical weathering - also breaks down solid rocks into smaller particles - which are chemically and physically new (and different). Existing minerals are transformed to new ones

The main agent of chemical weathering = water

Water reacts with high T (temperature) igneous and metamorphic minerals by providing a solvent and thus promoting the mobility of ions necessary for reactions. Ions are removed from structures of 'old' minerals and made available to form new chemical arrangements (new structures = new minerals)

Silicate minerals decompose by hydrolysis = reaction with H⁺ ion

1- example - potassium feldspar-->kaolinite

2KAlSi₃O₈ + 2H⁺ + 2HCO₃^- + H₂O-->Al₂Si₂O₅(OH)₄ + 2K⁺ + 4SiO₂ + 2HCO₃^-

H⁺ comes from CO₂ (atmosphere) + H₂O (rain) --> H₂CO₃ --> H⁺ + HCO₃^-

2- example - pyroxene to Fe oxide

4FeSiO₃ + H₂O + O² -->4FeO(OH) + 4SiO₂

Other common weathering reactions involve oxidation:

2Fe₃O₄ (magnetite) + O --> 3Fe₂O₃ (hematite)

2FeO + O₂ --> Fe₂O₃

Concept of maturity

The results of chemical weathering are sedimentary minerals.

Weathering of all types is faster when block or particle sizes are small or angular as more surface area is provided for chemical and mechanical processes to work on.

For a more in depth overview of this topic with pictures try Weathering from Pamela Gore.