Chemical Weathering
There are only a hand full of minerals that can exit on the earths surface and not succumb to the effects of weather. Nearly all minerals and the rocks that contain them are susceptible to chemical changes when exposed to water, heat, and dissolved salts.

Many water solutions can act as weak acids. Carbon dioxide in water produces carbonic acid, which is one of the weakest. The chemical decomposition of rock is carried out by the slow replacement of one atom for another, or by the very slow dissolution of portions of a mineral and conversion to another.

Hydrolysis
Potassium feldspar is converted to clay by the action of carbonic acid on the feldspar.

( H₂O + CO₂ --> H⁺¹ + HCO₃ ^-1 )

KAlSi₃O₈ + H⁺¹ + H₂O ---> K⁺¹ + Al₃Si₄O₁₀(OH)₈ + SiO₂

This converts the potassium feldspar (orthoclase) to the clay kaolinite. The resultant clay is very insoluble and can resist further hydrolysis. It also has far less strength that the original feldspar and is more susceptible to physical weathering.

Leaching
Leaching is the chemical partial dissolution and transportation of ions out of a mineral. Water percolating through sulfide ores may leach sulphur and the water ends up with both an unpleasant taste and smell.

The water can leach a mineral like marchasite (FeS₂) and remove part of the sulphur, producing one or more of the iron oxides.

Oxidation
Oxidation is the chemical process that causes an electron transfer between elements. The element loses an electron to become oxidized.

For instance: Fe⁺² ----> Fe⁺³ + e^-1 (electron)

In this case the iron is oxidized. The iron can come from pyroxenes, amphiboles, or micas. When it is hydrolyzed out of the structure it can then combine with oxygen and through the oxidation process it is converted to an iron hydroxide. Dehydration (loss of water) produces the mineral goethite (FeO (OH)). Goethite can be further dehydrated to hematite (Fe₂O₃).

Dissolution
Dissolution is the complete dissolving of a mineral by water. Halite (NaCl) can be entirely dissolved in water. CaCO₃ can be partially dissolved in water and form Ca ions and the resultant carbonate reacts with more water to form bicarbonate ion (HCO₃^-1) and a weak acid solution like the hydrolysis reaction above.