hydrolysis: 2 KAlSi3O8 + 2 H2CO3 + 9 H2O ⇌ Al2Si2O5(OH)4 + 4 H4SiO4 + 2 K+ + 2 HCO3− orthoclase (aluminosilicate feldspar) + carbonic acid + water ⇌ kaolinite (a clay mineral) + silicic acid in solution + potassium and bicarbonate ions in solution Oxidation Within the weathering environment, chemical oxidation of a variety of metals occurs. The most commonly observed is the oxidation of Fe2+ (iron) by oxygen and water to form Fe3+ oxides and hydroxides such as goethite, limonite, and hematite. This gives the affected rocks a reddish-brown coloration on the surface which crumbles easily and weakens the rock. Many other metallic ores and minerals oxidize and hydrate to produce colored deposits, as does sulfur during the weathering of sulfide minerals such as chalcopyrites or CuFeS2 oxidizing to copper hydroxide and iron oxides. Hydration Mineral hydration is a form of chemical weathering that involves the rigid attachment of water molecules or H+ and OH- ions to the atoms and molecules of a mineral. No significant dissolution takes place. For example, iron oxides are converted to iron hydroxides and the hydration of anhydrite forms gypsum. Bulk hydration of minerals is secondary in importance to dissolution, hydrolysis, and oxidation, but hydration of the crystal surface is the crucial first step in hydrolysis. A fresh surface of a mineral crystal exposes ions whose electrical charge attracts water molecules. Some of these molecules break into H+ that bonds to exposed anions (usually oxygen) and OH- that bonds to exposed cations. This further disrupts the surface, making it susceptible to various hydrolysis reactions. Additional protons replace cations exposed in the surface, freeing the cations as solutes. As cations are removed, silicon-oxygen and silicon-aluminium bonds become more susceptible to hydrolysis, freeing silicic acid and aluminium hydroxides to be leached away or to form clay minerals. Laboratory experiments show that weathering of feldspar crystals begins at dislocations or other defects on the surface of the crystal, and that the weathering layer is only a few atoms thick. Diffusion within the mineral grain does not appear to be significant. Biological weathering Mineral weathering can also be initiated or accelerated by soil microorganisms. Soil organisms make up about 10 mg/cm3 of typical soils, and laboratory experiments have demonstrated that albite and muscovite weather twice as fast in live versus sterile soil. Lichens on rocks are among the most effective biological agents of chemical weathering. For example,