Editing Weathering (section)

===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 oxide]]s are converted to [[iron hydroxide]]s and the hydration of [[anhydrite]] forms [[gypsum]].{{sfn|Boggs|1996|p=8}}

Bulk hydration of minerals is secondary in importance to dissolution, hydrolysis, and oxidation,{{sfn|Boggs|2006|p=9}} 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 on 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.{{sfn|Blatt|Middleton|Murray|1980|p=258}}{{sfn|Leeder|2011|pp=653-655}} 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.<ref>{{cite journal |last1=Berner |first1=Robert A. |last2=Holdren |first2=George R. |title=Mechanism of feldspar weathering: Some observational evidence |journal=Geology |date=1 June 1977 |volume=5 |issue=6 |pages=369–372 |doi=10.1130/0091-7613(1977)5<369:MOFWSO>2.0.CO;2|bibcode=1977Geo.....5..369B }}</ref>

[[File:Weathering 9039.jpg|thumb|A freshly broken rock shows differential chemical weathering (probably mostly oxidation) progressing inward. This piece of [[sandstone]] was found in [[Moraine|glacial drift]] near [[Angelica, New York]].]]