Editing Clay (section)

== Formation ==
[[File: Italian and African-American Clay Miners in Mine Shaft.jpg|thumb|Italian and African-American clay miners in mine shaft, 1910]]

Clay minerals most commonly form by prolonged chemical [[weathering]] of silicate-bearing rocks. They can also form locally from [[hydrothermal]] activity.{{sfn|Foley|1999}} Chemical weathering takes place largely by acid hydrolysis due to low concentrations of [[carbonic acid]], dissolved in rainwater or released by plant roots. The acid breaks bonds between aluminium and oxygen, releasing other metal ions and silica (as a gel of [[orthosilicic acid]]).){{sfn|Leeder|2011|pp=5-11}}

The clay minerals formed depend on the composition of the source rock and the climate. Acid weathering of [[feldspar]]-rich rock, such as [[granite]], in warm climates tends to produce kaolin. Weathering of the same kind of rock under alkaline conditions produces [[illite]]. [[Smectite]] forms by weathering of [[igneous rock]] under alkaline conditions, while [[gibbsite]] forms by intense weathering of other clay minerals.{{sfn|Leeder|2011|pp=10-11}}

There are two types of clay deposits: primary and secondary. Primary clays form as residual deposits in soil and remain at the site of formation. Secondary clays are clays that have been transported from their original location by water erosion and [[Deposition (geology)|deposited]] in a new [[sedimentary]] deposit.{{sfn|Murray|2002}} Secondary clay deposits are typically associated with very low energy [[Depositional environment|depositional environments]] such as large lakes and marine basins.{{sfn|Foley|1999}}