<P> Clay is a finely - grained natural rock or soil material that combines one or more clay minerals with traces of metal oxides and organic matter . Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure . Clays are plastic due to their water content and become hard, brittle and non--plastic upon drying or firing . Depending on the soil's content in which it is found, clay can appear in various colours from white to dull grey or brown to deep orange - red . </P> <P> Although many naturally occurring deposits include both silts and clay, clays are distinguished from other fine - grained soils by differences in size and mineralogy . Silts, which are fine - grained soils that do not include clay minerals, tend to have larger particle sizes than clays . There is, however, some overlap in particle size and other physical properties . The distinction between silt and clay varies by discipline . Geologists and soil scientists usually consider the separation to occur at a particle size of 2 μm (clays being finer than silts), sedimentologists often use 4--5 μm, and colloid chemists use 1 μm . Geotechnical engineers distinguish between silts and clays based on the plasticity properties of the soil, as measured by the soils' Atterberg limits . ISO 14688 grades clay particles as being smaller than 2 μm and silt particles as being larger . </P> <P> Clay minerals typically form over long periods of time as a result of the gradual chemical weathering of rocks, usually silicate - bearing, by low concentrations of carbonic acid and other diluted solvents . These solvents, usually acidic, migrate through the weathering rock after leaching through upper weathered layers . In addition to the weathering process, some clay minerals are formed through hydrothermal activity . 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 deposited in a new sedimentary deposit . Clay deposits are typically associated with very low energy depositional environments such as large lakes and marine basins . </P> <P> Depending on the academic source, there are three or four main groups of clays: kaolinite, montmorillonite - smectite, illite, and chlorite . Chlorites are not always considered to be a clay, sometimes being classified as a separate group within the phyllosilicates . There are approximately 30 different types of "pure" clays in these categories, but most "natural" clay deposits are mixtures of these different types, along with other weathered minerals . </P>

Where does clay come from how is it formed
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