1) soil particles with a typical grain size less than 0.004 mm. 2) a soil predominantly composed of such particles (contrast with sand and silt).

Merriam-Webster Online Dictionary
clay (noun)
a) an earthy material that is plastic when moist but hard when fired, that is composed mainly of fine particles of hydrous aluminum silicates and other minerals, and that is used for brick, tile, and pottery , specifically soil composed chiefly of this material having particles less than a specified size
b) - earth mud
a) a substance that resembles clay in plasticity and is used for modeling
b) the human body as distinguished from the spirit
c) fundamental nature or character - the common clay
- clay court
Clay (biographical name)
Henry 1777–1852 Am. statesman & orator
Clay (biographical name)
Lucius Du Bi*gnon 1897–1978 Am. gen. - bixcla18.wav du̇-ˈbin-yən
clay (Wikipedia)

Gay Head cliffs in Martha's Vineyard consist almost entirely of clay.

Clay is a finely-grained natural rock or soil material that combines one or more clay minerals with possible traces of quartz (SiO2), metal oxides (Al2O3 , MgO etc.) 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 particle size and geometry as well as 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.

Electron microscope photograph of smectite clay – magnification 23,500×

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.

Mixtures of sand, silt and less than 40% clay are called loam. Loam makes good soil and is used as a building material.

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