What Is Clay?
Clay is a special kind of earth. It is made up of the smallest particles of rock, tinier than those in most other kinds of soil. The minerals that make up ordinary clay are mostly silica and alumina. When wet, clay becomes a sticky, yet slippery, mud. The water acts as a lubricant and enables the clay particles to slip easily past one another.
Wet clay can be molded into almost any shape. When baked, clay becomes almost as hard as stone. So clay is used to make bricks, pots, dishes, and other useful things. Objects made of baked clay are called “ceramics.”
Clays are plastic due to their water content and become hard, brittle and non–plastic upon drying or firing. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure. Depending on the soil’s content in which it is found, clay can appear in various colors from white to dull gray or brown to deep orange-red.
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, and many naturally occurring deposits include both silts and clay. 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.
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.