Clay soil is the epitome of the saying, "One man's trash is another man's treasure." If you asked a gardener's feelings about clay, they would likely be directly opposite of what a pottery enthusiast would say. Regardless of your feelings toward clay, its formation is an interesting natural phenomenon that has important implications regarding its characteristics.
On a basic level, clay soil is composed of millions of clay particles which are 0.002 millimeters (0.0000787 inches) in diameter or smaller. These particles are tightly spaced, which is why clay is notorious for having poor water or air movement throughout. Furthermore, clay particles have a very strong affinity for water and, when exposed to water, they swell up and adhere to each other (a process known as cohesion).
According to the United States Geological Survey, clay deposits only form under a limited range of geological conditions. Soil horizons, continental and marine sediments, geothermal fields, volcanic deposits and weathering rock formations are the only environments under which clay soil deposits can be formed. Additionally, most clay formations occur when clay minerals are in contact with air, water or steam.
Clay soil is essentially composed of several minerals that deposit together and, over time, form a hardened clay deposit. Silicates, mica, iron and aluminum hydrous-oxide minerals are the most common minerals found in clay deposits. However, other minerals, such as quartz and carbonate, are also present in clay soils.
In order to form clay soil, the particles that compose the soil need to come from somewhere. Erosion is one source of particles for clay soils and it occurs when water rushes over the surface of rock. However, the largest source of clay particles is from weathering of rocks and soil. During weathering, both physical and chemical changes take place that create the small particles required to form clay soil. Lastly, diagenesis--the process that occurs when minerals that are stable in one environment destabilize because of compaction or burial--is another source of clay particles.
As discussed, clay soil is formed through the deposition of a variety of particles. Therefore, clay soils differ in their composition, usually based on the geological process that created the particles (erosion, weathering or diagenesis). Clay soil created by erosion is responsible for a large part of mudstone creation. Mudstone is a valuable part of the sedimentary environment, covering about 60 percent of marine continental shelves. In addition, Bentonite beds are clay soil beds that are created through the diagenesis of volcanic ash. Bentonite clay is used as pottery clay, as an absorbent for oil, drilling mud, and as a binder for bleaching liquids.