Montmorillonite in the context of "Montmorillon"

Tonstein (from the German "Ton", meaning clay, plus "Stein", meaning stone) is a hard, compact sedimentary rock that is composed mainly of kaolinite or, less commonly, other clay minerals such as montmorillonite and illite. The clays often are cemented by iron oxide minerals, carbonaceous matter, or chlorite. Tonsteins form from volcanic ash deposited in swamps.Tonsteins occur as distinctive, thin, and laterally extensive layers in coal seams throughout the world. They are often used as key beds to correlate the strata in which they are found. The regional persistence of tonsteins and relict phenocrysts indicate that they formed as the result of the diagenetic alteration of volcanic ash falls in an acidic (low pH) and low-salinity environment, consistent with a freshwater swamp. In contrast, the alteration of a volcanic ashfall deposit in a marine environment typically produces a bentonite layer.

The induration of tonsteins is in contrast to kaolin claystones that can be mined for kaolin clay, such as the ball clays found at Bovey Tracey which formed by the erosion of a nearby kaolinised granite. These deposits are generally softer, white, and plastic.

Bentonite (/ˈbɛntənaɪt/ BEN-tə-nyte) is an absorbent swelling clay consisting mostly of montmorillonite (a type of smectite) which can either be Na-montmorillonite or Ca-montmorillonite. Na-montmorillonite has a considerably greater swelling capacity than Ca-montmorillonite.

Bentonite usually forms from the weathering of volcanic ash in seawater, or by hydrothermal circulation through the porosity of volcanic ash beds, which converts (devitrification) the volcanic glass (obsidian, a volcanic glass with a chemical composition equivalent to rhyolite) present in the ash into clay minerals. In the mineral alteration process, a large fraction (up to 40–50 wt.%) of amorphous silica is dissolved and leached away, leaving the bentonite deposit in place. Bentonite beds are white or pale blue or green (traces of reduced Fe
) in fresh exposures, turning to a cream color and then yellow, red, or brown (traces of oxidized Fe
) as the exposure is weathered further.

The shrink–swell capacity of soils refers to the extent certain clay minerals will expand when wet and retract when dry. Soil with a high shrink–swell capacity is problematic and is known as shrink–swell soil, or expansive soil. The amount of certain clay minerals that are present, such as montmorillonite and smectite, directly affects the shrink-swell capacity of soil. This ability to drastically change volume can cause damage to existing structures, such as cracks in foundations or the walls of swimming pools.

A smectite (from Ancient Greek σμηκτός (smēktós) 'lubricated'; from σμηκτρίς (smēktrís) 'walker's earth, fuller's earth'; lit. 'rubbing earth; earth that has the property of cleaning') is a mineral mixture of various swelling sheet silicates (phyllosilicates), which have a three-layer 2:1 (TOT) structure and belong to the clay minerals. Smectites mainly consist of montmorillonite, but can often contain secondary minerals such as quartz and calcite.

The use of medicinal clay in folk medicine goes back to prehistoric times. Indigenous peoples around the world still use clay widely. Such uses include external application to the skin and geophagy. The first recorded use of medicinal clay goes back to ancient Mesopotamia.

A wide variety of clays are used for medicinal purposes—primarily for external applications, such as the clay baths in health spas (mud therapy). Among the clays most commonly used are kaolin and the smectite clays such as bentonite, montmorillonite, and Fuller's earth. However, their use is declining, and modern evidence-based medicine has ended the use of many types.