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Bentonite usually forms from the weathering of volcanic ash in seawater, or by hydrothermal circulation through the porosity of volcanic ash beds,[3][4] which converts (devitrification) the volcanic glass (obsidian, rhyolite, dacite) 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.[citation needed] Bentonite beds are white or pale blue or green (traces of reduced Fe2+) in fresh exposures, turning to a cream color and then yellow, red, or brown (traces of oxidized Fe3+) as the exposure is weathered further.[5]
As a swelling clay, bentonite has the ability to absorb large quantities of water, which increases its volume by up to a factor of eight.[5] This makes bentonite beds unsuitable for building and road construction. However, the swelling property is used to advantage in drilling mud and groundwater sealants. The montmorillonite / smectite making up bentonite is an aluminium phyllosilicate mineral, which takes the form of microscopic platy grains. These give the clay a very large total surface area, making bentonite a valuable adsorbent. The plates also adhere to each other when wet. This gives the clay a cohesiveness that makes it useful as a binder and as an additive to improve the plasticity of kaolinite clay used for pottery.[6]
One of the first findings of bentonite was in the Cretaceous Benton Shale near Rock River, Wyoming. The Fort Benton Group, along with others in stratigraphic succession, was named after Fort Benton, Montana, in the mid-19th century by Fielding Bradford Meek and F. V. Hayden of the U.S. Geological Survey.[4] Bentonite has since been found in many other locations, including China and Greece (bentonite deposit of the Milos volcanic island in the Aegean Sea). The total worldwide production of bentonite in 2018 was 20,400,000 metric tons.[7]
In geology, the term bentonite is applied to a type of claystone (a clay rock, not a clay mineral) composed mostly of montmorillonite (a clay mineral from the smectite group). It forms by devitrification of volcanic ash or tuff,[5] typically in a marine environment.[3][4] This results in a very soft, porous rock that may contain residual crystals of more resistant minerals, and which feels soapy or greasy to the touch. However, in commercial and industrial applications, the term bentonite is used more generally to refer to any swelling clay composed mostly of smectite clay minerals, which includes montmorillonite.[5] The undifferentiated reference to the weathered volcanic rock for the geologist or to the industrial mixture of swelling clays can be a source of confusion.
The montmorillonite making up bentonite is an aluminium phyllosilicate mineral whose crystal structure is described as low-charge TOT. This means that a crystal of montmorillonite consists of layers, each of which is made up of two T sheets bonded to either side of an O sheet. The T sheets are so called because each aluminium or silicon ion in the sheet is surrounded by four oxygen ions arranged as a tetrahedron. The O sheets are so called because each aluminium ion is surrounded by six oxygen or hydroxyl ions arranged as an octahedron. The complete TOT layer has a weak negative electrical charge, and this is neutralized by calcium or sodium cations that bind adjacent layers together, with a distance between layers of about 1 nanometer. Because the negative charge is weak, only a fraction of the possible cation sites on the surface of a TOT layer actually contain calcium or sodium. Water molecules can easily infiltrate between sheets and fill the remaining sites. This accounts for the swelling property of montmorillonite and other smectite clay minerals.[6]
The different types of bentonite are each named after the respective dominant cation.[8] For industrial purposes, two main classes of bentonite are recognized: sodiu