A highly plastic clay mineral related to montmorillonite (bentonite), more correctly, the name of the group on minerals that includes montmorillonite and other similar minerals.
The following was obtained from Southern Clay Minerals:
Smectite is a platy clay mineral. Three properties make smectite unique:
Shape - Smectite is a very lamellar material. Individual platelets resemble a business card in terms of morphology.
Size - Naturally occurring smectite ranges in size from 0.25 microns up to 0.75 microns across its broadest dimension.
Charge - Smectite platelets have a negative ionic charge within the body of the platelet and a positive charge on the edge of the platelet.
Due to its particular combination of properties, smectite can provide a host of benefits in water-based coatings. Due to its ability to structure in aqueous media, it is a thickener in low to mid shear ranges. Oftentimes it can be used to reduce or replace (in combination with other thickeners) cellulosic thickeners. It also prevents settling and syneresis (phase separation). Smectite will act as a pigment spacer (U.S. Patent 5,700,319) to improve hiding in titanium dioxide pigmented systems. Smectite improves sag resistance while not affecting flow and leveling to the extent other mineral thickeners might. This is due to smectite's true thixotropy which affords a delay in the recovery of viscosity when shear is released as opposed to the pseudoplastic tendencies of other minerals which cause an immediate recovery of viscosity.
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Standard porcelains used by potters and for the production of sanitary and table ware have surprisingly similar recipes. But their plasticities vary widely.
What is clay? How is it different that regular dirt? For ceramics, the answer lies on the microscopic level with the particle shape, size and how the surfaces interact with water.
One of many smectite micrographs at minersoc.org
Southern Clay Products Smectite Minerals website
Smectite on WikiPedia
The property of plasticity is evident in a wide array of materials of different mineralogy. In addition, many minerals are considered parent minerals to clays, they weather down to become these clays.