The group name for the 2:1 phyllosilicate minerals with a net negative layer charge between approximately -0.2 and -0.6 per formula unit on the layer. A generalized formula is X_0.3Y_2-3Z₄O₁₀(OH)₂ ^. nH₂O, where X is the exchangeable cation (e.g., Ca/2, Na, K, Mg/2), Y is a small to medium size cation (e.g., Al, Cr³⁺, Fe²⁺, Fe³⁺, Li, Mg, Ni, Zn), Z = Si, Al, and n is a rational number, not necessarily an integer). Smectite is often characterized by solvating polar organic molecules into the interlayer causing pronounced swelling between the layers [perpendicular to the (001) plane]. Smectite is commonly referred to as a “swelling clay”, as is vermiculite. The rock term, bentonite, refers to a smectite-rich material. See further discussion under Part 1, smectite and obsolete terms: Wyoming-type, Otay-type, Chambers-type, Tatatila-type, beidellite-type (ideal and non-ideal), and non-ideal montmorillonite.
Cf., bentonite, swelling clay, beidellite, hectorite, montmorillonite, nontronite, saponite, sauconite, stevensite, swinefordite, volkonskoite, yakhontovite

The conversion of smectite to illite involving a significant change in the chemical composition of the 2:1 layer from a smectite-like composition to an illite-like composition. The resulting net negative charge on the layer must become sufficiently large to preferentially fix potassium or other large cations and cause dehydration of the interlayer. The process of conversion may be one of apparent solid-state transformation or mostly in the solid state or apparent neoformation via a solution. The degree of conversion may be useful to understand crystallization parameters during burial diagenesis or metamorphism (e.g., Hower et al. (1976) or hydrothermal activity (e.g., Inoue et al., 1987).
Cf., illite/smectite

See suspension.

An obsolete varietal term for glauconite.

An obsolete term for calcic paragonite, calcic ephesite.

An obsolete term for paragonite.

An obsolete term for brammallite.

An obsolete term for aspidolite.

In soil science, plasma refers to fine-grained (too fine to resolve individual constituents by visible-light microscopy) crystalline or amorphous materials in soils, which includes saprolites.
Syn. pedoplasma
See saprolite, clay groundmass

In soil science, describing soil formed in place by in-situ weathering of rock parent material.
Cf., clay (sedimentary).