Smectite is derived from the Greek word “smecktos”, meaning “soap”. The term “smectite” is used to describe a family of expanding 2:1 phyllosilicate minerals that have permanent layer charge between 0.2 and 0.6 charges per half unit cell. The smectite family encompasses clay minerals such as montmorillonite, beidellite, nontronite, saponite, and hectorite. Smectites are constructed of a single octahedral sheet sandwiched between two tetrahedral sheets, with the octahedral sheet sharing the apical oxygens of the tetrahedral sheets . Smectites are divided into two subgroups: dioctahedral, when only two of the three octahedral sites per half unit cell are occupied, and trioctahedral, when all three sites are populated .

Smectites are typically identified by X-ray diffraction. All Mg-saturated smectites give a 1.5 nm peak at 54% relative humidity. Beidellite give a 1.4 nm peak after the glycerol solvation from the vapor, whereas montmorillonite expand to 1.8 nm. Both expand to 1.7 nm after ethylene glycol solvation. K-saturated smectites give a 1.25 nm peak at 54% relative humidity, which reflects the hydration of the K⁺ ion and corresponds to a monolayer of H2O in the interlayer space. Smectite heat at 110 ^oC and kept 0% relative humidity yield a 1.0 nm peak that collapses slightly to 0.98 nm on heating.

a method for distinguishing between montmorillonite and beidellite or nontronite by heating Li-saturated clay at 220 ^oC. Subsequent liquid glycerol solvation results in a 0.95 nm XRD peak for montmorillonite, while nontronite and beidellite expand to 1.77 nm. A recent update of the method involves heating at 250 ^oC overnight followed by glycerol vapor solvation for 16 h at 90 ^oC .

Dioctahedral and trioctahedral smectites can be distinguished by observing the 060 XRD peak in randomly oriented samples. The dioctahedral smectites common in soils produce a peak near 0.150 nm, whereas the trioctahedral smectites inherited from parent material produce a peak near 0.154 nm .

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