smectite

Smectite is a group of clay minerals within the phyllosilicate family characterized by a 2:1 layered structure consisting of an octahedral sheet sandwiched between two tetrahedral sheets. Isomorphic substitution within these sheets imparts a negative layer charge, which is balanced by exchangeable interlayer cations such as Na+, Ca2+, or Mg2+. The resulting clays have high surface area and a substantial cation exchange capacity, and they commonly occur as montmorillonite- or saponite-rich minerals.

Common smectites include montmorillonite, beidellite, nontronite, saponite, hectorite, and sauconite. They vary in dioctahedral versus trioctahedral character depending on which cations occupy the octahedral sheet (dioctahedral with Al3+ predominance; trioctahedral with Mg2+ or Fe2+).

Swelling properties: Smectites are known for their swelling behavior as water enters the interlayer space, increasing basal spacing when hydrated; montmorillonite shows strong swelling; the degree of swelling depends on the interlayer cation and ambient humidity.

Occurrences and uses: They form by weathering of primary silicate minerals and are abundant in soils and sediments; bentonite is largely montmorillonite-rich and has industrial uses such as drilling fluids, seals, liners, and filler.

Analytical notes: Identification by X-ray diffraction due to characteristic 2:1 layering; high cation exchange capacity; typical basal spacing around 1 nm when dry and about 1.2–1.5 nm when hydrated.