Smectites

Smectites are a family of expandable clay minerals within the phyllosilicate group. They share a 2:1 layered structure, consisting of two silica tetrahedral sheets sandwiching an alumina octahedral sheet. Substitution of ions within the sheets imparts a negative structural charge, balanced by exchangeable interlayer cations such as Na+, Ca2+, or K+. The most common member is montmorillonite; other smectites include beidellite, nontronite, saponite, and hectorite. A defining feature is their tendency to swell with water.

The 2:1 layers permit rapid uptake of water into interlayers, causing substantial swelling and high cation exchange capacity. Basal (interlayer) spacing varies with hydration, typically 9-12 Å in the dry state and enlarging to 12-20 Å when hydrated. The swelling behavior renders smectites highly plastic and capable of forming stable gels; their surface area is large and they can adsorb metals and organic molecules. Interlayer cations readily exchange with ions in surrounding solutions.

Smectites form by weathering of feldspars and volcanic glasses under low-temperature conditions, often during diagenesis or soil formation. They are common in bentonites, argillaceous sediments, and soils worldwide. Illitization, hydrothermal alteration, and the development of mixed-layer illite–smectite can modify their proportion in natural clays.

In soils, smectites influence plasticity, swelling pressure, and nutrient retention. In industry they are used in drilling fluids, as barriers or liners for waste containment, and in geosynthetic clay liners; their adsorptive properties also make them useful for pollutant capture and catalysis. Behavior of smectites is highly sensitive to water content and the identity of exchangeable cations.