smectita

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. Substitution of aluminum, magnesium, or iron in the sheets creates a negative lattice charge, which confers a high cation exchange capacity and the ability to hydrate and swell between layers. The best-known member is montmorillonite; other common smectites include beidellite, nontronite, saponite, and hectorite.

In structure, smectites are expandable clays: water and other solvents can enter the interlayer space, causing swelling. They are typically dioctahedral or trioctahedral, with variable layer charges and interlayer cations such as sodium, calcium, or potassium influencing swelling and dispersion.

Occurrence and formation: smectites form by weathering of volcanic glass, feldspars, and other silicates under neutral to mildly alkaline conditions, and they are common components of soils, clays in sediments, and bentonites. They are widely distributed in the continental crust and are especially important in soils with high rainfall and base saturation.

Uses and significance: due to their high cation exchange capacity, hydration, and plasticity, smectites are key in drilling fluids (as bentonite), as barrier materials for waste containment, and as adsorbents in environmental remediation and catalysis. Their swelling behavior can pose engineering challenges in foundations and earthworks, but also provides useful sealing and stabilization properties.

Identification and analysis: smectites are identified by X-ray diffraction of oriented mounts and by basal spacing that changes with hydration. Other methods include infrared spectroscopy and cation exchange tests. The broad group is defined more by structure and behavior than by a single mineral species.