Montmorillonites

Montmorillonites are a group of clay minerals within the smectite family, characterized by a 2:1 layered silicate structure in which an octahedral sheet is sandwiched between two tetrahedral sheets. They are typically dioctahedral and acquire a negative layer charge through isomorphic substitutions within the octahedral and/or tetrahedral sheets. The name derives from Montmorillon, a locality in France. Montmorillonites are known for high cation exchange capacity and their ability to swell markedly in the presence of water and certain solvents.

Chemically, montmorillonites carry a negative charge that is balanced by exchangeable interlayer cations such as Na+ or Ca2+. This, together with their expandable interlayer spaces, gives them high surface area and strong affinity for water and various organic molecules. Their cation exchange capacity is a defining property, enabling rapid ion exchange and sorption processes. They exhibit considerable swelling when hydrated, which affects their rheology and permeability.

Formation and occurrence: Montmorillonites form by the weathering and alteration of volcanic ash and glass in sedimentary environments, often producing bentonite deposits. They are common in soils and clay-rich sedimentary rocks and frequently occur with other clay minerals such as illite or kaolinite in interstratified assemblages.

Uses and applications: The swelling and adsorptive properties of montmorillonites make them valuable in drilling muds and as viscosifiers, sealants, and liners for containment and waste barriers. They function as adsorbents for pollutants, catalysts supports, and components in ceramics and cement. Their natural abundance and versatile chemistry also support various industrial and environmental applications.