phyllosilicate

Phyllosilicates, also known as sheet silicates, are a class of silicate minerals characterized by a two-dimensional arrangement of silicon-oxygen tetrahedra that form continuous sheets. Each tetrahedral sheet is linked to octahedrally coordinated cations (typically aluminum, magnesium, or iron) to create a layered structure. The common motif is tetrahedral–octahedral–tetrahedral (T–O–T), and the layers are held together by interlayer cations and often water. This sheet arrangement gives rise to characteristic, very soft minerals with strong basal cleavage and platy or micaceous habit.

Major groups within the phyllosilicates include the micas (such as muscovite and biotite), which consist of an octahedral sheet between two tetrahedral sheets and typically contain interlayer potassium; this yields excellent basal cleavage and elastic platy forms. Clay minerals (kaolinite, illite, smectite) comprise alternating tetrahedral and octahedral sheets with varying interlayer chemistry and hydration. Smectites (e.g., montmorillonite) are expandable due to water in the interlayers, illite is relatively non-expandable, and kaolinite is a non-expandable 1:1 type. Other important phyllosilicates include talc, serpentine, and chlorite, each with distinctive interlayer chemistries and structures.

Phyllosilicates form through weathering of silicate rocks, diagenesis of sediments, metamorphism, and hydrothermal alteration. They are major components of soils and sedimentary rocks, influencing properties such as soil structure and cation exchange capacity. Industrially, they are important as clays in ceramics, as drilling fluids, and as fillers and additives in various applications.