phyllosilicates

Phyllosilicates are a major class of silicate minerals characterized by two-dimensional sheets of linked silicon-oxygen tetrahedra. In these sheets each silicon tetrahedron shares three of its four oxygens with neighboring tetrahedra, leaving a basal oxygen site for interlayer cations or hydroxide groups. The sheets stack to form layered minerals that exhibit perfect cleavage along the basal plane.

Common phyllosilicate groups include the micas (for example muscovite and biotite), which have a sheet of octahedrally coordinated cations between tetrahedral sheets; the clay minerals such as kaolinite, illite, and smectite (montmorillonite), which are important in soils; chlorite, talc, and serpentine, which share variations in octahedral layers and interlayer water. Together they span a range from very soft to relatively hard and from platy to fibrous habits.

Formation and occurrence: Phyllosilicates form by weathering of silicate rocks, diagenesis and metamorphism, as well as hydrothermal processes. They are especially abundant in soils as clays and in sedimentary rocks as illite and smectite; chlorite and serpentine are common in metamorphic and ultramafic rocks.

Significance and uses: They influence soil properties such as moisture retention, cation exchange capacity and plasticity. Some phyllosilicates, notably mica and talc, have industrial uses in lubrication, cosmetics, and ceramics; clays are essential in ceramic industry and as drilling mud components.