Clays

Clays are a group of naturally occurring, fine-grained minerals composed mainly of hydrated aluminum silicates and related substances. They have a sheet-like (phyllosilicate) crystal structure, and most are composed of very small platelets less than 2 micrometers in their largest dimension. The mineral's chemistry and structure give clays characteristic properties such as plasticity when wet, high surface area, and charge-based interactions with water and ions.

Most crystalline clays fall into two structural families: 1:1 clays, like kaolinite, consisting of one tetrahedral sheet linked to one octahedral sheet; and 2:1 clays, such as illite and smectite, consisting of two tetrahedral sheets around one octahedral sheet. Interlayers in 2:1 clays host water and exchangeable cations, and some members (notably smectites) can swell markedly when hydrated.

Clays form primarily by weathering of aluminosilicate minerals in feldspar-bearing rocks, soils, and sediments. They can be primary minerals formed in rock as products of diagenesis or secondary minerals formed during weathering and hydrothermal alteration. They are ubiquitous in soils, contributing to fertility and structure.

Key properties include cation exchange capacity (CEC), plasticity, aging, shrink-swell behavior, and high specific surface area, which influence soil behavior, adsorption of contaminants, and, in industrial uses, rheology and permeability.

Industrial uses include ceramics and pottery (kaolinite for porcelain), brick and cement, drilling fluids, liners for landfills, and adsorbents and catalysts. In soils, clays affect nutrient availability, water retention, and erosion resistance.