- A large family of complex minerals containing the elements magnesium, aluminum, silicon and oxygen (magnesium, aluminum silicates) combined in a sheet-like structure. Clays are mined from surface pits as relatively pure deposits and used for bricks, pottery, foundry molds and in drilling fluids among other uses. Clays, as claystones, shales and intermixed with sands and sandstones make up the largest percentage of minerals drilled while exploring for oil and gas. Sodium bentonite is a useful additive for increasing the density of drilling muds, but other clay types are considered contaminants to be avoided and removed. Individual clay platelets can be viewed only with an electron microscope. Crystal structures are also determined by X-ray diffraction (XRD). The atomic structure of the clay group of layered silicate minerals varies from two-layer to three-layer or four-layer (mixed-layer) structures. One of the structural layers is a plane of silicon dioxide tetrahedra (silicon at the center and oxygen at all four corners of the tetrahedron). The other structural layer is a plane of aluminum hydroxide octahedra (aluminum at the center and hydroxides at all six corners). The tetrahedral and octahedral layers fit one on top of the other, with oxygen atoms being shared as oxide and hydroxide groups.
- smectite clay