a phyllosilicate
/ FILL-oh-SIL-ih-kate /
Let each tetrahedron share three of its four corners with neighbours, all in the same plane, and the chains widen into an endless flat sheet, a chicken-wire mesh of tetrahedra stretching in two directions. That is a phyllosilicate (from the Greek phyllon, leaf). This is the family of the micas, the talc you rub between your fingers, and, above all, the clay minerals that make traditional ceramics possible. Its signature is a book-of-pages habit: the sheets stack loosely and split apart into thin, flexible flakes.
In the ideal sheet, three shared corners per tetrahedron leave one non-bridging oxygen pointing up out of the plane, giving the repeating unit Si2O5 and a Si:O ratio of 2:5. Those upward-pointing oxygens are the hooks: they bond to a second, octahedral sheet of aluminium or magnesium hydroxide, gluing a tetrahedral sheet (T) to an octahedral sheet (O). Stack them one T to one O and you get a 1:1 layer (kaolinite); sandwich one O between two T sheets and you get a 2:1 layer (mica, talc, and the smectite clays). What sits between these stacked layers, hydrogen bonds, a locked-in cation such as K+, or loose swelling water, is what separates a stiff mica from a plastic clay.
The whole personality of a sheet silicate comes from that anisotropy: bonds are strong within the sheet but weak between the stacks, so the crystals cleave perfectly along the basal plane into flakes, feel soapy or greasy, and, when the interlayer holds water, can slide and swell. This is the structural secret behind the plasticity of clay, the peeling of mica, and the lubricity of talc, all from tetrahedra sharing three corners.
Peel a flake off a piece of mica with your fingernail: you are pulling apart two Si2O5-based sheets held together only by weak interlayer bonds, while the strong bonds inside each sheet stay untouched. Clay does the same on a microscopic scale, which is why wet clay slides.
Strong in the plane, weak between the planes: the two-dimensional sheet is why phyllosilicates cleave, slide, and, when hydrated, swell.
The 2:5 ratio is the ideal for a pure tetrahedral sheet. Real clays and micas replace some tetrahedral Si with Al and add octahedral Al, Mg, Fe and OH, so their measured formulas look busier than plain Si2O5.