A site on a clay mineral surface available for electrostatic attraction between oppositely charged species, including metal coordination bonding, and anion or polar molecule attractions, etc. Binding sites commonly occur: (1) where bonds are broken at particle edges (terminal edges) leaving a net positive charge and a possible under-bonded arrangement where bond lengths adjust accordingly to approach overall charge balance. Alternatively, any anion may compensate for the broken bond, with (OH)^– most common (and thus these sites may be affected by pH). (2) to charge balance isomorphous substitutions that occur within the 2:1 or 1:1 layer, for example, cations in the interlayer of smectite compensate for the (permanent) negative layer charge on the basal plane atoms.
Cf., edge site