Members of the phyllosilicates (sheet silicates) with related chemistry, all are hydrous aluminium silicates with layered structure; layers of [SiO₄] tetrahedra of composition [Si₄O₁₀]⁴⁻ are joined to Al-O layers (gibbsite-type layers) or (Mg,Fe)-O layers (brucite-type layers). 1:1 sheet silicates have one Si-O layer coupled to one brucite or gibbsite layer and include the serpentine group and the kaolinite or kandite group of clays; 2:1 sheet silicates have two Si-O layers joined to one brucite or gibbsite layer and include the smectite and illite groups of clays, bentonite and montmorillonite, as well as talc and the mica group; 2:2 sheet silicates have two Si-O layers joined to two brucite or gibbsite layers and include the chlorite group. It is difficult to distinguish clay minerals by hand or under the microscope, so sophisticated techniques of X-ray diffraction and scanning electron microscopy (SEM) are used to determine the precise clay mineral under investigation.