Clay minerals are composed of hydrous layered sili­cates that are part of the phyllosilicates family. The phyl — losilicates family is broad and is roughly separated by layer types, groups, subgroups and species (Brindley and Brown, 1980). Two basic units are important to build the clay minerals, the first is silicon atoms coordinated tetrahedrically to oxygen atoms (SiO4) and divalent or trivalent metals coordinated octahedrically to hydroxyls (M+2/M+3(OH)g). The silicon tetrahedral face can share the three corners with other silicon tetrahedral to build a hexagonal two-dimensional pattern, the tetrahedral sheet. A similar procedure can be adopted by the octahe­dral, where basically two differences can be obtained when M+2 or M+3 atoms occupy the center of the octahedral.

When M+2 is used and the octahedral share the edges, all octahedral sites are occupied and a two-dimensional unit is formed, the so-called octahedral sheet. This unit resembles the structure of brucite (Mg(OHD and the sheet is called trioctahedral. When M+3 is used and the octahedral share the edges, only 2/3 of the octahedral sites are occupied and the resulting two-dimensional unit resembles the structure of gibbsite Al(OHL; in this case, the sheet is named dioctahedral. In the ideal condition, the apical oxygen of the tetrahedral sheets can be linked to one octahedral, building the clay min­erals of the 1:1 layer type. The unshared hydroxyls of the octahedra lie at the center of the tetrahedra at the same "z" level of the shared apical oxygen.

Under ideal conditions, two clay minerals of the 1:1 layer type can be obtained. When M+2 occupies the center of the octahedral, the structure of chrysotile (Mg3(OH)4 Si2O5) is obtained and the replacement of M+2 by M+3 yields the structure of kaolinite (Al2(OH)4Si2O5).

As both sides of the octahedral have hydroxyls to share, one octahedral sheet can also be combined with two tetrahedral sheets, originating the 2:1 layer-type clay minerals. Again, when M+2 occupies the center of the octahedral, the structure of talc is obtained (Mg3(OH)2Si4O^) while its replacement by M+3 results in the structure of pyrophyllite (AL(OH)4Si4O10). Figure 16.1 shows the lateral (A) and top (B) views of the above-cited compounds.

In nature, the phyllosilicates are obtained through weathering, which is the phenomenon related to the

disintegration and chemical alteration of rocks and min­erals at the Earth’s surface in direct contact with the at­mosphere, water and organism. Through this process, many different isomorphic substitutions occur either in the tetrahedral (Si by Al or Fe+3) or in the octahedral sheets (Al or Mg by Fe+2/+3, Li, Ti, V, Cr, Mn, Co, Ni, Cu and Zn), mainly in clay minerals of the 2:1 type. The isomorphic substitution generates an excess of negative charge into the layers, which needs to be compensated with the intercalation of hydrated cations between the layers. Hence, this substitution generates

the cationic exchange capacity and the plastic properties of these clay minerals, particularly when they are dispersed in water.

Using the example of talc and pyrophyllite, these minerals can give origin to trioctahedral saponite ((Mty -nH2O)(M g3_y(A l, F e)y)(Si4_*A l*O 0(O H)2)), where Mg+2 is substituted by Al and Fe and Si, by Al. After this substitution, the excess of negative charges in the clay layers are compensated by the intercalation of hydrated cations (M+_y • nH2O). Another example of
trioctahedral mineral derived from pyrophyllite is the clay mineral hectorite ((M+•MH2O)(Mg3_yLiy)(Si4Oio (OH)2)). In the case of dioctahedral talc, the derived clay minerals are montmorillonite ((M+-n^O^A^_yMgy) (Si4Oio(OH)2)), beidellite ((M+-nH2O)Al2(Si4_* Al*) O1o(OH)2) and nontronite ((M+-:nH2O)Fe+3(Si4_xAlx) Oio(OH)2).