Chapter 2 Structures and Mineralogy of Clay Minerals

Phyllosilicates considered in this section ideally contain a continuous tetrahedral sheet. Each tetrahedron consists of a cation, T, coordinated to 4 oxygen atoms, and linked to adjacent tetrahedra by sharing three corners (the basal oxygen atoms, Ob) to form an infinite two-dimensional “hexagonal” mesh pattern along the a, b crystallographic directions. In the octahedral sheet, connections between each octahedron, M, to neighbouring octahedra are made by sharing edges. Edge-shared octahedra form sheets of hexagonal or pseudo-hexagonal symmetry. Common tetrahedral cations are Si4+, Al3+, and Fe3+. Octahedral cations are usually Al3+, Fe3+, Mg2+, Fe2+, but other cations, such as Li+, Ti4+, Cr3+, Ni2+, Mn2+, Cu2+, V5+, Co2+, and Zn2+ have been identified. Octahedra show two different topologies related to (OH) position, i.e. the cis- and the trans-orientation. The free corners (the tetrahedral apical oxygen atoms, Oa) of all tetrahedra points to the same side of the sheet and connect tetrahedral and octahedral sheets to form a common plane with octahedral anionic position (note that OH can be commonly substituted by O, F and Cl). One octahedral sheet is linked to either one or two tetrahedral sheets. The 1:1 layer structure consists of one tetrahedral and one octahedral sheet, while in the 2:1 layer structure one octahedral sheet is sandwiched between two tetrahedral sheets. In the 1:1 layer structure, the unit cell contains 6 octahedral sites with 4 cis- and 2 trans-oriented octahedra, and 4 tetrahedral sites, whereas 6 octahedral sites (usually 4 cis- and 2 trans-oriented) and 8 tetrahedral sites characterize the 2:1 layer unit cell. Structures with all 6 octahedral sites occupied are known as trioctahedral (Fig. 2.4a) but if only 4 of the 6 octahedra are occupied, the structure is referred to as dioctahedral. Often, the structural formula is reported on the basis of the half unit-cell content, i.e. it is based on 3 octahedral sites.In the 1:1 or TM phyllosilicates (e.g., dioctahedral kaolinite and trioctahedral serpentine) each layer is about 7 Å (0.7 nm) thick. One surface of such a layer consists entirely of oxygen atoms (Ob) belonging to the tetrahedral sheet, while the other surface is composed of OH groups of the octahedral sheet (Fig. 2.3). In the 2:1 or TMT layer the tetrahedral sheets are inverted and two-thirds of the hydroxyl are replaced by tetrahedral apical oxygen atoms. The periodicity along the c-axis varies from 0.91 to 0.95 nm in talc and pyrophyllite, to 1.40–1.45 nm in chlorite. The higher values for chlorite is due to interlayer occupancy. In talc the interlayer space is empty but in mica and illite it is occupied by anhydrous alkaline and alkaline-earth cations (layer periodicity about 1.0 nm). The interlayer space of smectite and vermiculite contains alkaline or alkaline-earth cations together with water molecules (layer periodicity of: i) about 1.2 nm when the interlayer position is occupied by cations with low field strength and water molecules; ii)1.5 nm when the interlayer is occupied by high field strength cations and water molecules; iii) more than 1.5 nm when water molecules are exchanged by polar molecules with different size) whereas that of chlorite showing TMTMint (where Mint is the octahedral interlayer sheet) sequence is occupied by a continuous octahedral sheet. The chlorite layer periodicity is about 1.4 nm.The lateral dimensions of the tetrahedral sheet are usually greater than those of the octahedral sheet. The lateral misfit between the two sheets requires adjustment in one or both sheets, causing the layer structure to deviate from ideal hexagonal symmetry. The lateral dimensions of tetrahedra can be reduced in three ways: i) by rotating adjacent tetrahedra in the opposite direction as evaluated by the rotation angle alpha measuring the deviation from 120° of each angle in the ring; ii) by thickening the tetrahedral sheet thereby reducing the basal area of each tetrahedron as evaluated by the angle tau (i.e., the deviation from 109° 28’ of Oa-T-Ob triads) and in this case, the tetrahedral sheet, frequently adjusts its lateral dimensions by decreasing (or increasing) its thickness; and iii) other structural arrangements, such as the tilting the tetrahedral sheet evaluated by deltaz which may be required to fit the tetrahedral and octahedral sheets. For more details see Brindley and Brown [1980], Bailey [1988a], de la Calle and Suquet [1988], Evans and Guggenheim [1988], Giese [1988], Güven [1988], Wicks and O’Hanley [1988], and Moore and Reynolds [1989], Brigatti and Guggenheim [2002].