BASIC FEATURES OF CLAYS
There are a number of features of the clay minerals that distinguish them as a group. The most important one is the chemical analysis which shows them to be composed of essentially silica, alumina water and frequently with appreciable quantities of iron and magnesium and lesser quantities of sodium and potassium. The upper limit of the size of clay particles is defined by geologists as 2 microns, with a mica like structure with the flakes composed of tiny crystal platelets, normally stacked together face-to-face. A single platelet is called a unit-layer.
2.1 Fundamental Building units
There are two simple building units from which the different clay minerals are constructed :
Octahedral Layer
This unit consists of two sheets of closely packed oxygen or hydroxyl atoms into which aluminium, iron or magnesium atoms are embedded in an octahedral structure. When aluminium is present, only two thirds of the ionic positions required to balance the structure are filled (Gibbsite Al(OH)3). When magnesium is present, all the positions are filled, thus creating a balanced structure (brucite, Mg(OH)).
Tetrahedral Layer
In each tetrahedral unit, a silicon atom is located in the centre of a tetrahedron, equidistant from four oxygen atoms, or hydroxyls. The base of the silica tetrahedral groups are arranged to form a hexagonal network, which is repeated infinitely to form a sheet of composition, Si406(OH)6.
These layers are tied together by sharing common oxygen atoms. It is the different combinations of these units and modification of the basic structure that give rise to the range of clay minerals with different properties. The two predominant units are the alumina octahedral sheet and the silica tetrahedral sheet.