Explain the basic structures of clay minerals.

Clay mineralogy is the science dealing with the structure of clay minerals on microscope, molecular and atomic Scale.The clay minerals are basically composed of tiny crystalline substances of one or more members of a small group of minerals-commonly known as clay minerals. These minerals are evolved mainly from the chemical weathering of certain Rock forming minerals.Chemically these minerals are hydrous aluminosilicates with other metallic ions.

The clay minerals Are divided into three main groups on the basis of their crystalline arrangement as shown in the following table:

Clay minerals are basically composed of two structural unit:

• Tectahedral unit

• Octahedral unit

Tetrahedral Units:

A tetrahedral units consists of a silicon atom surrounded by four oxygen atoms, forming the shape of tetrahedron. Oxygen atom are at the tips of the tetrahedron, whereas the Silicon atom is at its centre. Each of the oxygen ions at the base is common to 2 units. The arrangement is shown in the figure.

Each of the three oxygen ions at the base shares its charges with the adjacent tetrahedral unit. The sharing of charge leaves 3 negative charge at the base per tetrahedral unit and this along with two negative charges at the apex makes a total of five negative charges to balance  4 positive charges of silicon ion . The process of sharing the oxygen Ion at the base with neighbouring units leaves a net charge of -1 per units.

Octahedral units:

An octahedral unit consist of six hydroxyl forming a configuration of an octahedron and having one aluminium atom  at the centre. As the aluminium has 3 positive charges, an octahedral unit has three negative charges. Because of net negative charge, an octahedral unit cannot exist in isolation. Several octahedral units combine to form a gibbsite sheet.  The sheet is electrically neutral.

Both the structural units can be shown as: