In this article we will discuss about:- 1. Structure of Illite Mineral 2. Electrical Charge and Isomorphous Substitution 3. Morphology and Surface Area 4. Formation and Occurrence.
Structure of Illite Mineral:
The basic structural unit of illite mineral is identical to that of the montmorillonite mineral. It consists of an alumina sheet sandwiched between two silica sheets. However, the individual structural units are bonded, with one another, by non-exchangeable potassium (K+) ions between them.
This gives a stronger bonding between the structural units in illite than in montmorillonite. The potassium ions just fit into the space between the silica sheets of the neighboring structural units. The mineral is usually dioctahedral with only two-thirds of the octahedral positions of the alumina sheet occupied by aluminum, the remaining positions being left vacant.
The chemical formula of illite is (K,H3O)(Al, Mg, Fe)2(Si,Al)4O10[(OH)2,(H2O)] and its molecular weight is 389.34 g. It is usually gray-white to silvery-white, or greenish-gray in color. The specific gravity of the mineral varies from 2.6 to 2.9. Illite is also called clay mica or hydrous mica or hydrous muscovite.
There is often substitution of a silicon atom in the silica sheet by aluminum atoms, by isomorphous substitution that gives additional negative charge to the surface.
Morphology and Surface Area of Illite Mineral:
Because of the strong linkage between the structural units with potassium ions, the illite particles are much thicker than montmorillonite particles, although the lateral dimensions for the two are the same.
The thickness of illite particles is about 0.005 – 0.05 µm (or 0.000005 – 0.00005 mm), and the lateral dimensions are 0.1 – 0.5 µm (or 0.0001 – 0.0005 mm). This means that 5-50 structural units are combined to form the illite particle. The specific surface of illite is about 80 m2/g. the SEM image of the illite mineral.
The volume of a typical illite particle is about 1 × 10 –12 mm3 considering the particle to be of the shape of a rectangular prism. Taking the density of soil solids as 2.7 g/cc, the weight of a typical illite particle is about 2.7 × 10–15 g, which indicates how small is the weight of the particles and how the electrical forces of attraction and repulsion predominate the behavior of illite clays. However, here the influence of surface forces is much less than that in montmorillonite because of smaller surface area and more than that in kaolinite.
Illite swells or expands on the absorption of water. The magnitude of this swelling is more than that of kaolinite but less than that of montmorillonite. The bond between structural units in illite is stronger than that in montmorillonite and considerably weaker than that in kaolinite. Illite is generally associated with montmorillonite in clays.
Formation and Occurrence of Illite Mineral:
Illite is formed by the weathering of sedimentary rocks and is abundant in slate and shale. Illite is formed where silica is abundant and potassium is present. It is usually formed from igneous and metamorphic rocks and also by weathering of feldspar in cool climate. Montmorillonite may be converted to illite when potassium is present in significant quantities. It occurs in soils, formed by weathering of sedimentary rocks, in temperate and arid regions.