In this article we will discuss about:- 1. Structure of Kaolinite 2. Electrical Charge and Isomorphous Substitution 3. Morphology 4. Minerals in the Group 5. Formation and Occurrence 6. Commercial Uses.
Structure of Kaolinite:
The basic structural unit of kaolinite mineral consists of an alumina sheet combined with a silica sheet. The silica sheet and the gibbsite sheet are held together by hydrogen bonding between the oxygen atoms at the tips of the silica sheet and the hydroxyl ions of the alumina sheet, forming the common interface between the two sheets. The individual structural unit of kaolinite is shown in Fig. 3.3(a). A typical kaolinite particle is formed by stacking of several structural units of kaolinite mineral one over the other as shown in Fig. 3.3(b).
The bond between the unit layers is through the hydrogen bond between the silica sheet of one unit layer and the alumina sheet of the next unit layer. The bond is relatively strong, preventing hydration between the layers and allowing several layers to build up. The thickness of each structural unit of the kaolinite mineral is 7.2 Å. A kaolinite particle is made of 70-140 layers of individual structural units.
The chemical formula of kaolinite is Al2 Si2O5 (OH)4 and its molecular weight is 258.16 g. It is white, brownish white, grayish white, yellowish white, or grayish green in color. The specific gravity of the mineral is about 2.6. Figure 3.4 shows the typical X-ray diffraction pattern of kaolinite mineral.
The kaolinite mineral is electrically neutral. Only two-thirds of the octahedral positions in the gibbsite sheet are filled by aluminum and the remaining one-third is left vacant, and the mineral is called dioctahedral. However, some hydroxyl ions of the silica sheet dissociate in the presence of water and lose hydrogen, which gives the mineral a net negative charge.
The positively charged ions (cations) present in the pore water, such as Na+, Ca2+, Mg2+, and K+, are attracted to the negatively charged surface. The electrical force of attraction between the negatively charged surface and the cations holds the pore water in the intervening space between the cations. This pore water, which is under the influence of the electrical force of attraction, is known as adsorbed water.
Morphology of Kaolinite Mineral:
The particles formed by kaolinite mineral are hexagonal in shape with a thickness of about 0.05-0.1 µm (0.00005 – 0.0001 mm). Kaolinite particles have lateral dimensions of the order of 1-2 (am (or 0.001 – 0.002 mm). Figure 3.5 shows the scanning electron micrograph of a kaolinite particle. Each kaolinite particle has several stacks of hexagonal mineral units.
The volume of typical kaolinite particle is about 3 x 10–11 mm3, considering the particle as a hexagonal prism of equal sides of about 0.5 µm (0.0005 mm) size and about 0.05µm (0.00005 mm) thickness. Taking the density of soil solids as 2.7 g/cc, the weight of a typical kaolinite particle is about 8 ×10–14 g.
The surface area per unit weight, known as specific surface, is about 15 m2/g and is the least of all clays. Hence, the surface forces of attraction and repulsion are not very much significant in kaolinite, though the weight of kaolinite particles is small. Among all the clay minerals, kaolinites have low liquid limit, plasticity, and activity. This is due to the less significant influence of surface forces than that of gravity forces.
Minerals in the Group:
Because the basic structural unit of kaolinite is formed by combining one silica sheet and one alumina sheet, the kaolinite mineral is also called 1:1 clay mineral. Kaolinite, halloysite, and dickite are the important minerals in the kaolinite group. Out of these, kaolinite mineral is the most important in the group. A typical example of kaolinite mineral is china clay.
When a layer of water molecules exists between the basic structural units of kaolinite, the mineral is called halloysite. Halloysite does not rehydrate after losing water on heating. The dehydrated halloysite is known as metahalloysite. Metahalloysite shows a tubular structure in scanning electron micrograph.
Formation and Occurrence of Kaolinite:
Soils, formed by weathering in a warm moist climate, contain the kaolinite mineral. The kaolinite mineral is formed when alumina is abundant and silica is scarce, because it requires availability of alumina. Kaolinite is generally formed by leaching of acidic granite from feldspar and mica present in them. Kaolinite is formed by weathering of rocks in the areas of high rainfall having good drainage conditions resulting in effective leaching of cations.
Low pH, low electrolyte concentration, and removal of Ca, Mg, and Fe ions by leaching are favorable conditions for the formation of kaolinite. Kaolinite is not formed when calcium is present in a significant proportion.
Kaolinite can be identified in a soil by the X-ray diffraction (XRD) peaks of 7.13 Å in the first-order reflection and 3.56 Å peak in the second-order reflection. The quantity of kaolinite present in a soil can be determined from differential thermal analysis (DTA).
Halloysite is formed in volcanic areas of high rainfall by leaching of feldspar by hydrogen sulfide, which is formed by the oxidation of pyrite.
Commercial Uses of Kaoline:
Kaolin is used in ceramics, medicine, coated paper, cosmetics and as a food additive in toothpaste, and also as a light diffusing material in white incandescent electric bulbs. It is generally the main component in porcelain.
It is also used as filler for paint, rubber, and plastics as it is relatively inert and is long lasting. However, the greatest demand for kaolinite is in the paper industry to produce a glossy paper. Natural kaolinite usually contains small amounts of uranium and thorium. Hence, it is useful in radiological dating.