6 Main Types of Clay Minerals

This article throws light upon the six main types of clay minerals. The types are: 1. Kaolinite 2. Smectite Group 3. Illite Group 4. Vermiculite 5. Chlorite 6. Allophanes.

Type # 1. Kaolinite:

The most prominent member of the 1: 1 type of clay minerals is kaolinite, in which one silica tetrahedral layer is joined with one aluminum octahedral layer when the top oxygen of the silica tetrahedral layer occupies the position of the oxygen of the aluminum octahedral layer and is common to the tetrahedral and octahedral layer as shown in Fig. 7.1.

Practically no isomorphous substitution of ions taken places. A weak negative charge develops on the surface of the basal oxygen of the silica tetrahedral layer and a weak positive charge develops on the surface of the hydrogen of the hydroxyl of the aluminum octahedral layer.

The positive charge on the hydroxyl surface attracts the negative charge on the oxygen surface. Hence the kaolinite units are tightly bound by hydrogen bonds. Two-thirds of the octahedral positions are occupied by aluminum ions and one-third is empty as shown in Fig. 7.1. The cation exchange capacity is 3 to 15 mill-equivalents per 100 gm. The total surface area is 37 to 45 sq. metres/gm. The external surface area is 30 to 35 sq. metre/gm. internal surface area is 7 to 10 sq. metres/gm.

Type # 2. Smectite Group:

One central layer of aluminum octahedron is attached with two layers of silical tetrahedra, one at the top and another at the bottom. The octahedral layer is joined with the tetrahedral layer when the top oxygen of the tetrahedral layer occupies the position of the hydroxyl of the octahedral layer as shown in Fig. 7.2.

Two-third of the octahedral positions is occupied by aluminum ions, and one-third is empty as shown. The smectite group has several members like Montmorillonite, beidellite, nontronite, saponite etc. In Montmorillonite the major replacement is Mg⁺⁺ for Al⁺⁺⁺ in the octahedral layer.

In saponite the major replacement is Al⁺⁺⁺ for Si⁺⁺⁺ in the tetrahedral layer and Mg⁺⁺ for Al⁺⁺⁺ in the octahedral layers. Major replacement is Al⁺⁺⁺ Si⁺⁺⁺⁺ in the tetrahedral layer in beidelliste and Fe⁺⁺ for Al⁺⁺⁺ in the octahedral layer in nontronite. Thus we find that there is considerable isomorphous ionic substitution mainly in the octahedral layer in various members of the smectite group.

Consequently negative charges develop on the smectite group of clay minerals. A weak negative charge also develops on the oxygen surface of the smectite group of clay minerals units, which actually repel each other.

Consequently the interlayer space between the units increases which means that the smectite group of clay mineral expands and there is a considerable / internal surface between the plate-like units, where positive cations and water molecules are attracted.

So they enter there the cation exchange capacity of Montmorillonite is 80 to 150 mill equivalent per 100 gm. The total surface area is 580 to 750 square metre per 100 gm. The external surface area is 80 to 150 sq. m. /gm. The internal surface area is 500 to 600 sq. metres/gm.

Type # 3. Illite Group:

The structure of illite is the same as that of mica. One octahedral layer occurs between the two tetrahedral layers of silica, one at the top and the other at the bottom. The octahedral layer is joined with the tetrahedral layer when the top oxygen of the tetrahedral layer occupies the position of the hydroxyl of the octahedral layer as shown in Fig. 7.3 and Fig. 7.4.

One-sixth of the silicon ions Si⁺⁺⁺ have been replaced from tetrahedral positions by aluminum ions, Al⁺⁺⁺. Hence the potassium ions K⁺ occur in the hexagonal cavity between the two units of illite and bind the illite units tightly.

Illites for this are non-expanding. When two-thirds of the octahedral positions are occupied by aluminum ions and one-third is empty, then it is called dioctahedral illite (Fig. 7.3).

When all the octahedral positions are occupied by magnesium Mg⁺⁺ and ferrous, Fe⁺ ions, then it is called trioctahedral illite (Fig. 7.4). The cation exchange capacity of illite is 15 to 40 milliequivalents per 100 gm. Total, external and internal surface areas are 120 to 170, 50 to 70 and 70 to 100 square metre per gm. respectively.

Type # 4. Vermiculite:

Vermiculite consists of two silica tetrahedral layers and one octahedral layer, joined together when the top oxygen of the silica tetrahedral layer occupies the position of hydroxyl ion of the octahedral layer. Some silicon

Ions are isomorphously substituted by aluminum ions and therefore magne­sium ions occur between the sheets of vermiculite to balance the structure electrically. Most vermiculite is of the dioctahedral type where the aluminum ion is isomorphously substituted by Mg⁺⁺, Fe⁺⁺ etc. (Fig. 7.5).

Magnesium occu­pies all the octahedral position in the trioctahedral vermiculite (Fig. 7.6). The basal oxygen surface of the tetrahedral layer has a weak negative charge. A negative charge also develops due to isomorphous ion substitution in the octahedral layer and in the tetrahedral layer. Consequently the negatively charged vermiculite units actually repel each other.

Hence the space between vermiculite units increases. But the magnesium ions which occur between units of vermiculite hold them together, so that the expansion of vermiculite is somewhat limited. Hence it expands less than the smectite group of clay minerals.

Each magnesium ions is surrounded by six outer molecules which are called bound water. The rest of the water molecules occurring between units of vermiculite are called unbound water, as shown in Figs 7.5 and 7.6. The Cation Exchange Capacity of Vermiculite is 100 to 150 mill equivalent per 100 gm. The total external and internal surface areas are 780 to 900, 80 to 100 and 700 to 800 square metres gm. respectively.

Type # 5. Chlorite:

The most common member is chlorite. Chlorite consists of one unit of a vermiculite layer and one unit of a magnesium octahedral layer called Brucite as shown in Fig. 7.7.

In the vermiculite layer, one central magnesium and ferrous dominated octahedral layer is attached with two silica tetrahedral layers as shown in Fig. 7.7. The top oxygen of the silica tetrahedral layer occupies the position of the hydroxyl of the Aluminum octahedral layer and is common to both the tetrahedral and octahedral layers.

Some silicon ions are isomorphously substituted by aluminum ions. All the octahedral positions are occupied by magnesium, and ferrous ions as shown in Fig. 7.7. All the octahedral positions are occupied by magnesium ions in the brucite layer as shown in Fig. 7.7.

Chlorite is also called a 2:2 type of clay mineral. The Cation Exchange capacity is 15 to 40 milliequivalents per 100gms; total, external and internal surface areas are 130 to 180, 70 to 100 and 60 to 80 square metres/gm. respectively.

Type # 6. Allophanes:

Allophanes are amorphous hydrated aluminosilicates which are random asymmetrical arrangement of silica tetrahedra and aluminum octahedra. Its approximate general formula is Al₂O₃.2SiO₂.H₂O. It has high cation adsorption capacity and considerable capacity to adsorb anions.