Fundamentals of Soil Science (1+1)

Types of layer Silicates

Types of layer Silicates

• On the basis of the number and arrangement of tetrahedral (silica) and octahedral (alumina-magnesia) sheets contained in the crystal units or layers, silicate clays are classified into three different groups:

(a) 1 :1 Type clay minerals
(b) 2:1 Type clay minerals
(c) 2: 1: 1 (or) 2:2 Type clay minerals (Fig.1)

a)1:1 layer Silicates:

• In soils, kaolinite group is the most prominent 1:1 clay mineral, which includes kaolinite, hallosite, nacrite and dickite.
• These have one Si tetrahedral and one Al octahedral sheet per crystalline unit
• The tetrahedral and octahedral sheets in a layer of a kaolinite crystal are held together tightly by oxygen anions, which are mutually shared by the silicon and aluminum cations in their respective sheets.
• These layers, in turn, are held together by hydrogen bonding. Consequently, the structure is fixed and no expansion ordinarily occurs between layers when the clay is wetted.
• Cations and water do not enter between the structural layers of a 1:1 type mineral particle. The effective surface of kaolinite is restricted to its outer faces or to its external surface area.
• Kaolinite crystals usually are hexagonal in shape.
• Because of the strong binding forces between their structural layers, kaolinite particles are not readily broken down into extremely thin plates.
• Kaolinite exhibits very little plasticity, cohesion, shrinkage, and swelling.

b) 2:1layer Silicates

• The crystal units (layers) of these minerals are characterized by an octahedral sheet sandwiched between two tetrahedral sheets. Three general groups have this basic crystal structure. Two of them, smectite and vermiculite are expanding type minerals, while the third mica group (illite), is non-expanding

Expanding Minerals:

• The smectite group of minerals includes montmorillonite,beidellite,nontronite and saponite
• This group of minerals are noted for their interlayer expansion and swelling when wetted. The water enters the interlayer space and forces the layers apart. Montmorillonite is the most prominent member of this group in soils.
• The flake-like crystals of smectite are composed of an expanding lattice 2:1 type clay mineral. Each layer is made up of an octahedral sheet sandwiched between two tetrahedral (silica) sheets.
• The layers are loosely held together by very weak oxygen – oxygen and cation-to-oxygen linkages.
• Exchangeable cations and associated water molecules are attracted between layers causing expansion of the crystal lattice.
• The internal surface exceeds the external surface of clay crystal. In montmorillonite, magnesium replaces replaced aluminum in some sites of octahedral sheet.
• Likewise, some silicon atoms in the tetrahedral sheet may be replaced by aluminum. These substitutions give rise to a negative charge.
• These minerals show high cation exchange capacity, swelling and shrinkage properties. Wide cracks commonly form in smectite dominated soils (e.g., Vertisols) when dried. The dry aggregates or clods are very hard, making such soils difficult to till.

Vermiculites

• Vermiculites have structural characteristics similar to those of montmorillonite in that an octahedral sheet is found between two tetrahedral sheets.
• In the tetrahedral sheet of most vermiculite, aluminum is substituted by silicon in most of the sites. This accounts for most of the very high net negative charge associated with these minerals.
• Water molecules, along with magnesium and other ions, are strongly adsorbed in the interlayer space of vermiculites. They act primarily as bridges holding the units together rather than as wedges driving the units apart.
• The degree of swelling is less for vermiculites than for smectite. Therefore, vermiculites are considered limited expansion clay minerals, expanding more than kaolinite but much less than the smectite.
• The cation exchange capacity (CEC) of vermiculite is higher than all other silicate clays because of very high negative charge in the tetrahedral sheet. Vermiculite crystals are larger than those of the smectite but much smaller than those of kaolinite.

Non-expanding minerals:

• Micas are the type of minerals in this group-muscovite and biotite.
• Weathered minerals similar in structure to these micas are found in the clay fraction of soils.They are called fine-grained micas or illite.
• The basic structure of illite or micaceous mineral is similar to that of montmorillonite. However, the particles are much larger than those of the smectite.
• Some of the silicon ions are replaced by aluminium ions in the tetrahedral sheet (20% ). This results in a net negative charge in the tetrahedral sheet which is compensated by potassium ions
• The potassium as a binding agent, preventing expansion of the crystal. Hence, fine-grained micas are quite non-expanding.
• The properties such as hydration, cation adsorption, swelling, shrinkage and plasticity are less intense in fine grained micas.The specific surface area varies from 70 to 100 m² g^-1, about one eighth that for the smectite.

c) 2:1:1 layer Silicates :

• This silicate group is represented by chlorites. Chlorites are basically iron magnesium silicates with some aluminum present.
• The crystal unit is composed of one 2:1unit like mica or montmorillonite and one octahedral unit, Brucite, Mg₃(OH) ₆ layer.
• Mg dominates the octahedral sheet in the 2:1 unit.
• There is no water adsorption between the chlorite crystal units, which accounts for the non expanding nature of this mineral.