Silicate clay Properties, Classification and Structure

The electro-microscopic clay minerals with a diameter of less than 2 microns are known as silicate clay. The electro-microscopic clay minerals with a diameter of less than 2 microns are known as silicate clay.

(i) Chemical composition and Size:

According to a chemical study, clay contains silica, alumina, iron, and mixed water. Between 90% and 98% of the colloidal clay is made up of these. Plant nutrients like Ca, Mg, and K, among others, are present in the soil colloidal matter. Clay is made up of several types of hydrated aluminum-iron silicates combined, in certain situations, with an excess of sesquioxides or silica.

(ii) Shape:

By using an electron microscope to analyse silicate clay minerals, it was discovered that the particles are laminated, consisting of layers of plates, flakes, or even rods. Each clay particle is composed of many plates that resemble structural components.

Depending on the kind of clay mineral, variable amounts of power are used to hold the various units or flakes of clay minerals together. Some clay flakes have sharp edges, while others have ragged or fluffy edges. In every instance, the horizontal axis develops clay minerals more than the vertical axis.

(iii) Surface Area:

When clay is in an aqueous solution, its surface area is often defined as the portion of the particle that is accessible to ions or molecules. A sizable portion of the exterior surface must be exposed by every clay particle (finer percentage of soil).

There are also significant interior surfaces in certain clay. This interior space lies between each particle’s plate-like crystal components (Fig. 9.1). Clay colloids’ enormous surface area is therefore owing to both their plate-like structure and fineness.

For completely dispersed clay suspensions, the surface areas of the clay particles may be determined using cetylpyridinium bromide (which dissolves in water). Surface area for clays such sodium montmorillonite ranges from 700 to 800 m2/g; vermiculites and other mixed layer clays range from 300 to 500 m2/g; micaceous clays range from 100 to 300 m2/g; and kaolinitic clays range from 5 to 100 m2/g.

(iv) Electronegative Charge

Since clay micelles (micro cells) have negative charges, various ions (cations) with opposing charges are drawn to each colloidal clay crystal. The colloidal clay particles have both an exterior and an interior ionic layer, which have surfaces with a strongly negative charge (cations swarming layer).

(v) Adsorbed Cations:

Clay micelles are capable of binding a variety of cations, including H+, Al3+, Ca2+, Mg/+, Na+, and K+. The amount of these cations that clay can hold varies depending on its kind. The physical and chemical characteristics of the soil are frequently determined by cations adsorbed (if dominant) on the clay colloids, which in turn affects plant development.

Classification Of silicate clay

Tetrahedral and octahedral sheet numbers and arrangements are used to divide silicate clays into three groups: 1. 1:1 Type Clay Minerals 2. 2:1 Type Minerals 3. 2: 1:1 Type Minerals.

1. 1:1 Type Clay Minerals:

One silica sheet and one alumina sheet together make up this silicate clay. Kaolinite is a major component of the 1:1 type group in soils. The others are Dickite, Nacrite, and Halloysite.

Oxygen anions (O2-), which are shared by Si4+ and Al3+ in their respective sheets, hold the two sheets together. These units in turn are tightly bound together by H- bonding.

The lattice is stable as a result of strong bonding, which prevents expansion between two units when they are wet, the absence of cations and water between the units, the minimal amount of isomorphic substitution (CEC), and the low levels of plasticity, cohesion, shrinkage, and swelling. A high degree of intensity colloidal characteristics are not present in kaolinite. Kaolinite units range in size from 0.1 to 50 m in width, with a majority having 0.2 to 2.0 m. They are pseudohexagonal in form. The kaolinite group is larger than the others.

Due to the tubular crystals and sheets of water in between these layers, halloysite’s plasticity, shrinking, and swelling are somewhat greater than those of kaolinite. There is only 15 m2/g of total surface area per unit mass.

2. 2: 1 Type Minerals:

Two tetrahedral (silica) sheets are surrounded by an octahedral (alumina) sheet.

3. 2:1:1 Type (or 2:2) Type Mineral:

Example – chlorites.

Essentially, chlorites are ferro-magnesium silicates with a little amount of aluminium.

Like Brucite [Mg (OH)2], chlorites feature an additional layer of alumina that is dominated by magnesium. However, Mg2+ also predominates in 2:1 type minerals’ alumina (trioctahedral) sheet. Thus, the crystal unit consists of two tetrahedral silica sheets and two alumina sheets with a magnesium composition. It is frequently referred as as 2:2 type clay mineral for this reason. In other words, chlorites are mostly magnesium silicates with a small amount of iron and aluminium. The CEC is similar to illite in that it is not growing.

Example:

A sodium-dominated alkali soil that covers the clay micelle’s surface is what gives the soil its poor physical state.

Amorphous Clays:

They are silica and aluminium mixes that haven’t crystallised into well-oriented structures. Since they don’t crystallise, they are technically not minerals. These clays are found in areas where there were a lot of weathered materials present but not enough time or favourable circumstances for crystal formation. Amorphous clays are frequently found in volcanic ash-derived soils.

They exhibit contradictory characteristics, such as strong positive charges (high anion exchange capacities) or even high cation exchange capacities. These clays have a changeable charge that is dependent on how much H+ is in solution since practically all of their charge comes from hydroxyl (OH) ions, which can obtain a positive ion or lose the H+ attached (the soil acidity).

Sesquioxide Clays (Metal Oxides and Hydrous Oxides):

Most of the silica and a large portion of the aluminium are dissolved and slowly washed away in humid, warm climes under conditions of widespread leaching by rainfall and long-term severe weathering of minerals. Sesquioxides are the remaining substances, which have reduced solubilities.

Sesquioxides (metal oxides) are combinations of iron oxide (Fe2O3) or iron hydroxide (Fe(OH)3) and aluminium hydroxide (Al(OH)3). Due to the fact that the formulas for iron and aluminium clays are Al2O3.xH2O and Fe3O3, respectively, and that xH2O contains 1.5 times as much oxygen as Al or Fe, these compounds are referred to as sequioxides. These clays, which might be crystalline or amorphous, do not swell. They don’t attach to things and don’t act like silicate clays do.

Structure of Silicate Clays:

The majority of silicate clays are composed of planes of oxygen atoms held together by silicon and aluminium atoms by the attraction of positively and negatively charged atoms known as ionic bonding. A layer is made up of three or four planes of oxygen atoms with silicon and aluminium ions in between.

Like a deck of cards, a clay particle is made up of numerous layers. Aluminosilicates make up most silicate clays (aluminium and silicon components of the clay structure). In Fig. 9.2, these two fundamental molecular parts are depicted.

Due to its four-sided shape, one silicon atom is surrounded by four oxygen atoms to form a silica tetrahedron, and the unit possesses tetrahedral coordination. A silica tetrahedral sheet is a term used to describe the planes of oxygen bound together by Si4+. The aluminium octahedron is an eight-sided building unit made up of six hydroxyls or oxygen atoms around the core aluminium atom (Fig. 9.2).

A large number of aluminium octahedra are placed in a plane to produce an aluminium octahedral sheet. These aluminium octahedra are connected to one another by shared oxygen atoms in an octahedral layer. A 1:1 lattice is made of one silica sheet and one aluminium sheet. Two silica sheets and one aluminium sheet make up a 2:1 lattice.

The structures of numerous layer silicates, including mica, vermiculite, montmorillonite, chlorite, kaolinite, and other interstratified and intergradient layer silicates, are made up of various combinations of these two basic structural units (tetrahedral and octahedral sheets).

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