Introduction

Clay minerals are classified as hydrated aluminous phyllosilicates (from Greek word phyllon, meaning leaf) or sheet silicates, where the silicon tetrahedra (silicon coordinated with 4 oxygen) and typically aluminum or magnesium octahedra (aluminum or magnesium coordinated with 6 oxygen) form a 2D sheet like structure (Figure 1).^{[1, p.493, 458]} However, there seems to be some discrepancies with this common clay mineral definition, since minerals such as talc (Mg₃Si₄O₁₀(OH)₂) contain aluminum only in trace amounts,^[2] and talc is still categorized as a clay mineral. Also, other minerals such as micas share a similar layered structure. One of the distinguishing factor of clay and typically clay minerals is that the particle size is smaller than 0.002 mm,^[3] whereas individual mica crystals can be as large as 7 m²,^[4] although micas with very small particle size can be considered as clay minerals. This however does not mean that a clay mineral necessarily has a small particle size. Other cations that are typical in clay minerals are iron, alkali metals and earth alkali metals.

Clay minerals are products of physical and chemical weathering of soils and rocks which are typically high in aluminum and silicon bearing minerals.^[1][5] During weathering processes earlier formed minerals such as feldspar are slowly destroyed by chemical reactions after which various ions in solution recombine forming clay minerals as well as other oxides and hydroxides.^{[1, p.290]}

Figure 1. a) Structure of Kaolinite (Al₂Si₂O₅(OH)₄) showing the two layers of silicon tetrahedra (blue color) and aluminum octahedra (gray color). In Kaolinite there are no interlayer cations between the two sheets. The silicon tetrahedra are bonded to four oxygen atoms (red color) and the aluminum octahedra share two oxygen ions with the silicon tetrahedra. The other four corners of the octahedra are hydroxyl (OH) anions.  In b) the crystallographic direction is [001]. Figure: Aleksi Rantanen.

Basic Structure

The basic structural component of a clay mineral and all phyllosilicates is the infinitely extending tetrahedral silicate sheet (Figure 2 a).^{[1, p.162-163]} Each silicate tetrahedra are connected to three other tetrahedra leaving one of the oxygens to bond with other ions or polyhedra. The silicates are also classified by the sheet structure of the octahedral layers (Figure 2 b and c).^[6] In trioctahedral sheet silicates each O or OH ion is surrounded by three divalent cations (typically Mg⁺² or Fe⁺²) and in the dioctahedral sheets each O or OH ion is surrounded by two trivalent cations (typically Al⁺³).

Figure 2. a) A single layer of silicon tetrahedra, which is also a defining characteristic of phyllosilicate minerals. The smallest formula unit in phyllosilicates is (Si₂O₅)^2- b) A single layer of aluminum octahedra. The octahedra containing aluminum(III) ions are arranged in a hexagonal pattern which is called the dioctahedral layer. c) Trioctahedral layer containing magnesium(II). The octahedral sheet is continuous and each oxygen is shared between three octahedrons.^[6] Figure: Aleksi Rantanen.

Phyllosilicates are also structurally separated based on the stacking of the tetrahedral (T) and octahedral (O) layers.^[7] The stacking of T and O sheets are typically written as 1:1 (repeating TO layers), 2:1 (repeating TOT layers), and 2:1:1 (repeating TOT-O layers), where the first and the second index indicate the number of the T and the O sheets and the third number indicates that there is a separated O sheet between the TOT sheets (Figure 2).

Figure 3. Phyllosilicate sheet structures showing the a) 1:1 or TO, b) 2:1 or TOT, and c) 2:1:1 or TOT-O sheet structures. Figure: Aleksi Rantanen.

Classification

Silicate minerals are classified into six different groups, based on how the silica tetrahedra are connected to each other.^{[1, p.162-163]} These groups are neso-, soro-, ino-, cyclo-, phyllo-, and tectosilicates. Clay minerals belong to phyllosilicates and these are further divided to micas and clay minerals. Clay minerals are classified based on the layered structure of the polyhedra (TO, TOT or TOT-O), the chemical composition, and the type of the octahedral layer (dioctahedral or trioctahedral).^[8] Clay minerals are typically separated into Kaolinite, Smectite, Vermiculite, Illite and Chlorite groups,^[3] as well as others such as pyrophyllite-talc, palygorskite and sepiolite.^[9] These groups contain many different minerals, but the minerals within a group share similar structural, chemical or physical characteristics. For example the Chlorite group contain minerals such as Borocookeite LiAl₄(Si₃B)O₁₀(OH)₈, Chamosite (Fe,Mg)₅Al(Si₃Al)O₁₀(OH)₈, Clinochlore (Mg,Fe²⁺)₅Al(Si₃Al)O₁₀(OH)₈, etc., but the layered structure of all chlorite minerals is 2:1:1 or TOT-O. Clay mineral classification is shown in Table 1.^[8] Some clay minerals such as Smectite, Vermiculite, Halloysite, and even some micas can swell as they absorb water.^[8] In these minerals water and other ions are absorbed to the surface of the grain or between the interstices of individual TO or TOT layers.

Table 1. Clay mineral classification with groups, minerals and their chemical composition, and structural characteristics. The structural data and the structure are shown for the mineral in bold. The swelling clays are color coded in red. The Table is based on a Figure from Pavon and Alba 2021. ^[8] All Figures within the table are by Aleksi Rantanen.

*The table does not include all the minerals of each group.

**Vermiculite is sometimes classified as its own group and sometimes it is under the smectite group.^[2]

***Many of the minerals have multiple polymorphs, but only one is show in the Table

Applications

Clay minerals have interesting chemical and physical properties, which makes them ideal for both already well established industries, such as paper making, ceramics, and drilling mud, etc.,^[1] and for basic building blocks of new functional materials.^[13] Clay minerals are also cheap and they come in large quantities, which makes their acquisition easy.^[11]

Bentonite is a term that is used of a material that is almost entirely composed of montmorillonite clay mineral. Montmorillonite swells as water is incorporated between the TOT layers, but also because the structure of montmorillonite is electrically unbalanced, having a slight overall negative charge, the fine clay particles are able to exchange cations around the edges of the individual minerals. Because of these physical and chemical properties bentonite has many industrial applications such as in drilling muds, as desiccant and absorbent, in foundry molds for metal casting, as an absorbing material for industrial spills and in iron ore enrichment.^{[1, p.458]}

Vermiculites are used for their ability to expand with the introduction of water. As vermiculite is heated it exfoliates and creates a material that has high thermal stability and is suitable for thermal insulation.^{[1, p.458][13]} Vermiculites are also used as absorbent materials for environmentally hazardous fluids, as well as agents for soil conditioning in agriculture where vermiculite is used as a fertilizer carrier, herbicide, and insecticide.^{[1, p.458]} As vermiculite is heated it exfoliates to layered nano particles with a thickness of approximately 1.4 nm.^[13] This has the potential of being used as a basic building block of different types of nanomaterials. The hydrophobic and hydrophilic properties of vermiculite can be changed by different techniques, such as chemical crafting or surface coating. These properties might come useful in different fields, for example, adsorption separation, polymer composites, catalysis, energy, and biomedicine.

Kaolin is a term commonly used for clay that contains almost exclusively kaolinite. Kaolin is desirable, because it is low in contaminants, such as iron, it has a fine and even particle size, and it is pure white in color. Kaolin has many useful applications, such as in paper, plastic, and rubber industry, and it is also used in ceramics, tile, porcelain, and enamel. In paper industry kaolin is used to fill the surface irregularities and enhance inc absorption, opacity, brightness, and smoothness of the paper. Kaolin can be used as a filler in plastics and rubber and as an additive in paint.^{[1, p.457-458]} Kaolinite is the mineral used typically in the ceramic industry and it is the main component in the bathroom sink and toilet, since the mineral does not absorb any water.^{[14][1, p.457-458]}

Halloysite is a kaolinite group clay mineral with the same chemical composition as kaolinite. Halloysite forms nanotube structures which can be used to make different composite materials. Vinokurov V.A. et al. 2017 intercalated the Halloysite nanotubes with furfuraldehyde converting it to tetradentate ligands which bind to Ru(III) ions. Figure 4 shows transimission electron microscope images (TEM) of the nanocomposite material. These types of composite materials have high surface area working well as catalysts and can also be used for sorption of metal ions from aqueous solutions. Halloysite as well as many other clay minerals are ideal raw material because they are inexpensive and are available in large quantities.^[11]

Figure 4. Left: TEM images of the Ru containing Halloysite nanotubes. Right: steps for the composite material and the proposed structure of the Ru intercalated Halloysite nanotube. ^[11] Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/).

Other applications where clay minerals are used include cement, adhesive, asphalt, food, and health-care industry.

References