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The crystalline structure of clay minerals is built
up from different types of sheets or layers. The fundamental building
blocks of these sheets are the tetrahedron and the octahedron units. The
tetrahedron is composed of either a central silicon or aluminium surrounded
by four oxygen ions in a tetrahedral coordination 1.
The octahedron is composed usually of a central poly-valent cation surrounded
by six oxygen (O) or hydroxyl (OH) ions in an octahedral coordination.
Whether a cation forms tetrahedral or octahedral coordination with oxygen
depends on the relative size of the cations and anions involved. This
is commonly expressed as the radius ratio rr or ρ (the greek letter
rho). Where, ρ is equal to the cation radius divided by the anion
radius. All the radius ratios given here are with respect to the oxygen
anion (Pauling, 1960; Huheey,
1983; West, 1984).
When 0.414 < ρ < 0.732, then this reults in a coordination number of 6 and an octahedral geometry; for magnesium, ρ = 0.47; ferrous, ρ = 0.57 and ferric, ρ = 0.31. Note that aluminium ρ = 0.41 is borderline, hence, it has the ability to fit in either octahedral or tetrahedral coordination.
In summary, the cation is stable in a particular coordination environment, as long as it is able to keep the oxygen anions from touching, thereby preventing repulsive forces from destabilizing the structure. For example, silicon is small enough that only 4 oxygen’s are able to fit around it and the most stable arrangement of these oxygen’s is in tetrahedral coordination. Cations such as Mg, Fe(II) and Fe(III) are larger and thus able to accommodate 6 oxygen’s in their coordination environment. Aluminium's size is in between silicon and, iron and magnesium, therefore, it has the ability to fit in either octahedral or tetrahedral coordination.(top)
The tetrahedron and octahedron make up the basic building block. As previously mentioned, the sheet silicates are composed from the tetrahedral and octahedral basic building blocks, as shown in figure 1. The tetrahedral layer is comprised of Si-tetrahedron joined at their basal oxygen's (the oxygen's at the base of the tetrahedron). The linked Si-tetrahedron in the tetrahedral layer forms a network of hexagonal holes as demonstrated by the hyperlink above. This will become important as the '2:1 clay minerals' are discussed below. All the tetrahedral bases are in the same plane; hence their apices (or tips) point in the same direction and form a common layer with one face of an octahedral sheet (Grim, 1968). The tetrahedral sheet is therefore asymmetrical with respect to surface morphology. Since the three basal oxygens are shared, the tetrahedral layer is considered to have a unit cell formula of Si2O5 – derived from one tetrahedron: one Silicon plus half of three shared oxygens 1.5 plus the apex or tip oxygen 1 which is equivalent to SiO2.5 but; normal considerations of stoichiometry double the formula unit to give integer values, Si2O5 (Blatt et al, 1972).(Top)
figure 1: the Si-tetrahedron and Al-octahedron fitting together to form layers. (Source: Monash University, Department of civil engineering)(top)
The octahedral layer comprises two sheets of closely packed oxygen or hydroxyl ions with aluminium, magnesium or iron cations embedded within it; each in octahedral coordination and equidistant from six oxygen or hydroxyl ions (Grim, 1968). These octahedral unit cells share their oxygen or hydroxyl ions with adjacent cells forming an extensive, symmetrical sheet. Due to the nature of the octahedral sheet being able to accomodate different cations, the octahedral layer can be either di-octahedral or tri-octahedral. The di-octahedral layer is where 2 out of 3 of the octahedral sites are occupied by a trivalent cation, such as aluminium or ferric cations. Here, 2 cations multiplied by the 3+ ionic charge on each cation = 6. In the case of the tri-octahedral layer 3 out of 3 sites are occupied by a divalent cation , that is, a +2 ion such as ferrous or magnesium. Here, 3 cations multiplied by the 2+ ionic charge on each cation = 6. To emphasise, the di- and tri- prefixes refers to the number of octahedral sites occupied and NOT the charge on the cation.
If only aluminium is present, as in kaolinite (Mason and Berry 1968), the structure is called a gibbsite layer (or sheet or structure) with a unit cell formula of Al2(OH)6 (Grim, 1968), often abbreviated as the stoichiometric equivalent Al(OH)3 (Klein and Hurlbut, 1993). Hence gibbsite (and thus kaolinite) is di-octahedral. The alternative, with only a divalent central ion, is tri-octhedral; with magnesium as a divalent, central ion, the structure is called a brucite layer (or sheet or structure) with a unit cell formula of Mg3(OH)6 (abbreviated as Mg(OH)2). Gibbsite or hydrargillite (hydrous aluminium oxide, Al2(OH)6 or Al2O3•3H2O) and brucite (hydrous oxide of magnesium, Mg(OH)2) are usual minerals in their own right in literature on clay chemistry, mineralogy and structure.
Before moving on to construction of the clay minerals using the fundamental building blocks discussed, charge development must first be understood. Isomorphic substitution is the substitution of similar sized cations (but of a different species) for those present in the crystal structure without changing the crystal structure. This is common in clay minerals and is important in the differentiation of clay minerals and their behaviour (Goldman et al, 1990); particularly in relation to charge development on the surface of the clay crystal and its consequent effects on Cation Exchange Capacity (This is discussed further under Properties). For example, if aluminium replaces silicon, excess negative charge will develop on the tetrahedral layer as a trivalent cation (+3) has replaced a tetravalent cation (+4) yet the same negative charge still resides on the surrounding oxygens. Similarly if ferrous or magnesium replaces aluminium, excess negative charge develops on the octahedral layer since a divalent cation (+2) has replaced a trivalent cation (+3) without changing the surrounding negative charge on the oxygen or hydroxyl ions. This charge development necessitates that some clays include interlayer cations to balance the charge, such as potassium cations in Illite, and sodium or calcium in smectites (Blatt et al, 1972) - hence terminology such as sodium bentonite or calcium bentonite. Bentonite is a widely used clay material. Bentonite is not strictly a clay mineral but a generic name for rock or clay deposits predominantly composed of the clay mineral montmorillonite (Goldman et al, 1990). It is a highly colloidal, plastic clay originally found in cretaceous strata near Fort Benton in Wyoming, formed in situ by alteration of volcanic ash (Grim, 1968). A semi-arid climate, alkaline groundwater and alkaline soil formed the sodium bentonite found in Wyoming; calcium bentonite generally forms in coastal plain environments (Goldman et al, 1990). The consequences of excess negative charge will become apparent in the following sections.(top)
The first clay mineral that we will build is kaolinite, the simplest clay mineral. Overlaying a tetrahedral and octahedral layer forms the 1:1 or T:O (Tetrahedral: Octahedral) structure of kaolinite. The tetrahedral and octahedral layers are bonded together by sharing oxygen anions between Al and Si on the tips of the tetrahedra. The composite T:O layers, called platelets, stack up like an open sandwich or the pages in a book to from a crystal or micelle of the 1:1 mineral. Since Al is in the octahedral layer, as discussed earlier, kaolinite is a di-octahedral mineral.
The 1:1 platelets of kaolinite are held together strongly via hydrogen bonding between the OH octahedral layer hydroxyls (OH) and the tetrahedral layer oxygens (O). Due to this strong attraction, the platelets do not expand when hydrated and kaolinite only has external surface area. This affects several important properties of kaolinite: it has a lower affinity for water; it has a lower dispersivity; and it does not achieve as low a permeability on compaction as other clays. In addition, the paucity of isomorphic substitution creates a low charge development and a low CEC, typically 3 to 15 meq/100 g (Goldman et al, 1990) . (top)
Thickening the same kaolinite T:O platelet constructed above by adding a second tetrahedral layer below the octahedral layer with the tips of both tetrahedral layers pointing inwards towards the central octahedral layer (Grim, 1968) creates a Tetrahedral : Octahedral : Tetrahedral T:O:T layer. It is a 2:1 mineral because a single octahedral layer is sandwiched by two tetrahedral layers. If the di-octahedral sheet is gibbsite Al2(OH)6, the mineral is called pyrophyllite Al2Si4O10(OH)2 (Klein and Hurlbut, 1993). Note that it isn’t a simple addition of one gibbsite sheet Al2(OH)6 plus two silica tetrahedral sheets 2 multiplied by the Si2O5 equals Si4O10, since the oxygens predominate in the shared plane and displace the excess hydroxyls.
Similarly, if the tri-octahedral sheet is brucite Mg3(OH)6, the mineral is called talc Mg3Si4O10(OH)2. The bonding between the 2:1 platelets of talc and pyrophyllite are Van Der Waals bonds and are weaker than the hydrogen bonds that hold the 1:1 platelets of kaolinite together. These 2:1 layers don’t expand when hydrated, thus only have external surface area similar to kaolinite and essentially no cation exchange capacity (CEC). All of the rest of the Clay minerals are based on these two sandwiches of octahedral and tetrahedral layers (T:O and T:O:T), with various substitutions within the layers, different arrangement in the stacking and different matching of the layers.
Starting again with the same basic sandwhich structure of the 2:1 mineral pyrophyllite, isomorphic substitution of Al for Si in the tetrahedral layer produces muscovite or the basic mica group. In this mineral, there is a good deal of isomorphic substitution which consequently means the mineral has a large amount of excess negative charge.
The significance of negative charge due to isomorphic substitution becomes more apparent This excess negative charge is usually balanced by an interlayer ion, in this case, potassium. Because of potassium's ionic radii, it fits almost perfectly in the hexagonal shaped basal holes of the tetrahedral layer. Consequently, the interlayer's collapse and hold this potassium tightly. The 2:1 layers are held together due to this electrostatic attraction between the Si tetra layer and potassium. Since the layers collapse upon the interlayer potassium, pure micas are non-expanding and therefore also have only external surface area. They also have no available CEC since the inter-platelet potassium ions entirely satisfy the excess negative charge created by the isomorphic substitution.
Starting again with pyrophyllite, isomorphic substitution of Mg (a divalent cation) for Al (a trivalent cation) in the octahedral layer produces montmorillonite (Al,Mg)8(Si4O10)4(OH)8•12H2O. The isomorphic substitution is less than in muscovite hence the overall charge created is less. In addition, the isomorphic substitution is primarily in the octahedral layer (compared to the tetrahedral layer for muscovite). Since it is effectively buried or shielded within the octahedral layer (as opposed to the exposed tetrahedral layers of muscovite), montmorillonite can expand when hydrated. This is a basic demonstration of Coulomb’s Law where the force of attraction (repulsion) between charges is directly proportional to the product of the charges and inversely proportional to the square of the distance between them (Isaacs, 1996).
Montmorillonite is the aluminous (and most important) member the smectite group of clay minerals (originally called montmorillonites) which only occur as extremely small particles. A salient feature of smectite is that water or any polar liquid can enter between the layers of the platelet causing separation; these minerals can expand significantly when wetted and contract when dried 2(Grim, 1968). The intra-absorption of water (with other chemicals) allows that Montmorillonite is the most active of clay minerals, exhibiting a high CEC (80 to 150 meq/100 g); it has a very high specific surface area (50 to 120 m2/g primary, 700 to 840 m2/g secondary)3 Montmorillonite is susceptibility to swelling and is subsequently also particularly sensitive to chemical attack; interlayer cations strongly influence swelling behaviour; sodium montmorillonite can swell 15 to 20 times its volume; calcium montmorillonites, 0 to 5 (Goldman et al, 1990).
Vermiculite has less isomorphic substitution than muscovite but more than montmorillonite; so it’s properties lie between muscovite and montmorillonite. In addition its isomorphic substitution is mostly in the tetrahedral layer. Hence, this mineral is semi-expanding. Due to the expanding ability of these two minerals, both montmorillonite and vermiculite have internal and external surface area and significant CEC (Montmorillonite: 80 - 150 cmol/kg; Vermiculite: 100 - 200 cmol/kg).(top)
Reconsider the 2:1 (T:O:T) mineral muscovite again; it has isomorphic substitution of Al for Si in the tetrahedral layer. Now adding an octahedral sheet made of Al and Mg and place it in the interlayer between the 2:1 layers. We just constructed the 2:1:1 or 2:2 mineral, it is termed 2:1:1 (or 2:2) because it has another octahedral sheet in the interlayer. So, two octahedral sheets are now sandwiched by tetrahedral sheets. The extra octahedral sheet is held together in the interlayer because we have Al (trivalent) substituted isomorophically for Mg (divalent). Chlorite is an example of a 2:1:1 mineral. This is opposite to the isomorphic substitution we have been previously discussing. This isomorphic substitution gives rise to a net positive charge on the interlayer octahedral sheet. Therefore, the platelets are held together electrostatically and this mineral is non-expanding. These minerals give rise to the CEC that is essential for holding nutrient elements in an available form for agriculture.
To summarise, elements with the same valence and coordination number frequently substitute for one and other in a silicate structure, this is called isomorphic substitution. However, when elements of the same coordination number but different valency are exchanged, there is an imbalance of charge. Clay species develop because of this and the molecular formula is usually an average over the whole sample. For example bentonite, due to its phenomenal swelling abilities, makes an excellent sealant for bore holes and nuclear waste disposal sites. Bentonite is a generic name but is consistently used when sodium is substituted in the inter-layer of montmorillonite Na0.3(Al1.7Na0.3)Si3O10(OH)2.nH2O. Where R is the interlayer ion and n is dependant on the state of hydration of the clay. (top)
The first classification of phyllosilicates is based primarily on the number of tetrahedral and octahedral sheets that make up a clay micelle. Secondly, the occupancy of the octahedral layer, whether or not it is di- or tri- octahedral. Finally, charge per formula unit for each layer. Many clay minerals do occur as the dominant mineral in some areas and almost all of them occur as minor constituents with other clay minerals. Discrete clay particles may consist of alternating layers of two or more different clay minerals, which are referred to as mixed-layer clays. Common combinations contain the expandable clay mineral montmorillonite or beidellite, inter-layered with chlorite or with a mica. The most common clay minerals can be classified into five main groups:
1 The charged atoms (or radicals) within a crystal are considered as stably charged structures with their orbitals full, and are referred to as ions, either cations or anions. Cations are positively charged; eg Si4+, Al3+, Fe3+ and Fe2+ which may be written as Si(IV), Al(III), Fe(III) or Fe(II). Anions are negatively charged; eg O-2, OH-1. The nature of the bonding is such that most bonds are not purely ionic but involve a portion of covalent bonding since ever cation is surrounded by many anions and every anion is surrounded by many cations. A deal of charge sharing occurs and many texts (quite validly) regard these ions as ions.(back).
2 When the platelet structure is completely collapsed (i.e. platelet thickness or c-axis dimension about 9.6 Angstrom Units), re-expansion may proceed with difficulty and usually takes considerable time (typically 10 to 16 hours – Pers. Comm: Hayden Gardner – Rheochem Pty Ltd 2002). The Angstrom Unit symbolised as a capital with a small circle at the apex Å, often simply referred to as an Angstrom, is 10-10 metres or one ten thousandth of a micron or one tenth of a nanometre (nm =10-9 metres = 10 Å ) or one hundred picometres (pm = 10-12 metres, 1A = 100 pm). All the atoms of the periodic table vary between 0.3 and 3 Angstrom Units in radius (West, 1984, p294). An old but useful unit; X-Rays typically have a wavelength of 1 Å or less so XRD works is effective with atom arrangements and crystals. The Angstrom is still generally used in clay mineralogy and crystallography.(back).
3 Primary adsorption means the absorption on the outer, exposed surface of the platlets. Secondary absorption occurs within the platlets, along with the substituted water molecules.(back).
This web site was constructed by Benjamin.K. Galton-Fenzi. Last Updated: 21 February, 2004 11:02 AM