Structure of Clay Minerals

The outermost layer of our planet, the crust, contains the accessible mineral wealth of the planet. The eight most abundant elements in the crust (Table 1) make up 98.5% of the mass of the crust [10]. The most common metal, silicon, is never found in its elemental form in nature. Instead, silicon is combined in silicate minerals, which make up more than 90% of the mass of the Earth’s crust [11]. Depending on the composition and formation conditions, silicate minerals have structures that range from individual clusters (orthosilicates) to three-dimensional networks (tecto – silicates) [11]. These minerals can be contained in relatively pure single mineral deposits or, more commonly, in rocks such as granite that are made up of one or more mineral species.

The term clay refers to fine-grained aluminosilicates that have a platy habit and become plastic when mixed with water [11]. Dozens of minerals fall under the classi­fication of clays and a single clay deposit can contain a variety of individual clay minerals along with impurities. Clay minerals are classified as phyllosilicates because of their layered structure [12]. The most common clay mineral is kaolinite, although others such as talc, montmorillonite, and vermiculite are also abundant. Each of the

Table 1 Chemical composition of the Earth’s crust

Element

Percent by Weight

O

50

Si

26

Al

7.5

Fe

4.7

Ca

3.4

Na

2.6

K

2.4

Mg

1.9

All others

1.5

clay minerals is composed of a unique combination of layers that are made up of either tetrahedral or octahedral structural units that form sheets [13]. Tetrahedral sheets are made up of oriented corner-shared Si-O tetrahedra (Fig. 3) [14]. Each tetrahedron shares three of its corners with three adjacent tetrahedra, resulting in a structural formula of (Si2O5)n for the sheet [15]. Likewise, octahedral sheets are composed of Al bonded to O or Oh anions, resulting in an effective chemical formula of AlO(OH)2 [15,16]. The structure of this sheet is shown in Fig. 4 [14]. The simplest clay mineral, kaolinite, is produced when each of the Si-O tetrahedra in the tetrahedral sheet shares an oxygen with an Al-O/OH octahedron from the octahedral sheet, shown as a perspective drawing in Fig. 5. The repeat unit or layer in the resulting structure is

Structure of Clay Minerals

Fig. 3 A single Si-O tetrahedron and the structure of the tetrahedral sheet (Reproduced by permis­sion of the McGraw-Hill companies from R. E. Grim, Applied Clay Mineralogy, McGraw-Hill, New York, 1962) [14]

Structure of Clay Minerals

Fig. 4 A single Al-O octrahedron and the structure of the octahedral sheet (Reproduced by permis­sion of the McGraw-Hill companies from R. E. Grim, Applied Clay Mineralogy, McGraw-Hill, New York, 1962) [14]

Structure of Clay Minerals

Fig. 5 Perspective drawing of the kaolinite structure taken from Brindley (Reproduced by permis­sion of MIT Press from G. W. Brindley, “Ion Exchange in Clay Minerals,” in Ceramic Fabrication Processes, Ed. by W. D. Kingery, John Wiley, New York, 1958, pp. 7-23) [13]

composed of alternating octahedral and tetrahedral sheets. Bonding within each repeat unit is covalent, making the layers strong. In contrast, the bonding between repeat units is relatively weak, allowing the layers to separate when placed in an excess of water or under a mechanical load. The chemical formula for kaolinite, as determined by site occupancy and charge neutrality requirements, is Al2Si2O5(OH)4, which is commonly expressed as the mineral formula Al2O3-2SiO2-2H2O. The structure and properties of kaolinite are summarized in Table 2. The repeat units for clay minerals other than kaolinite are produced by altering the stacking order of the octahedral and tetrahedral sheets or by isomorphous substitution of cations such as Mg2+ and Fe3+ into the octahedral sheets [17].

Structure of Clay Minerals
Подпись: Kaolinite Structure of Clay Minerals

Conceptually, the next simplest clay mineral is pyrophyllite, which is produced by attaching tetrahedral sheets above and below an octahedral layer (Fig. 6), compared with just one octahedral sheet for kaolin [15]. The resulting chemical composition of pyro – phyllite is Al2Si4O10(OH)2, which is equivalent to the mineral formula Al2O34SiO2H2O. The structure and properties of pyrophyllite are summarized in Table 2.

Table 2 Composition and crystallography of common clay minerals

Kaolinite

Pyrophyllite

Mica (Muscovite)

Chemical formula

Al2Si2O5(OH)4

A^OJOH^

KA^OJOH^

Mineral formula

Al2O3.2SiO2.2H2O

Al2O3.4SiO2.H2O

K2O.3Al2O3.6SiO2.2H2O

Crystal class

Triclinic

Monoclinic

Monoclinic

Space group

Pi

C2/c

C2/c

Density

2.6 g cm-3

2.8 g cm-3

2.8 g cm-3

c-Lattice parameter

7.2 A

18.6 A

20.1 A

• Silicon ® Aluminum Q Oxygen © Hydroxyl

• Si or Al іЦї Potassium ss Al, Fe, or Mg

Fig. 6 Schematic representation of the structure of koalinite, pyrophyllite, and mica (muscovite) after Brindley [13]

More complex clay minerals are produced when Mg2+ or Fe3+ substitute onto the octahedral Al3+ sites in either the kaolinite or the pyrophyllite structures [17]. Along with the substitution onto the octahedral sites, Al3+ can substitute onto the tetrahedral sites. These substitutions produce a net negative charge on the structural units, which, in turn, can be compensated by alkali (Na+, K+) or alkaline earth (Ca2+, Mg2+) cations that attach to the structure either between the layers of the structural units or within the relatively large open space inside the Si-O tetrahedra [13]. Families of clay minerals that contain isomorphous substitutions on Al3+ and/or Si4+ sites are micas and chlorites. The structure of a potassium compensated mica-type mineral is shown in Fig. 6. The charge-compensating cations in these clays are relatively mobile, giving some clays significant cation exchange capacity [15]. In addition to the distinctly dif­ferent minerals produced by altering the arrangement of the structural units or by sub­stituting cations into the structure, some clays are susceptible to hydration of the interlayer cations, which can cause swelling in the c-direction. An almost infinite number of clay minerals can be conceived by varying site occupancy and layer orders. These structures can be complex and difficult to determine by experimental methods such as X-ray diffraction. Further complication arises due to the fact that some clays are made up of layers with different structural units (e. g., a random sequence of pure or partially substituted pyrophyllite – and kaolinite-type layers).

An additional structural variant for clay minerals is the chlorite-type structure. Chlorites are similar to the pyrophyllite-type structures with two tetrahedral sheets and an octahedral sheet making up each layer. Instead of alkali or alkaline earth inter­layer cations, chlorites contain a brucite (Al-Mg hydroxide) layer between successive pyrophyllite-type layers [18].

The major mineralogical classifications associated with clays are summarized in Fig. 7 [18]. Fortunately as ceramists, we are more concerned with the properties of clays than their mineralogy and most often we classify them by use.