6 Oh

Figure 4-5. Atom arrangement in the unit cell of a 2-layer mineral (schematic). (Courtesy of John Wiley & Sons.)

minerals. The fundamental difference between the two types of minerals is that the structures of the prototypes are balanced and electrostatically neutral, whereas the clay mineral crystals carry a charge arising from isomorphous substitutions of certain atoms in their structure for other atoms of a different valence '' For example, if an atom of Al + 3 is replaced by an atom of Mg ' a charge deficiency of one results. This creates a negative potentiai at the surface of the crystal, which is compensated for by the adsorption of a cation, in the presence of water, the adsorbed cations can exchange with cations of another species in the water, and they are therefore known as the exchangeable canons. Substitutions may occur in either the octahedral or tetrahedni! sheets, and diverse species may be exchanged, giving rise to innumerable groupings and sub-groupings of clay minerals.

The degree of substitution, the atoms involved, and the species of exchangeable cations are of enormous importance in drilling fluids technology because of the influence they exert on such properties as swelling, dispersion, and Theological and filtration characteristics. The clay mineral groups of interest, and their characteristics, are discussed below.

The Smectites*

As mentioned earlier, pyrophyllite and talc are the prototype minerals tor the smectite group. In their crystal lattice, the tetrahedral sheet of one layer is adjacent to the tetrahedral sheet of the next, so that oxygen atoms are opposite oxygen atoms. Consequently, bonding between the layers is weak, and cleavage is easy.7 Partly because of the weak bonding, and partly because of high repulsive potentials on the surface of the layers, arising from isomorphous substitutions, water can enter between the layers, thereby causing an increase in the c-spacing. Thus smectites have an expanding lattice, which greatly increases their colloidal activity, because it has the effect of increasing their specific surface many times over. All the layer surfaces, instead of just the exterior surfaces, are now available for hydration and cation exchange, as shown in Figure 4—6.

Members of the smectite group are differentiated on the basis of the prototype mineral, the relative amounts of substitutions in the octahedral or tetrahedral layer, and on the species of atoms substituted. Table 4-1 lists the principal members of the group.8

Note that the convention for writing the formulas of clay minerals is ;is follows:

Suppose the prototype mineral is pyrophyllite, which has the formula:

2[Al2Si4O10(OH)2].

* Formerly called montmoriUinoids.

if one aluminum atom in six in the octahedral sheet is replaced by one atom of magnesium, and one atom of silicon in eight in the tetrahedral sheet is replaced by one atom of aluminum, then the formula would be written:

Montmorillonite is by far the best known member of the smectite group, and has been extensively studied because of its common occurence and economic importance. It is the principal constituent of Wyoming bentonite, and of many other clays added to drilling fluids. It is the active componen! in the younger argillaceous formations that cause problems of swelling and hea ving when drilled.

The predominant substitutions are Mg+2 and Fe+3 for A1' 1 in the octahedral sheet, but A1 +3 may be substituted for Si + 4 in the tetrahedral sheet. If the substitutions in the tetrahedral sheet exceed those in the octahedral, the mineral is termed a heidelliieso that montmorillonite and beidellite may be regarded as end members of a series.

The charge deficiency varies over a wide range, depending on the degree of substitution. The maximum is approximately 0.60, and the average 0.41 ; " The specific surface may be as much as 800 m^g.11

Like other smectites, montmorillonite swells greatly because of iis expanding lattice. The increase in c-spacing depends on the exchangeable

Table 4-1 Smectites*

Principal Substitutions

Prototype (no substitutions)

Practically all octahedral

Predominantly octahedral

Predominantly tetahedral

Tr ¡octahedral Minerals

Dioctahedral Minerals

Pyrophvllite Al ,Si4

Volchonskoite (Al,G'ri:

"From Brindley and Roy (8). API Projcct 55. Copyright (957 by API.

+To cach formula the group O10 (OH), should be added as well as the exchangeable ail ion

Figure 4-7. Electron micrograph of montmorillonite. Magnification: x 87,500

{Courtesy of J.L, McAfee, Baylor University.)

cations. With certain cations (notably sodium), the swelling pressure is so strong that the layers separate into smaller aggregates and even into individual unit layers (see Figure 4-7). A number of attempts have been made to determine the particle size of sodium montmorillonite, but the determination is difficult because of the flat, thin, irregular shape of the platelets, and because of the wide range of sizes. In a comprehensive study, Kahn'2 separated sodium montmorillonite into five size fractions in an ultracentrifuge. Using a combination of methods, he then determined the maximum width and the thickness of the platelets in each fraction. The results, summarized in TabSe

Figure 4-7. Electron micrograph of montmorillonite. Magnification: x 87,500

{Courtesy of J.L, McAfee, Baylor University.)

4-2, show that both the width and thickness decrease with decrease in equivalent spherical radius. If the c-spacing in the aggregates is assumed to be 19 A (see Figure 4-14), then the number of layers in the coarsest fraction was S. and the average was a little over one in the three finest fractions, which represented 57% by weight of the sample.

Small angle X-ray diffraction studies'3 of the same three fine fractions also indicated monolayers. Light scattering studies14 indicated one to two layers per aggregate in fractions with less than 60 A esr, and somewhat smaller maximum widths than found by Kahn. An electron micrograph"5 of the edge of a flake of sodium montmorillonite, which was taken from the coarse fraction in an ultracentrifuge, showed aggregrates of three to four layers each, stacked together to form the flake (See Figure 4-8).

Table 4-2

Dimensions of Sodium Montmorillonite Particles in Aqueous Suspension*

Width by electro-

Equivalent optical Average ±

Spherical birefring- hy electron Number

Fraction Percent Radius ence microscope Thickness per

Number by Weight (microns) (microns) (microns) À Particle

0 0

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