4 I

'Alter Kahn (12). ¡"10.000 Angstroms = 1 micron. \svuming a c spacing of 19A.

11 lite4»

Mites are hydrous micas, the prototypes for which are muscovite (dioctahedral mica) and biotite (trioctahedral mica). They are three-layer clays, with a structure similar to montmorillonite, except that the substitutions are predominately aluminum for silicon in the tetrahedral sheet. In many cases, as much as one silicon in four may be so replaced. Substitutions may also take place in the octahedral sheet, typically magnesium and iron for aluminum. The average charge deficiency is higher than that of montmorillonite (0.69 vs 0.41 ),y and the balancing cation is always potassium.

lllites differ sharply from montmorillonite in that they do not have an expanding lattice and in that no water can penetrate between the layers. The strong interlayer bonding is probably because of the higher layer charge, because the site of the charge is nearer the surface in the tetrahedral sheet, and because the size of the potassium ion is such that it just fits into the holes in the oxygen network, and forms secondary valance links between adjacent

Figure 4-8. Edge view of a flake of sodium montmorillonite. The dark parallel lines, which are 10A ± thick, are the unit layers. (Electron micrograph by Barclay and Thompson.15 Courtesy of Nature.)

layers. Thus, the potassium normally is fixed, and cannot be exchanged, ion exchange can, however, take place at the exterior surfaces of each aggregate. Since hydration is also confined to the exterior surfaces, the increase in volume is much less than that caused by the hydration of montmonllonitc.

Illites disperse in water to particles having an equivalent spherical radius of about 0.15 micron, widths of about 0.7 micron, and thickness of about 720 A

Some illites occur in degraded form, brought about by leaching of potassium from between the layers. This alteration permits some interlaver hydration and lattice expansion, but never to the degree attained by montmorillonite.

Kaolinite

Kaolinite is a two-layer clay with a structure similar to that shown in Figure 4 5. One tetrahedral sheet is tied to one octahedral in the usual manner, so that the octahedral hydroxyls on the face of one layer are juxtaposed to tetrahedral oxygens on the face of the next layer. In consequence, there is strong hydrogen bonding between the layers, which prevents lattice expansion. There is little, if any, isomorphous substitutions, and very few, if any, cations are adsorbed on the basal surfaces.

Not surprisingly, therefore, most kaolinites occur in large, well-ordered crystals which do not readily disperse to smaller units in water. The width of the crystals range from 0.3 to 4 microns, and the thickness from 0.05 to 2 microns.

Dickite and nacrite are two other members of the kaolinite group. They differ from kaolinite in their stacking sequences.

Chlorites

Chlorites are a group of clay minerals whose characteristic structure consists of a layer of brucite alternating with a three-sheet pyrophyllite-type layer, as shown in Figure 4-9. There is some substitution of A1+ 3 for Mg+ 2 in the brucite layer, giving it a positive charge which is balanced by a negative charge on the three-sheet layer, so that the net charge is very low. The negative charge is derived from the substitution of Al + 3 for Si + 4 in the tetrahedral sheet. The general formula is:

The members of the chlorite group differ in the amount and species of atoms substituted in the two layers, and in the orientation and stacking of the layers. Normally, there is no interlayer water, but in certain degraded chlorites, part of the brucite layer has been removed, which permits some degree of interlayer hydration and lattice expansion.

Tetrahedral sheet-* Octahedral sheet

Tetrahedral sheet-*

Brucite layer -

Figure 4-9. Diagrammatic representation of chlorite.

Chlorites occur both in macroscopic and in microscopic crystals. In the latter case, they always occur in mixtures with other minerals, which makes determination of their particle size and shape very difficult. The c-spacing, as determined from macroscopic crystals, is 14 A, reflecting the presence of the brucite layer.

Mixed-Layer Clays

Layers of different clay minerals are sometimes found stacked in the same lattice. I nterstratilled layers of illite and montmorillonite, and of chlorite and vermiculite, are the most common combinations. Generally, the layer sequence is random, but sometimes the same sequence is repeated regularly. Usually, mixed-layer clays disperse in water to smaller units more easily than do single mineral lattices, particularly when one component is of the expanding type.

Attapulgite

Attapulgite particles are completely different in structure and shape from the mica-type minerals discussed so far. They consist of bundles of laths, which separate to individual laths when mixed vigorously with water (See Figure 4-10). The structure of these laths has been described by Bradley.86

There are very few atomic substitutions in the structure, so the surface charge on the particles is low. Also, their specific surface is low. Consequently, the rheological properties of attapulgite suspensions are dependent on mechanical interference between the long laths, rather than on electrostatic jnterparticle forces. For this reason, attapulgite makes an excellent suspending agent in salt water.

Sepiolite is an analagous clay mineral, with different substitutions in the structure, and wider laths than attapulgite. Sepiolite-based muds are recommended for use in deep wells because their rheological properties are not affected by high temperatures.17

Figure 4-10. Electron micrograph of attapulgite clay showing open mesh structure magnified 45,000 times. (Courtesy of Attapulgus Minerals and Chemical Corporation.)

Origin and Occurrence of Clay Minerals

Clay minerals originate from the degration of igneous rocks in situ. The parent minerals are the micas, which have already been discussed: the feldspars, [(CaO) (K,0)Al2036Si02]; and ferromagnesium minerals, such as horneblende [(Ca, Na2)2 (Mg, Fe. Al); (Al, Si)8022 (OH,F)2j. Bentonite is formed by the weathering of volcanic ash.

The weathering process, by which the clay minerals are formed from the parent minerals, are complex and beyond the scope of this paper. Suffice it to say that the main factors are climate, topography, vegetation, and time of exposure.18 Of major importance is the amount of rainfall percolating downwards through the soil, and the soil's pH. The pH is determined by the parent rock, the amount of carbon dioxide in the atmosphere, and the vegetation. Silica is leached out under alkaline conditions, and alumina and lhe ferric oxides under acid conditions. Leaching and deposition lead to the various isomorphous substitutions discussed previously.

Clays formed in situ are termed primary days. Secondary clay s are formed from primary clays carried down by streams and rivers, and deposited as sediments in fresh water or marine environments. Their subsequent burial and transformation by diagenesis is discussed in Chapter 8.

The various species of clay minerals are not distributed evenly throughout the sedimentary sequence. Montmorillonite is abundant in Tertiary sediments, less common in Mesozoic, and rare below that. Chlorite and illite are the most abundant clay minerals: they are found in sediments of all ages, and predominate in ancient sediments. Kaolinite is present in both young and old sediments, but in small amounts.

Montmorillonite occurs in its purest form in primary deposits of bentonite. Wyoming bentonite is about 85% montmorillonite. Sodium, calcium and magnesium are the most common base exchange ions. The ratio of monovalent to divalent cations varies over a range of approximately 0.5 to I 7,1" even within the same deposit. Other montmorillonites, of various degrees of purity, have been found in many places all over the world. They seem to be particularly abundant in formations of Middle Tertiary and Upper Cretaceous.20

Note that the term bentonite was originally defined as a clay produced h\ in situ alteration of volcanic ash to montmorillonite, but the term is now used tor any cla> whose physical properties are dominated by the presence of a smectite.

Ion Exchange

As already mentioned, cations are adsorbed on the basal surfaces of clay crystals to compensate for atomic substitutions in the crystal structure. Cations and anions are also held at the crystal edges, because the interruption of the crystal structure along the c axis results in broken valence bonds. In aqueous suspension, both sets of ions may exchange with ions in the bulk solution.

The exchange reaction is governed primarily by the relative concentration of the different species of ions in each phase, as expressed by the law of mass action For example, for two species of monovalent ions, the equation may be written;

where [A]s and [B]s are the molecular concentrations of the two species of ions in the solution, and [A]c and [B]c are those on the clay. K is the ion exchange equilibrium constant, e.g., when K is greater than unity, A is preferentially adsorbed.

When two ions of different valencies are present, the one with the higher valence is generally adsorbed preferentially. The order of preference usually

H ' > Ba" > Sr++ > Ca++ > Cs+ > Rb4 > K f > Na+ > Li '

but this series does not strictly apply to all clay minerals: there may be variations. Note that hydrogen is strongly adsorbed, and therefore pH has a strong influence on the base exchange reaction.

The total amount of cations adsorbed, expressed in milliequivalents per hundred grams of dry clay, is called the base exchange capacity (BEC), or the cation exchange capacity (CEC). The value of the BEC varies considerably, even within each clay mineral group, as shown in Table 4- 3. With montmorillonite and illite, the basal surfaces account for some 80" 0 of the BEC. With kaolinite, the broken bonds at the crystal edges account for most of the BEC.

The BEC of a clay and the species of cations in the exchange positions arc a good indication of the colloidal activity of the clay. A clay such as montmorillonite that has a high base exchange capacity, swells greatly and forms viscous suspensions at low concentrations of clay, particularly when sodium is in the exchange positions. In contrast, kaolinite is relatively inert, regardless of the species of exchange cations.

Table 4-3

Base Exchange Capacities of Clay Minerals*

Meq/lOOg of Dry Clay

Montmorillonite

Vermiculite Illite

Kaolinite Chlorite

Attapulgite-Sepiolite

0 0

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