Clay Minerals (1968) 7, 373.

I N V I T E D R E V I E W

I N F R A R E D S P E C T R O S C O P Y IN C L A Y M I N E R A L

S T U D I E S

V. C. F A R M E R

Macaulay Institute ]or Soil Research, Aberdeen

(Received 23 February 1968)

I N T R O D U C T I O N

Infrared absorption by a mineral arises from the vibrations of its constituent atoms, and the frequencies of these vibrations are dependent on the mass of the atoms, the restraining forces of the bonds, and the geometry of the structure. The resultant spectrum of absorption frequencies is a characteristic property of the mineral which can not only serve as a fingerprint for its identification, but also give, in favourable cases, unique information on features of the structure, including the nature of isomorphic substituents, the differentiation of molecular water from constitutional hydroxyl, the degree of regularity of the structure and an indication of the family of minerals to which an unknown mineral is related.

In many applications, the surface properties and reactivity of clays are of greater importance than their bulk composition, and infrared spectroscopy has a peculiar contribution to make in this field. Changes in spectra of molecules adsorbed on the clay surface provide direct information on the mechanism and sites of adsorption.

Many of the present applications of infrared spectroscopy to clay studies have been obviously possible, in principle, since the first exploratory studies of Coblentz, over fifty years ago. Progress was limited till recently by the lack of satisfactory equipment and convenient techniques of sample preparation. Modern spectro- meters and techniques permit the whole spectrum of mineral vibrations to be routinely studied, and a body of information has been built up which makes such studies increasingly profitable. But even today infrared studies of the adsorption properties and thermal behaviour of clays may be limited by sample preparation and handling problems.

This review will be restricted to aspects of theory and practice relevant to clay

374 V. C. Farmer

studies. For more general treatments of infrared methods the texts of White (1964) and Potts (1963) are recommended. Bellamy (1958) and Colthup, Daly & Wiberley (1964) treat interpretation of spectra, principally of organic compounds, and Nakomoto (1963) deals with the spectra of the simpler inorganic and co-ordination compounds. Applications to mineralogy have been recently reviewed by Lyon (1967).

T E C H N I Q U E S A N D I N S T R U M E N T A T I O N

The infrared spectrometer Fundamental vibrations of minerals can extend from 4000 cm -1 down to

100 cm -1 or less, and spectra should cover as much of this range as possible. The minimum coverage to be aimed at for silicate minerals is 4000-400 cm -1 with a resolution of 2-3 cm -1 "throughout the range. These standards are readily achieved with modern grating instruments. Prism instruments give inadequate resolution in the important 4000-2000 cm -1 region where hydroxyl stretching vibrations occur. In choice of an instrument, especially for research purposes, consideration should be given to the space available for ancillary equipment such as vacuum cells, beam condensers and polarizers.

Sample handling The alkali-halide pressed disk technique (White, 1964) is the most generally

useful as a first step in characterizing minerals. Clay samples separated by sedimentation are already of satisfactory size (<2/~) for use in this technique, but require either freeze-drying, or washing with alcohol followed by benzene, to prevent the sample drying out in an intractable form. Minerals which require grinding should be moistened with water or alcohol (Truddenham & Lyon, 1960; Farmer, 1964a), as dry grinding can destroy crystal structure. Spectra should be recorded at more than one sample concentration so that both strong and weak bands are recorded under optimum conditions. Suitable concentrations for most silicate spectra are 0.3 mg and 2 mg of sample in a 12-mm diameter disk. Provided the mineral is stable, the prepared disk can be heated to 100 ~ C or higher to reduce the amount of water adsorbed on the KBr and the sample (Farmer, 1966). Samples with cation exchange properties should be saturated with potassium or sodium ions, as polyvalent cations retain co-ordinated water to high temperatures. Various pretreatments, such as de-ferration and alkaline extractions may assist in the concentration of certain clay components. T h e removal of organic matter associated with clays has been discussed by Farmer & Russell (1967).

Potassium bromide disks are inapplicable in studying the surface properties of clays, which are highly dependent on the exchangeable cation. Such studies are conveniently made on smectites and vermiculites using self-supporting films, pre- pared by evaporation of salt-free suspensions onto polyethylene or polyester film, from which they are readily stripped by drawing the film over a sharp edge (Serratosa, 1960; Fripiat, Chaussidon & Touillaux, 1960; Farmer & Mortland,

Infrared spectroscopy in clay mineral studies 375

1965; Walker & Garrett, 1967). Materials which do not show layer expansion properties generally form films too fragile to handle, and these must be supported on infrared transmitting windows (e.g. Ledoux & White, 1964). Self-supporting films prepared by pressure are widely used in surface studies on silica, silica-alumina catalysts, and zeolites (McDonald, 1958; Angell & Schaffer, 1965; Hambleton, Hockey & Taylor, 1966); this technique may have application in clay studies, possibly of allophanes.

All these film-forming techniques lead to orientation of layer silicates parallel to the plane of the film, as may also mull techniques (Potts, 1963), which find occasional application. The alkali-halide pressed disk technique, in contrast, gives more nearly random orientation.

Study of the thermal behaviour of clays, of adsorbed species, and deuteration procedures may require exclusion of atmospheric moisture during treatment and subsequent examination. Cells designed by Angell & Schaffer (1965) and Uytter- hoeven (described by Granquist & Kennedy, 1967) have been found particularly convenient for this purpose. The former permits identical treatment of two or three samples in comparative studies while the latter allows sample orientation relative to the incident beam to be adjusted. Other cell designs are described by Little (1966). Thermal decomposition of minerals at temperatures up to 700 ~ C can also be followed in KBr pressed disks with little readsorption of water, although some interaction with the KBr may occur during decomposition (Farmer et al., 1968).

S P E C T R A - S T R U C T U R E R E L A T I O N S H I P S

The vibrations of complex solid structures are seldom localized in any one type of atom or bond. An exception is the vibrations of hydrogen, especially in bond- stretching modes. Accordingly, the vibrations of hydroxyl groups will be treated separately. These vibrations are essentially localized on the proton, but are affected by the nature of cations to which the OH group is directly co-ordinated, by hydrogen bonding, by static electric fields originating in other ions in its immediate environment, and by the oscillating fields induced by neighbouring vibrating hydroxyl groups. Both stretching and bending vibrations of OH groups are there- fore valuable diagnostic indicators of their environment.

OH stretching

In talc (Vedder, 1964; Wilkins & Ito, 1967) and in amphiboles (Burns & Strens, 1966), OH groups co-ordinated to the octahedral groupings (Mg,Mg,Mg), (Mg,Mg,Fe2+), (Mg,Fe~+,Fe 2+) and (Fe2+,FeZ+,Fe 2+) give clearly resolved, distinct absorption bands. In biotites and phlogopites these bands are broadened by the effect of Al-for-Si substitution, and so overlap (Wilkins, 1967). They are also displaced to higher frequencies by the electrostatic field of interlayer potassium ions: this effect was established by study of K-saturated saponite (Farmer & Russell, 1966). Most phlogopites show two OH stretching bands corresponding

376 V. C. Farmer

to the octahedral groupings (Mg,Mg,Mg) and (Mg,Mg,A1) (Vedder, 1964) and all biotites show an additional group of bands in the 3550-3620 cm -1 region (vacancy bands) due to OH co-ordinated to only two octahedral cations (Vedder, 1964; Farmer et al., 1967). The high intensity of these vacancy bands in oxidized vermi- culites and biotites indicates loss of octahedral ions following oxidation of octahedral Fe 2+ (Farmer et al., 1967).

In the dioctahedral group of minerals (Farmer et al., 1967), celadonites and glauconites show up to four distinct OH bands corresponding to the four octahedral pairs (Mg,A1) (Mg,Fe3+), (Fe2+,A1) and (Fe2+,Fe3+); but in montmorillonites, OH co-ordinated to (A1,AI) and (Mg,AI) does not give separate bands, and in the muscovite-phengite series, a distinct band from OH co-ordinated to (Mg,A1) has been found only when Mg substitution is high and Al-for-Si substitution is low, i.e. close to the Al-celadonite end-member. Substitution of Fe 3+ for A1 ~+ in mont- morillonites and beidellites lowers OH stretching frequencies, from 3630 cm -1 in aluminous species to 3560 cm -1 in nontronite.

Well-crystallized kaolinite shows four distinct OH stretching frequencies between 3700 and 3620 cm -1. Deuteration of the surface hydroxyls of each layer following expansion with hydrazine has established that the inner hydroxyl, which is not deuterated at room temperature, absorbs at 3620 cm-Z---considerably lower than the corresponding OH group of pyrophyllite (3675 cm-1). The observation that two of the three higher frequency bands arise from dipole changes in the plane of the layers was initially taken to be inconsistent with the accepted structure of kaolinite, in which the surface hydroxyls all lie nearly perpendicular to the layers (Ledoux & White, 1964; Wada, 1967), but no inconsistency need arise if the effects of dipole-dipole coupling between neighbouring hydroxyls is allowed for (Farmer, 1964b). Dickite and nacrite give absorption patterns similar to each other but distinct from kaolinite, while b-axis disordered kaolinites give patterns con- sistent with a random succession of kaolinite-type and dickite-type stacking (Farmer & Russell, 1964). Typically, halloysites are highly disordered, and their spectra are more diffuse than those of well-crystallized kaolinite, but Chukhrov & Zvyagin (1966) have presented evidence for the existence of well-crystallized halloysite with spectra nearly identical to kaolinite.

The brucite layer of chlorites gives two, strong, more or less resolved bands below 3620 cm -1, whose frequency decreases with increasing iron content (Kodama & Oinuma, 1963), and which arise from OH groups perpendicular to the layers (Serratosa & Vinas, 1964). Interlayers of Mg(OH)2 and AI(OH)3 introduced into montmorillonite give much weaker bands at higher frequencies than do natural chlorites; indeed, interlayer Mg(OH)2 absorbs at a higher frequency than brucite (Ahlrichs, 1968; Russell, 1965).

OH-bending

The frequencies of OH bending overlap those of other lattice vibrations, and are distinguished by studying minerals, synthetic or natural, in which OH has been replaced by OD, or by analogy with related minerals. Their frequency is markedly

Infrared spectroscopy in clay mineral studies 377

dependent on the ion to which they are co-ordinated, and their environment. In dioctahedral minerals the (A1,A1,OH) grouping absorbs in the range 915-950 cm -1 (Stubican & Roy, 1961a; Vedder & McDonald, 1963; Farmer & Russell, 1964; Farmer et al., 1967). Inner and surface OH groups of kaolinite absorb at 915 cm -1 and 938 cm -1, respectively (Wada, 1967). Two different bending vibrations (in-plane and out-of-plane) might be expected in layer silicates; an assignment for the second in muscovite (Vedder & McDonald, 1963) is not, however, consistent with the direction of dipole change of the suggested band (Vedder, 1964). Clearly resolved bending vibrations of OH associated with (A1,A1), (Fe3+,A1) and (Mg,A1) have been identified in montmorillonite, although the corresponding OH stretching vibrations are not resolved (Farmer et al., 1967). Observations on the (Fe~+,A1,OH) bending vibration have clearly established reversible reduction and oxidation of octahedral iron at room temperature (Farmer & Russell, 1967). Nontronites show, in addition to the principal (Fe~+,Fe~+,OH) vibration at 818 cm -1, a subsidiary band at 850 cm -1 which is eliminated by reduction. A suggestion that this band arises from a (Fe~+,A1,OH) grouping (Farmer & Russell, 1964, 1967) is now un- likely, as the 850 cm -z is found to be present in nontronites of very low A1 content (Russell, private communication).

In saponite and hectorite, a band at 655 cm -~, previously assigned to an Si-O vibration (Farmer, 1958), has been identified as the OH bending vibration (Farmer et al., 1968), and similar assignments seem likely for bands in this region in talc, chrysotile, antigorite, phlogopite and chlorite (Naumann, Safford & Mumpton, 1966). This frequency is considerably higher than the MgOH bending vibration in brucite (400-500 cm -*, Buchanan, Caspers & Murphy, 1963).

Lattice vibrations

Some of the general problems associated with the assignment of the vibrations of the cation-oxygen lattice of oxides and silicates have been discussed by Tarte (1967), and Farmer (1964a), and the particular case of the layer silicates by Farmer (1958), Saksena (1961), Vedder (1964), and Farmer & Russell (1964, 1966). Gener- ally, silicates show two main regions of strong absorption, near 1000 cm -1 (due to Si-O stetching) and near 500 cm -~ (due to Si-O bending, often coupled with other cation-oxygen vibrations). A detailed assignment for the four bands of talc, saponite and hectorite in the region 600-1100 cm -x (Farmer, 1958), requires modification in view of the reassignment of a band at 650-670 cm -~ to OH bending (see above). The Si-O bending vibration ascribed to this absorption must lie below 500 cm -1. Substitution of AI for Si in the silicon-oxygen lattice modifies Si-O vibrations, and causes new bands, which must be ascribed to Si-O-A1 vibra- tions, to appear in the 600-900 cm -1 region (Stubican & Roy, 1961a, b; Farmer & Russell, 1964). Some other effects of isomorphous substitutions on the spectra of layer silicates are summarized by Stubican & Roy (1961c). Vibrations of O H groups, moving as a whole, can couple with other lattice vibrations, and probably account for bands of kaolin minerals in the 700-800 cm -~ region (Farmer & Russell, 1964).

378 V. C. Farmer

A P P L I C A T I O N S

Identification o] clay minerals and o] clay components

The empirical identification of clay minerals by infrared spectra requires access to spectra of well characterized specimens. For this purpose Lyon's (1962a) and Lawson's (1961) bibliographies of published mineral and inorganic spectra cover the earlier years well. Valuable collections of mineral spectra have been pub- lished by Moenke (1962) and Lyon (1962b), each containing about 370 spectra. Some selected references to spectra of clay minerals and possible associated minerals are listed in Table 1.

Generally, infrared examination of a mineral can yield useful information supplementary to that given by X-ray and thermal studies, giving, for example, information on the octahedral occupancy of glauconites and montmorillonites, and serving to distinguish the various members of the smectite family (beidellites, montmorillonites, nontronites, and saponites) without recourse to analysis. Infrared spectra appear to be particularly sensitive to disorder in crystal structures, whether arising from poor crystallinity, or from random isomorphous substitution. This probably explains some divergence between the spectra of synthetic layer silicates (Stubican & Roy, 1961a, b) and those of natural specimens. Marked differences in spectra between well-ordered and disordered structures have been reported for spinels (Hafner & Laves, 1961; Tarte, 1963) and felspars (Laves & Hafner, 1956; Hafner & Laves, 1957) and have been used to distinguish sillimanite and mullite (Tarte, 1959). The rather diffuse spectra of many fireclays and ball clays (Beutel- spacher &Marel, 1961) indicate that the disorder in the structure of these kaolinitic minerals does not consist solely of stacking faults.

The sensitivity of infrared spectroscopy to different components of a mixed species is often very different from that of other methods. Gibbsite and kaolinite can be detected at lower levels by their OH stretching bands than by X-ray tech- niques (Wilson, 1966; Kodama & Oinuma, 1963), and small amounts of chlorites in vermiculites are readily detected by this means (author's observation). As each layer of a layer silicate absorbs infrared radiation independently, their detection does not depend on the regular successive stacking required to give X-ray diffraction, and a few layers of kaolinite or chlorite randomly distributed between those of another species absorb in proportion to their number. Thus hydrothermal con- version of montmorillonite to kaolinite is detectable at a very early stage, before three-dimensional order has been achieved (Poncelet & Brindley, 1967). The high sensitivity of infrared methods to such well-ordered species as kaolinite can, how- ever, be a disadvantage, as the sharp absorption bands of kaolinite often obscure the more diffuse bands of more poorly ordered species, although the latter may form the major part of a sample.

Amorphous substances give more featureless spectra than do well crystallized minerals, but absorb as strongly, so that their presence in admixture with crystalline components is less likely to be overlooked in infrared than in X-ray examination

Infrared spectroscopy in clay mineral studies 379

TaBI_~ 1. Selected references to infrared spectra of clay minerals, and of some related and ancillary minerals

Silicate minerals

1 : 1 Layer silicates (a) Kaolinite minerals and halloysite. Beutelspacher & Marel (1961); Chukhrov & Zvyagin (1966);

Farmer & Russell (1964, 1966); Moenke (1962); Sadtler Inorganic Spectra Y168. (b) Serpentine minerals and septechlorites. Brindley & Zussman (1959); Kodama & Oinuma

(1963); Moenke (1962); Montoya & Baur (1963); Stubican & Roy (1961a, b). 2 : I : 1 Layer silicates

Chlorites. Hayashi & Oinuma (1965, 1967); Kodama & Oinuma (1963); Moenke (1962). 2 : 1 Layer silicates

(a) Talc. pyrophyllite. Farmer (1958); Farmer & Russell (1964); Vedder (1964). (b) Montmorillonite, beidellite, rectorite, nontronite. Farmer & Russell (1964); Farmer et al.

(1967); Grim & Kulbicki (1961); Stubican & Roy (1961a, b). (c) Saponite, hectorite, vermiculite. Farmer (1958); Farmer & Russell (1964); Farmer et al. (1967);

Marel (1966). (d) Dioctahedralmicas and lepidolite. Arkhipenko et aL (1965); Farmer & Russell (1964); Farmer

et al. (1967); Lyon (1962b); Marel (1966); Vedder & McDonald (1963). (e) Trioctahedral micas. Arkhipenko (1963); Farmer & Russell (1964); Farmer et al. (1967); Liese

(1963, 1967a); Lyon (1962b); Vedder (1964). Chain-lattice silicates

Palygorskite, sepiolite. Marel (1961); Ovcharenko (1966). Disordered and amorphous silicates

Allophane, imogolite. Fieldes & Furkert (1966); Mitchell et aL (1964); Wada (1967). Rock-forming silicates. Lyon (1962b); Moenke (1962).

Oxide minerals

Silica (crystalline, opaline and hydrated). Benesi & Jones (1959); D'Or et al. (1955); Lippincott et aL (1958); Lyon (1962b); Plendl et al. (1967); Sun (1962).

Aluminium oxides and hydroxides. Caill6re & Pobeguin (1966); Duffin & Goodyear (1960); Frederickson (1954); Fripiat, Bosmans & Rouxhet (1967); Kolesova & Ryskin (I 959, 1962); Lyon (1962b); Marel (1966); Moenke (1962); Takamura & Koezuka (1965) ; Tarte (1963, 1967).

lron oxides and hydroxides. Fripiat, Bosmans & Rouxhet (1967); Hartert & Glemser (1956); Liese (1967b); Moenke (1962); Suetaka (1964.)

Manganese oxides and hydroxides. Gattow (1962); Pablo (1965); Schwarzmann & Marsmann (1966); Shiratori & Aujama (1965).

Other ancillary minerals

Carbonates. Adler & Kerr (1963a, b); Baron et al. (1959); Huang & Kerr (1960); Moenke (1962). Sulphates. Adler & Kerr (1965); Moenke (1962); Omori & Kerr (1963); Wiegel & Kirchner (1966). Phosphates. Arlidge et al. (1963); Baddiel & Berry (1966); Fowler, Moreno & Brown (1966);

Moenke (1962).

380 V. C. Farmer

(Mitchell, Farmer & McHardy, 1964). On the other hand, a proportion of a crystal- line species such as vermiculite, whose OH stretching bands are weak, and whose lattice vibrations give rather diffuse absorption, is more readily detected by X-ray methods than by infrared when associated with allophane. Infrared absorption of

�9 amorphous compounds is, as with crystalline compounds, a function of the atomic composition and atomic bonding, and can give information on, for example, the presence of a discrete silica phase in association with a silica-alumina phase, and point to structural differences in allophane and silica-alumina gels of similar composition (Mitchell, Farmer & McHardy, 1964; Wada, 1967).

Thermal studies

In this field, infrared studies can distinguish loss of water from loss of hydroxyl (Fripiat, Chaussidon & Touillaux, 1960; Russell & Farmer, 1964; Newman, 1967), can follow decomposition of interlayer NH~ +, and detect hydroxyl groups generated by the proton liberated on decomposition of NH4 + (Russell & Farmer, 1964; Russell & White, 1966; Farmer & Russell, 1967). New information on the collapsed phase formed on heating Li-, Mg- and NH4-montmorillonite has been obtained (Tettenhorst, 1962; Farmer & Russell, 1967), and the products of dehydroxylation and rehydroxylation of montmorillonite, beidellite and pyrophyllite characterized (Serratosa, 1960, 1962; Stubican & Roy, 1961d; Heller et al., 1962; Farmer & Russell, 1967).

Dehydroxylation of kaolin minerals has been the subject of numerous studies (Stubican, 1959; Arkhipenko & Plekhanova, 1961; Miller, 1961; Stubican & Roy, 1961d; Plantz & Muller-Hesse, 1963; Pampuch & Wilkos, 1965; De Keyser, Wollast & De Laet, 1965). From this work it has been concluded that aluminium in metakaolinite is in four-fold co-ordination; that both surface and inner hydroxyls are lost simultaneously; and that some hydroxyls persist in the metakaolinite phase, although evidence for this is confused by the readiness with which hydroxyl groups reform when metakaotinite is exposed to water vapour. The phases formed when metakaolinite recrystallizes at higher temperatures have been examined by Stubican (1959) Plantz & Muller-Hesse (1963), De Keyser (1965), and Freund (1967).

Infrared observations of decreased intensity of OH vibrations at elevated tem- peratures have played a major part in Fripiat's arguments for a 'pre-dehydroxylation state', characterized by high proton mobility, in layer silicates and hydroxides (Fripiat & Toussaint, 1963; Fripiat, Rouxhet & Jacobs, 1965; Fripiat, Bosmans & Rouxhet, 1967). Some doubt remains that these observations are in part due to radiation from the heated sample, although the effect has been reported indepen- dently by Vedder (1964).

The processes of loss of adsorbed water, followed by condensation and loss of structural OH have been studied in silica gels, aluminium hydroxides, and silica-alumina catalysts; surface OH which remains after heating these materials in vacuum at high temperatures have been detected by infrared methods, and their accessibility and reactivity explored. In zeolites, surface OH, water co-ordinated to cations, and decationated forms (following thermal decomposition

Infrared spectroscopy in clay mineral studies 381

of exchangeable NH4 + ions) have received considerable attention. These investi- gations have been recently fully reviewed by Little (1966).

Surface properties and interlayer complexes of clays

Studies in this field have concentrated on the smectites, and, to a lesser extent, on vermicufite, because of their high internal surface area and the availability of convenient methods of sample preparation. These minerals, in their natural state, contain interlayer water which can be replaced, in whole or in part, by other polar molecules. Infrared study of interlayer water in smectites indicates that this water forms weaker hydrogen bonds with oxygens of the silicate lattice than with other water molecules, and that water directly co-ordinated to the smaller and more highly charged exchangeable cations forms stronger hydrogen bonds than water in outer spheres of co-ordination (Farmer & Russell, 1967). Water has been shown to be retained to high temperatures, especially by the more highly polarizing cations (Fripiat, Chaussidon & Touillaux, 1960; Russell & Farmer, 1964; Tarasevich & Ovcharenko, 1966). Loss of water in outer spheres of co-ordination is marked by a shift of O H stretching to higher frequencies, and a change in the relative intensities of the bending and stretching absorption bands of the water (Russell & Farmer, 1964; Farmer & Russell, 1967).

A very wide range of molecules, adsorbed in the interlayer space of expanding layer silicates, has now been studied by infrared methods (Table 2). These have

TABLE 2. Infrared studies of molecules adsorbed in interlayer positions of smectites and vermiculites

Acids. Kohl & Taylor (1961); Larson & Sherman (1964); Yariv, Russell & Farmer (1966); Harter & Ahlrichs (1967).

Alcohols. Dowdy & Mortland (1967, 1968); Ovcharenko, Tarasevich & Radul (1967); Tarasevich, Radul & Ovcharenko (1967).

Aldehydes and ketones. Tensmeyer, Hoffmann & Brindley (1960); Hoffmann & Brindley (1961); Kohl & Taylor (1961); Larson & Sherman (1964).

Aluminium hydroxide. Brydon & Kodama (1966); Weismiller et al. (1967). Amides. Tahoun & Mortland (1966a, b). Amines. Fripiat, Servais & Leonard (1962); Bodenheimer et al. (1965); Farmer & Mortland (1965);

Chaussidon & Calvet (1965). Amino-acids. Fripiat et al. (1966). 3-Aminotriazole. Russell, Cruz & White (1968). Ammonia. Mortland et al. (1963); Russell (1965). Ethyl N, N, di-n-propylthiolcarbamate. Mortland & Meggit (1966). Guanidine and derivatives. Beck & Brunton (1960). Magnesium hydroxide. Russell (1965). Nitric oxide. Mortland (1965). Nitrobenzene. Yariv, Russell & Farmer (1966). Polymers. Holmes & Toth (1957); Kohl & Taylor (1961); Aripov, Sarukhanov & Akhmedov (1964); Taymuratzova & Ignat'yeva (1965). Pyridine. Serratosa (1965); Farmer & Mortland (1966). Urea. Mortland (1966).

382 V. C. Farmer

considerably amplified our understanding of the physical and chemical processes involved, and have shown that the nature of the saturating cation plays a key role in the adsorption process. The interactions observed include:

(1) Direct co-ordination to inorganic cations (all polar molecules under suitable conditions, particularly with transition metal, alkali metal, and alkaline earth metal cations).

(2) Direct co-ordination through a hydrogen bond to a protonated organic or + +

inorganic cation, e.g. R3N: HNR3, or D: HNR~, where D is an electron donor (ammonia, organic amines, pyridine, urea, amides).

(3) Indirect co-ordination of polar molecules to cations through a linking water molecule (pyridine, nitrobenzene, benzoic acid, and possibly acetone, with inorganic cations).

(4) Displacement of one base by another (e.g. NH4++ pyridine---->NH3 + pyridinium ion).

(5) Precipitation of inorganic cations as hydroxides or carbonates (e.g. (H20)2. Mg 2+ + 2NH3 --~ Mg(OH)2 § 2NH~ + : all basic molecules with suitable cations).

(6) Formation of protonated cations without precipitation of inorganic hydroxides (e.g. NH3 and organic bases (a) in H-montmorillonite and (b) in Na-montmorillonite).

(7) Formation of organic anions (benzoic acid). (8) Space filling or co-ordination in outer spheres of co-ordination. Any one species of molecule may be adsorbed in a variety of ways depending

in part on the cation, and in part on the conditions of adsorption. Thus pyridine co-ordinates to Mg ~+ and Ca ~+ through a linking Water molecule at normal humidity. On removal of the linking water molecules by heat and evacuation, pyridine becomes directly co-ordinated to Ca 2+, but pyridinium ion and Mg(OH)2 are formed with Mg 2+. More than one mechanism may operate in the same system: for example, some Ca z+ ions in montmorillonite co-ordinate NH3 directly, while others are precipitated as hydroxide with formation of NH4 +, so indicating that exchangeable cations in montmorillonite do not all have the same environment; the effect of environment is also indicated by a higher proportion of co-ordinated NH3 in Ca- and Mg-saponite, compared with the corresponding montmorillonite systems.

In reaction (5) above, the amount of adsorbed base which is converted to the protonated cation is often greater than would be expected from the known basicity of the inorganic hydroxides formed. This phenomenon has been ascribed to in- creased ionization of interlayer water in expanding layer silicates, compared with bulk water systems (Fripiat, 1964; Mortland, 1966; Harter & Ahlrichs, 1967). On the other hand, Farmer & Russell (1967) have argued that the effect is due, at least in part, to preferential adsorption of the organic cation formed in the inter- layer space, so that the equilibrium

+

M+n(H20)~ + n: B ~ nil : B + M(OH),~ + (x--n)H20

Infrared spectroscopy in clay mineral studies 383

is shifted to the right (here M +n is an organic cation, and :B an organic base). In all the reactions listed, except (6b), the exchangeable cation plays a deter-

mining role, but oxygens of the silicate lattice also interact with adsorbed molecules as acceptors for hydrogen bonds (water, alcohols, ammonia, acids) and through combined ionic and Van der Waals forces (e.g. with aromatic cations such as pyridinium). Lattice OH in smectites appear to play little part (Russell, 1965), and evidence for strong interaction leading to total loss of lattice OH absorption is, as yet, unconvincing (Holmes & Toth, 1957; Taymurazova & Ignat'yeva, 1965).

In interlayer complexes of kaolin minerals, hydrogen bonds formed by surface hydroxyl on one layer to the interlayer compound, and either hydrogen bonding or a dipole-dipole interaction with the surface oxygens on the contiguous layer, appear to play a key role in the penetration of hydrazine, urea, dimethyl sulphoxide, and potassium acetate (Ledoux & White, 1966; Olejnik et al., 1968; Sanchez Camazano & Gonzalez Garcia, 1966) and in the stability of secondary interlayer complexes involving water only, or ammonium chloride (Wada, 1965a, b).

In addition to the interactions noted above which involve dipole interactions, hydrogen bonding, co-ordinate linkages, and proton transfers, more far-reaching hydrolytic and oxidative reactions have been found to be induced in molecules adsorbed on clay surfaces; these include decomposition of alkylammonium to give ammonium ion (Chaussidon & Calvet, 1965), hydrolytic decomposition of hexam- minocobalt (III) ion (Chaussidon et al., 1962), hydrolysis of urea (Mortland, 1966), of EPTC (Mortland & Meggit, 1966), and of substituted triazines (Russell et al., 1968), and an unidentified reaction of pyridine (Farmer & Mortland, 1966).

Studies of adsorbed species on silica, silica-alumina, alumina, and zeolites have been principally concerned with the highly dehydrated systems of interest in catalytic applications. The principal mechanisms identified include hydrogen bonding by surface hydroxyls, proton transfer from surface hydroxyls (Br6nsted sites), co-ordination of electron-donor molecules to exposed cations (Lewis sites) either in the surface layer or on exchange positions, condensation reactions leading to Si-O-C or AI-O-C bonds, and oxidation of adsorbed alcohols to give carboxylate ion. This field has been recently reviewed by Little (1966).

C O N C L U S I O N

This review reveals very clearly that infrared studies play a significant role in many fields of clay science. The contribution of such studies will increase as our back- ground of knowledge and technique develops. There are several fields in which careful study of well-characterized series of natural or synthetic minerals must add to our understanding of ion distribution in minerals (e.g. in chlorites and biotites) and it is to be hoped that journal editors will welcome publication of good quality spectra of new varieties of mineral species even though few con- clusions can be drawn from them at present.

As science is the art of the soluble (Medawar, 1967) progress will probably be fastest in fields selected to suit the potentialities and limitations of infrared

384 V. C. Farmer

methods, but even in studies where few firm positive conclusions can be drawn, monitoring by infrared spectroscopy can often exclude possible interpretations of results given by other methods. Use of infrared spectroscopy in the study of clay structures and clay reactions can therefore take its place beside X-ray diffraction, thermal methods, and chemical analysis, in a combination which is inevitably more powerful than any one alone.

R E F E R E N C E S

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