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Soil Components

Volume 2

Inorganic Components

Edited by

John E. Gieseking

Springer-Verlag New York . Heidelberg . Berlin

1975

(

Chapter 17

The Characterization of Soil Minerals by Infrared Spectroscopy

V. C. Farmer and F. Palmieri

Contents

page A. Introduction 574

B. The origin of the infrared spectrum 575

C. Techniques and instrumentation 576

I. The infrared spectrometer 576

II. Sample preparation 577

III. Sources of error in infrared spectrometry 581

D. Identification and characterization of minerals 581

I. Identification of minerals and mineral mixtures 581

11. Structure-spectra relationships 590

I. Vibrations of hydroxyl and water 590

a. Lattice hydroxyl 590

b. Water absorption 597

2. Discrete polyatomic anions and cations; the orthosilicates 601

3. The condensed silicates 603

E. Some applications of infrared spectroscopy 607

1. The relationship between coordination number and spectra 608

II. Thermal studies on clay minerals 608

III. Surface properties of clays and related colloids 610

IV. Applications to soil clays 614

I. Review of published work 614

2. The routine study of soil clays 616

F. Conclusions 620

Appendix: a bibliography of inorganic and mineral spectra 621

References 641

1975 by Springer-Verlag New York, Inc.

573

574 V. C. Farmer and F. Palmieri

A. Introduction

The infrared spectrum of a mineral is a characteristic feature, which permits the identification of mineral species. The absorption bands arise from vibrations of the atoms or ions in the structure, and the frequencies of vibrations are dependent on the mass of the atoms, the restraining forces of the bonds, and the geometry of the structure. As a result, the spectrum of a mineral is sensitive to isomorphous replacements in its structure, as these affect both bond strengths and atom masses. The symmetry and regularity of a structure play an important part in determining the intensity and frequency of its vibrations, so that the infrared spectrum is often a sensitive indicator of the degree of order of a crystalline mineral. Amorphous structures absorb infrared radiation as strongly as crystalline structures, although their absorption bands are broader and show fewer distinctive features. Nevertheless, infrared spectroscopy is one of the few techniques which can yield information on the structure and composition of amorphous phases.

Many applications of infrared spectroscopy are basically empirical, depending on comparison of the spectrum of the sample being investigated with those of more fully characterized samples. However, with increasing understanding of the factors affecting the vibrational frequencies of minerals, information can be obtained from the infrared spectrum that could not readily be obtained by other means. The relationship between structure and spectrum is most direct for the vibrations of protons, but even here the relationship is not a simple one, and it is still more complex for the lattice vibrations; much remains to be done before all the factors involved are fully understood. New factors arise in the study of silicates, which are not important in the more fully explored field of organic compounds. Some understanding of the theory of crystal spectra and its complexities is essential, however, to assist interpretation, and to avoid rash extrapolation of empirical correlations.

In soils, the surface properties and reactivity of the clay fraction are of greater importance than its bulk composition, and infrared spectroscopy has a pecular contribution to make in this field. In materials of bigh surface area, the vibrations of surface groups can be directly observed. Studies of changes in these vibrations when organic and inorganic molecules are adsorbed on the surface provide information on the mechanism of adsorption. Further information can be obtained from changes in the spectrum of the adsorbed molecule.

Instrumentation and techniques are also of the highest importance. Many of the present applications of infrared spectroscopy to the study of minerals have been obviously possible since Coblentz's exploratory work 50 years ago. In the event, progress has been slow, limited first by the performance of early infrared spectrometers, and later by the lack of convenient and satisfactory techniques of sample preparation. Modern spectrometers and techniques permit the whole spectrum of mineral vibrations to be studied in a routine manner, but study of the surface properties and the thermal behavior of minerals can still depend on the development of sample preparation and handling techniques appropriate to the particular problem being investigated.

It is the purpose of this chapter to review the applications and techniques of mineral and inorganic infrared spectrometry with particular reference to the clay fraction. The research worker in infrared spectrometry will require a wider and more detailed knowledge of theory, instrumentation, and techniques than can be given here. He is referred to such standard texts as those of BEAVEN et al. [1961], CROSS [1960], CONN and AVERY [1960], SMITH et at. [1957], BRUGEL [1962], and LECOMTE [1958]. HERZBERG [1945] and WILSON et al. [1955] deal with the theoretical aspects of molecular vibrations. POTTS [1963] and MILLER [1965] are particularly useful for information on techniques and instrumentation, and WHITE'S [1964] book is a mine of information for the practising spectroscopist. Inevitably, these texts concentrate on the

575 Soil Minerals and Infrared Spectroscopy

infrared spectra of organic and the simpler inorganic compounds. Similarly, modern treatments of the theory of crystal spectra (VEDDER and HORNIG [1961]; MITRA and GIELISSE [1964]) deal principally with molecular crystals and the simpler ionic compounds. An older and useful introductory test dealing with the theory of crystal spectra is that of MATHIEU [1945]. COLTHUP et at. [1964], BELLAMY [1958], and JONES and SANDORFY [1956] are valuable texts treating the characterization of organic compounds by their infrared spectra.

B. The Origin of the Infrared Spectrum

Absorption occurs when infrared radiation excites the vibrations of atoms or groups of atoms in a molecule or crystal. For this to occur, the vibrations must be associated with a changing dipole moment, and the intensity of absorption is dependent on the effective strength of the oscillating dipole. Many vibrations will, therefore, produce weak or no absorption, when there is little or no change in the dipole moment during the vibration. The principal change in dipole moment occurs, of course, at the same frequency as the vibration and absorbs infrared radiation of this frequency, which is termed the fundamental frequency. But vibrations that are not strictly harmonic or linear can develop weaker dipole oscillations at twice, or higher multiples of, the fundamental frequency, and it is not always easy to distinguish these from weaker fundamental absorption bands. An obvious physical explanation of such overtone bands can be seen in the bending vibration of a hydrogen-bonded hydroxyl group (Figure 1), where the movement of the hydrogen causes the length of the hydrogen bond to

o I I I I I

---H--

1 Figure 1. Bending (libration) of a hydrogen-bonded hydroxyl group.

change at twice the frequency of the fundamental vibration, and this gives rise to a dipole change at twice the fundamental frequency along the direction of the hydrogen bond. Overtone absorption is commonly fairly strong for bending frequencies of hydrogen-bonded hydroxyl groups. Absorption can also occur at frequencies that are sums or differences of fundamental frequencies (combination frequencies), and here again less obvious physical models can often account for this absorption. Such models are, of course, of limited application, as the absorption process is, in fact, a quantum effect not fully explicable in terms of classical mechanics.

An isolated light atom in a crystal lattice of heavy atoms vibrates independently of the rest of the lattice. In general, it has three frequencies corresponding to vibration in three perpendicular directions, given, for example, by

v=~!kx x 27T '-./;;

where m is the mass of the atom; Vx is the vibrational frequency; and k is the force constant in

576 v. C. Farmer and F. Palmieri

dynes/cm, i.e., the force that the atom experiences when displaced from its equilibrium position . The concept of the vibrations of an isolated atom, can, however, seldom be applied, as the atom will, in general, have frequencies that are close to those of its neighbors, so that its vibrations couple with theirs. Nevertheless, a unit cell containing n atoms has 3n modes, even though the atoms do not vibrate independently. Three of these correspond to motion of the unit cell as a whole and contribute to the acoustic modes. The remaining 3n - 3 are termed optical modes, in which the atoms in the unit cell move relative to one another. Infrared radiation has wavelengths long relative to unit-cell dimensions, and so excites crystal vibrations in which adjacent unit cells are in the same phase over a considerable crystal volume. It is, therefore, sufficient to consider only the 3n - 3 vibrations of the primitive unit cell. Thus, sodium chloride, containing two ions in its primitive unit cell (this is not the face-centered cell usually depicted) has 3 x 2 - 3 = 3 vibrations. Because of the cubic symmetry of the crystal, these vibrations form a triply degenerate set associated with only one fundamental absorption band. The number of vibrations and the complexity of the spectrum increase rapidly with increasing number of atoms in the unit cell. It is sometimes possible to consider separately certain groups of atoms in the unit cell that are more tightly bound to one another than to the other atoms or ions in the unit cell. Examples of these are ionic hydroxyl groups, water of crystallization, ammonium ions, and most of the common oxyanions such as sulphate, nitrate, and carbonate. Limitations to this approximation will be discussed more fully later.

The formula for the vibrational frequency of an isolated atom, given above, does, in spite of its limitations, indicate two important factors-atomic mass and force constant-which determine the frequency of vibration. The highest fundamental frequencies found in minerals are those of the stretching vibrations of hydroxyl groups in the 3750 to 2000 cm - 1 frequency range; in these, the vibrations can be considered as localized in the proton. Motion of the proton perpendicular to the OH bond (variously termed "rocking" or "Iibration" for the more ionic OH groups, and "bending" for the more covalent X-OH bands) occurs at lower frequencies, from about 1630 cm - 1 in H 20, down to 419 cm - 1, for example, in LiOH (BUCHANAN et al. [1964]). Here the vibration of the proton may couple with lattice or group vibrations, which involve the oxygen to which it is attached. Such vibrations include the stretching vibrations of oxyanions, in the 1500 to 700 cm - 1 region, or the motion of the OH group as a whole in an ionic lattice [e.g., Mg(OHh and Ca(OH)2, BUCHANAN et al. [1963]]. Similarly, coupling may occur between the bending vibrations of oxyanions in the 600 to 300 cm - 1 region and the motion of polyvalent cations. The vibrations of lowest frequency are those of the heavier and more weakly bound atoms in ionic lattices, and the translational or librational (rotary) motions of oxyanions. Muscovite, for example, has bands at 188, 165, and 109 cm - 1, and phlogopite has bands at 151 and 86 cm - 1, which may correspond to motion of the potassium ion (VEDDER [1964]).

C. Techniques and Instrumentation

I. The Infrared Spectrometer

The majority of commercial infrared spectrometers were until very recently equipped with sodium chloride prisms, and in consequence a majority of published spectra cover only the frequency range 4000 to 650 cm - 1. A proportion of spectra extend down to 400 cm - 1, a region covered by the use of a KBr prism, and some to 270 cm -1, using CsBr. In general, such prism instruments give sufficient resolution, sensitivity, and frequency precision in the 2000 to 270 cm - 1 region, but are inadquate to resolve fully OH-stretching absorption in the 3000 cm - 1 region. Adequate resolution in this region could be achieved by using a lithium fluoride prism,

Soil Minerals and Infrared Spectroscopy 577

but this was seldom used. This situation is changing and will change more rapidly in the future with the advent of commercial grating spectrometers. Moderately priced spectrometers are available, covering the 4000 to 400 cm - 1 region with a resolution of about I cm - 1 or better, and must now be considered the minimum equipment for mineral studies. More expensive instruments extend the frequency coverage down to 230 to 200 cm - 1, and a few permit spectra to be obtained to about 50 cm - 1. As the study of the region below 400 cm - 1 requires strict exclusion of water vapor from the spectrometer, there may well be advantages in having a separate instrument to cover this region. Although the high resolution of grating spectrometers may not be required over much of their range, this resolution can be exchanged for increased radiant energy by opening the slits. This reserve of energy permits more rapid recording and is a considerable advantage in studying small specimens that do not occupy the whole infrared beam, or when energy is necessarily lost by sample scattering or in such ancillary equipment as microscope attachments, polarizers, hot cells, and accessories for reflection studies. In high resolution instruments, the size of the source or detecting element may limit the extent to which the slits can be usefully opened, especially near the lower limits of its frequency coverage. There may then be advantages in instruments that do not aim at the highest possible resolution. In the choice of an infrared spectrometer, especially for research studies, consideration should be given to the space available for introducing the ancillary equipment mentioned above. For example, it may well become routine procedure to study the spectra of soil clays in an evacuable hot cell to assist in the differentiation of their various components.

An alternative method of covering the 400 to 40 cm - 1 region in a single scan is by the interferometric method developed at the British National Physical Laboratory. Commercial instruments are available and will be attractive especially to those who have computer facilities necessary to convert the interferometric intensity pattern into the form of a normal absorption spectrum.

II. Sample Preparation

The majority of mineral samples must be examined in powder form, and it is then essential to reduce the particle size below 2 /Lm . Coarse particles scatter infrared radiation, and so reduce the energy transmitted. Scattering varies sharply in the region of strong absorption bands and distorts their shape. Dipole coupling effects, discussed more fully later, can cause bands to shift and broaden as the particle size increases, and with strong absorption bands these effects may persist in particle sizes below I /Lm. Clay minerals separated by sedimentation are already of suitable size for infrared study, but larger mineral crystals must be ground. A fine fraction can be separated by sedimentation when working with a single mineral species, but this process can scarcely be applied where the amount of sample is limited and is not acceptable with a mixed mineral sample, as it may lead to a selective sampling of the more easily ground components. The grinding process must be carried out under the gentlest conditions possible. Vigorous grinding can partially or wholly destroy crystal structures by plastic deformation; the surfaces differ from the body of particles and can, as has been shown for quartz, become hydroxylated (SODA [1962] ; TAKAMURA et al. [1964]). Marked changes in the spectrum of talc and saponite on vigorous grinding have been noted and ascribed to edge effects on the vibrations of small particles (FARMER [1958]). PERKINS et al. [1955] found that prolonged grinding of kaolinite and montmorillonite gave an amorphous aluminosilicate. Changes in the spectrum of calcite on grinding (STERZEL [1964]) have been ascribed to lattice distortions . The appearance of carbonate absorption bands in ground saponite (FARMER [1958]) and xonotlite (FARMER [1964a]) indicates chemical changes following structural degradation. These deleterious effects of grinding can be avoided or considerably moderated


by moistening the sample. Any nonreactive liquid may well be suitable, but alcohols have been recommended (TUDDENHAM and LYON [1960]), and the authors have found water effective. Small samples of softer minerals can be rapidly and easily ground in an agate mortar; harder minerals may require a vibratory grinder of the type shown by FARMER [1957], now commercially available from Research and Industrial Instruments, Ltd., London. Some contamination of hard samples by the materials of the balls and container of the grinder (steel, agate, or tungsten carbide) may then occur.

The advantages of small particle size will be lost, unless the sample is uniformly dispersed over the area of the infrared beam. For this purpose, the alkali halide pressed-disk technique, recently reviewed by WHITE [1964], has considerable advantages in convenience and reproducibility and is, where applicable, the recommended technique. Suitable concentrations for silicate spectra are 2 to 3 mg and 0.1 to 0.3 mg in 170 mg KBr to give a 12 mm diameter disk, two concentrations being used to record both weak and strong absorption bands under optimum conditions. Smaller disks containing micrograms of material can be examined with or without the use of beam-condensing equipment (ANDERSON and WOODALL [1953]; BISSET et al. [1959]; DINSMORE and EDMONDSON [1959]; HOWE et al. [1961]). Ground samples can be quantitatively incorporated in the disk by adding the KBr to the mortar or capsule in which the sample has been ground. Two minutes in a vibratory grinder is sufficient to give a satisfactory uniform dispersion. Various forms of dies have been described in the literature to press the KBr powder and sample to give a clear transparent disk (WHITE [1964]). It is particularly convenient when the disk is formed within a sleeve in which it can be handled and used (FARMER [1957]).

The pressed disk is applicable to all water-insoluble, stable minerals. It cannot generally be used for water-soluble salts, where ion exchange in the disk can, but by no means always does, occur (MELOCHE and KALBUS [1958]). Ion exchange can also occur with zeolite minerals and clays, so those features that are a function of the ion, such as the hydration state or the absorption of ammonium ion on the exchange sites, cannot be examined in disks. The unstable 14 A form of tobermorite has been found to lose water and decompose to the II A form under the vacuum conditions used in preparing pressed disks (FARMER et al. [1966]). Alkali halide disks cannot safely be used in studying the vibrations of surface groups or adsorbed molecules. The author has found, for example, that hydroxyl groups on the surface of silica gel become hydrogen bonded to the halide ions in disks. Another disadvantage of the pressed-disk technique is that finely divided alkali halides are hygroscopic, and this adsorbed water can cause uncertainties in the study of the hydroxyl and water absorption bands of the mineral itself. This interference can be minimized by avoiding excessive grinding of the alkali halide, by choosing a concentration of the mineral such that its hydroxyl absorption is strong compared with that of adsorbed water on the disk, and by heating the disk to remove adsorbed water. Alkali halide disks are porous and lose most of their adsorbed water at 1000 Should this treatment cause any change in the mineral itself, this will be indicated by changes in its spectrum other than simple loss of water absorption bands. The use of silver chloride and thallous bromide has been advocated for disk material, as these are not hygroscopic (SMALLWOOD and HART [1963]; THOMPSON [1963]; PYTLEWSKI and MARCHESANI [1965]).

Where some interaction between the alkali halide and the sample is likely or suspected, mineral powders can be examined as mulls in medicinal paraffin (Nujol). Regions of the spectrum observed by oil adsorption can be examined using hexachlorobutadiene or fluorinated oils. Mulling techniques are generally less convenient, less easily reproducible, and less economical of material than alkali halide pressed disks; these advantages can be recovered by preparing disks in which the sample is dispersed in paraffin wax (DEANE [1965], polyethylene (MAY and SCHWING [1963]; SMETHURST and STEELE [1964]), or polytetraftuoroethylene (SAUMAGNE and JOSIEN [1959]), and the first two are commonly used in far infrared studies.


Light scattering is higher in such disks than in alkali halide disks, as the refractive index of these organic materials does not match those of minerals so well. The absorption bands of the disk material make them unsuitable for general use, but they have much to commend them in comparison with mulls, when facilities for the preparation of pressed disks are available.

A microtechnique whereby 4 to 35 pog of solid material is squeezed between diamond or sapphire windows has been used to study minerals under pressure (WEIR et al. [1959]) and may have more general application (LIpPINCOTT et al., [1961]; WEIR and LIPPINCOTT [1961]).

In studying the absorption of surface groups and of adsorbed molecules, it is necessary to use either self-supporting films or dry deposits on a transparent window, and such preparations are also desirable or essential for following reversible dehydration and dehydroxylation reactions of minerals in the infrared. Such preparations, and possibly also nujol mulls, give the layer silicates a preferred orientation, so that absorption bands whose dipole change is perpendicular to the layers appear only weakly when the infrared beam is incident at right angles to the plane of the deposits (FARMER [1958]; FARMER and RUSSELL [1964]).

Special techniques are applicable to certain minerals . Thus, the montmorillonite and vermiculite groups of minerals yield excellent self-supporting films, which do not scatter light excessively (FRIPIAT et al. [1960a]; RUSSELL and FARMER [1964]; FARMER and MORTLAND [1965] ; WALKER and GARRET [1967]). A suitable technique is to grind to mg of a well-washed montmorillonite ( < 2 pom) with a drop of water in an agate mortar to give a creamy consistency. More water is then added slowly to give a uniform dispersion. One ml of this dispersion is pipetted onto thin polythene film held flat on a glass surface by the capillary action of a drop of water. Evaporation in a dry atmosphere at room or slightly higher temperature gives a film about an inch in diameter, which can be separated from the polythene by drawing it over a sharp edge (Figure 2) . Not all montmorillonite samples give satisfactory films by this

/ClaYfilm

'----~/) \'-----------'

1Perspex

_-- Polythene

Figure 2. Preparation of montmorillonite films.

technique. Many soil clays containing hydrous oxides shrink excessively on drying, causing the film to break up. Kaolinite films do not cohere.

Finely divided preparations of amorphous silica, silica-alumina gels, and synthetic zeolites give satisfactory films by pressing the powders between polished steel faces generally at about 10,000 Ib/in. 2 (McDONALD [1958]). Separation of the sample from the surface is facilitated by introducing aluminum foil between the sample and the steel faces. The film can then be separated from the foil by the technique shown in Figure 2. These self-supporting


films are adequately transparent in the 3 fkm region, especially when the particle size is of the order of 0.1 fkm ANGELL and SCHAFER [1965]), and have permitted studies of surface hydration and dehydoxylation, which would not otherwise be possible. They are sufficiently porous to permit the adsorption of gases and liquids on the constituent particles. Results obtained on pressed films may not, however, be exactly comparable to those on free powders, as it has been shown that some surface hydroxyl groups, which can exchange their hydrogen for deuterium in the free powder, are not accessible in the pressed films (HAMBLETON et al. [1965]). Highly transparent plates of alumina aerogel suitable for surface studies have been prepared by slow gelation of alumina sols (PERI and HANNAN [1960]; PERI [1965a]).

Such self-supporting films are too thick to follow changes in the stronger absorption bands during dehydration reactions, and alternative techniques must also be used for minerals that do not yield films. Sometimes reversible dehydration reactions can be followed by heating alkali halide or polytetrafluoroethylene disks containing the sample, as such disks are sufficiently porous to allow water vapor to escape, and rehydration is sufficiently slow to permit the spectrum of the anhydrous state to be examined (FARMER [1966]). Otherwise, the sample can be examined as a dry deposit on a suitable window material in a hot cell. Dispersions of the finely divided sample in alcohols or nonpolar liquids are evaporated on such substrates as alkali halide, silver chloride, or Irtran (ZnS) windows. Quartz or even microscope cover slides are sufficiently transparent to examine the 2000 to 4000 cm - 1 region. The possibility of ion exchange when samples are deposited on alkali halide substrates must be considered. The principal difficulty in this technique is to avoid coagulation of the powdered sample, when the advantages of fine size will be lost. Coagulation may be avoided by stirring the dispersion with a fine steel wire as it evaporates. Some workers have sprayed dispersions onto hot windows so that evaporation of the liquid is rapid (NICHOLSON [1960]). In general, it is difficult to prepare satisfactory thick deposits to study weak absorption bands in the 3000 cm - 1 region, where scattering is most severe. Hydroxyl absorption bands may pass undetected even with deposits that appear to have considerable transmission (SZYMANSKI et al. [1960]), but this transmission is actually between particles, rather than through the sample.

The attenuated total reflection technique, in which the sample is brought into good optical contact with a surface at which the infrared radiation undergoes total reflection, gives spectra similar to, but not identical with, absorption spectra (FAHRENFORT and VISSER [1962]). This technique gives excellent spectra for powdered minerals (HARRICK and RIEDERMAN [ 1965]).

Reflection spectra can be obtained from the polished faces of minerals and rocks. Here reflection is high in the region of strong absorption bands, but weak absorption bands may not be detected. The reflectivity is a complex function of both refractive index and absorption coefficient, and both these characteristics can be mathematically derived from reflection spectra (VEDDER and HORNIG [1961]; MITRA and GIELISSE [1964]). Using polarized radiation, the direction of the dipole change associated with absorption bands has been derived by reflections off suitable polished faces of single crystals of several minerals, including muscovite (VEDDER [1964]). This information can also be obtained by absorption techniques, but it is difficult to obtain thin enough sections of most minerals (less than 1 fkm) to examine the stronger absorption bands by this technique (ANGELL [1964]). The use of reflection and also emission spectra in characterizing rocks and minerals has been examined by LYON [1964a]. Powdered minerals generally give spectra of poor contrast by these techniques, but developments in instrumentation (HOVIS [1965]; Low and COLEMAN [1966]) promise to increase their applicability.

It should be noted that the stronger absorption bands of minerals become dependent on particle size and shape when the particle size is less than the wavelength of the infrared radiation, or more accurately, than the wavelength of the crystal vibration, which is considerably


shorter than the wavelength in air, although of the same frequency. The vibrating ions in a crystal give rise to significant oscillating electric fields, and each ion is subjected to the fields due to the motion of all the other ions in the crystal that participate in the vibration, and has its vibrational frequency modified by them. This interaction between vibrating ions is termed dipole coupling, and for very small crystals < I /km, the coupling forces are dependent on crystal shape, so that the position of the stronger absorption bands would differ for platy, needle-shaped, or spherical particles of the same mineral. In large crystals, for all but those of cubic or tetragonal symmetry, the position and width of strong absorption bands depend on the face from which reflection observations are made. These effects have been discussed with particular reference to the kaolin minerals by FARMER and RUSSELL [1966] and may account for discrepancies found between the spectra of powdered hydrargillite and that of thin sections of a single crystal (T AKAMURA and KOEZUKA [1965]).

Ill. Sources of Error in Infrared Spectroscopy

Infrared spectroscopy, like other experimental procedures, can give highly misleading results, and the practising spectroscopist must be alive to possible sources of error. Instruments are not always in perfect adjustment, and the user must assure himself of the accuracy and reproducibility of his recordings by the use of such standard test substances as indene (JONES et al. [1961]), polystyrene film (PLYLER et al. [1957]), and atmospheric and other gases (DOWNIE et al. [1953]). The International Union of Pure and Applied Chemistry (1961) has published a set of provisional wavelength standards covering the region 4000 to 600 cm - 1, which is particularly valuable in calibrating grating spectrometers.

Spectra often display spurious bands. Some of these arise from imperfect compensation of atmospheric absorption bands. Even with good compensation, the spectrometer becomes insensitive when scanning regions of strong atmospheric absorption, so that band contours are distorted . It is worthwhile to purge the spectrometer with dry air or nitrogen.

Spurious bands also arise from unsuspected contaminants in samples, and a useful summary of some, which have been encountered in general infrared applications, has been compiled by Launer (see MILLER [1965]; WHITE [1964]). Contamination of clays is particularly common because of their high surface areas and high adsorptive capacity. Clay samples exposed to the laboratory atmosphere frequently develop ammonium absorption bands between 3270 and 2700 cm - 1 and near 1400 cm - 1. Ammonia arises not only from chemical operations, but is a common constituent of polish formulations used for floors and benches. Organic contaminants can be picked up from the atmosphere and by contact with plastic containers and tubing. Their presence gives C-H stretching absorption bands at 2950 and 2930 cm - 1, and sometimes carbonyl absorption in the 1600 to 1750 cm - 1 region.

Spurious peaks, which almost certainly arise from one or others of these causes, have been ascribed to clays or their associated absorbed water.

D. Identification and Characterization of Minerals

The use of infrared spectroscopy to identify minerals and to provide information on their structure has been discussed in several reviews (LAUNER [1952]; LECOMTE [1958] ; LEHMANN and DUTZ [1959]; AKHMANOVA [1959]; SETKINA [1959]; LAWSON [1961]; MOENKE [1961, 1962a, 1964a]; KLEBER and MOENKE [1964]; LYON [1962a, I 964b, 1967]; STUBICAN [1963]; TARTE [1963a]; FARMER [1964a]) and is the subject of this section.

I. Identification of Minerals and Mineral Mixtures

The empirical identification of minerals by infrared spectroscopy requires a collection of


spectra obtained from well-characterized specimens, and such spectra are widely scattered through the literature. Most of the earlier spectra, and some of the more recent, are of poor quality, largely due to excessive particle size of the sample. Many minerals will require reexamination with modern instruments and techniques to give better resolution of hydroxyl absorption in the 3000 cm - 1 region and to extend the spectrum below the 630 cm - 1 limit imposed by sodium chloride prism spectrometers. References to published spectra are given in a bibliography at the end of this chapter, which includes carbonates, phosphates, aluminates, oxides, hydroxides, and silicates. The latter are classified according to structure. A valuable

Lepidolite

553

.. u c:

(Si4-Y Aly) ro .D (R Stevens) o

B Y = 0.18- 0.72


bibliography of mineral spectra has also been compiled by LYON [1962a]. NAKAMOTO'S book [1970] and LAWSON'S bibliography [1961] cover a wider field of inorganic substances including gases, coordination compounds, and semiconductors as well as the simpler ionic compounds and some minerals. MOENKE'S [1962a] collection includes 355 spectra of minerals, extending down to 400 cm- I. LYON'S [1962b] publication is valuable for its 370 spectra, including several of wel1-characterized isomorphous series of silicate minerals, such as the lepidolites (Figure 3). muscovite (Figure 4), and feldspars (Figure 5). Other small collections of spectra are those of

MgVI

(Coleman)

Muscovite

339-11

3144 (Anal)

(Chinnerl

(Y=0.96)

Muscovite

3098

(Hemley)

Hydromuscovite

3064 (Anal)

(Threadgold)

(Y=0.97)

., uc: .0 '" o B <

Vcm l

1018

Figure 4. Spectra of muscovites (LYON [1962b]).

LEHMANN and DUTZ [1959], SAKSENA [1961], LAUNER [1952], HUNT et at. [1950], ADLER et at. [1951], and KELLER et at. [1952]. MILLER and CO-WORKERS [1952, 1960] have published spectra extending down to 300 cm - 1 of a variety of salts. In Sadtler's collection of inorganic spectra (4000 to 250 cm - 1), salts and coordination compounds predominate (TURNER [1966]). Some groups of compounds that have received individual attention are listed in Table 1.

V. C. Farmer and F. Pal

mieri

KRC(1987)

AlbiteAN2(1987)

"645 ~529

allall

AN28 5381(1990) 619 AN33(1991)

AN43(1992)

AN52(1999)

AN67(2000)

AN69(3065)

AN85(2001)

AN94(2003)

Anorthite

~1011JI


Table 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: BWTELSPACHER and MAREL [1961a], CHUKROV and ZVYAGIN [1966], FARMER and RUSSELL [1964, 1966], MOENKE [1962a], Sadtler Inorganic Spectra Y168. (b) Serpentine minerals and septich/orites: BRINDLEY and ZUSSMAN [1959], KODAMA and OINUMA [1963], MOENKE [1962a], MONTOYA and BAUR [1963], STUBICAN and Roy [1961a, b] .

2:1:1 Layer Silicates Ch/orites: HAYASHI and OINUMA [1965, 1967], KODAMA and OINUMA [1963], MOENKE [1962a].

2:1 Layer Silicates (a) Talc, pyrophyllite: FARMER [1958], FARMER and RUSSELL [1964], VEDDER [1964]. (b) Montmorillonite, beidellite, rectorite, nontronite: FARMER and RUSSELL [1964], FARMER et al. [1967], GRIM and KULBICKI [1961], STUBICAN and Roy [1961a, b]. (c) Saponite, hectorite, vermiculite: FARMER [1958], FARMER and RUSSELL [1964], FARMER et al. [1967], MAREL [1966]. (d) Dioctahedral micas and lepidolite: ARKHIPENKO et al. [1965], FARMER and RUSSELL [1964], FARMER et al. [1967], LYON [1962b], MAREL [1966], VEDDER and McDoNALD [1963]. (e) Trioctahedral micas: ARKHIPENKO [1963], FARMER and 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 and FURKERT [1966], MITCHELL et at. [1964], WADA [1967a].

Rock-forming Silicates : LYON [1962b], MOENKE [I 962a].

Oxide Minerals

Silica (Crystalline, Opaline, and Hydrated): BENESI and JONES [1959], D'OR et al. [1955], LIPPINCOTT et al. [1958], LYON [1962b], PLENDL et al. [1967], SUN [1962].

Aluminum Oxides and Hydroxides: CAILLERE and POBEGUlN [1966], DUFFIN and GOODYEAR [1960], FREDERICKSON [1954], FRIPIAT et al. [1967], KOLESOVA and RYSKIN [1959, 1962], LYON [1962b], MAREL [1966], MOENKE [1962a], TAKAMURA and KOEZUKA [1965], TARTE [1963a, 1967].

Iron Oxides and Hydroxides: FRIPIAT et al. [1967], HARTERT and GLEMSER [1956], LIESE [1967b], MOENKE [1962a], SUETAKA [1964].

Manganese Oxides and Hydroxides: GATTOW [1962], PABLO [1965], SCHWARZMANN and MARSMANN [1966], SHlRATORI and AIYAMA [1965].

Other Ancillary Minerals

Carbonates : ADLER and KERR [1963a, b], BARON et at. [1959], HUANG and KERR [1960], MOENKE[1962al Sulphates: ADLER and KERR [1965], MOENKE [1962a], OMORI and KERR [1963], WIEGEL and KIRCHNER

[1966]. Phosphates: ARLIDGE et al. [1963], BADDIEL and BERRY [1966], FOWLER et al. [1966], MOENKE [1962a].

The identification of a mineral and the recognition of its presence in mixtures are more certain when its absorption bands are numerous and sharply defined and if they appear in distinctive regions of the spectrum. Sharp absorption bands usually reflect a high degree of crystallinity and regularity in the structure, so that random isomorphous substitution, which is a common feature in silicates, tends to broaden absorption bands and reduce detail in the spectrum; this is particularly marked with aluminum-for-silicon substitution. Nevertheless, as is clear from the spectra of feldspars (Figure 5), lepidolites (Figure 3), and other layer silicates (Figure 6), the spectrum of a mineral can not only define a mineral species, but place it


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accurately within its range of compositional variation, and this with a sample which can, with suitable techniques, be less than I jLg.

Infrared characterization will be more specific than X-ray characterization, where isomorphous substitution gives rise to small changes in lattice parameters, but causes larger changes in vibrational frequencies . In the dioctahedral montmorillonite group, beidellite, nontronite, and montmorillonite are readily distinguished, and Fe-for-AI substitution in the octahedral layer of montmorillonite can be established (STUBICAN and Roy [1961a, b]; FARMER and RUSSELL [1964]). Other examples will be found in the references of Table I.

The presence of hydroxyl absorption bands and their pattern are important distinguishing features, and a grating spectrometer is desirable for their examination. The absorption bands in the 1600 to 650 cm - 1 region arise principally from the stretching vibrations of the anion groups in the structure, and minerals with a common anion are distinguished by the effect on these anion vibrations of the cations, hydrogen bonding by water of crystallization, and of other features of the crystal environment. Such changes are often marked, as can be seen, for example, in the spectra of carbonates (BARON et at. [1959]) and of the various anhydrous and hydrated forms of AIP04 and FeP04 (ARLIDGE et at. [1963]), but there are clearly advantages in extending the spectra below 600 cm - 1, where the vibrations of the cations in the structure should affect the vibrational pattern more profoundly (TARTE [1963a, b]).

It is unfortunate that the strongest stretching absorption bands of silicate, phosphate, and sulphate anions overlap to a considerable extent in the 1200 to 900 cm - 1 region. Carbonate absorption bands near 1400 cm - 1 are clearly separated from these, so that the infrared technique is probably the most sensitive for identifying carbonate ion in the presence of silicates and other minerals, and the position of its absorption bands in this region, together with its weaker absorption bands near 870 and 700 cm - 1, can fully characterize the carbonate present. Nitrate, borate (tricoordinated), and ammonium, however, also absorb near 1400 cm - 1 . Overlapping absorption bands may, in certain cases, be distinguished by making use of differing chemical or thermal stability of the minerals likely to contribute to this absorption. Thus, borate absorption in tobermorite, a natural calcium silicate, becomes apparent following thermal decomposition of the carbonate also present (FARMER et at. [1966]) .

There is some indication that the infrared spectrum is particularly sensitive to disorder in crystal structure, whether arising from poor crystallinity, or from random isomorphous substitution. The spectra of synthetic preparations of layer silicates (STUBICAN and Roy [1961a, b]) frequently diverge from those of well-crystallized natural specimens; similarly, spectra of synthetic iron phosphates were found to show marked spectral differences compared with well-crystallized specimens of the components indicated by X-ray studies (ARLlDGE et at. [1963]). Marked differences in infrared spectra between well-ordered and disordered structures have been reported for spinels (HAFNER and LAVES [1961]; TARTE [1963a]; Roy and FRANCIS [1953]) and feldspars (MILKEY [1960]; LAVES and HAFNER [1956]; HAFNER and LAVES [1957]) and have been used to distinguish sillimanite and mullite (TARTE [1959a]; Roy and FRANCIS [1953]).

Amorphous substances give very much more featureless spectra than do well-crystallized compounds. Nevertheless, infrared spectroscopy is a unique tool in deriving information on the structure of such substances, and this aspect will be discussed more fully later. Amorphous substances absorb as strongly as crystalline substances, and so their presence in admixture with crystalline components is less likely to be overlooked in infrared than in X-ray examination. On the other hand, small amounts of crystalline components in the presence of larger amounts of amorphous material of similar composition will often be more readily detectable by their X-ray diffraction pattern.

Semiquantitative analysis of mineral mixtures with distinctive absorption bands is readily


achieved, particularly by using the alkali halide pressed-disk technique. High accuracy requires that the particle size of the sample be reduced below the level at which it affects the intensity of absorption bands (DUYCKAERTS [1959]). The rapidity with which results can be obtained (30 min) is attractive, as is the sensitivity of the technique for certain components, for example, quartz, carbonates, and kaolinite. Quantitative infrared analysis of minerals has been discussed by LEHMANN and DUTZ [1959], TUDDENHAM and LYON [1960], LYON et al. [1959], HUNT and TURNER [1953], CORBRIDGE [1956], and MIDGLEY [1964]. Difficulties arise where the degree of crystallinity has a profound effect on the intensity of absorption bands, and this problem has been discussed particularly in the estimation of kaolin minerals (LYON and TUDDENHAM [1960a]; MAREL [1960]; BEUTELSPACHER and MAREL [196la]). For this problem, as for many others, it is clear that a combination of X-ray and infrared methods will give more information than can either alone. Indeed, the full resources of modern ir:strumental and chemical methods

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Figure 7. OH stretching vibrations in various phlogopites and talc. (a) Canadian phlogopite; d = 100 lim; total iron content from X-ray spectroscopy 0.015/3; (b) Madagascar amber ; d = 100 lim; total iron 0.30/3; (c) Canadian amber; d = 100 lim; total iron content 0.34/3; (d) Madagascar amber; d = 100 lim ; total iron content 0.50/3; (e) biotite; d = 100 lim; total iron content 0.62/3; (f) biotite; d = 10 lim; total iron content 0.76/3; (g) talc (Eden, N.H.); fluorolube mull; total iron content 0.32/3. Redrawn from VEDDER [1964].


(MACKENZIE [1957, 1961]; MAREL [1961]) may still fail to fully define the structure, composition, and properties of complex systems.

II. Structure-Spectra Relationships

I. Vibrations of Hydroxyl and Water

a. Lattice Hydroxyl

The use of infrared spectroscopy for the characterization of water of crystalization and hydroxyl groups has been reviewed by YUKHNEVICH [1963]. The stretching vibration of an OH bond is essentially localized on the proton, and so gives information on its environment. The

Wavelength ().1m) Wavelength ().1m) Wavelength ().1m)

270 275 280 285 270 275 280 270 275 280 285

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3750 3700 3650 3600 3550 3500 3750 3700 3650 Frequency (em}) Frequency (em-}) Frequency (em})

Fig. 8 Fig. 9 Fig. 10

Figure 8. Hydroxyl absorption of randomly oriented samples of A-pyrophyllite; B-beidellite; C-rectorite; D-muscovite or paragonite; E-margarite, F-Wyoming montmorillonite; G-Skyrevdalen montmorillonite; H-Woburn montmorillonite; l-nontronite; and K-ferric celadonite. (FARMER and RUSSELL [1964]).

Figure 9. Hydroxyl absorption of A-kaolinite; B-dickite; C-nacrite; D-Puga kaolon. R denotes randomly oriented specimens, and N films at normal incidence (FARMER and RUSSELL [1964].)

Figure 10. Hydroxyl absorption of A-taIc; B-hectorite; C-saponite; D-phlogopite (126 Jlm); E-green biotite (115 Jlm); and F-dark-brown biotite (25 Jlm). The mica spectra are of single flakes of the thickness indicated at normal (N) and 45 incidence (R) (FARMER and RUSSELL [1964]).


most important factor determining the frequency of vibration is the strength of hydrogen bonding, but other factors certainly playa part. The influence of some of these factors can be seen in the hydroxyl absorption of the layer silicates (Figures 7 to 10) studied by VEDDER [1964], FARMER and RUSSELL [1964], KODAMA and OINUMA [1963], SERRATOSA and BRADLEY [1958a, b] and SERRATOSA et al. [1962, 1963]. In pure talc, the hydroxyl ion is coordinated to three magnesium ions and is oriented perpendicular to the silicate layers. This hydroxyl ion absorbs at 3677 cm - I. The presence of a ferrous ion impurity gives rise to subsidiary bands at 3663 cm- I (OH coordinated to 2Mg+ Fe2+) and at 3645 cm- I (OH coordinated to Mg+2Fe2+) (Figure 7, VEDDER [1964]; see also WILKINS and ITO [\967]). Substitution of Li+ for Mg2+ in the octahedral layer or of Al for Si in the tetrahedral layer of talc gives the clay minerals, hectorite and saponite, respectively. These substitutions cause the OH absorption to broaden, but do not change its frequency [Figure lO(b), (c)] (FARMER and RUSSELL [1966]). Anhydrous monovalent exchangeable ions in the interlayer space of saponite lie directly over some of the hydroxyl groups, and the positive electric field of the ions raises the frequence of these groups, which then absorb at 3710 to 3720 cm -I (FARMER and RUSSELL [1966]). This perturbation disappears when the ions are fully hydrated. Ca 2+ and Mg2+, even when anhydrous, do not give rise to a perturbed OH vibration, and this indicates that these cations are not situated close to hydroxyl groups. In the mica phlogopite, all hydroxyl groups are perturbed by inter layer K +, and the hydroxyl absorption appears at 3708 cm - I. According to VEDDER [1964], the electric fields due to AI-for-Si substitution in its vicinity cause the hydroxyl group to tilt slightly from the perpendicular (about 11.5), and considerably broaden the band . Substitution of Fe 2+ for Mg2+ in phlogopite does not give rise to a separate band, but probably contributes to its breadth. Substitution of AI 3 + for Mg2 + can account for a band at 3665 cm - I in biotites (Figures 7 and 10), which still shows good perpendicular orientation (VEDDER [1964]). With increasing substitution of trivalent ions for divalent ions in biotites, vacancies occur in the octahedral layer, so that some hydroxyl groups are coordinated to two cations instead of three, and these hydroxyl groups are tilted at a considerable angle to the perpendicular. Their absorption bands occur at lower frequencies [Figures 7(c), (d), (e), and (f)], and their intensity is not affected by turning a mica flake at an angle to the beam (Figure 10). VEDDER [1964] has tentatively assigned the band at 3625 cm- I to OH coordinated to two divalent ions. FARMER et al. [1967] have presented evidence indicating that bands near 3600 cm- I and 3560 cm- I

arise from OH coordinated to (AI, Fe2+) and (Fe3+, Fe2+) pairs, respectively. They found, in oxidized ferruginous biotites and vermiculites, a strong broad band at 3550 to 3560 cm - I, which they ascribed to OH linked to two trivalent ions.

In the dioctahedral 2: 1 layer silicates, the hydroxyl ion lies at a small angle to the plane of the sheets, and its intensity is little affected by the angle between the infrared beam and the layer plane. VEDDER and McDONALD [1963] have found (from infrared observations on cleaved plates) that in muscovite the OH group is inclined at about 16 to the cleavage plane. Some of the factors that affect the OH frequency in dioctahedral minerals can be seen in Figure 8 and have been discussed by FARMER and RUSSELL [1964]. The presence of Fe3+ in the octahedral layer of nontronite and celadonite causes a marked displacement of the OH bands to lower frequencies (near 3550 cm - I) compared with minerals containing only aluminum (3620 to 3675 cm- I ), and this effect of Fe3+ can also be seen on comparing a high-iron montmorillonite (spectrum H, Figure 8) with the low-iron forms (spectra F, G, Figure 8). The narrow bands of pyrophyllite and celadonite can probable be correlated with low AI-for-Si substitution in these minerals and suggests that montmorillonites, in which there is no AI-for-Si substitution, would also give sharp hydroxyl absorption. The displacement of the OH vibration from 3675 cm - I in pyrophyllite to near 3630 cm - I in muscovite cannot be ascribed to a perturbing effect of the interlayer potassium ion, as the corresponding band of beidellite is


present even when the interlayer cations are fully hydrated. This band does weaken when interlayer water is removed from beidellite, particularly when the excha ngeable cation is divalent. Perturbed OH vibations at 3500 to 3560 cm - 1 are found when beidellite or montmorillonite containing exchangeable Ca 2 + or Mg2+ are dehydrated (RUSSELL and FARMER [1964]; FARMER and RUSSELL [1966]) .

Natural and synthetic celadonites (FARMER el al. [1967]) reveal up to four discrete bands, at 3602, 3577, 3557, and 3534 cm- 1 , due to OH associated with the ion pairs (MgAI), (Fe2+ AI), (MgFe3+), and (Fe 2+ Fe3+). Three of these are given by the ferric celadonite shown in Figure 8(k). Glauconites examined by the authors show much broader absorption bands than this celadonite, probably due to AI-for-Si substitution in the lattice, but the separate absorption bands can still be seen unless octahedral AI 3 + is high, when the spectrum in this region approaches that of aluminous illites, with a broad undifferentiated band in the 3600 to 3630 cm - 1 region.

In the above discussion, it has been generally assumed that different hydroxyl absorption bands in a spectrum correspond to different types of hydroxyl group, but this is not always true. Thus, where a unit cell contains two symmetrically equivalent hydroxyl groups related by a plane of symmetry (Figure 11), two separate absorption bands can appear. In the symmetrical

lal (bl

Figure 11. (a) The symmetrical and (b) antisymmetrical stretching modes of vibration of two hydroxyl groups related by a plane of symmetry.

stretching mode (a), the dipole change lies in the plane of symmetry, while in the antisymmetrical mode (b), the dipole change is perpendicular to this plane. The frequencies of these modes can differ when there is significant coupling between the vibrations of the two hydroxyl groups. This coupling can be either through proximity of the protons, or through the oscillating electric field produced by the vibrations. In the special case of the water molecule, there is direct coupling through the common oxygen atom, and here the frequencies differ by 104 cm - 1 in the isolated gaseous molecule (HERZBERG [1945]). The 2: 1 (micalike) layer silicates do contain two hydroxyl groups in the primitive layer unit cell (FARMER and RUSSELL [1964]). but as these are related by a center of symmetry, the symmetric mode is active in the Raman spectrum, but not in the infrared spectrum. Raman observations have not been made on the layer silicates, but in the similar situation found in brucite (Mg(OHh). the Raman and infrared frequencies differ by 45 cm - 1 (BUCHANAN el al. [1963]).


Coupling is likely to occur between the vibrations of the hydroxyl groups in the kaolin minerals. LEDOUX and WHITE [1964a, b] have conclusively established, by exchange of deuterium for hydrogen, that the 3620 cm - 1 band of the kaolin minerals arises from the inner hydroxyl group of the kaolin layers, while the outer layer of hydroxyl groups (the gibbsite layer) gives rise to the three absorption bands at higher frequencies [Figure 9(a)]. FARMER and RUSSELL [1964] and FARMER [1964b] have pointed out that if coupling is assumed, the effects of orientation of the three bands shown by kaolinite, at 3697, 3669, and 3652 cm - 1, can be accounted for by the generally accepted structure in which the surface hydroxyl groups are oriented at a high angle to the layer, pointing toward the oxygens of the next kaolinite layer in which they are in contact. The pattern shown by nacrite and dickite [Figure 9(b), (c)] suggests that the surface hydroxyl groups in these minerals are not so nearly equivalent as in kaolinite (FARMER and RUSSELL [1964]). If coupling is not assumed, the spectra appear to require that some of the surface hydroxyl groups are oriented nearly parallel to the layers (SERRATOSA et af. [1962, 1963]; WOLFF [1963]; LEDOUX and WHITE [1964a]). The differences between the frequencies of the inner hydroxyl groups in the kaolin minerals (3620 cm - 1) and in pyrophyllite (3675 cm - 1) have not yet been explained and are a clear warning against assuming that frequencies found in one structure will necessarily hold in another apparently similar structure.

The absorption bands due to the surface hydroxyl groups of the kaolin layers weaken when polar organic molecules or salts are introduced between the layers, and a new band due to the hydrogen-bonded OH groups, or possibly water, appear in the 3570 to 3600 cm - 1 region (WADA [1965a, b]; LEDOUX and WHITE [1964a]). This band also appears in some halloysite samples (SERRATOSA et af. [1963]; KODAMA and OINUMA [1963]).

Hydrogen bonding can give rise to considerably greater shifts in frequency than those discussed above and causes a marked progressive broadening of the band, increasing with the displacement in frequency. The strength of hydrogen bonding is a function of the acidity of the proton and the basicity of the electron donor, between which the bond is formed. A number of curves have been published relating the OH stretching frequency to the 0-0 distance in the bond (YUKHNEVICH [1963]), but these must be used with caution, as the 3620 to 3630 cm - 1 band of muscovite and the kaolin minerals does not fit well on these curves (LEDOUX and WHITE [1964b]), and the marked shift to lower frequencies found for hydroxyls coordinated to Fe 3 + in nontronite and celadonite is certainly not entirely due to hydrogen bonding (FARMER and RUSSELL [1964]). With very strong hydrogen bonds, the OH stretching absorption can split into two bands near 2400 and 1800 cm - 1, or, in what is thought to be a symmetrical hydrogen bond, give rise to a very strong, broad, diffuse band extending from about 1600 cm - 1 to below 600 cm - 1. For such strong bonds, the correlation between OH absorption and 0-0 distance is again poor (BLINC et af. [1960]).

Hydroxyls coordinated to divalent or monovalent ions are very weakly acidic and form only weak hydrogen bonds; typical OH stretching frequencies are Mg(OH)z 3697 cm - 1, Ca (OH)z 3644 cm - 1, LiOH 3678 cm - 1, and LiOH H 2 0 3574 cm - 1. Hydroxyls coordinated to Fe3+ or AI3+ are more acidic. Thus Al-OH vibrations appear at 3620 to 3697 cm - 1, when weakly perturbed in the layer silicates (Figures 8 and 9), at 3380 to 3620 cm - 1 in gibbsite (Figure 12), and at 2900 to 3290 cm - 1 when strongly hydrogen bonded in boehmite and diaspore (Figure 12). Similarly, the absorption of Fe2+0H ranges from about 3550 in layer silicates to 2900 to 3230 cm - 1 in goethite and lepidocrocite (Figure 12). In the chlorite minerals, silicate layers similar to those in phlogopite and the biotites alternate with hydroxide layers of the brucite structure, which carry a positive charge due to substitution of Fe3+ or AI3+ for Mg2 +. Infrared studies show that the OH groups of the brucite layer are oriented perpendicular to the sheets (SERRATOSA and VINAS [1964]). These groups absorb very strongly in the region


3380 to 3574 cm - 1 and give a doublet (Figure 12), whose frequency decreases with increasing iron content of the chlorite (KODAMA and OINUMA [1963]). Absorption of the OH groups of the silicate layer can be resolved near 3700 cm - 1, as in phlogopite and biotites, and persists after the brucite layer has been decomposed by heating (FARMER [unpublished]). It is noteworthy that Mg(OH)2 precipitated between the layers of montmorillonite absorbs at a frequency of 3711 cm - 1, higher even than in brucite (RUSSELL [1965]), and much higher than in natural chlorites. SiOH groups are still more acidic; isolated hydroxyl groups on silica surfaces absorb at 3750 cm - 1 (McDONALD [1958]). Absorption of ether displaces these to 3300 cm - 1 (McDONALD [1958]) and of NH3 to near 3000 cm- 1 (CANT and LITTLE [1965]). Afwillite and IX-dicalcium silicate hydrate, which contain 03SiOH tetrahedra, show two or three bands in the region 2400 to 2800 cm - 1, indicating strong hydrogen bonds (RYSKIN [1959]; R YSKIN and

Wavenumbers (em-I) 3500 3000 2500

(b)

(g)

(h ),==,--=-,-==-,_~-=-,-,-I-,-I 2.65 3.05 3.45 3.85

Wavelength (/lm)

Figure 12. Hydroxyl absorption of aluminum and iron hydroxides of a chlorite and of scarbroite. (a) Gibbsite; (b) bayerite; (c) diaspore; (d) boehmite; (e) scarbroite; (f) chlorite; (g) goethite; (h) lepidocrocite.


STAVITSKAYA [1960, 1963]; PETCH et af. [1956]). Potassium hydrogen metasilicate has an additional band at 1840 cm- I (STAVITSKAYA et af. [1967]). Anhydrous tobermorite, the only other mineral in which SiOH groups have been identified, absorbs at 3490 cm - I (FARMER et af. [ 1966]).

The acid phosphates and sulphates all show more or less strong hydrogen bonding. Symmetrical hydrogen bonds occur in monohydrogen phosphates and in hydrogen carbonates and show the typical profound perturbation of the OH vibration. More usually, the acid phosphates give a broad band typical of moderately strong hydrogen bonds near 2600 cm - I (CORBRIDGE [1956]). This last band, together with other features of the spectrum, was used to identify the presence of acid phosphate groups in various crystalline aluminum and iron phosphates (ARLIDGE et af. [1963]). Its presence in fresh amorphous precipitates show the presence of acid phosphate groups, which disappear as the gels crystallize on digestion to give crystalline (AI, Fe)P04 2H 20 (FARMER [unpublished]). Infrared evidence for the presence of acid phosphates in calcium-deficient apatites has been reported (POSNER et af. [1960]).

The frequency of vibration of OD groups is lower than those of OH groups by a factor of 1.35 to 1.36, so the accessibility and rate of exchange of hydroxyl groups with D 20 can be readily followed by infrared spectroscopy. The technique also serves to distinguish overlapping hydroxyl absorption bands, where the groups concerned differ in the ease of exchange of their protons. Thus, only the few hydroxyl groups on the surface of kaolinite crystals are accessible to D 20 at room temperature, but the outer hydroxyl groups of the individual layers are readily exchanged when potassium acetate or hydrazine enters between the layers (LEDOUX and WHITE [1964a, b]). The hydroxyl groups of montmorillonites (RoY and Roy [1957]) and of muscovites (VEDDER and McDONALD [1963]) exchange with D 20 at elevated temperatures, and MORTLAND et af. [1963] found that ND3 in montmorillonite also reacted with the lattice hydroxyl above 200.

Comparison of the spectra of minerals containing OD and OH are particularly valuable in identifying the bending vibrations of XOH bonds and the rocking or librational frequencies of hydroxyl groups. A hydroxyl group should have two mutually perpendicular vibrations of this type, which will necessarily occur at the same frequency when the OH group lies along a threefold or higher axis of symmetry. Rocking frequencies of hydroxyl groups have been detected in a number of hydroxides and basic salts (GLEMSER and HARTERT [1953]; HARTERT and GLEMSER [1956]; TARTE [1958]). In general, the more covalent the X-OH bond, the higher is the OH rocking or bending frequency. Hydrogen bonding of the OH group also tends to raise the frequency, and GLEMSER and HARTERT [1956] have derived a formula relating the OH rocking frequency to the metal-oxygen distance and the OH stretching frequency. In the layer silicates, the in-plane rocking vibration of OH coordinated to AI3+ has been positively identified by deuterium substitution in muscovite (925 cm - I, VEDDER and McDONALD [1963]), kaolinite (938 and 915 cm- I , STUBICAN and Roy [196Ib]), and montmorillonite (FARMER et af. [1967]). The 938 cm- I band of kaolinite shifts to a higher frequency (970 cm- I ) when salts are intercalated between the layers, showing that it arises from the surface layer of hydroxyl groups (W ADA [l965a, b]). This assignment has been confirmed by selective deuteration (W ADA [1967b]). Bands in the 950 to 915 cm - 1 region in pyrophyllite, beidellite, and montmorillonite can also be confidently ascribed to AIOH. In the aluminum hydroxides (Figure 13), these bands appear between 910 and 1075 cm- 1 (GLEMSER and HARTERT [1953]; FREDERICKSON [1954]). A band at 405 cm - 1 in muscovite has been ascribed to an out-of-plane AI-OH bending vibration (VEDDER and McDONALD [1963]), but the change in dipole moment is not, as would be expected, normal to the plane (VEDDER [1964]).

In montmorillonites, the (AIMgOH) grouping absorbs at 845 cm - I and the (AIFe3+0H) grouping at 880 to 890 cm - 1 (FARMER et af. [1967]). The association of the latter band with


Boehmite

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3176

Gibbsite a-trihydrate 3177 (shaded)

Bayerite

f}.trihydrate

3178

'Y-aJumina AJ 20 3 3299

.. (J

'" '" c .0 o .0 ~

825 747

Figure 13. Absorption spectra of y-alumina and its hydrates (LYON [1962bJ).

ferric ion was established by its sensitivity to reducing conditions (FARMER and RUSSELL [1964, 1967]). A band of nontronite at 848 cm - 1 is also eliminated by reduction with hydrazine, but the main OH deformation of Fe2~OH, at 818 cm- 1 in nontronite, is little affected by reducing conditions (FARMER el al. [1967]). On lepidocrocite and goethite (Figure 14), two OH librations appear between 750 cm- 1 and 1030 cm- 1 (HARTERT and GLEMSER [1956]; MOENKE [1962a]), and two libration frequencies in the range 700 to 900 cm - 1 also appear in the basic ferric phosphate, leucophosphite (ARLIDGE el al. [1963]).

In saponite and hectorite, a band at 655 cm - 1 has been identified as the OH bending vibration (FARMER el al. [1968]), and similar assignments seem likely for bands in this region of talc, chrysotile, antigorite, phlogopite, and chlorite.

The greater covalent character of the Si-O bond raises the SiOH bending frequency. Bending in the plane of the SiOH bond gives a band at 1282 cm - 1, while out-of-plane bending appears at 712 cm - 1 in a-dicalcium silicate hydrate. The corresponding bands of silica gels are not obvious; a band near 950 cm - 1 in gels sometimes ascribed to SiOH bending is, in fact, an Si-O stretching vibration of the SiOH group (TAKAMURA el al. [1964]). POH bending also appears in the 1200 to 1400 cm- 1 region (CORBRIDGE [J956); RYSKIN and STAVITSKAYA [1960]).


b. Water Absorption

The most distinctive OH bending frequency is the deformation of the HOH angle of water in minerals and salts, which most commonly occurs near 1630 cm - 1 and has a normal range of 1590 to 1670 cm - 1, although bands as high as 1705 cm - 1 and as low as 1560 cm - 1 have been ascribed to this vibration (YUKHNEVICH [1963]). The higher frequencies appear to be associated with strong hydrogen bonding. Thus, in liquid water, the bending frequency is near 1640 cm - 1, compared with 1595 cm - 1 for the gaseous molecule. Water in the interlayer space of expanding layer silicates also normally absorbs near 1640 cm -1 (FRIPIAT et al. [1960a]; RUSSELL and FARMER [1964]), but water, which is coordinated to magnesium ions and hydrogen bonded to pyridine in the interlayer space of montmorillonite, absorbs at 1700 or 1680 cm - 1 according to whether both or only one of the water protons is bonded to pyridine (FARMER and MORTLAND [1966]). It is by no means established, however, that hydrogen bonding is the only factor that affects the HOH bending vibration (HARTERT [1956]; ANGELL and SCHAFFER [1965]).

D%

h /, 80

~ ..,,- -/ \ I \ 60- , '"

~ \ I \ I \ !---- '-./ l \ \

" ~ \ 40

r\ =---' 20

1600 1400 1200 1000 800 700 600 500

(a)

D%

80 V""'\

"\ / "\. [ .......

,-~ - \

V V "I\-

'\ 60

40'" I'-../ "'"'v 20

1600 1400 1200 1000 800 700 600 500

(b)

Figure 14. Absorption spectra of (a) goethite and (b) Iepidocrocite (MOENKE, [1962aD.


The OH stretching frequencies of water molecules are much more sensitive to hydrogen bonding than is the bending frequency, and YUKHNEVICH [1963] has found in the literature an overall range of 3620 to 2700 cm - I for crystal hydrates; recently, water absorption at 3690 to 3720 cm - I in zeolites has been reported (BERTSCH and HABGOOD [1963]; ANGELL and SCHAFFER [1965]). As with isolated hydroxyl groups, the strength of hydrogen bonding is a function of the acidity of the proton and the basicity of the atom to which the proton is hydrogen bonded. Water coordinated to highly polarizing cations such as Be 2+ , AI3+, or Fe3+ is acidic; the precipitation of hydroxides of these metals in weakly acid solutions can be considered as due to ionization of the coordinated water molecules. The relation of the 0-0 distance to OH stretching frequency for water molecules is the same as that found for other types of hydroxyl, and the O-X - distance in hydrogen bonds to halide ions falls on the same curve when the greater radius of these anions is allowed for (GLEMSER and HARTERT [1955, 1956]). Water, however, does not appear to form the strongest hydrogen bonds of the symmetrical or doubleminimum energy types (BLINC et a/. [1960]), since transfer of a proton from water to the electron donor, with formation of a hydroxide ion and a protonated cation, is more likely.

A detailed correlation of the OH stretching absorption bands with the environment of water molecules in crystals is not always straightforward ; interpretation is hindered by the breadth and overlapping of bands, and by the presence of combination frequencies giving subsidiary maxima (HARTERT and GLEMSER [1956]), which might be interpreted as arising from distinct types of hydroxyl group. Liquid water itself still presents difficulties in detailed interpretation (YUKHNEvICH [1963]). However, a fairly clear-cut interpretation has been made for the Mg2 + -H20-pyridine system in montmorillonite and saponite (FARMER and MORTLAND [1966]). In the 14.7 A complexes, magnesium is coordinated to six water molecules, each of which is hydrogen bonded through one proton to pyridine. The vibration of this strongly hydrogen-bonded proton occurs at 2780 cm - I, while that of the free proton is at 3627 cm - I, a band seen only in saponite where the absorption of the lattice hydroxyl group does not interfere. In complexes of higher spacing (about 23 A), both protons of water are hydrogen bonded to pyridine giving a broad band at 3060 cm - I . In this case, the hydrogen bonds to pyridine are weaker as the positive charge arising from the polarizing effect of the cation is distributed over both protons of the water molecule, instead of being localized on one. The high frequency of the free proton in the 14.7 A complex indicates that most, if not all, of the water molecules can form only very weak hydrogen bonds with the silicate layers.

The interpretation of the water absorption bands in the usual hydrated forms of the layer silicates is more difficult, as the absorption of the more acidic water molecules directly coordinated to the cations overlies that of water molecules not directly coordinated. With the more polarizing cations, a strong band or shoulder appears at lower frequencies, which must be ascribed to hydrogen bonds between the acidic, directly coordinated water and water in outer spheres of coordination. This band lies at 2940 cm - I for AI3+, and 3220 cm - I for Cu 2+, It appears as a well-defined shoulder near 3250 cm- I for Mg2+ and Ca 2+, but a weaker shoulder near 3260 cm - I in Na- and K-montmorillonite corresponds to one found in liquid water and ascribed to an overtone of the 1640 cm - I band (Figure 15; RUSSELL and FARMER [1964]). The principal absorption band ofinterlayer water lies at 3400 to 3450 cm - I, close to that of liquid water, and can be ascribed to water in outer spheres of coordination, and to less strongly hydrogen-bonded water coordinated to Na + and K +. A band near 3610 cm - I is seen in saponites, but is obscured in montmorillonite by lattice OH absorption. It is particularly clearly seen in Na + and K + saponite (Figure 15). This band must arise from the weaker hydrogen bonds between water and the silicate lattice, and in agreement with this interpretation, the dipole moment has a significant component perpendicular to the layers (FARMER and RUSSELL [1967]). Removal of the more loosely held water in outer spheres of


coordination by heating in vacuum causes the main 3420 cm - I band to decline much more than the 1630 to 1640 cm - I band (Figure 15) (FRJPJAT et al. [1960] ; RUSSELL ana FARMER [1964]).

Simultaneously, the maximum of the H 20 stretching band shifts to higher frequency, but a broad shoulder near 3250 cm - I persists in spectra of Mg-saponite and Mg-vermiculite after water in outer spheres of coordination has been lost. This shoulder, absent from spectra of Mg-hectorite and Mg-montmorillonite, is indicative of strong hydrogen bonding of coordina ted water to surface oxygens involved in AI-O-Si linkages, which are more negatively

Frequency (em-})

3750 3500 3250 3000 3750 3500 3250 3000

250

250 Na

2.6 2.8 3.0 3.2 3.4 2.6 2.8 3.0 3.2 3.4 Wavelength (/-1m)

Figure 15. OH stretching absorption of water in saponite films containing various exchangeable cations and heated to the indicated temperatures (RUSSELL and FARMER [1964]).

charged than those involved in Si-O-Si linkages. The stronger hydrogen bonding is also reflected in the fact that water is more difficult to remove from saponite and vermiculite than from montmorillonite and hectorite, which contain little or no tetrahedral AI. (RUSSEL and FARMER [1964] ; FARMER and RUSSELL [1971]).

In addition to the stretching and bending vibrations of the free molecule, vibrations corresponding to rotary or translational vibrations of the water molecule as a whole appear in crystals, and their frequency is highly dependent on the forces exerted on the water molecule by coordination to cations and by hydrogen bonding to negative sites. The highest frequency of this type in liquid water is a broad band near 710 cm- I , shifting to 800 to 850 cm- I in


ice (GIGUERE and HARVEY [1956]); water in expanding layer silicates also contributes a very broad band in the 900 to 600 cm - I region. Libration of coordinated water in crystals give sharper absorption bands, which generally lie at higher frequencies for stronger coordinate bonds (NAKAGANA and SHIMANOUCHI [1964]). Rocking of water coordinated to the smaller divalent ions ranges from about 600 cm - I Mg2 + to 887 cm - I for Cu 2 +, while wagging ranges from 460 cm- I for Mg2+ to 645 cm- I for Ni2+. Metal-oxygen stretching of the coordinate bond was found to lie in the 300 to 400 cm - I region for divalent ions, rising to 490 cm - I for [Cr(H20)6]3 +. Librational frequencies for water in Na, Sr, and Ba halides have also been reported and lie at rather lower frequencies (ELSKEN and ROBINSON [1961]).

The hydronium ion (OH;) has been studied in aqueous solutions and in crystalline acid hydrates, where it has been found to give four bands in the regions 3380 to 2650 cm - I, 2150 to 2100 cm- I , 1750 to 1670 cm- I , and 1200 to 1130 cm- I (YUKHNEvICH [1963]). The two higher-frequency bands are very broad and diffuse, and the lowest, though narrow, is weak. The 1700 cm - I band is moderately strong, but its frequency range overlaps that of water. The presence of hydronium has been postulated from infrared evidence in vermiculite (BoKu and ARKHIPENKO [1962]) and in an expanded muscovite (WHITE and BURNS [1963]), but for neither is the evidence strong, and an alternative assignment of the 3470 cm - I band, which was ascribed to (OH3)+ in muscovite, has been made for rectorite and beidellite, in which similar bands were detected (RUSSELL and WHITE [1966]; FARMER and RUSSELL [1967]).

It is clear from the above discussion of the various forms that water may take in minerals that the presence of OH stretching frequencies with little or no absorption near 1640 cm - I establishes the presence of constitutional hydroxyl groups, and that a band near 1650 cm - I, in the absence of any organic contaminants, establishes the presence of water in the mineral. Where water is present, however, it must, in general, be removed to determine whether more tightly bound constitutional hydroxyl groups are present. This presents no difficulties where the anhydrous product is stable, but difficulties arise when the product is hygroscopic. Ideally, a self-supporting disk or flake of material, or a deposit on a transparent window, can have its spectrum recorded after heating in an evacuated cell with transparent windows, and this technique has been applied to silica gels (McDoNALD [1958]; BENESI and JONES [1959]); zeolites (SZYMANSKI et al. [1960]; ANGELL and SCHAFFER [1965]), amorphous aluminosilicates (BASILA [1962]), alumina (PERI [1965a]), montmorillonite, saponite, and vermiculite (RUSSELL and FARMER [1964]; FRIPIAT et al. [1960a]; BUSWELL and DUDENSOSTEL [1941]). Convenient vacuum cells for this purpose have been described by LITTLE [1966], ANGELL and SCHAFFER [1965], and GRANQUIST and KENNEDY [1967].

Sometimes it is acceptable to seal the dehydrated powder from atmospheric moisture with mineral oil (SZYMANSKI et al. [1960]; STUBICAN [1959]; FROHNSDORFF and KINGTON [1958]), and this also reduces scattering. Use can be made of the porosity of potassium bromide pressed disks; dehydration of a sample incorporated in a disk proceeds more slowly and at a higher temperature than exposed powder, but rehydration, where reversible, is sufficiently slow to permit spectra to be obtained at atmospheric temperatures and humidities. This technique has been applied to the calcium silicates reyerite (CHALMERS et al. [1964]), and tobermorite (FARMER et al. [1966]), to various aluminum and ferric phosphates (ARLIDGE et al. [1963]), and to soil clays (MITCHELL and FARMER [1962]), but can sometimes lead to interaction between the alkali halide and the sample (FARMER [1966]). Generally, it is possible to distinguish between adsorbed water on the one hand and water of crystallization or zeoli tic water that enters into the structure on the other, since loss of the latter modifies the vibrations of other parts of the crystal structure, and so causes changes in the spectrum in addition to simple loss of water absorption bands. Such changes have been reported for expanding layer silicates (TETTENHORST [1962]; RUSSELL and FARMER [1964]), calcium silicates (CHALMERS et al. [1964]),


aluminum and ferric phosphates (ARLIDGE et al. [1963]), and calcium sulphates (RAZOUK et al. [1960]; FISCHER [1963]; WIEGEL and KIRCHNER [1966]).

2. Discrete Polyatomic Anions and Cations; the Orthosilicates

Like the hydroxyl ion and water molecule in crystals, the vibrations of the simpler polyatomic anions and cations can be treated to a first approximation as isolated molecules, as the bond forces within the ion are generally considerably stronger than the electrostatic forces between ions. This group of inorganic compounds, which includes coordination compounds, have been fully discussed by NAKAMOTO [1970]. Such polyatomic ions as carbonate, nitrate, sulphate, phosphate, and ammonium have characteristic frequencies, which do not differ greatly with the countercation, and which permit the recognition of their presence in minerals. Thus, ammonium ion has been detected by its infrared absorption in feldspars (ERD et al. [I 964]) and in muscovite (VEDDER [1965]). This characteristic absorption permits the distinction and estimation of ammonium ions in the presence of adsorbed ammonia, in clays treated with ammonia gas (MORTLAND et al. [1963]; RUSSELL [1965]). Similarly, carbonatoapatites are readily distinguished from hydroxyapatites (TUDDENHAM and LYON [1960]), and carbonate ion can be detected and estimated in scapolite (SCHWARCZ and SPEELMAN [1965]).

Deviations of the absorption pattern from that expected for a free molecule can provide information on the crystal environment. A polyatomic ion containing n atoms has 3n - 6 internal modes of vibration and six external modes, corresponding to rotary and translational vibrations of the ion as a whole. Thus, the carbonate and nitrate ions have six internal modes. Owing to the symmetry elements of such ions, one vibration (vI' Figure 16) is inactive in the

x

Figure 16. Vibrations of a planar XYJ molecule or ion; and V4 are doubly degenerate VJ (NAKAMOTO [1963]).

infrared, but active in the Raman spectrum and occurs near 1050 cm - I. The out-of-plane vibration (v 2 ) near 850 cm - I, which is a pure bending vibration, is active in the infrared and not in the Raman. The in-plane stretching modes V3 and the in-plane bending modes V4 are active in the infrared and Raman spectrum. Because of the threefold axis of symmetry, these are degenerate vibrations; that is, V3 and V4 can each be excited to give a dipole moment in any direction in the plane of the ion without change in frequency. Anyone vibrational mode of this type can be represented as the sum of two mutually perpendicular vibrations of identical


frequency, and so these vibrations are termed doubly degenerate . As represented in Figure 16, "3 is a pure stretching vibration, and "4 is purely bending. However, these vibrations are of the same symmetry species, so that some mixing can, and does in fact, occur (WILSON et al. [1955)). The higher-frequency vibration near 1400 cm - I is principally stretching, and the lower near 700 cm - I is principally bending. In the crystal environment, these ions may no longer have their full symmetry. With the loss of the plane of symmetry in the plane of the molecule, "I can become active in the infrared, and with loss of the threefold axis of symmetry, "3 and "4 are no longer degenerate, so that two distinct frequencies may be observed for each doubly degenerate vibration of the isolated carbonate or nitrate ion. Thus, "I is active, and "4 is doubled in aragonite, but not in the more symmetrical calcite crystal (ADLER and KERR [1962)). On the other hand, the degree of perturbation may be insufficient to produce any observable effect, and this is true for the "3 vibration in aragonite. The perturbation caused by the crystal surroundings, leading to loss of the full symmetry and to some changes in the force constants of the isolated ion, is termed the static field effect.

Similar considerations apply to tetrahedral ions, such as ammonium, sulphate, phosphate, and orthosilicate. The isolated ions have only two absorption bands, arising from triply degenerate stretching and bending vibrations ("3 and "4 in Figure 17). Perturbation by the

Figure 17. Vibrations of an isolated X0 4 molecule or ion. Vibration V2 is doubly degenerate ; VJ and V4 are triply degenerate (FARMER [1964a]).

crystal environment can cause the infrared inactive vibrations ("I and "2) to become active. Hydrogen bonding of ammonium ion is one of the stronger perturbing forces arising from the crystal environment, causing the N-H stretching frequency to vary between 3100 and 3330 cm - I, and the N-H bending frequency to vary between 1396 and 1484 cm- I (WADDINGTON [1958]; MATHIEU and POULET [1960)). Attachment ofa proton to one or more oxygens as in the acid phosphate and 03SiOH ions leads to a strong pertubation of the x- o vibrations (RYSKIN and STAVITSKAYA [1960)). The X-OH bond acquires greater single-bond character and falls in frequency, while the other x-o bonds acquire greater double-bond character, and may be further perturbed by the effects of hydrogen bonding by the XOH group, or by water of crystallization. As a result, acid phosphate groups give more complex spectra than the P0 3- anion (CORBRIDGE [1956]).

If there is more than one crystallographic environment for a given anion, then these different types of anion can give distinct absorption bands. Even when the anions in a unit cell are symmetrically equivalent, coupling between them can again increase the number of


possibly observable bands, in the same way as was illustrated for the simpler case of the hydroxyl ion (Figure II). This coupling is termed the dynamic field effect, because it arises from the effect of one vibrating anion on another; it differs according to the relative phase of the vibrations in these ions. In considering how the different ions will interact with one another, the symmetry of the unit cell as a whole must be taken; it is generally sufficient to consider the point group from which the space group of the crystal is derived. Reference to correlation tables given by FARMER [1974] permit the number of vibrations of each symmetry species to be calculated . The symmetry classification, in turn, determines whether the vibrations are infrared active, Raman active, or totally inactive, and also, for some symmetry groups, the direction of the dipole change associated with the vibration. MITRA and GIELISSE [1964] have discussed the particular case of gypsum in some detail.

In addition to the internal vibrations of these polyatomic ions, there are three librational and three translational vibrations for each such ion in the unit cell, and three translational vibrations for each monoatomic ion in the unit cell. Although these are often of much lower frequency than the internal vibrations and can be considered separately, vibrations of the lighter cations do interact with and modify the internal vibrations of anions. These modifying effects of the crystal environment are particularly clearly seen in the orthosilicates. With the heavier divalent cations, there is still a clear separation of Si-O stretching bands around 1000 cm - 1 from bending vibrations around 500 cm - 1 with little or no absorpti0n in the 600 to 800 cm - 1 region. Absorption does appear in this region in the spectra of the beryllium orthosilicate, phenakite, (LEHMANN and DuTZ [1959]), and the aluminum orthosilicates, kyanite, andalusite, and sillimanite (TARTE [1959a]), because of the higher frequencies associated with the cation-oxygen bonds, and the mixing of Si-O stretching and bending modes imposed by strong cation-oxygen forces. The sillimanite spectrum is particularly remarkabl

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