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Clay Minerals (19g0) 15, 321-335. THE APPLICATION OF ELECTRON SPIN RESONANCE SPECTROSCOPY TO STUDIES OF CLAY MINERALS: I. ISOMORPHOUS SUBSTITUTIONS AND EXTERNAL SURFACE PROPERTIES PETER L. HALL Department of Chemistry, University of Birmingham, P.O. Box 363, Birmingham B15 2TT (Received 26 July 1979; revised 19 May 1980) ABSTRACT: Electron spin resonance (ESR) spectroscopy has contributed significantly to the identification and characterization pf paramagnetic impurities associated with clays, Following a brief discussion of the general principles of the technique, a review is given of the application of ESR to the study of those paramagnetic species (chiefly iron and radiation-induced lattice defects) located either within the aluminosilicate structure or present as an external impurity phase. In recent years a number of relatively modern physical techniques have been increasingly utilized in investigations of structural and physicochemical properties of clay minerals. One of these, electron spin resonance (ESR) spectroscopy, has now become a significant tool in clay mineral research and a large number of papers have been wholly or partially devoted to the interpretation of ESR spectra arising from paramagnetic ions or radicals associated with clays. Though the ESR method is restricted to a certain range of paramagnetic species, it can yield a wide range of detailed information relating to: (a) the occurrence and location of paramagnetic substitutional impurities and lattice defects (trapped hole or electron centres) in clays; (b) the extent of structural disorder and nature of thermal phase transitions of clay minerals; (c) the nature of paramagnetic impurities associated with clay surfaces, including organic radicals and transition metal oxides and hydroxides; (d) the mobility of hydrated ions in the interlamellar space of expanding lattice clays utilizing transition metal ions or organic radical cations as 'spin probes'; (e) the distribution of lattice charge sites in clays; (f) the reactions of adsorbed molecules on the external and interlamellar surfaces involving the formation and stabilization of free radicals. It should not be forgotten, however, that natural materials are far from the spectro- scopist's ideal of well-characterized, pure single crystals. Apart from work reported here on micas and vermiculites, most studies have been made on powder samples containing paramagnetic species either isomorphously substituted in the aluminosilicate lattice or present as a surface-adsorbed or dispersed impurity phase. In these circumstances care is required in the interpretation of experimental data. Moreover, it is desirable to compar e 0009-8558/80/12~)0-0321502.00 1980 The Mineralogical Society
Transcript

Clay Minerals (19g0) 15, 321-335.

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

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

I. I S O M O R P H O U S S U B S T I T U T I O N S A N D E X T E R N A L

S U R F A C E P R O P E R T I E S

PETER L. H A L L

Department of Chemistry, University of Birmingham, P.O. Box 363, Birmingham B15 2TT

(Received 26 July 1979; revised 19 May 1980)

ABSTRACT: Electron spin resonance (ESR) spectroscopy has contributed significantly to the identification and characterization pf paramagnetic impurities associated with clays, Following a brief discussion of the general principles of the technique, a review is given of the application of ESR to the study of those paramagnetic species (chiefly iron and radiation-induced lattice defects) located either within the aluminosilicate structure or present as an external impurity phase.

In recent years a number of relatively modern physical techniques have been increasingly utilized in investigations of structural and physicochemical properties of clay minerals. One of these, electron spin resonance (ESR) spectroscopy, has now become a significant tool in clay mineral research and a large number of papers have been wholly or partially devoted to the interpretation of ESR spectra arising from paramagnetic ions or radicals associated with clays.

Though the ESR method is restricted to a certain range of paramagnetic species, it can yield a wide range of detailed information relating to:

(a) the occurrence and location of paramagnetic substitutional impurities and lattice defects (trapped hole or electron centres) in clays;

(b) the extent of structural disorder and nature of thermal phase transitions of clay minerals;

(c) the nature of paramagnetic impurities associated with clay surfaces, including organic radicals and transition metal oxides and hydroxides;

(d) the mobility of hydrated ions in the interlamellar space of expanding lattice clays utilizing transition metal ions or organic radical cations as 'spin probes';

(e) the distribution of lattice charge sites in clays; (f) the reactions of adsorbed molecules on the external and interlamellar surfaces

involving the formation and stabilization of free radicals.

It should not be forgotten, however, that natural materials are far from the spectro- scopist's ideal of well-characterized, pure single crystals. Apart from work reported here on micas and vermiculites, most studies have been made on powder samples containing paramagnetic species either isomorphously substituted in the aluminosilicate lattice or present as a surface-adsorbed or dispersed impurity phase. In these circumstances care is required in the interpretation of experimental data. Moreover, it is desirable to compar e

0009-8558/80/12~)0-0321502.00 �9 1980 The Mineralogical Society

322 P. L. Hall

the ESR data with that provided by other techniques (e.g. M6ssbauer or photoelectron spectroscopy) and to utilize physical or chemical methods of sample refinement, e.g. chemical pretreatment or magnetic separation.

A review of ESR studies on clays to the end of 1973 has been published (Che et al., 1974).

Part I of the present review is confined to ESR phenomena associated with paramag- netic species located within the aluminosilicate lattice, or on the external surfaces of non-expanding clays. The application of ESR to studies of phenomena associated with the interlamellar region of expanding-lattice clays, including the mobility, structure and reactivity of hydrated transition metal ions, forms a distinct area which is covered in Part II.

P R I N C I P L E S OF ESR S P E C T R O S C O P Y

This section is devoted to a qualitative introduction to the principles of ESR spectro- scopy. For a more comprehensive treatment of both the theory and experimental practice of ESR, the reader is referred elsewhere (Ingram, 1967; Abragam & Bleaney, 1970).

The ESR technique depends upon the property that any atomic system which contains unpaired electrons possesses a net magnetic moment which will interact with an applied magnetic field. Consider the simplest possible case, i.e. a free atom containing a single unpaired electron. The electron has one of two possible spin directions, corresponding to the allowed values of the spin quantum number (S = +�89 or _1). In the absence of an external magnetic field the energies of the two spin states are equal. In the presence of an applied magnetic field, however, the energies of the two spin states are reduced and increased respectively by an amount k g~H, where H is the magnetic field,/~ is the Bohr magneton (eh/2mc) and g is the spectroscopic splitting factor. For a completely free spin, g = 2.0023. Thus at any given magnetic field the separation between the'two spin energy levels (the 'Zeeman splitting') is equal to g~H. Transitions between these levels (i.e. flipping of spin direction) can, in principle, be induced by radiation of frequency v, such that hv = g~H. This is known as the resonance condition and is illustrated in Fig. 1.

From the resonance condition one could in principle survey the ESR spectrum by varying the radiation frequency, as in optical spectroscopy, and keeping the magnetic field constant. In practice it is more convenient to do the reverse, i.e. to utilize a fixed microwave frequency and scan the ESR spectrum by varying the magnetic field. In addition, for reasons of detection sensitivity, most ESR spectra normally appear, not as

Energy ~ s=-�89

I S:-+2 I : g / 3 H I I

~ s - +! 2

Doublet [H Zero) H I I 9:ncreas:n-

FIG. 1. The resonance condition: case of a single unpaired electron in an external magnetic field.

ESR of clays." Part I 323

J

g,,

gx I I I g y

/ t / t igz

H ~ "

g•

FIG. 2. Typical ESR first derivative lineshapes. (a) Isotropic g-value. (b) Axially symmetric case in the polycrystalline average. (c) Orthorhombically symmetric case in the polycrystalline average.

the direct absorption curve, but as its first derivative. A typical resonance lineshape is illustrated in Fig. 2a.

So far we have considered only a single unpaired electron whose interaction with its environment may be neglected. In most cases, however, the unpaired electrons are influenced by other interactions, such as the electric fields arising from neighbouring atoms in the crystal, and magnetic interactions due to the proximity of nuclei having non-zero spins. These effects influence the number and position of resonance lines observed. The following discussion outlines a few general principles illustrative of the features of the ESR spectra of some of the more common paramagnetic ions.

For reasons discussed elsewhere (Abragam & Bleaney, 1970), with iron-group transi- tion ions one can frequently neglect the influence of the higher orbital energy levels as a first-order approximation, and consider only the spin energy levels associated with the orbital groundstate (L = 0). There are 2 S + 1 such spin levels, where S is the spin quantum number of the ion. In a free ion, as for the simple case discussed above where S = �89 these levels will be of the same energy until split by an applied magnetic field. For an ion in a crystal lattice, however, there are mechanisms which may split the 2 S + 1 spin levels even in the absence of an external magnetic field. These splittings, termed zero-field splittings (ZFS), are attributable to the anisotropy of the crystal field, i.e. the local electric field in the vicinity of the paramagnetic ions due to the charges on neighbouring atoms.

324 P. L. Ha l l

The overall effect of the electric and magnetic interactions is to produce a series of spin energy levels whose separations depend on both the magnitude and direction of the applied magnetic field. If the crystal field is highly symmetrical (octahedral or cubic symmetry) the ZFS will be zero or very small, and no significant deviations from g-values of 2.0 would be expected. For lower symmetries (axial or orthorhombic) significant g-shifts may occur, resonances being in general anisotropic. In a single crystal this results in a single ESR line associated with each possible transition, whose g-value, and hence magnetic field position, will be dependent on the orientation of the magnetic field with respect to the symmetry axes of the crystal field. In a polycrystalline material the spectra will be averaged over all possible crystallite orientations.

Resonances in powders and amorphous materials are most easily resolved when the resonance field value varies only slowly with angle, i.e. near a maximum or minimum. In fact, stationary values of g (or H) occur when the magnetic field lies along one of the principal symmetry axes of the crystal field (Dowsing & Gibson, 1969; Aasa, 1970). In a single crystal, location of these maximum and minimum g-values as the sample is rotated determines the orientation of the crystal field symmetry axes with respect to the crystallo- graphic axes. In a powder, this is not generally possible, but broader resonances are usually observed whose features are related to the principal g-values. These are conven- tionally designated gt~ and g~ in the axial case and gx, gy and gz in the rhombic case. The appearance of powder spectra arising from paramagnetic species of axial and rhombic symmetry are illustrated in Figs 2b and 2c respectively (Searl et al., 1961; Kneubuhl, 1960).

For clay minerals it is frequently possible to obtain some orientational information, even when single crystals are unavailable, by exploiting the natural tendency of clay platelets to adopt preferential orientation. Thus ESR measurements on sedimented films or compressed powders with the magnetic field orientated at different angles to the sample may be expected to show some anisotropy. Recently, Swartz et al. (1979) have shown that ESR lineshape simulations can provide a quantitative determination of the degree of orientation in clays and other lamellar structures.

More complex ESR spectra arise in cases where the paramagnetic ion (or its nearest neighbour) possesses a non-zero nuclear spin. Here, interactions between the electronic and nuclear spins produce a further small splitting of the energy levels of the unpaired electrons. This splitting, termed hyperfine splitting (HFS), separates individual ESR transitions into multiplets. For example, the interaction of an unpaired electron with a nucleus of spin I results in an ESR line having 21+ 1 equally spaced components. The magnitude of the HFS is a direct measure of the strength of the electron-nucleus interaction, and can be related to the degree of ionic (or covalent) character of metal- ligand bonds (Boucher et al., 1969).

A brief survey will now be given of the typical features of the ESR spectra of a few paramagnetic species which commonly occur in association with clay minerals.

Iron

Fe 2+ (3d 6) is a non-Kramers ion (see Abragam & Bleaney, 1970) and is generally unobservable by ESR. Fe 3+ (3d 5) has been extensively studied in a wide range of materials. In environments approximating to cubic symmetry, resonances at or near g = 2.0 are expected, having a maximum of five fine-structure components due to the

ESR of clays: Part I 325

selection rule AS = + 1. In axial symmetry where the ZFS terms are relatively weak, resonances close to g = 2.0 of the form of Fig. 2b have been observed in oxide powders (Lunsford, 1965). In cases of axial or rhombic symmetry with relatively large ZFS terms, powder spectra are frequently described by means of the spin Hamiltonian

= g f lH.S+D[Sz2-1S(S+ 1)] +E[Sx 2 - Sy 2]

where the first term describes the Zeeman splitting and the second and third terms represent the axial and rhombic components of the ZFS. The latter split the six-fold ground state into three Kramers doublets. The g-values are dependent on both the ratio 2 = E/D and the ratio D/hv, where hv is the energy of the microwave quantum. The parameter 2 is directly related to the symmetry of the environment of the ion, 2 = 0 representing axial symmetry and 2 = ] the 'completely' orthorhombic case in which the three Kramers doublets are equally split by the crystal field. When D > hv, it can be predicted that for ). = 0 one expects resonances at gl = 2.0 and gi = 6.0 for the lowest doublet. For 2 = �89 an isotropic resonance atg = 4-3 is predicted for the central doublet, and extremely anisotropic resonances are predicted for the upper and lower doublets (Aasa & Vanngard, 1965; Wickman et al., 1965; Dowsing & Gibson, 1969). All symme- tries can be described by 2 values lying between 0 and �89 depending on the definition of the axes (Blumberg, 1967; Hall et al., 1974a).

Manganese

Like Fe 3+, the Mn 2+ ion has the unpaired electron configuration 3d 5, so that in general there are six spin levels (5: = + 25, + 23, + �89 between which a maximum of five allowed transitions are possible according to the selection rule AS -- + 1. In addition, however, the spin of the 55Mn nucleus (I = ~-) results in a splitting of each fine-structure line into 2 I+ 1, i.e. six, hyperfine components. The ZFS is usually smaller than in the case of Fe3+; frequently in a powder or solution spectrum the five AS transitions are unresolved due to their small anisotropy and only six lines are therefore usually observed near g = 2.0. The value of the hyperfine splitting constant may vary between approximately 55 and 95 x 10 -4 cm-1 depending on the nature of the ligands surrounding the Mn 2+ ion.

Copper

The Cu + ion has the configuration 3d l~ and is therefore diamagnetic. The Cu 2+ ion (3d 9) has been most commonly observed in axially symmetric crystal fields (tetragonal or trigonal) in which the ground state is a spin doublet. Typical g values are glt ~ 2.3-2.4 and gi ~2.0-2.2. Furthermore, both 63Cu and 65Cu nuclei have spin I = 3, which leads to the observation in some cases of a weak four-line hyperfine structure.

Vanadium

The divalent and tetravalent oxidation states are Kramers ions. V 2+ has configuration 3d 3, while V 4+, and the vanadyl oxycation, VO 2+, have the configuration 3d 1. Here only the tetravalent case will be considered, for which g-values close to 2-0 are invariably observed, frequently with axial symmetry. The 51V nucleus has spin I = 5, which gives rise to 8 hyperfine components associated with each principal g-value. The HFS is usually

326 P. L. Hall

markedly anisotropic, the splitting constants being approximately AII ~ 150 x 10 _4 cm - and At ~50 x 10 -4 cm-1, but can be averaged to produce an isotropic spectrum in cases where the ion is rapidly tumbling in a solution-like environment (see Part II).

Trapped radicals or defect centres

In addition to transition ions, a wide variety of organic and inorganic free radicals, as well as paramagnetic lattice defects such as trapped holes or electrons, exhibit ESR spectra. Defects are often produced when materials are subjected to natural or artificial irradiation, which may result in the breaking of bonds or the ejection of electrons or protons. The paramagnetic centres thus produced may be stabilized by existing local charge imbalance in the crystal lattice. Thus the isomorphous replacement of a cation by one of lower charge (or of an anion by one of higher charge) leads to a total positive charge deficiency which may stabilize a positive hole, producing a paramagnetic hole centre. Conversely the replacement of a cation by one of higher charge (or of an anion by one of lower charge) may stabilize a paramagnetic electron centre. The two types of defect centre described may also arise from cation or anion vacancies. A wide range of hole and electron centres have been found to occur in halides, oxides and silicates, and have been characterized by ESR and optical spectroscopy. Both types of centre exhibit g-values close to 2.0, hole centres usually having g>~2-0 and electron centres having g~<2.0. In oxides and silicates the charge is frequently located on oxygen atoms adjacent to the substituent atom or vacancy. The ESR spectra may exhibit HFS where there are neigh- bouring nuclei possessing zero spins (Marfunin & Bershov, 1970; O'Brien & Pryce, 1955; Ioffe & Yanchevskaya, 1967).

ESR S T U D I E S OF C L A Y M I N E R A L S

Single crystal studies." micas and vermiculite

Novozhilov et al. (1970) studied a synthetic fluorphlogopite doped with vanadium, manganese and iron. In the first two cases the ESR spectra could be assigned to the divalent cations (V 2+ and Mn z+) substituting isomorphously for Mg 2+ in octahedral sites. For Fe 3+, evidence for both 4-fold and 6-fold coordination was obtained, the ESR spectra of the six-fold coordinated Fe 3+ exhibiting doublet or triplet HFS due to the interaction of the unpaired electrons with either one or two 19F nuclei in octahedra of the type FeO5F and FeO4F2 respectively.

ESR studies of natural muscovite and phlogopite have been reported by Kemp (1971, 1972, 1973). The spectra were attributed to Fe 3+ ions in sites of orthorhombic symmetry. In muscovite (Kemp, 1973) two centres were observed, one producing an isotropic line at g = 4 (2 = 31) and the other giving an anisotropic resonance whose angular dependence led to a value of 2 = 0.17. Two distinct sets of resonance lines associated with the second centre were observed, identical in behaviour but having a relative displacement of 120 ~ about the c-axis of the muscovite 2M~ structure. In the case ofphlogopite, it was suggested that four orthorhombic Fe 3 + centres were present, of which only one gave a well-defined ESR spectrum which could be fitted to a value of 2 = 0.09.

More detailed studies of the ESR spectra o fFe 3 + in micas have been reported by Olivier et al. (t976a, 1977). In addition to resonances at g = 2.0 possibly associated with

ESR of clays: Part I 327

magnetic inclusions, evidence for four substitutional sites was obtained, two having six-fold coordination (Oa and Oh) and two having four-fold coordination (T, and Tb). The site Ob was of maximum rhombic character (2 ~ 3 ~) and associated with octahedra having the two hydroxyl groups in adjacent sites. The site Oa approximated more closely to axial symmetry, having 2 = 0-11, and was associated with octahedra having the two OH groups opposite. One of the tetrahedral sites, Ta, was of approximately axial symmetry, with the symmetry axis lying along an Fe-O bond. The angular dependence of the spectrum led to a value of 2 = 0.06. The other tetrahedral site exhibited rhombic (C2v) symmetry.

For Llano vermiculite, the ESR spectra were also interpreted in terms of the presence of two octahedral and two tetrahedral Fe 3+ sites (Olivier et al., 1976b). Differences in the intensities of the resonances between natural and weathered micas were consistent with the oxidation of Fe 2+ to Fe 3+ under neutral or acidic conditions, together with the conversion of hydroxyl to oxide ions (Olivier et al., 1976a).

Kaolinite

Most kaolinites exhibit a characteristic ESR spectrum consisting of two main groups of resonances: (i) a group of lines at low magnetic field values, centered at g = 4-2, and (ii) overlapping groups of lines at higher field values, centered at g = 2.0. Heating the clays in air at 500~ causes the collapse of the low-field resonances to a single line while the features at g = 2-0 disappear, as illustrated in Fig. 3 (Angel & Hall, 1973). Although the first observation of ESR in kaolinite dates back to the work of Boesman & Schoemaker (1961) and later reports by Friedlander et al. (1963) and Wauchope & Haque (1971), the detailed identification of the species responsible for the observed spectral features has only been achieved recently as a result of systematic and detailed investigations of a wide range of natural and doped synthetic clays (Angel & Hall, 1973; Jones, 1974; Jones et al.,

Resonance B. Resonance A.

20~ 3 0 0 ~ 4 0 0 ~ 5 0 0 ~

400~ c b

c

FIG. 3. ESR spectra of a Cornwall kaolinite heated for 24 h at various temperatures (Angel & Hall, 1973).

328 P. L. Hall

1974; Meads & Malden, 1975; Angel et al., 1974, 1976, 1977). Discussion of the results of these studies falls into two parts.

Low-fieMresonances. The ESR spectra of most kaolins exhibit three peaks in the region of g = 4, together with a peak at lower magnetic field corresponding to a g-value of approximately 9.0. Considerable variations in the overall lineshape occur between differ- ent samples, which can only be partially explained by contributions from Fe 3 + resonances due to relatively iron-rich mineralogical impurity phases such as micas (Angel & Hall, 1973).

Based both on experimental studies and theoretical considerations, the X-band spectra are assigned to two distinct Fe 3 + substitutional sites, whose resonances partially overlap:

(i) Centre I, giving an isotropic line at g = 4-2 (requiring 2 = ~ and D >~ 0-8 cm-1); (ii) Centre II, giving three lines corresponding to g-values gz = 4-9 gx = 3.7 and

gy = 3-5, the latter two frequently being unresolved in powder spectra. Those resonances are consistent with partially orthorhombic symmetry having 2 = 0.22 + 0.01.

From X-band measurements Jones et al. (1974) found, in addition, two higher field resonances whose g-values could only be explained on the assumption that Centre II itself is a composite feature arising from two Fe 3+ sites differing slightly in symmetry (IIa and IIb). Both centres appeared to be consistent with spin Hamiltonian parameters in the range E/D = 0-22 • 0-01 and D =0.45 _ 0.02 cm- i. From a combination of X-band and Q-band measurements, Meads & Malden (1975) also observed three distinct Fe 3§ sites in kaolinite, although their data indicated slightly different best-fit parameters correspond- ing to Centres IIa and l ib (2 =0-234, D=0.48 cm-1; and 2 - 0-207, D =0-32 cm- I).

The results are, however, essentially in good agreement. Taking the ESR data in conjunction with M6ssbauer studies (Malden & Meads, 1967) and phosphorescence studies (Jones et al., 1974) it appears that all three sites can be attributed to octahedrally coordinated Fe 3+ ions.

A correlation between the lineshape of the composite Fe 3+ spectrum and the crystal- linity index as defined by Hinckley (1963) was observed (Jones et al., 1974; Meads & Malden, 1975). The relative population of Centre I in comparison with Centre II increases as the crystallinity index of the kaolinite decreases. Centre I is thus attributable to Fe 3+ ions in sites adjacent to layer stacking disorders such as random nb/3 displacements or 120 ~ rotations (Noble, 1971). Centres IIa and IIb, in contrast, are associated with Fe 3+ ions in regions of high crystallinity and regular stacking. The difference between Centres IIa and IIb may be attributed to the two distinct orientations of surface OH groups suggested by the calculations ofGiese & Datta (1973). The low-field resonances of natural clays can be identically reproduced in Fe 3 + doped synthetic kaolinite, as illustrated in Fig. 4 (Jones et al. , 1974; Angel et al., 1976). Intercalation of kaolinite with urea, dimethylsul- phoxide or potassium acetate causes the three-line Fe 3 + resonance to collapse to a single line at g = 4-2. Thus, disruption of the regular interlayer bonding accompanying inter- calation of such molecules reduces the symmetry of the local environment of the ferric ions from that of Centre IIa or IIb to that of Centre I.

Further studies of the relationship between the ferric ion ESR spectra and the iron content and crystallinity of kaolinites have been made by Mestdagh et al. (1980). Their results indicate an inverse relationship between the intensity of the single isotropic g = 4.3 resonance and the Hinckley crystallinity index. The clear association of this resonance with crystalline imperfection and stacking disorder is in good agreement with

ESR of clays: Part I

[g--4-O

O ~

g=2-O

J Natural kaolinite

Mg doped kaolinite (no signals)

Fe 3+ doped kaolinite

Mg doped kaolinite X- irradiated

Mcj doped kaolinite - X-irradiated and annealed

329

~ F'eS+and Mg doped F . ~ kaolinite

X-irradiated and annealed

FIG. 4. ESR spectra of natural and doped synthetic kaolinites (Angel et al., 1976).

the observation of the single-line resonance in metakaolin (Angel & Hall, 1973) and in iron-containing silicate glasses (Castner et al., 1960; Loveridge & Parke, 1971). Similar resonances occur also in a wide range of amorphous materials, including natural organic residues such as humic acids containing traces of iron (Hall et al., 1974b). There is an apparent difficulty in reconciling the very common occurrence of this resonance in structurally disordered or amorphous materials with the theoretical explanation of its origin. The latter requires a rather specific crystal field symmetry described by the spin Hamiltonian parameter 2 = k. This lends support to the ideas of Peterson et al. (1974) whose calculations indicate that a resonance of this type in glassy systems might well arise from a broad statistical distribution of site symmetries in which there are little or no correlations between the principal g-values.

High-field resonances (g ~2.0). Numerous features have been observed in the ESR spectra of kaolins in the region of g --- 2.0. These are attributable to both lattice defect centres (labelled below as A, B ! and B2) and also transition metal impurities other than substitutional Fe 3 +. In the latter category may be included resonances due to iron oxide or

330 P. L. Hal l

hydroxides, manganese (Mn2+), vanadium (V 4+ or VO 2+) and organic free radicals. The experimental results may be summarized as follows:

1. Centre A. An asymmetric two-line spectrum having gl = 2.05 and gl = 2.00 (Angel & Hall, 1973; Meads & Malden, 1975) characteristic of the powder-averaged spectrum of an axially symmetric species (Fig. 2b). Orientation studies indicate that the unique axis lies close to the kaolinite c-axis. Annealing at 400~ removes this resonance, while the Fe 3+ spectra collapses to a single line a tg -- 4.2 similar to that of Fe 3+ in glasses (Castner et al., 1960). This centre is absent in Fe3+-doped synthetic kaolinites, but has been observed in synthetic kaolinite doped with Mg 2+ (Angel et al., 1974, 1976) or Fe 2+ (Angel et al., 1977) following irradiation with 35 keV X-rays. The ESR spectra are illustrated in Fig. 4. Centre A has therefore been attributed to a trapped positive hole of the type Si-O + Mg or Si-O + Fe(II), although a trapped 02 ion has been suggested as an alternative explanation (Jones et al., 1974). Similar resonances at g = 2.0 were observed in dickite and pyrophyllite (Hall, 1973). In kaolins the intensity of the resonance increases with increasing crystallinity but decreases with increasing non-extractable iron content (Meads & Malden, 1975; Herbillon et al., 1976). By a numerical integration method (Hall, 1972) the intensity was found to lie between 1018 and 1019 spins per gram in a range of natural kaolins, which corresponds to a maximum level of octahedral substitution of Mg 2+ or Fe 2+ for AP + of approximately two atoms per thousand (Hall, 1973). This is considerably lower than the concentrations of Mg or Fe found by chemical analysis. Similar findings have been reported recently by Mestdagh et at. (1980). Angel & Vincent (1978) observed that hydrogen pretreatment inhibited the formation of the centre in doped synthetic kaolinites. Though all the available evidence suggests that the resonance is attributable to a hole centre associated with a low level of substitution of octahedral AP + by divalent ions, the precise nature of the centre and the explanation of the relationships between its concentration and crystallinity and chemical composition remain unclear. However, recent ESR and M6ssbauer studies of a 57Fe-doped synthetic kaolin have led to the suggestion that the defect is located at boundaries between 'trioctahedral' Fe 2+ clusters and regular aluminium regions of the octahedral sheet, rather than individual divalent substituents (Currier, 1980).

2. Centres B1, B2. At least one other defect centre occurs in kaolinite, the ESR signals of which can be difficult to resolve as they partially overlap with the resonance due to Centre A. These are characterized by the presence of HFS due to the interaction of the unpaired spins with 27A1 nuclei ( I= 5/2). These defects have quite low concentrations in natural kaolinites, but are greatly enhanced by X-irradiation, and are thermally stable up to approximately 200~ They can be reversibly created and destroyed by irradiation and annealing (Angel & Hall, 1973). Meads & Malden (1975) suggest that there are two defect centres of this type, the first having parameters gll= 2.028, gt = 1.988 and A = 7.6 gauss, and the second having gll = 2-047 and an uncertain value of ~ due to overlap of the hyperfine components. It is probable that these centres are both associated with tetra- hedral AP + substitution, the first being most likely a hole located on a surface oxygen atom which bridges normal and charge-deficient tetrahedral sites (Aliv~O+-Si). The second, whose g-values are close to those of Centre A, could then be assigned to a hole on an inner oxygen atom linking tetrahedral and octahedral aluminium atoms (Al~v-O +- Alvi). In this case, one would anticipate at least eleven HFS components, which is not inconsistent with the experimental data (Angel & Hall, 1973; Meads & Malden, 1975).

E S R o f clays: Part I 331

The temperature sensitivity of this resonance suggests a possible application as a natural geothermometer.

3. Iron oxides and hydroxides. Broad resonances at g = 2.0, in some cases more than 1000 gauss in width, occur in many kaolinites. In many cases these are substantially reduced in intensity by magnetic separation, and may be attributed to superexchange coupling between Fe -O-Fe pairs in iron-rich impurity phases, probably micas (Meads & Malden, 1975). In other cases these resonances seem to be associated with impurities coating the kaolinite surface. Angel & Vincent (1978) have investigated the nature of these resonances in a range of American (Georgia) and British (South West Peninsula) clays. These workers concluded that in American clays the resonances appear to be principally due to hematite-like or goethite-like phases resistant to the deferrification procedure of de Endredy (1963). The British clays were primarily associated with a lepidocrocite-like phase removable by de Endredy's method. The association of iron with kaolinite appears, therefore, to be heterogeneous, consisting of both surface-adsorbed and lattice-substi- tuted components. Herbillon et al. (1976) have reported ESR studies of tropical soil kaolinites subjected to acid attack which also indicate the heterogeneity of the association of iron with kaolinite, the major non-structural Fe 3+ component being dissolved at the same rate as aluminium, whereas a smaller component (responsible for part of the broad resonance at g = 2-0) was resistant to acid treatment.

4. Manganese. In some kaolins six-line hyperfine spectra, having a splitting constant A = 90-100 gauss, have been observed (Meads & Malden, 1975), which are attributable to an Mn2+-containing impurity phase.

5. Vanadium. In many kaolins, especially Georgia kaolins, the eight-line isotropic (or sixteen line anisotropic) spectra characteristic of tetravalent vanadium are observed, usually of relatively low intensity. Vincent & Angel (1980) have concluded that these features are due to isomorphously substituted V 4+ ions, rather than to surface adsorbed V 4+ or VO 2+ ions.

6. Organic free radicals. In some kaolins containing organic matter a weak contribution to the ESR spectra at g = 2.0 arises from an organic free radical similar to that occurring in soil organic matter, particularly in humic acids (Rex, 1960). Usually, however, this only contributes a small fraction of the signal intensity in this region (Hall et al., 1974b).

Illite, smectites and other minerals

Matyash et al. (1967) studied the temperature dependence of the ESR spectra of an illitic mica. They observed a resonance at g = 4.37, which increased in intensity on heat treatment, and attributed it to the oxidation of Fe 2 + to Fe 3 +. These workers also observed a broad feature due to clusters of neighbouring Fe 3+ ions, similar to the resonances discussed above in connection with kaolinite.

Friedlander et al. (1963) reported narrow g = 2.0"signals in illite and montmorillonite which at that time could not be fully characterized. Wauchope & Haque (1971) also observed sharp g = 2.0 resonances in these minerals, which were found to persist after treatment with acids or oxidizing agents. They suggested that the resonances could not be attributed to organic free radicals associated with the clay surfaces, but were probably due to some unspecified type of defect centre located within the aluminosilicate lattice. Similar resonances were observed in montmorillonite and beidellite by Olivier et al. (1975) which may be attributable to hole centres of a similar nature to those observed in kaolinite.

332 P. L. Hal l

4 . 4 8

T 9 2

I I I I ~ 1 I IOOQ

Ob /

Oa Centre V

T~ / f ~ "q' . 2 - 0 0

J l I p I I I J ~ 2 0 0 0 3000 H (oe)

FIG. 5. ESR spectra o f ( l ) Wyoming and (2) Camp Berteau montmorillonites (Olivier et al . , 1975).

Olivier et al. (1975) also examined the ESR spectra of chlorite and a variety of smectites. Detailed spectra were observed in all cases, dominated by broad features at g = 4-3 together with numerous additional lines. ESR spectra of two montmorillonites (Wyom- ing and Camp Berteau) are illustrated in Fig. 5, these indicating the presence of Fe 3+ ions in two distinct octahedral sites and two tetrahedral sites. It is not clear, however, whether the difference between the symmetries of the two octahedral sites arises from the two alternative (cis and trans) configurations of the hydroxyl groups in the FeO4(OH)2 octahedra, or from the nature of the cations in the neighbouring octahedral sites.

McBride et al. (1975a, 1975b) observed that the g = 4 resonances in smectites were influenced by both the nature of the exchange cations and the degree of hydration of the minerals. A weaker signal on the high-field shoulder of the main g = 4.3 line was observed, which could be reversibly removed and restored by dehydration and resolva- tion respectively for sodium- and calcium-exchanged montmorillonites. The weaker resonance was attributed to Fe 3+ ions occupying octahedral sites adjacent to Mg 2+ ions, while the more intense resonance was attributed to ferric ions in octahedra adjacent to A13+. For Li-montmorillonite, the weaker signal did not reappear on resolvation. This effect was attributed to the migration of the small Li + ions into vacant octahedral sites during the thermal dehydration, where they provided local charge compensation. The ESR spectra are illustrated in Fig. 6.

Other relevant recent ESR studies have been reported by Elsass & Olivier (1978) and Goodman (1978). The former workers studied a range of natural clays and classified the

Na

I

2 J / '

S

FIG. 6. ESR spectra of sodium, lithium and calcium montmorillonites, l. Hydrated under ambient conditions. 2. After heating at 205~ for 24 h. 3. Resolvated in ethanol (McBride et al . , 1975b).

ESR of clays: Part I 333

sites of iron substitution and the nature of the defect centres present. Goodman (1978) investigated the mode of occurrence of iron in some iron-deficient montmorillonites by a combination of ESR and M6ssbauer studies. He concluded that a part of the iron content of these clays was in the form of iron-rich clusters probably associated with external particle surfaces, giving very broad ESR lines. In addition, sharper lines were observed indicative of the presence of isolated substitutional Fe 3+ ions.

S U M M A R Y A N D C O N C L U S I O N S

The ESR technique has provided a substantial amount of data regarding isomorphous substitutions in clay minerals. It has been established that ferric ions can occupy up to three distinct octahedral sites in kaolinite, and up to four distinct sites in micas, smectites and vermiculite, two octahedral and two tetrahedral sites. Three distinct paramagnetic defect centres occur in kaolinites, which are 'positive hole' centres associated with charge-deficient isomorphous substitution. The first is associated with the replacement of aluminium by magnesium or ferrous ions in the octahedral sheet. The other two are attributable to aluminium for silicon substitution in the tetrahedral sheet. Similar centres appear to occur in other minerals, but have not yet been fully characterised. It is clear that ESR studies of a wide range of minerals, both natural and synthetic, can provide detailed information regarding the nature and distribution of substituent atoms in these materials.

R E F E R E N C E S

AASA R. (1970) Powder line shapes in EPR spectra of high-spin ferric complexes. J. Chem. Phys. 52, 3913 3930. AASA R. • VANNGARD T. (1965) ESR single crystal study of Fe 3+ with large rhombic crystal field splitting.

Arkiv. Kemi 24, 331-339. ABRAGAM A. & BLEANEY B. (1970) Electron Paramagnetic Resonance of Transition Ions. Clarendon Press,

Oxford. ANGEL B.R. & HALL P.L. (1973) Electron spin resonance studies of kaolins. Proc. Int. Clay Conf. Madrid, 4260. ANGEL B.R. & VINCENT W.E.J. (1978) Electron spin resonance studies of iron oxides associated with the surface

of kaolins. Clays Clay Miner., 26, 263-272. ANGEL B.R., JONES J.P.E. & HALL P.L. (1974) Electron spin resonance studies of doped synthetic kaolinites I.

Clay Miner. 10, 247-256. ANGEL B.R., RICHARDS K. & JONES J.P.E. (1976) The synthesis, morphology and general properties of kaolinites

specifically doped with metallic ions and defects generated by irradiation. Proc. Int. Clay Conf Mexico City, 297 3O4.

ANGEL B.R., CUTTLER A.H., RICHARDS K. & VINCENT W.E.J. (1977) Synthetic kaolinites doped with Fe 2+ and Fe 3+ ions. Clays Clay Miner. 25, 381-383.

BLUMBERG W.E. (1967) The EPR of high spin Fe 3+ in rhombic fields. Pp. 119-132 in: Magnetic Resonance in Biological Systems (A. Ehrenberg and T. Vanngard, editors). Pergamon, New York.

BOESMAN E. & SCHOEMAKER D. (1961) R~sonance paramagnetique de l'ion Fe 3 + dans la kaolinite. Compt. Rend. Acad. Sci. 252, 1931-1933.

BOUCHER L.J., TYNAN E.C. & YEN T.F. (1969) Spectral properties of oxovanadium (IV) complexes: IV. Correlation of ESR spectra with ligand type. In: Electron Spin Resonance of Metal Complexes (T.F. Yen, editor). Hilger, London.

CASTNER T., NEWELL G.S., HOLTON W.C. & SLICHTER C.P. (1960) Note on the paramagnetic resonance of iron in glass. J. Chem. Phys. 32, 668~573.

CHEM., FRAISSARD J. 8~; VEDRINE J.C. (1974) Applications de la RPE et de la RMN/ t l'6tude des silicates et des argiles. Bull. Grpe Ft. Argiles 26, 1 53.

CUTTLER A.H. (1980) The behaviour of a synthetic 57Fe doped kaolin: M6ssbauer and electron paramagnetic resonance studies. Clay Miner. 15, 429-443.

334 P. L. H a l l

DOWSING R.D. & GmSON J.F. (1969) Electron spin resonance of high-spin d5 systems. J. Chem. Phys. 50, 294~303.

DE ENDREDY A.S. (1963) Estimation of free iron oxides in soils and clays by a photolytic method. Clay Miner. Bull. 5, 209-217.

ELSASS F. & OLIVIER D. (1978) Infrared and electron spin resonance studies of clays representative of the sedimentary evolution of the Basin of Autun. Clay Miner. 13, 299 308.

ERIEDLANDER H.Z., FRtNK C.R. & SALDICK J. (1963) Electron spin resonance spectra in various clay minerals. Nature 199, 61-62.

GIESE R.F. & DATTA P. (1973) Hydroxyl orientation in kaolinite, dickite and nacrite. Am. Miner. 58, 471 479.

GOODMAN B.A. (1978) An investigation by M6ssbauer and EPR spectroscopy of the possible presence of iron-rich impurity phases in some montmorillonites. Clay Miner. 13, 351 356.

HALL P.L. (1972) Errors in the determination of ESR spin concentrations by numerical double integration. J. Phys. D, 5, 673-675.

HALL P.L. (1973) Electron spin resonance studies of aluminosilicate minerals and associated organic substances. PhD thesis, Univ. London.

HALL P.L., ANGEL B.R. & JONES J.P.E. (1974a) Dependence of the spin Hamiltonian parameters E and D, on labelling of magnetic axes: application to ESR of high-spin Fe 3 +. J. Mag. Resonance 15, 64-68.

HALL P.L., ANGEL B.R. & BRAVEN J. (1974b) Electron spin resonance and related studies of lignite and ball clay from South Devon, England. Chem. Geol. 13, 97-I 13.

HERBILLON A.J., MESTDAGH M.M., VIELVOYE L. & DEROUANE E.G. (1976) Iron in kaolinite with special reference to kaolinite from tropical soils. Clay Miner. 11,201-219.

HINCKLEY D.N. (1963) Variability in "crystallinity" values among the kaolin deposits of the coastal plain of Georgia and South Carolina. Clays Clay Miner. 11,229-235.

INGRAM D.J.E. (1967) Spectroscopy at Radio and Microwave Frequencies. Butterworths, London. IOFFE V.A. & YANCHEVSKAYA I.S. (1967) Thermoluminescence and electron paramagnetic resonance of irra-

diated aluminosilicates. Opt. Spectrosc. 23, 494-496. JONES J.P.E. (1974) Electron spin resonance studies of doped synthetic kaolinites. PhD thesis, Univ. London. JONES J.P.E., ANGEL B.R. & HALL P.L. (1974) Electron spin resonance studies of doped synthetic kaolinites I I.

Clay Miner. 10, 257-270. KEMP R.C. (1971) Orthorhombic iron centres in muscovite and phlogopite micas. J. Phys. C, 4, LI I L I3 and 5,

792 (corrigendum). KEMP R.C. (1972) Electron spin resonance of Fe 3+ in phlogopite. J. Phys. C, 5, 3566-3572. KEMP R.C. (1973) Electron spin resonance of iron (3 +) in muscovite. Phys. Stat. Sol. B57, K79-K81. KNEUBUHL F.K. (1960) Lineshapes of electron paramagnetic resonance signals produced by powders, glasses

and viscous liquids. J. Chem. Phys. 33, 1074-1078. LOVERIDGE D. & PARKt S. (1971) Electron spin resonance of Fe 3+, Mn 2+ and Cr 3+ in glasses. Phys. Chem.

Glasses 12, 19 27. LUNSFORD J. (1965) Polycrystalline EPR spectrum of Fe 3+ ions in MgO. J. Chem. Phys. 42, 26122618. MALDEN P.J. & MEADS R.E. (1967) Substitution by iron in kaolinite. Nature 215, 844-846. MARFUNIN A.S., & BERSHOV L.V. (1970) The true structure and electron-hole centres of minerals. Sovrem.

Kristall. Miner. 186-206. MATYASH I.V., POLSmN E.V. & KALINCHENKO A.M. (1969) Investigation of the effect of thermal treatment on

hydromica by radiospectroscopy. Geokhimiya 7, 840-845. MCBRIDE M.B., PlNNAVAIA T.J. & MORTLAND M.M. (1975a) Perturbation of structural Fe 3+ in smectites by

exchange ions. Clays Clay Miner. 23, 103 108. McBRIDE M.B., MOrtrLAND M.M. & PINNAVAh T.J. (1975b) Exchange ion positions in smectite: effect on

electron spin resonance of structural iron. Clays Clay Miner. 23, 162 164. MEADS R.E & MALDEN P.J. (1975) Electron spin resonance in natural kaolinites containing Fe 3+ and other

transition metal ions. Clay Miner. 10, 313 345. MESTDAGtt M.M.. VmLVOYE L. & HERBILLON A.J. (1980) Iron in kaolinite: II. The relationship between

kaolinite crystallinity and iron content. Clay Miner. 15, 1 13. NOBLE F.R. (1971) A study of disorder in kaolinite. Clay Miner. 9, 71-81. NOVOZHILOV A.I., SAMILOVICH M.I., ANKIN I.M. & SERGEEV-BOBR A.A. (1970) Paramagnetic V, Fe, Mn

impurities in crystals of fluorophlogopite, a synthetic mica. Izv. Akad. Nauk. SSSR. (Neorg. Mater) 6, 108-112.

E S R o f clays: Par t I 335

O'BRIEN M.C.M. & PRYCE M.H.L. (1955) Paramagnetic resonance in irradiated diamond and quartz: an interpretation. Pp. 88-91 in: Report of Conf on Defects in Cryst. Solids, Bristol. Physical Soc., London.

OLIVIER D., VEDRINE J.C. & PEZERAT H. (1975) Application de la RPE it la localization du Fe 3+ dans les smectites. Bull. Grpe Fr. Argiles 27, 153-165.

OLIVlER D., VEt)RINE J.C. & PEZERAT H. (1976a) Resonance paramagn6tique el+ctronique du Fe 3+ darts les argiles alt+res artificiellement et dans le milieu naturel. Proe. Int. Clay Conf. Mexico City, 231-238.

OLIVIER D., LAUGINIE P. & FRIPIAT J.J. (1976b) Relationship between the longitudinal relaxation rates of water protons and of well defined paramagnetic centres at low temperature in hydrated vermiculite. Chem. Phys. Letters, 40, 131-133.

OLIVIER D., VEDRINE, J.C. & PEZERAT H. (1977) Application de la RPE ft, la localization des substitutions isomorphique dans les micas: Fe 3 + dans la muscovite et la phlogopite. J. Solid State Chem. 20, 267-269.

PETEgSON G.E., KURKJIAN C.R. & CARNEVALE A. (1974) Random structure models and spin resonance in glass. Phys. Chem. Glasses 15, 52 58.

REX R.W. (1960) Electron paramagnetic resonance studies of stable free radicals in lignins and humic acids. Nature 188, 1185 1186.

SEhrte J.W., SMITH R.C. & WVARt) S.J. (1961) Electron spin resonance absorption for polycrystalline sub- stances. Proc. Phys. Soc. 78, 1174-1176.

SWARTZ J.C., HOFFMAN B.M., KRIZET R.J. & ATMATZIDIS D.K. (1979) A general procedure for simulating EPR spectra of partially oriented paramagnetic centres. J. Mag. Resonance 36, 259 268.

VINCENT W.E.J. & ANGEL B.R. (1980) Electron spin resonance studies of vanadium impurities in kaolinite. Clays Clay. Miner., 28, (in press).

WAUCHOPE R.D. & HAQUE R. (1971) ESR in clay minerals. Nature Phys. Sei. 233, 141. WICKMAN H.H., KLEIN M.P. & SHIRLEY D.A. (1965) Paramagnetic resonance of Fe 3+ in polycrystalline

Ferrichrome A. J. Chem. Phys. 42, 2113-2117.

RI~SUMI~: La spectroscopie de r+sonance paramagn&ique 61ectronique (RPE) a apport+ une contribution significative ~ l'identification et it la caract6risation des impuret6s paramagn&iques associ6es aux argiles. Apr6s une br6ve discussion des principes g~n6raux de la technique, on passe en revue l'application de la RPE ~, l '&ude de ces esp6ces paramagn6tiques localis6es soit dans la structure de l'aluminosilicate, soit dans une phase 6trang6re. I1 s'agit essentiellement de fer et de d6fauts de r6seau induits par rayonnement.

KURZREFERAT: Die Elektronenspin-Resonanz-Spektroskopie (ESR) hat zur Bestimmung und Charakterisierung von paramagnetischen Verunreinigungen in Tonen einen bedeutenden Beitrag geliefert. Nach einer kurzen Diskussion der haupts~ichlichen Prinzipien dieser Technik, wird ein Oberblick gegeben fiber die Anwendung der ESR-Spektroskopie zur Untersuchung solcher paramagnetischer Spezies (haupts/ichlich Eisen und strahlungs-induzierte Gitterdefekte), die entweder innerhalb der Aluminosilikat-Struktur vorliegen oder als externe Verunreinigungs- phasen vorhanden sind.

RESUMEN: La espectroscopia de resonancia del espin de los electrones ha contribuido signifi- cantemente a la identificaci6n y caracterizaci6n de impurezas paramagn~ticas relacionadas con las arcillas. Despu6s de una breve discusi6n de los pfincipios generales de esta t~cnica se pasa revista a la aplicaci6n de la misma al estudio de las especies paramagn~ticas (principalmente hierro y defectos de la red cristalina inducidos por la radiaci6n) localizados dentro de la estructura de aluminosilicato o presente como fase de impurezas externa.


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