Download the application of electron spin resonance

Clay Minerals (19g0) 15, 321-335.
THE APPLICATION
OF ELECTRON
SPIN RESONANCE
SPECTROSCOPY
TO STUDIES
OF CLAY MINERALS:
I. I S O M O R P H O U S
SUBSTITUTIONS
AND EXTERNAL
SURFACE
PROPERTIES
P E T E R 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 spectroscopist'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 paramagnetic 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.
PRINCIPLES OF ESR SPECTROSCOPY
This section is devoted to a qualitative introduction to the principles of ESR spectroscopy. 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 = +89or _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
I
~
s=-89
S:-+2
I
:g/3H
I
I
~
s
-
Doublet
[H Zero)
+!
2
H 9:ncreas:nI
I
FIG. 1. The resonancecondition:case of a singleunpaired electronin an externalmagneticfield.
ESR of clays." Part I
323
J
g,,
g•
gx
I
I
I
/
t
gy
/
H
t
igz
~"
FIG.2. TypicalESR firstderivativelineshapes. (a) Isotropicg-value. (b) Axiallysymmetriccasein
the polycrystallineaverage. (c) Orthorhombicallysymmetriccase in the polycrystallineaverage.
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 transition 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. H a 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 crystallographic 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 conventionally 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 metalligand 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
E S R 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 l H . S + D [ S z 2 - 1 S ( S + 1)] +E[Sx 2 -
Sy2]
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 symmetries can be described by 2 values lying between 0 and 89depending 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 c m 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 neighbouring nuclei possessing zero spins (Marfunin & Bershov, 1970; O'Brien & Pryce, 1955;
Ioffe & Yanchevskaya, 1967).
ESR STUDIES OF CLAY MINERALS
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 (V2+ 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 H F S 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 f F e 3+ in micas have been reported by Olivier
et al. (t976a, 1977). In addition to resonances at g = 2.0 possibly associated with
E S R 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 o f 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 o f 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
Resonance B.
20~
400~c
300 ~
A.
400 ~
500 ~
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 different 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 c m - 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 corresponding to Centres IIa and lib (2 =0-234, D = 0 . 4 8 cm-1; and 2 - 0-207, D =0-32 c m - 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 crystallinity 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, dimethylsulphoxide 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 intercalation 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
329
ESR of clays: Part I
g=2-O
[g--4-O
J
O
Natural kaolinite
Mg dopedkaolinite
(no signals)
~
Fe3+doped kaolinite
Mg dopedkaolinite
X- irradiated
Mcj dopedkaolinite
- X-irradiated
and annealed
F
. ~
~
F'eS+andMg doped
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 o f 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. Hall
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 atg -- 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 tetrahedral 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 H F S 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 F e - O - F e 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-substituted components. Herbillon et al. (1976) have reported ESR studies o f 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 o f 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. Hall
4.48
9T
Ob
/
T~ /
Oa
f
~
Centre V
"q' . 2 - 0 0
2
I
I
I
I
~
1
I
IOOQ
J
l I p
2000
I
I
I J ~
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 (Wyoming 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 o f 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 resolvation 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
G o o d m a n (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.
SUMMARY
AND CONCLUSIONS
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.
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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.
K U R Z R E F E R A T : 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 Verunreinigungsphasen vorhanden sind.
RESUMEN: La espectroscopia de resonancia del espin de los electrones ha contribuido significantemente 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.