Bergaya F. Handbook of Clay Science

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Laperche, V., Prost, R., 1991. Assignment of the far-infrared absorption bands of K in micas.

Clays and Clay Minerals 39, 281–289.

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vol. 13, The Clay Minerals Society, Aurora, CO, pp. 65–98.

Madejova

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phlogopite. European Journal of Mineralogy 9, 501–508.

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of talc: a near infrared (NIR) spectroscopy study. American Mineralogist 89, 319–326.

Petit, S., Righi, D., Madejova

, J., Decarreau, A., 1998. Layer charge estimation of smectites

using infrared spectroscopy. Clay Minerals 33, 579–591.

Petit, S., Righi, D., Madejova

, J., Decarreau, A., 1999. Interpretation of the infrared NH

spectrum of the NH

-clays: application to the evaluation of the layer charge. Clay Min-

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thodes spectroscopiques a

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References 917

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10–18.

Chapter 12.6: Fourier Transform Infrared Spectroscopy918

Handbook of Clay Science

Edited by F. Bergaya, B.K.G. Theng and G. Lagaly

Developments in Clay Science, Vol. 1

919

Chapter 12.7

NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY

J. SANZ

Instituto de Ciencia de Materiales de Madrid, CSIC, Campus de Cantoblanco, ES-

28049 Madrid, Spain

Nuclear magnetic resonance (NMR) spectroscopy provides information on both

the structural and dynamic aspects of solids, including clay miner als. In NMR

spectroscopy, the interaction of nuclear magnetic moments, m

¼ðh=2pÞ g

I, with an

external magnetic ﬁeld, B

, induces the splitting of the 2I+1 energy levels of the

nuclear spin I (Zeeman interaction). In the last express ion, g

stands for the mag-

netogyric ratio of nuclei and h for the Planck constant. Irradiation of the sample

with the radio frequency, o

( ¼ g

), produces the absorption of the resonant

energy between adjacent energy levels (detection of NMR experiment) (Fig. 12.7.1a).

NMR spectroscopy is sensitive to the local environment of atoms. Nuclei in

different structural environments ‘see’ slightly different magnetic ﬁelds and conse-

quently absorb photons of different frequencies. Since these magnetic ﬁelds are

generally dependent on the crystal orientation with respect to B

, the NMR spectra

of powder samples show low resolution. Besides chemical shift, there are other

anisotropic interactions that reduce spectral resolution: (i) dipolar interactions, H

between magnetic moments of nuclei; (ii) paramagnetic interactions, H

, between

magnetic moments of nuclei and paramagnetic centers; and (iii) quadrupolar inter-

actions, H

, between quadrupolar moments of nuclei and electric ﬁeld gradients at

occupied sites (Slichter, 1990).

The spectral resolution of solids can be improved by: (1) rotating the sample

around an axis inclined at 54144

with respect to the magnetic ﬁeld B

; this ‘magic-

angle spinning’ (MAS) technique averages the angular dependence, and (2) applying

decoupling techniques that reduce the spin term of hamiltonians describing magnetic

interactions.

In MAS experiments, (Fig. 12.7.1b ) the spinning frequency must be larger than

the width of the NMR signal expressed in cycles per second (cps). If this is smaller,

side bands spaced by the same quantity (spinning rate) are detected in spectra. The

analysis of spinning side band patterns provides useful information about sites oc-

cupied by nuclei, distortions of polyhedra, and local symmetry of occupied sites. The

DOI: 10.1016/S1572-4352(05)01033-0

position of the central peak of these patterns (isotropic chemical shift) is indicative of

the chemical environment of nuclei .

Chemical shift anisotropies and ﬁrst-order quadrupolar interactions (I>1/2) de-

crease with the MAS technique. However, for systems in which second-order quad-

rupolar interactions are important, the position an d shape of the central components

can be seriously affected (Samoson and Lippmaa, 1983). In this case, second-order

quadrupolar effects can be attenuated by using higher magnetic ﬁelds (B

) or double

a) b)

Fig. 12.7.1. (a) Energy levels for a nucleus with I ¼ 3=2 in a magnetic ﬁeld B

with the

allowed transitions. The ﬁgure shows the effect of the ﬁrst-order quadrupolar interactions. (b)

Experimental arrangement of samples in MAS-NMR experiments (spinning around

y ¼ 54144

). Inﬂuence of increasing spinning rates on the

P CP-MAS spectra of trip-

henylphosphine oxide. From Wasylishen and Fyfe (1982).

Chapter 12.7: Nuclear Magnetic Resonance Spectroscopy920

rotation (DOR, DAS) techniques. Further, a separated evaluation of chemical shift

and quadrupole contributions was achieved by the application of two-dimensional

multi-quantum techniques (Massiot et al., 1996). As a result, the spectral features of

individual sites can now be determined. How ever, in very anisotropic environments

signals can be strongly broadened, making quantitative analysis difﬁcult (Alma

et al., 1984; Massiot et al., 1990).

On the other hand, decoupling techniques can average spin terms of heteronuclear

dipolar interactions, giving improved spectral resolution (Duer, 2002). Spectral

broadening is often caused by

H atoms. In such cases, irradiation of protons at the

Larmor frequency during recording of the NMR signal can reduce linewidth of

components. If the sample is rotated during decoupling, the spectral resolution can

be further improved (DR-MAS technique). A modiﬁed form of this technique is

termed ‘cross-polarization (CP)-MAS’. Its application can improve the detection of

atoms located near OH/H

O groups through transfer of polarization from hydrogen

atoms to analysed nuc lei (Pines et al., 1973).

12.7.1. NMR SPECT ROSCOPY OF CLAY MINERALS

In the following sections, several examples were chosen to illustrate the appli-

cability of the NMR technique to investigating the structural and dynamic aspects of

clay minerals (Sanz and Serratosa, 2002).

A. Structural Characterization

Cation Coordination

For some phyllosilicates the location of Al is difﬁcult to assess from chemical or

structural analyses. In particular, the distinction of Al and Si in tetrahedral sites by

X-ray diffraction (XRD) is problematic because of the similarity of their atomic

scattering factors. In this case,

Al MAS-NMR spectroscopy can provide direct

information on Al coordination, as the signals corresponding to octahedral (0 ppm)

and tetrahedral (70 ppm) Al are well resolved. Fig. 12.7.2a shows the

Al-NMR

spectra of three representative phyllosilicates: pyrophillite, muscovite, and phlogopite

(Sanz and Serratosa, 1984). The spectra consist of one or two principal compo-

nents and a series of small side bands associated with the spinning of the sample.

Pyrophyllite contains only Al

, while muscovite has both Al

and Al

. Although

the ideal composition of phlogopite contains only Al

, the sample analysed contains

a small amount of Al

(substituting for Mg) in the octahedral sheet (see Chapter 1).

In these spectra the resolution is very good and, in principle, a quantitative de-

termination of site occupancy is feasible. To this end , side bands of all nuclear

transitions should be considered (Alma et al., 1984 ; Massiot et al., 1990). Also, small

ﬂip angles, strong magnetic ﬁelds, and high-rotation frequencies must be used

(Samoson and Lippmaa, 1983). The

Al MAS-NMR spectra can also be used to

12.7.1. NMR Spectroscopy of Clay Minerals 921

analyse distortions on Al polyhedra. To accomplish this, spinning side band patterns

must be analysed in terms of anisotropy and asymmetry parameters that describe

quadrupolar interactions.

Crystallographic Sites

The

Si MAS-NMR spectra of sepiolite, Mg

, show three

resonances at 92.5, 95.0, and 98.5 ppm, associated with the three crystallo-

graphic sites of this mineral (Fig. 12.7.2b). The resonance at 98.5 ppm decreases

when a new component at 86 ppm is detected. This led Barron and Frost (1985) to

assign both components to Si atoms in Si–O–S i and Si–O–H groups at the edges of

individual ribbons. The two remaining signals at 92.5 and 95.0 ppm may be

a) b)

Fig. 12.7.2. (a)

Al MAS–NMR spectra of pyrophillite, muscovite, and phlogopite showing

the signals corresponding to tetrahedral and octahedral Al. Spinning side bands are labelled as

sb. From Sanz and Serratosa (1984). (b)

Si MAS-NMR spectra of three samples of sepiolite

with different degree of crystallinity. In this mineral, three different crystallographic sites are

detected. From Sanz (1990).

Chapter 12.7: Nuclear Magnetic Resonance Spectroscopy922

identiﬁed by cross-polarization and two-dimensional techniques. However, the

assignment of the three signals is not yet deﬁnitive (Barron and Frost, 1985; Sanz,

1990; d

Espinose de la Caillerie and Fripiat, 1994; Weir et al., 2002).

In the case of kaolinite, Si

(OH

), the

Si-NMR spectrum displays two

components, associated with Si tetrahedra interacting in a different way with con-

tiguous layers (Thompson, 1985; Thompson and Barron, 1987 ). The two compo-

nents in the

Al MAS-NMR spectrum of kaolinite were also interpreted in terms of

the presence of two octahedral aluminium sites (Rocha and Pedrosa de Jesu´ s, 1994).

Cation Distribution

In examining a series of aluminosilicates with well-deﬁned structures, Lippmaa et al.

(1980) found that the chemical shift of the

Si MAS-NMR signal depends on the

degree of tetrahedral polymerization. In the tetrahedral sheet of 2:1 phyllosilicates,

substitution of Al

for Si

in neighbouring tetrahedra causes the

Si line to shift to

a lower ﬁeld. This allows the Si atoms that are surrounded by 3Si, 2Si1Al, 1Si2Al,

and 3Al to be identiﬁed (Sanz and Serratosa, 1984)(Fig. 12.7.3).

The chemical shift of the Si signal is sensitive to layer distortion arising from

matching of tetrahedral and octahedral sheets (see Chapter 2). In particular, the

chemical shifts of the four Si components of Na

-saponites and micas change with

respect to the twisting angle, a, between contiguous tetrahedra (Sanz and Robert,

1992). A parallel analysis of chemical shift values versus TOT angles shown that, in

agreement with other silicates, these values become more negative when TOT angles

increase (Engelhardt and Mitchell, 1987).

Analysis of the

Si MAS-NMR spectra of phyllosilicates enables the tetrahedral

composition (Si

4x

) of samples with Si>Al to be estimated. When Loewenstein’s

rule (Al

–O–Al

avoidance (Loewenstein, 1954) is satisﬁed, the tetrahedral Si/Al

ratio can be derived from the expression (Sanz and Serratosa, 1984):

ð1=3Þ

1  x

where I

stands for relative intensity of Si-NMR components associated with nAl

and (3n)Si.

The distribution of Si and Al in t he tetrahedral sheets of 2: 1 phyllosilicates may

be estimated from the intensity of the four Si-NMR components. In particular, the

dispersion of Al an d Si is shown to favour the existence of Al–O–Si at the expense

of Al–O–Al and Si–O–Si associations (Herrero et al., 1985, 1987, 1989). Monte

Carlo simulations indicated that Al dispersion is intermediate between that

deduced from the Loewenstein and MDC (maximum dispersion of charges) mod-

els. The

Si-NMR spectra of a large number of samples are reproducible when the

number of Al in hexagonal r ings of the tetrahedral sheet is assumed to be as close as

possible to the average value deduced for each composition from the HDC (homo-

geneous dispersion of charges) model (Herrero et al., 1987, 1989).Inthecaseof

12.7.1. NMR Spectroscopy of Clay Minerals 923

Fig. 12.7.3.

Si MAS-NMR spectra of phyllosilicates 2:1 as a function of tetrahedral Al

fractional content, x

¼ Al=ðSi þ AlÞ. The number that identiﬁes each component represents

the number of Al, which surround a given Si tetrahedron. From Sanz and Robert (1992).

Chapter 12.7: Nuclear Magnetic Resonance Spectroscopy924

phyllosilicates with Al>Si, Loewenstein’s rule is not obeyed but the HD C model

remains valid (Sanz et al., 2003).

A parallel study was carried out with respect to the octahedral sheet of trioctahedral

micas. In this case, the distribution of Mg

and Fe

around OH



and F



ions is

undertaken on monocrystalline samples formed by superposition of platelets with axes

a and b disposed in the same way. By recording the

Hand

F-NMR spectra as a

function of sample orientation, four possible associations Mg

,Mg

Fe, MgFe

,and

around the OH groups can be identiﬁed (Sanz and Stone, 1983a). On the contrary,

a direct coordination of F



ions to Fe is not found (Sanz and Stone, 1979). Quan-

titative analysis of the spectra indicates that F



ions are preferentially coordinated to

,whileOH



groups coordinate to the remaining Mg

and Fe

cations. As a

result, the Mg/Fe ratio (deduced from the

H signal) decreases as the F



content

increases. It is concluded that ﬂuoro-magnesium and hydroxyl domains are locally

segregated in the octahedral sheet of micas (Sanz and Stone, 1983b).

B. Adsorption Phenomena

Clay– Water Inter actions

In hydrated Cs-Vermiculites, residual motions are cancelled out at low temperatures.

This allows the following two peaks to be resolved: (i) associated with Cs

in the

Stern layer near the basal oxygen planes of the layers (29 ppm); and (ii) associated

with Cs

ions in the Gouy diffuse layer surrounded by their hydration shells

(8 ppm) (Weiss et al., 1990; Sullivan et al., 1998). Laperche et al. (1990) studied the

NMR signals of

Na,

111

Cd, and

133

Cs in the interlayer space of Llano vermiculite

as a function of water content. Each hydration state, corresponding to two, one, or

zero water layers, is characterized by one speciﬁc value of the isotropic chemical shift

of cations. As the number of water molecules coordinated to the cations increases,

the lines shift downﬁeld (Fig. 12.7.4a).

The

H-NMR spectrum of hydrated 2:1 layer clay minerals consists of a

central line due to structural OH groups, and a doublet due to sorbed water

(Hougardy et al., 1976). By changing the angle between the magnetic ﬁeld (B

)and

the c-axis of the clay mineral aggregate, a dependence (3cos

y1) is obtained

for the doublet separation. This indicate s that the water molecules maintain a

preferential orientation with respect to the clay mineral layers (Fig. 12.7.4b).

The NMR spectra of the two-layer hydrate of Na-vermiculite suggest that water

molecules rotate around their c

symmetry axis, while the hydration shells

rotate around the c



-axis of the phyllosilicate (Fig. 12.7.4c). The angle deduced

between the c

and c



-axes is close to 651 . According to this model, the t wo

minima in the T

curves at 225 and 360 K are associated with the rotation of

O molecules around their c

axis, and the rotation/diffusion of [Na

]

hydration shells (Hougardy et al., 1976). In both cases, the values deduced

for the activation energy are between 25 and 35 K J/mol. A similar model

was proposed for the one-layer hydrate of Li-hectorite (basal spacing ¼ 1.26 nm).

12.7.1. NMR Spectroscopy of Clay Minerals 925

Fripiat et al. (1980) suggested that each Li

is coordinated to three water mole-

cules arranged in a dynamic pyramidal conﬁguration showing two rotational mo-

tions (around the c

and c



axes) s imilar to those found for the two-layer hydrate of

-vermiculite (Hougardy et al., 1976).

Hofmann and Klemen (1950) proposed that when a Li

-montmorillonite is

heated at 300 1C the exchangeable Li

ions move irreversibly into the layer structure.

b) c)

Fig. 12.7.4. (a)

MAS-NMR spectra of Na-vermiculite at different hydration levels: (i)

two layers (1.475 nm); (ii) one layer (1.175 nm); (iii) biphasic, one, and zero layer, and (iv)

dehydrated. Spinning sidebands are indicated by (

). From Laperche et al. (1990). (b)

spectrum of the two-layers hydrate of Na

-vermiculite at room temperature showing the

doublet and the central line. (c) Octahedral arrangement of water molecules around the Na

ions. From Hougardy et al. (1976).

Chapter 12.7: Nuclear Magnetic Resonance Spectroscopy926