Bergaya F. Handbook of Clay Science

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3 5 7 9 11 13 15

-7.5

-3.5

0.5

4.5

Fe, d Fe-O = 202 pm

Fe, d Fe-O = 195 pmÅ

Fe, 95%

20%

Fe, 80%

60%

Fe, 40%

80%

Fe, 20%

(Å

−1

)

6.5

3.5

0.5

-2.5

-5.5

3 5 7 9 11 13

Fe-O, Spokane nontronite

10%

Fe, 90%

15%

Fe, 85%

Wavevector k (Å

−1

)

Wavevector k (Å

−1

)

(k)

Fig. 12.3.9. Application of the FeyO contributions of powder Fe K EXAFS in determining

the tetrahedral Fe content of nontronites. (A) The amplitude and phase changes of the FeyO

wave vector as a function of mixing different amounts of

Fe at d ¼ 195 pm to

Fe at

d ¼ 202 pm. (B) Example least squares ﬁt for the FeyO contribution of Spokane nontronite

assuming d ¼ 195 pm for tetrahedral Fe and d ¼ 202 pm for octahedral Fe. Modiﬁed after

Gates et al. (2002).

Chapter 12.3: X-ray Absorption Spectroscopy816

At the opposite extreme, the pre-edge amplitude of the ferruginous smectite SWa-1 is

enhanced, yet the other methods indicated that SWa-1 has nil tetrahedral Fe(III). In

this case, the enhanced amplitude is related to a high degree of distortion of Fe

octahedra caused by the presence of neighbouring Al and Mg octahedra. The dis-

tortion is sufﬁcient to decrease the t

-e

splitting, whi ch is better resolved than for

the NG-1, NAu-2 and Spokane nontronites (Fig. 12.3.3B). The South Australian

nontronite, NAu-1, also has an enhanced pre-edg e structure, but this enhancement

seems to be primarily due to its increased Al content compared with the Garﬁeld

sample (Gates et al., 2002). Using other methods, Gates et al. (2002) found that

the Garﬁeld an d NAu-1 nontronites may contain 2–3% of total Fe in tetrahedral

coordination, essentially at the detection limit of pre-edge XAFS spectroscopy

(Manceau and Gates, 1997).

Finally, the presence of different sites in layer structures affects the pre-edge

XAFS spectra. The pre-edge spectra of heat-treated Garﬁeld nontronite (G500) is

shown in Fig. 12.3.3B. The pre-edge amplitude is enhanced, in this case due to the

presence of 30% ﬁve-fold coordinated Fe (

Fe) in G500. Additionally, the pre-

edge peak of G500 is broadened by as much as 30% of the non-heated samples. Heat

treatment causes dehydroxylation and migration of some M2 cations to M1 sites

(Drits et al., 1998b). The M1 site is slightly larger than the M2 site, so distortion is

introduced in a new average of interatomic angles and distances. The broadening of

G500 is 30% compared with that of the Spokane sample, reiterating just how

uniform the Fe(III) octahedra are in the end member nontronite.

XANES Spectroscopy

Ildefonse et al. (1994, 1998) could estimat e the amount of tetrahedral Al in layer

silicates by decomposition of the Al K XANES spectra (Fig. 12.3.4). By comparison

with a variety of reference minerals, they found that all Al-bearing layered structures

had three amplitude maxima in the near-edge structure related to Al coordination

environment. The ﬁrst maxima near 1564 eV showed up as an inﬂection in the rising

edge of references containing no

Al, but became well-resolved and quite pro-

nounced for layer silicates containing appreciable

Al. The two peaks near 1566 and

1569 eV were related to

Al. By decomposing the three peaks in the Al K XANES,

Ildefonse et al. (1994, 1998) estimated accurately the

Al content of several mont-

morillonites and illite–smectite interstratiﬁcations. Thus, Al K XANES can be used

to determine tetrahedral Al content in smectites where such techniques as NMR fail

due to the presence of too much Fe (Gates et al., 1996).

The substitution of Ga for Al in synthetic kaolins and smectites was investigated

by Martin et al. (1998), using Ga K XANES, IR, electron microscopy and XRD

analyses. They found that Ga substitution in kaolins was limit ed presumably because

of the size difference between Ga and Al. When the ratio Ga/(Ga+Al) o0.1, Ga

incorporation into the kaolin struc ture was conﬁrmed by an increase in the d(06-33)

spacing. At Ga=ðGa þ AlÞ40:1, kaolin-smectite and kaolin-oxide precipitates oc-

curred, but Ga was predominatly associated with the smectite pha ses as observed by

12.3.2. XAFS Studies on Smectite Structure 817

IR. Ga K XANES spectra of Ga-kaolin and Ga-smectite showed that Ga substituted

for Al in the octahedral sheet of kaolin, but in both octahedral and tetrahedral sites

in smectite, indicating the ﬂexible nature of the 2:1 phyllosilicate structure.

For the Fe-bearing phyllosilicates, the form of the edge crest in the Fe K XANES

spectra pro vides information on oxidation state and degree of crystallinity (Gates

et al., 1997; Dyar et al., 2001). A shoulder within 4 eV above E

in the edge crest is

observed in reduced montmorillonite and nontronite, the resolution of which is

improved with increasing levels of Fe(II) (Gates et al., 1997). Similar features were

observed in the Fe K XANES spectra of biotite micas (Dyar et al., 2001) but here the

edge crests are better resolved due to greater crystallinity, and are probably related to

multiple-scattering contributions.

D. Octahedral Cation Distributions and Isomorpho us Substitutions in Smectites

In dioctahedral smectite structures, two types of octahedral cation arrays are

present. Most beidellites, ferruginous smectites, nontronites and illites are composed

of trans-vacant (centrosymmetric) octahedra. Here each (mostly) dival ent octahedral

cation occupies M2 sites, shares its two hydroxyl anions with one adjacent cation in

an M2 site along one edge (2OH edge), and two oxygen anions (2O edge) along edges

with two other M2 cations at 1201 (Fig. 12.3.10). Fe-poor smectites and mont-

morillonites have octahedral cations in both cis (M2) and trans (M1) sites (non-

centrosymmetric structure) (Drits et al., 1998b; Sains-Diaz et al., 2001). In the latter

structure, one M2 site is vacant, and any given octahedral cation (M1, M2) will share

two edges containing both O and OH (O,OH edge) and a 2O edge with adjacent

different (M2, M1) cations. In trioctahedral structures, where all octahedral sites are

ﬁlled with divalent cations, each M1 cation shares four O,OH edges and two 2O

edges with M2 cations. Each M2 cation shares one 2OH edge and two 2O edges with

other M2 cations, but two O,OH edges and a 2O edge with M1 cations. Such

distinctions are important as they affect the number of shells, and the number of

atoms within shells that are calculated based on the symmetry of the octahedra sheet

(Manceau et al., 1998; Da

hn et al., 2003; Vantelon et al., 2003).

Table 12.3.3 displays the number of atoms in shells and the distances of the shells

from the absorber (Al in this case) for a hypothetical Ni-saturated and Ni-

substituted Texas montmorillo nite STx-1, as calculated by TKATOMS (Ravel,

1999) in two different symmetries. Note that more oxygen atoms within particular

shells are calculated for c2/m (trans vacant, centrosymmetric) symmetry than for the

correct cis-vacant c2 symmetry. Also, more atoms make up the tetrahedral shells,

which show spli tting due to the makeup of the octahedral sheet. Thus, while an

apparently subtle difference exists between cis- and trans-vacant structures, it can

have a large impact on the parameters used by EXAFS analysis programmes in

calculating scattering paths.

Chapter 12.3: X-ray Absorption Spectroscopy818

Ni Bearing Layer Silicates and Minerals

The occurrence of Ni in various layer silicates attracted considerable attention given

the industrial importance of these minerals as Ni ore sources. Manceau and Calas

(1985, 1986, 1987) and Decarreau et al. (1987) found that Ni associ ated with lateritic

smectite and other layer silicate structures always occurs in Ni-enriched clusters of at

least 2–3 nm diameter. Associated infrared studies (Ge

rard and Herbillon, 1983)

indicated a predominance of NiNiMg-OH over NiMgMg-OH absorption stretching

bands, suggesting that Ni substitutes within the octahedral layer near sites initially

having charge deﬁcit.

M2 M2

M2 M1

M2 M2

M2 V

Fig. 12.3.10. Representation of different conﬁgurations of the octahedral sheets in diocta-

hedral smectites showing (A) the location of M1 and M2 sites in trioctahedral sheets and the

location of vacancies (V) in (B) trans vacant and (C) cis vacant, dioctahedral sheets. Solid

circles represent hydroxyls and hollow circles represent oxygens.

12.3.2. XAFS Studies on Smectite Structure 819

Ni Substitution in Montmorillonite Using the Hofmann– Klemen Effect

On heating Ni

-saturated Camp Bertaux montmorillonite at 300 1C, Muller et al.

(1997) observed changes in the Ni K EXAFS spectra consistent with the formation

of NiyM(MQFe, Al, Mg) shells at  290 pm from the absorber (Fig. 12.3.11A).

Increases in the b parameter (d(06-33) diffraction peak) indicate that interlayer Ni

migrates into vacant octahedral sites, presumably those closest to sites of Mg

substitution where octahedr al charge is locat ed. The extraordinary amplitude of this

new shell in the EXAFS spectra clearly relates to the close proximity of Fe

to the

substituted Ni

(recall the p-difference in phase-sh ift between heavy and light at-

oms at similar interatomic distances). Muller et al. (1997) interpreted this ﬁnding in

terms of the preferential migration of Ni

into vacant M2 (cis) octahedra between

and pairs of Fe

octahedra. From changes in the OH bending region of the

IR spectrum they furt her deduced that Fe–Mg clustering (Fig. 12.3.11B) occurred

within the octahedral sheet of the montmorillonite.

The IR spectrum of Camp Bertaux montmorillonite showed a band near

800 cm

–1

, assigned by Muller et al. (1997) to OH bending mode of pairs of OH-

sharing octahedral Fe (FeFe-OH). However, Gates (2005) attributed bands at

790–800 cm

–1

in the IR spectra of 32 dioctahedral smectites (including Wyoming

Table 12.3.3. Number of neighbours, N, and interatomic distances calculated for centro-

symmetric and non-centrosymmetric structures of a hypothetical Ni-saturated, Ni-substituted

montmorillonite using chemistry reported by Da

hn et al. (2003) for the montmorillonite STx-1

and unit cell parameters (as determined by XRD) of a ¼ 518 pm, b ¼ 989 pm, and assuming

c* ¼ 967 pm (collapsed interlayer). Chemistry



and symmetry

were input into TKATOMS

(Ravel, 1999) enabling scattering paths to be determined by FEFF (Rehr, 2003). Only single

scattering events were taken to determine R

eff

in IFEFFIT (Newville, 2003)

Centrosymmetric (c2/m) Non-centrosymmetric (c2)

eff

(pm) NR

eff

(pm)

NiyO

6 201 (2), 204 (2), 213 (2) 6 185 (2), 198 (2), 226 (2)

NiyOct

3 307 (2), 330 (1) 3 307 (2), 329 (1)

NiyTet

4 324(2), 331 (2) 4 244 (2), 389 (2)

NiyO

4 346 (2), 368 (2) 2 325

NiyO

6 393 (2), 403 (2), 406 (2) 4 353 (2), 378 (2)

NiyO

8 4.21 (2), 457 (2), 462 (2), 463 (2) 6 396 (2), 426 (2), 467 (2)

NiyTet

4 445 (2), 467 (2) 4 532 (4)

NiyInt 2 516 2 534

NiyOct

2 518 2 518

NiyTet

8 534 (2), 557 (2), 569 (2), 578 (2) 4 532

NiyOct

4 558 4 558



Al and Mg concentrations were combined for calculations. Ni was assumed to occupy 10% of M2 sites.

Al was taken at the origin of an M2 site for both c2 and c2/m symmetry.

Chapter 12.3: X-ray Absorption Spectroscopy820

montmorillonites) containing between 0 and 4

Fe per unit cell, to FeMg-OH in-

dicating that OH sharing between Fe–Mg pairs is common. Since the chemical

composition and IR spectrum of Camp Bertaux montmorillonite are not dissimilar

from those of Wyomi ng montmorillonite, the OH bending band near 800 cm

–1

in the

IR spectr um of Muller et al. (1997) should probably be assigned to FeMg-OH. The

octahedral cation distributions in Camp Bertaux montmorillonite is thus likely to

look like that shown in Fig. 12.3.11C with Fe–Mg pairs as OH-sharing cations. This

model is consistent with the EXAFS analysis of Muller et al. (1997), and similar to

that proposed by Vantelon et al. (2003) for other comparable smectites.

The pioneering work by Muller et al. (1997) made available to clay science a

valuable new approach to quantifying octahedral occupancies in smectites. Although

limited to montmorillonites and those smectites where the layer charge is dominantly

located in the octahedral sheet, this method serves as a model for future researchers

who wish to improve their understanding of the distribution of octahedral cations in

dioctahedral smectites.

Octahedral Cation Distribution in Fe-rich Layer Silicates

The P-EXAFS study by Manceau et al. (1990a) on the distribution of Fe and Mg in

the octahedral sheet of biotite indicated that Fe–Mg clustering is also common

among biotites, in agreement with NMR data (Sanz and Stone, 1983). However,

P-EXAFS can provide additional information that is not attainable by NMR. The

amplitudes and phases of the FeyM

shell contribution in the reverse FT of Fe K

EXAFS spectra are nearly identical for biotite minerals with widely differing oc-

tahedral Fe/Mg ratios. This would indicate that in biotites the local environment

around Fe is the same regardless of chemical composition. In other words, Fe and

Mg are not randomly distributed (Manceau, 1990; Dyar et al., 2001).

Using powder XRD, Manceau et al. (2000a) showed that the octahedral sheets

of nontronite were trans-vacant (centrosymmetric) within 5% error. By assuming

complete centrosymmetric layer structures, the calculated (DVLS–FEFF) partial P-

EXAFS spectra were in excellent agreement with those extracted from experimental

spectra. A recent EXAFS study also indicated that a centrosymmetric layer structure

for the ferruginous smectite, SWa-1, was appropriate (Vantelon et al., 2003).

P-EXAFS spectra indicated that the distribution of octahed ral cations in SWa-1

was random with respect to FeyFe and Fey(Al,Mg) cation proxim ities (Manceau

et al., 2000a). This contrasted with probability determ inations by Madejova

et al.

(1994) using IR spectroscopy. They indicated that FeyFe and AlyMg pairings

were more likely at the expense of FeyAl and FeyMg pairings than would be

allowed by simple random distribution. In a centrosymme tric structure, the OH

stretching and bending bands (in the IR spectra) preferentially probe octahedral

cation pairing along the b cell dimension, whereas all directions within the octahedral

plane are probed by EXAFS. The EXAFS data ( Manceau et al. 2000a) can be

reconciled with IR data (Madejova

et al., 1994) by arranging the cations in such a

12.3.2. XAFS Studies on Smectite Structure 821

way that the octahedral chemical composition, IR absorbances and EXAFS data are

satisﬁed (Fig. 12.3.12).

Octahedral Cation Distributions in Fe-poor Smectites

Vantelon et al. (2001, 2003) applied IR an d powder Fe K EXAFS to the study of

octahedral occupancy in smectites with Fe content s ranging from 0.1 to 0.6

atoms per O

(OH)

. In order to acceptably ﬁt their EXAFS data, they used cis-

vacant (no n-centrosymmetric) octahedral sheets for all Fe-poor smectite samples

(Table 12.3.3).

The results (Vantelon et al., 2003) would suggest a strong departure from random

distribution of cation ordering within the octahedral sheet. On the basis of their

EXAFS spectra the smectites may be classiﬁed into four distinct groups in agreement

with IR (Vantelon et al., 2001) and a host of previous analyses (e.g., Gu

ven, 1988).

For Wyoming montmorillonites (SWy-1 and SWy-2) the octahedral cation distri-

bution indicated Fe–Fe avoidance. For the other smectites, the distribution indicated

varying degrees of Fe clustering, but not necessarily as OH-sharing pairs (as inferred

from IR). The FTs of the k

weighted Fe K EXAFS spectra revealed a splitting

of the second peak between 220 and 350 pm (Fig. 12.3.13 ). The FEFF-modelled

EXAFS spectra ( Vantelon et al., 2003) showed that splitting of the ﬁrst cation shell

was due to the presence of ocahedral Al. Thus, splitting is strongly suppressed for

octahedral arrays where each Fe

has as neighbours on average 2 other Fe

and 1

(or Mg

) cations, as for the ferruginous smectite SWa-1. With increasing

and Mg

content the splitting increases, with a shoulder growing on the

shorter R* side of the FeyM

shell (Fig. 12.3.14). This splitting is a result of

interference from the p-phase difference between light (Mg, Al) and heavy (Fe)

backscattering atoms (Fig. 12.3.5 and Section 12.3.1E).

The models proposed by Vantelon et al. (2003) for the distribution of cations in

the octahedral sheets of Fe-poor smectites, are in general agreement with previous

studies indicating clustering of certain cations in smectites (Decarreau et al., 1987;

Muller et al., 1997). However, since FeMg-OH bands occur in the IR spectra of

many smectites, including Wyoming (SWy-2) (Madejova

et al., 1994) and Arizona

(SAz-1) (Gates, 2005) montmorillonites, the models of Vantelon et al. (2003) appear

to neglect OH-sharing Fe–Mg octahedra. In Fig. 12.3.15, the octahedral cation

Fig. 12.3.11. (A) Powder Ni K EXAFS spectra of Ni-saturated Camp Bertaux montmorillo-

nite before and after heating to 250 1C, redrawn from Muller et al. (1997). Labels for atomic

shells from interlayer (unheated) or octahedral (heated) Ni derived from TkATOMS and

FEFF scattering calculations of montmorillonite with chemistry reported by Muller et al.

(1997). (B) Two-dimensional cation distribution in the octahedral sheet of Camp Bertaux

montmorillonite based on Muller et al. (1997). (C) Rearrangement of the octahedral cation

array based on re-assignment of the IR band near 800 cm

1

as FeMg-OH bending (Gates,

2004).

Chapter 12.3: X-ray Absorption Spectroscopy822

-montmorillonite, unheated

-montmorillonite, heated to 250 °C

Normalized Intensity

Distance (Å)

Ni-O

Ni-Fe,Ni

Ni-Si

Ni-O

Ni-O(water)

Ni-Si

Ni-Fe,

Si,Al

Ni-O

23456

12.3.2. XAFS Studies on Smectite Structure 823

distributions, deduced from EXAFS analysis (Vantelon et al., 2003), were modiﬁed

by taking the IR data into account (Gates, 2004).

Insightful studies such as those by Vantelon et al. (2003) have the potential to

provide important information on the variety of structures associated with smectites

that may be used to advantage in nanotechnology or other industrial or environ-

mental applications. An obvious and rich area of study is to extend such analyses

(incorportating P-EXAFS techniques) to smectites with compositions intermediate

between SWy-2 (5% Fe

) and SWa-1 (25% Fe

), and to correlate octahe-

dral compositions with geochemical information on formation and weathering of

smectites.

12.3.3. ORIENTATION OF INTERCALATED ORGANIC MOLECULES

Only limited use was made of synchrotron-based techniques to study the orien-

tation of intercalated organic species in layer silicates, and all analyses were carried

out with micas or related minerals (Ha

hner et al. 1996a, 1996b; Lin et al., 1997;

Fischer et al., 1998; Brovelli et al., 1999; Fenter and Sturchio, 1999). In majority of

the cases, the alkyl and aromatic groups of organic molecules are oriented at high

angles with respect to the interlayer surface (Bedzyk and Cheng, 2002; Fenter, 2002).

This is partly due to the high (negative) layer charge, and partly to co-sorption of the

organic cation–inorganic anion (in the case of alkylammonium halides), giving rise

to a high packing density (Slade and Gates, 2004a).

Brovelli et al. (1999) used polarised C K NEXA FS spectroscopy to determine the

orientation of monolayers of octadecyltrimethylammonium (ODTMA) and di-

octadecyldimethylammonium (DODMA) cations on the external surface of mica

crystals (the layer charge, X, was unspeciﬁed but assumed to be 2e

–

per O

(OH)

For DODMA, a well-ordered self-assembled monolayer was observed with the alkyl

SWa-1

Fig. 12.3.12. Model for the two-dimensional cation distribution in the octahedral sheet of

SWa-1, based on Manceau et al. (2000a), but modiﬁed using updated chemical and IR data

(Gates et al., 2002; Gates, 2005).

Chapter 12.3: X-ray Absorption Spectroscopy824

1054

Bavaria

SWy-1

Milos

SWy-2

Georgie

North

Africa

SAz-1

STx-1

(k)

Fe per

Sample N

FeFe

SWy-1 0.47 0.0

SWy-2 0.45 0.0

China 0.10 0.4

STx-1 0.14 0.5

Georgie 0.36 0.5

Milos 0.46 0.7

SAz-1 0.17 0.9

North Africa 0.28 1.0

Bavaria 0.57 1.3

(OH)

China

R* (Å)

Fig. 12.3.13. Fourier transforms of k

weighted Fe K EXAFS spectra of Fe-bearing dioc-

tahedral smectites. Inset: the number of Fe atoms per O

(OH)

reported by Vantelon et al.

(2003) for each sample and the number of Fe neighbours in the FeyM

shell (N

FeFe

) de-

termined by least squares FEFF analysis. Modiﬁed and redrawn from Vantelon et al. (2003).

12.3.3. Orientation of Intercalated Organic Molecules 825