Bergaya F. Handbook of Clay Science

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Handbook of Clay Science

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

Developments in Clay Science, Vol. 1

1097

Chapter 13.2

PARALLELS AND DISTINCTIONS BETWEEN CLAY

MINERALS AND ZEOLITES

D.L. BISH

Department of Geological Science, Indiana University, Bloomington,

IN 47405-1405,USA

Clay minerals and zeolites are among the most hydrated minerals on the Earth’s

surface and are found in a myri ad of environments, including soils, seaﬂoor deposits,

hydrothermal alteration products, altered vo lcanic deposits, sediments, etc. Al-

though they often occur together and have many similar attributes, many of the

properties of clay minerals and zeolites are distinctive due to important differences in

their cryst al structures. The similarities and distinctions hold true for a variety of

clay minerals and zeolites, but for the purpose of this discussion, I will compare and

contrast smectites and natural zeolites.

13.2.1. SMECTITES AND ZEOLITES: SIMILARITIES

It is well known that smectites and zeolites both often occur as ﬁne-grained,

authigenic phases, and they are good records of low-temperatu re secondary alter-

ation. Smectites and zeolites also have open structures, containing exchangeable

cations and water molecules. Both classes of minerals have very high cation exchange

capacities (C EC) and exhibit useful cation selectivities. Smectite CEC are approx-

imately 90–130 cmol(+)/kg (e.g., Weaver and Pollard, 1973), and the exchange ca-

pacity results from substitutions in both the tetrahedral and octahedral sheets of the

2:1 layers (Chapter 12.10). Theoretical exchange capacities for zeolites (calculated

based on structural formulae) are higher and exhibit wider variation, 180–300 for

clinoptilolite and mordenite (two of the most common natural zeolites), and from

300 to 400 meq/100 g for other common zeolites such as chabazite, stilbite, phill ip-

site, erionite, and laumontite (Barrer, 1978; Colella, 1996). Zeolite cation exchange

capacity originates solely from substitutions in the tetrahedral framework. Both

smectites and zeolites are generally selective for large, univalent ions (e.g., Cs

) over

smaller, more hydrated cations such as Li

or Mg2

(Chapter 12.10.), and this

DOI: 10.1016/S1572-4352(05)01040-8

selectivity has been exploited in removing contaminants from natural and process

waters (Chapter 11.1).

As emphasised a bove, smectites and zeolites are some of the most hydrated min-

erals. Both smectites and zeolites can contain large amounts of water, up to 20% in

zeolites and in excess of 30% in some smectites (at 100% relative humidity (RH), not

in liquid water, where smectites osmotically swell). Note that measur ements of

maximum water content of both sets of minerals cannot be made reliable by simply

drying a sample at high temperature and recording the weight loss. Such a meas-

urement would only provide information on the hydration state of the mineral under

room conditions of humidity. Both zeolites and smectites continue to take up water

as the partial pressure of water (RH) is increased, up to a point in zeolites that is

structurally constrained. The water in zeolites and smectites can reversibly leave and

re-enter the structures, giving rise to anomalous thermal behav iour unlike that of

most silicates. Unlike most solids, smectites and zeolites contract on heating (and/or

dehydration) and expand on cooling and/or hydration. It is seldom that the effects of

thermal expansion are seen with zeolites below about 300 1C, as they are overshad-

owed by the larger effects of dehydration-induced contraction.

13.2.2. SMECTITES AND ZEOLITES: DISTINCTIONS

In spite of the many similarities between smect ites and zeolites, many of their

properties are distinctive due to major differences in their crystal structures.

Smectites and phyllosilicates in general have layer structures, with exchangeable

cations and variable amounts of water molecules in the interlayer region. In clay

minerals with lower layer charges, such as smectites, the attractive forces between the

individual layers are approximately balanced by expansive forces due to hydration of

interlayer cations. As a result of the weak layer–layer bonding, there is often little or

no crystallographic order from layer to layer. Water enters the interlayer region to

interact with the interlayer surface and hydrate the interlayer cations. Water mol-

ecules can only en ter the interlayer region in integral amounts, that is, one at a time

and not in fractional amounts. Thus, smectites hydrate in a layer-by- layer manner,

going from a zero-water layer structure to a four-water-layer structure (Chapter 5).

These structures have basal spacings of approximately 1.0, 1.25, 1.5, 1.7, and 2.0 nm.

The measured smectite basal spacings often only approximately reﬂect such a step-

wise increase in layer repeat distances due to the existence of interstratiﬁc ations of

adjacent hydrates. As a result of this stepwise hydration, many smectite phenomena,

speciﬁcally those related to interlayer hydration, are quantised.

In contrast to the two-dimensional layer structures of smectites, zeolites have

three-dimensionally connected alumosilicate framework structures, and their frame-

works are negatively charged due primarily to Al

- for Si

substitutions. Unlike

smectite structures, zeolite structures are typically ordered in three dimensions al-

though stacking faults are known to occur in some zeolites like mo rdenite (Camp bell

Chapter 13.2: Parallels and Distinctions Between Clay Minerals and Zeolites1098

and Cheetham, 2002). These aluminosilicate frameworks form large structural cav-

ities known as extrafram ework sites in which exchangeable cations and H

O mol-

ecules occur (Fig. 13.2.1). Structural studi es have shown that H

O interacts with

both the framework and exchan geable cations, an d H

O commonly enters the ex-

traframework sites progres sively as the partial pressure of water is increased (in-

crease in RH). There is also evidence for a more or less continuous rearrangement of

extraframework cations and H

O molecules and concomit ant structural relaxation

as hydration or dehydration proceed. As a result of this progressive hydration (or

dehydration), associated phenomena are considerably less quantised than with

smectites. However, structural studies have also shown that H

O molecules occur in

multiple sites with potentially very different attraction energies, see review by Bish

and Carey (2001). Indeed, if interaction energies for distinct H

O molecules are

signiﬁcantly different, or if hydration/dehydration-induced structural changes are

large, stepwise hydration/dehydration reactions may occur, as in the zeolite lau-

montite, Ca

[Al

]16H

O(Fig. 13.2.2).

13.2.3. CRYSTAL STRUCTURAL TRANSITIONS

As a result of the layer-by-layer hydration/dehydration behaviour of smectites,

their T- or P(H

O)-induced structural transitions are typically quantised, that is, they

Fig. 13.2.1. Clinoptilolite structure, (Na,K, 1/2Ca)

[Al

]20H

O, down c, showing

extraframework cations in the large structural cavities.

13.2.3. Crystal Structural Transitions 1099

occur in a stepwise fashion. As H

O molecules enter or leave the interlayer region,

large structural transitions occur, as illustrated in Fig. 13.2.3 for a Na

-exchanged

nontronite. This ﬁgure illustrates several impor tant aspects of smectite hydration

and dehydration. First, the stepwise expansion of the smectite layers as RH increases

is apparent, with ‘‘steps’’ centred around 1.0, 1.2, and 1.5 nm. Equally notewor-

thy is the hysteresis between the hydration and dehydration branches; note that the

increasing-RH path is distinct from the decreasing-RH path. Although this feature

has not yet been quantitatively explained, it is a result of the large structural changes

associated with hydration and dehydration, namely expansion/contraction on the

order of 0.25 nm. Hysteretic hydration/dehydration be haviour is generally the rule

for smectites. The accompanying X-ray diffraction data in Fig. 13.2.3 illustrate the

changes in patterns throughout the hydration/dehydration reaction. Important

points to note from these data include the increasing breadth of the (0 0 1) reﬂections

between ‘‘steps’’ and the large changes in intensity that occur wi th changes in hy-

dration state. The increased reﬂection breadth between ‘‘steps’’ results from the

existence of interstratiﬁcations of adjacent hydrates.

Similar data are shown in Fig. 13.2.4 for three cation-exchanged varieties of the

Clay Minerals Society source SAz-1 smectite. The data for the Na

-exchanged

variety illustrate the stepwise hydration/dehydration reaction very well, with distinct

steps near basal spacings of 1.25 nm and 1.5 nm. This ﬁgure also shows that the

Fig. 13.2.2. Laumontite structure,  down c, showing the positions of extraframework Ca

ions and water molecules.

Chapter 13.2: Parallels and Distinctions Between Clay Minerals and Zeolites1100

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

0 102030405060708090100

relative humidity (%)

3 4

5 6

7 8

9 10

11 12

2-Theta (CuKa)

decreasing RH

increasing RH

basal spacing (nm)

Fig. 13.2.3. Basal spacing for Na

-exchanged NG-1 nontronite as a function of relative

humidity. Data are shown for both increasing-RH and decreasing-RH directions. Also shown

are the X-ray powder diffraction patterns, taken with Cu Ka radiation, beginning with an RH

of 0% at the bottom, progressing through an RH of 100% in the middle, and ending with an

RH of 0% at the top.

-exchanged

increasing %RH

decreasing %RH

1.4

1.9

1.4

0.9

1.9

1.4

0.9

basal spacing (nm)

relative humidity (%)

Fig. 13.2.4. Basal spacing for cation-exchanged SAz-1 smectite as a function of relative hu-

midity. Data are shown for both increasing-RH (triangles pointing up) and decreasing-RH

(triangles pointing down) directions.

13.2.3. Crystal Structural Transitions 1101