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Applied Clay Science 2

Thermal loading of smectite-rich rocks: Natural processes vs.

laboratory experiments

I. KolarıkovaT, R. Prikryl, R. Hanus, E. Jelınek

Charles University, Ins. of Geochemistry, Mineralogy and Mineral Resources, Albertov 6, Prague 2, 128 43 Czech Republic

Received 13 April 2004; received in revised form 28 December 2004; accepted 11 January 2005

Available online 3 March 2005

Abstract

Changes of physico-chemical properties of smectite-rich rocks (used as an engineered barrier in the nuclear waste

repository) are studied. Two different genetic types of clays are evaluated—bentonite representing the residual weathering type

and sedimentary montmorillonite-rich clay. Cation exchange capacity and specific surface area were determined for bulk

samples (after heating experiments) and results were compared to a natural analogue (Ishirini deposit, Libya).

Structural changes of clay minerals were studied using XRD, TG and DTA. Two main transformation processes (illitization

and kaolinization) of smectite-rich rocks followed by deterioration of physico-chemical properties were observed after heat

treatment. Cation exchange capacity drops significantly with increasing temperature and so does Smicro. However, SBET values

increased when heat treatment was applied.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Bentonite; Montmorillonite-rich clay; Natural analogue; Structural change; Physico-chemical properties; Repository

1. Introduction

Smectite-rich rocks (bentonites) have been studied

as a main component of engineered barriers in nuclear

waste storage facilities for several decades (Lindblom,

1978, NAGRA, 1985, SKB, 1995, Savage et al.,

1999). The favourable behaviour of these clays (low

hydraulic conductivity, high swelling, favourable

sorption properties and cation exchange capacity—

see e.g. Elzea and Murray, 1994 or Kendall, 1996) is

0169-1317/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.clay.2005.01.002

T Corresponding author.

E-mail address: jaro@natur.cuni.cz (I. Kolarıkova).

influenced by the presence of montmorillonite, a

member of the smectite-clay mineral family (Mazurek

et al., 2003).

The physico-chemical properties of smectites

deteriorate mainly due to the increase of temperature

causing changes of crystal structural properties and

gradual transformation of smectite to illite as docu-

mented in natural geological process-diagenesis (Pytte

and Reynolds, 1989).

The aim of this paper is to determine trans-

formation processes followed by changes in phys-

ico-chemical properties of clays exposed to high

temperatures.

9 (2005) 215–223

I. Kolarıkova et al. / Applied Clay Science 29 (2005) 215–223216

2. Ishirini (Libya)—a natural analogue area

2.1. Geological setting of the Ishirini bentonites

The Ishirini area belonging to the NE part of

Jabal al Hasawnah (Libya) was selected due to the

presence of extensive bentonite bodies affected by a

thermal front (Alhodiari, 1995, Kolarıkova and

Hanus, 2003).

Five geological units can be distinguished in the

studied region—the Hasawnah formation (Cambrian),

a thin sequence of Miocene sediments (the Ahurfin

formation), a volcanic region represented by a

number of andesite bodies (Miocene), the Pliocene

Hishar formation (bentonites) and Plio-Pleistocene

basalts.

The Hasawnah formation transgressed the pene-

planed Precambrian by basal conglomerates and

continues as a monotonous sandstone sequence

(Collomb, 1962). The total thickness of the Hasawnah

formation is 350–400 m (Jurak, 1978). The sedimen-

tological character of the Hasawnah formation corre-

sponds to fluviatile continental or in places to

subcontinental conditions (Jurak, 1978). They consist

of basal conglomerates and a thick sequence of

sandstones with sporadic thin interbeds of sandy

siltstones and quartzites.

The basal part of the Ahurfin formation consists of

marly calcilutites with calcarenites, displaying diago-

nal bedding. The upper part of Ahurfin formation

begins with white-grey gypsiferous claystones which

are followed by dolomites. The andesite bodies

intruded this formation in the 20.2–15.9 Ma interval

(Abdelkader, 1997).

The bentonite deposit of Pleistocene age (Hishar

formation) is concentrated in SE part of Ishirini.

Clay deposits were formed by the alteration of

pyroclastic rocks of andesitic composition. The

bentonites are composed mainly of montmorillonite

(determined by X-ray diffraction and infrared

spectrometry with Fourier transform) with Ca2+ as

the dominant interlayer ion. The crystallochemical

formula was calculated from chemical analysis

using the recalculation method according to Rieder

(1977):

(Na0,01K0,02Mg0,01Ca0,27)[(Al1,42Mg0,18Fe3+0,32

Ti0,08)(Si3,66Al0,34)O10(OH)2] (Kolarıkova and Hanus,

2003). Quartz and kaolinite are subordinate.

2.2. Thermal effect

Some of the bentonite sequences were crosscut

by sub-volcanic basaltic domes and small intrusions

which injected the Hishar formation. These intru-

sions produced a large metamorphic aureole due to

the high temperature of the intrusion.

The age relation between the andesites and

basalts could not be determined solely from their

terrain relationships, but also by radiometric age

determination of the basalt dike-1.8 Ma (Abdel-

kader, 1997), which indicates that the dates of the

origin of both these rocks are not close to each

other.

The intrusion temperature of basalt dike

has been calculated using pyroxene thermom-

etry (Lindsley, 1983). The temperature ranges

from 830 to 900 8C (Kolarıkova and Hanus,

2003).

3. Laboratory thermal loading

3.1. Samples

Two sets of experimental material were used in the

experiments. The first set represents bentonites s.s.,

i. e. argillised pyroclastic rocks.

This material was sampled at the Rokle deposit

(western part of the Czech Republic). Rokle benton-

ite consists mainly of montmorillonite with Ca and

Mg as the dominant interlayer ions. Kaolinite, illite

and quartz are subordinate.

The second type of experimental material is

composed of sedimentary montmorillonitic clays from

the Zelena deposit (Czech Republic). These clays

contain illite–smectite (I–S) mixed layers, kaolinite

and accessory minerals (quartz, K-feldspar and

calcite).

The mineralogical composition of studied

clays was determined by X-ray diffraction.

Analysis of smectites was facilitated by prepara-

tion of oriented and glycolated samples. Semi-

quantitative mineralogy of selected samples was

determined using the CQPA (Chemical Quantita-

tive Phase Analyses) recalculation program (Klika

and Weiss, 1993), XRD and chemical analyses

data.

Fig. 1. XRD profiles of 001 diffraction of Ca/Mg montmorillonite

from the Rokle deposit.

Fig. 2. XRD patterns of oriented aggregates of glycolated I–S at

different temperatures (Zelena deposit).

I. Kolarıkova et al. / Applied Clay Science 29 (2005) 215–223 217

3.2. Experimental heating

Experiments on the fraction less than 2 Am(obtained by suspension, sedimentation and filtra-

tion) were done using a high-temperature XRD

camera. X-ray patterns were taken 15 min after

reaching each of the following temperatures 25 8C,50 8C, 70 8C, 80 8C, 90 8C, 110 8C and 200 8Cusing a Siemens D5000 diffractometer with the

following measurement conditions: radiation CuKa,

voltage 40 kV, current 30 mA and step size 0.028per 10 s. The X-ray camera was not evacuated

during exposure because the vacuum dehydrates the

samples and evacuation would cause changes

similar to those induced by heating (Weiss et al.,

1997).

Experimental simulation of hydrothermal alter-

ation processes in clays was performed on I–S rich

material from the Zelena deposit. Clay samples

reacted with heated solutions (pH 6.5 and salinity 5

wt.% NaCl). No further agitation of the samples

took place during the 10 day (1 month) experiment.

Low salinity should insure that temperature would

play the key role in possible transformation

processes.

Thermal characteristics of the fine-fraction samples

have also been determined using thermogravimetric

(TG) analyses (PL Thermal TGA-1000 apparatus,

scan rate of 10 8C/min, temperatures between 23 and

900 8C).Powder bulk samples for cation exchange capacity

(CEC) and specific surface experiments were heated

in a muffle furnace to 50 8C, 60 8C, 70 8C, 80 8C, 908C, 100 8C, 150 8C, 200 8C, 250 8C, 300 8C, 350 8C,400 8C, 450 8C and 500 8C and kept for 60 min at

each temperature. After dehydration–rehydration

cycles, CEC and specific surface area were recorded.

The tests were performed on 5 sets of homogenized

samples.

The analyses performed on bulk/fine-fraction

samples were performed at the Clay Institute (Labo-

ratoire de Mineralogie-Cristallographie) in Paris.

Libyan bentonites were analysed at the Charles

University (Institute of Geochemistry, Mineralogy

and Mineral Resources) in Prague.

Fig. 4. TG analysis of kaolinite from the Zelena deposit.

I. Kolarıkova et al. / Applied Clay Science 29 (2005) 215–223218

4. Evaluation of thermal effect

4.1. Mineralogical changes

The changes of montmorillonite in bentonite

(Rokle deposit) due to heating experiments were

determined using XRD monitoring of the first basal

diffraction (001). The profile intensity decreased

together with a shift of the maximum from 15.36 2to 10.20 2 as the temperature increased from 25 8C to

200 8C (Fig. 1). After Ca/Mg montmorillonite reached

100 8C, the profiles became very diffused and

collapsed to 10.20 2 when heated to 200 8C.Fig. 2 shows I–S transformation due to the high

temperature (10 day, 1 month experiment). X-ray

analyses were conducted and interpreted by Prof.

Picembon. The degree of illitization is directly

proportional to increasing temperature.

Changes of illite–smectite mixed layers were

studied also using the TG analysis. The TG curve

(Fig. 3) shows important peaks at 70 8C and 130 8C,corresponding to the loss of interlayer water–outer

hydration shell (at 70 8C) and remaining interlayer

water (at 130 8C). The third loss event corresponds to

the dehydroxylation process and the last peak (at 625

8C) can be interpreted as the beginning of structural

changes in the clay (Chipera and Bish, 2002).

The TG analyses of kaolinite show that the

evolution of volatiles began at ~400 8C, peaked at

~515 8C and terminated at ~705 8C (Fig. 4). These

temperatures represent the dehydroxylation of the

Fig. 3. TG analysis of illite–smectite from the Zelena deposit.

kaolinite with the onset of the transformation to

metakaolin and final destruction of the metakaolin

structure at ~705 8C (Chipera and Bish, 2002).

Natural analogue study revealed the following

mineralogical change: the smectite content in the host

bentonite remained almost unchanged at distances of

several meters or more (Fig. 5) from the intrusion, but

decreased continuously in close proximity to the

volcanic intrusion. Knowing thermal history of the

clay (based on isotopic and fluid inclusion studies),

the transformation of smectites was assumed. A small

amount of montmorillonite (several cm from the

contact zone) was transformed to kaolinite due to

the high temperature (~470 8C) and high salinity

(~39.5 wt.% NaCleq.) (compare Figs. 5 and 6).

Fig. 5. QKM diagram showing the proportion of major components

of bentonite at 2 m from the volcanic contact.

Fig. 6. QKM diagram showing the proportion of major components

of bentonite collected at increasing distances from the volcanic

contact: ! 15 cm from the contact; " 20 cm from the contact; #

30 cm from the contact; $ 40 cm from the contact; % 50 cm from

the contact; & 90 cm from the contact.

I. Kolarıkova et al. / Applied Clay Science 29 (2005) 215–223 219

4.2. High resolution transmission electron microscopy

(HRTEM)

Lattice fringe images were obtained using a

HITTACHI HRTEM at 150 kV and 30 AA with a

CIME EPEL cold field emission source.

Smectite interlayers can be dehydrated during

HRTEM analyses due either to ionic bombardment

during the sample preparation or high vacuum

conditions during analysis, or both (Kubler, 1989).

The cross-fringes demonstrate the coherence of the

stacked layers (Fig. 7).

Fig. 7. HRTEM lattice fringe image of I–S from the studied sample

(without heat treatment). The brighter spots are attributed to

smectite interlayers.

4.3. Cation exchange capacity

Cation exchange capacity (CEC) was determined

using substitution of exchangeable cations by solu-

tions (1 mmol/l CsCl, 50 mmol/l Ca2+ and 50 mmol/l

Mg2+) and by measurement of concentrations of

sorption before and after experiments using AES

and ICP-MS (Clay Institute–Laboratoire de Miner-

alogie-Cristallographie, Paris and Institute of Chem-

ical Technology, Prague).

The decrease in CEC appears to be continuous

with increasing temperature (Fig. 8).

The CEC of montmorillonitic clays (Zelena

deposit) decreases slowly, whereas values recorded

for bentonites (Rokle deposit) drop rapidly after

reaching 120 8C.The CEC of divalent cations revealed a small and

slow decrease with increasing temperature (Fig. 8).

High temperature also influenced Libyan benton-

ites and their chemical parameters. The CEC

decreases closer to the volcanic contact (Fig. 9). Six

samples were tested with two measurements made on

each. The values are high (90.8 mmol/100 g) at a 150

cm distance from the heating source, and correspond

to an exchange capacity of a bentonite rich in smectite

(Mazurek et al., 2003). However, the closer to the

volcanic/bentonite contact, a lower value of CEC was

observed. This rapid decrease can be partly explained

by the appearance of kaolinite which has a very

limited exchange capacity.

Fig. 8. CEC of monovalent and divalent cations of bentonite (Rokle

deposit) and montmorillonite-rich clay (Zelena deposit).

Fig. 9. CEC (M++M++) of Ishirini bentonite in respect to the distance from the bentonite/volcanic intrusion contact.

I. Kolarıkova et al. / Applied Clay Science 29 (2005) 215–223220

4.4. Surface area and porosity

Parameters characterising the porous structure of

Libyan clays were obtained from the sorption analyses

using a SORPTOMATIC 1800 apparatus Carlo Erba

(Institute of Rock Structure and Mechanics, Academy

of Sciences of the Czech Republic). The samples used

were outgassed until they reached a constant weight,

at a pressure of b10�6 Pa, at a temperature of 333 K.

The specific surface area SBET, which represents

predominantly the surface area of meso- and macro-

Fig. 10. Specific surface area of Libyan bentonites in respect to the

distance from the bentonite/volcanic intrusion contact.

pores (radiiN2 nm), was determined according to the

BET method (Brunauer et al., 1938) from the N2

adsorption isotherm at 77 K.

The microporous structure (pore radii b2 nm)

parameters were determined from CO2 isotherms at

the temperature of 298 K within a pressure range from

0 to 1000 Torr. The isotherms were evaluated

according to Dubinin’s theory of volume filling.

Dubinin’s and Medek’s equations (Dubinin, 1967,

Medek, 1977) were applied in the calculation of the

basic texture parameters.

Specific surface area of Czech clay samples used

during the experiment was determined using a

BELSORT 28 (Clay Institute, Paris).

Fig. 10 showing the natural analogue results

documents the decreasing trend of Smicro, whereas

nearly no changes in SBET values were observed.

Experiments revealed the decrease in Smicro with

increasing temperature (see Figs. 11 and 12). Smicro

decreases significantly after reaching 90 8C (mont-

morillonitic clays, Zelena deposit) or 150 8C (benton-

ites, Rokle deposit). However, SBET increases rapidly

with increasing heat treatment (especially after

exceeding 100 8C).

5. Discussion

Experimental simulations of hydrothermal illitiza-

tion showed the solid-state clay transformation mech-

anism. This mechanism is proposed for materials with

Fig. 11. Specific surface area of montmorillonite-rich clay from the Zelena deposit.

I. Kolarıkova et al. / Applied Clay Science 29 (2005) 215–223 221

low permeability such as bentonites (Altaner et al.,

1984; Elliot et al., 1991). Solid state transformation

(Shutov et al., 1969, Dunoyer de Segognac, 1970;

Hower et al., 1976) involves illitization with gradual

replacement of smectite by illite on a layer-by-layer

basis.

In this process, which involves fluids that can act

as catalysts and transport media, the replacement of

smectite by illite occurs in natural analogues close to

the heating source—most often volcanic intrusion

(Bouchet et al., 1999).

Fig. 12. Specific surface area of ben

The transformation process in the closed (labora-

tory) systems was described by Herbert and Kasbohm

(2004) and is characterised by Al3+–Mg2+ substitution

in the octahedral layer causing an increased octahedral

layer charge.

On the other hand, in the open geological system

studied (Ishirini bentonite deposit), kaolinization was

observed. In montmorillonite particles a substitution

of Mg2+ by Al3+ took place. The reduction of Si4+/

Al3+ ratio from 2.4 in montmorillonite to 1 in kaolinite

is a clear indication of removal of Si during the

tonite from the Rokle deposit.

I. Kolarıkova et al. / Applied Clay Science 29 (2005) 215–223222

reaction in the open systems (Herbert et al., 2004).

These processes were confirmed by laboratory experi-

ments (Herbert and Kasbohm, 2004).

In the Ishirini area, temperature was not the only

one factor which caused the transformation of benton-

ite. High salinity fluids (Kolarıkova and Hanus, 2003)

operating in the system assisted in the mineralogical

change. Illitization was not observed, probably due to

the fact that no K source existed in the system.

The observed structural changes indicate that

changes in physico-chemical properties should occur.

Experiments confirmed the anticipated decrease in

CEC with increasing temperature.

This is accompanied by an increase of SBET values

and a decrease of Smicro. Pore space represents a very

important factor which can influence mechanical and

sorption properties (Gens et al., 2002). Such a rapid

decrease in Smicro can reduce the ability of limiting

and retarding the escape of radionuclides from the

repository.

Results from the natural analogue study contra-

dict previously published data on bentonites (Bar-

akat, 1998; Techer, 2002), where no specific relations

to the distance from the heating source were observed.

In the Ishirini deposit, Smicro decreases closer to the

volcanic contact, but SBET values did not reveal any

major change. Increasing Smicro with increasing

distance from the volcanic contact can be explained

by a higher amount of montmorillonite with high

microporosity.

The decreasing trend in CEC appears to be similar

to the experimental results.

6. Conclusions

Experimental and natural analogue studies revealed

two main transformation processes which may be

operating in the environment of the deep nuclear

waste repository:

(1) Illitization (confirmed by experimental hydro-

thermal alteration) occurred after 10 days.

Transformation of illite–smectite mixed layer

minerals was caused by temperature factors.

(2) Kaolinization was observed at the Ishirini

deposit in close proximity to the volcanic/

bentonite contact. Transformation of montmor-

illonite into kaolinite was caused by high

tempered (~470 8C), hypersaline (~39.5 wt.%

NaCleq.) fluids affecting bentonites near the

contact.

As a result of step heating experiments, a continual

decrease in ion exchange ability and Smicro of the clay

material due to the temperature increase was

observed. On the other hand, SBET values increased

with heat treatment.

Heat treatment has changed the structure of clays

and their physico-chemical properties. Bentonites

exposed to high temperatures (N100 8C) are not stableand transformations followed by deterioration of

favourable chemical and physical properties may be

expected in bentonite clay barriers in the long run.

Acknowledgements

This work was carried out as a part of Clay

microstructures Project (1/14/9k64). Financial support

was provided by a grant from GACR No. 104/02/

1464, a grant from the Ministry of Education, Czech

Republic MSM:113100005 and project GAAV IAA

3046401.

Special thanks are given to Prof. Picembon for

conducting the XRD and TG analyses.

RAWRA kindly allowed publication of results

obtained during the project bVerification of substitu-

tion of bentonites by montmorillonitic claysQ (contractNo. 10/2002/Wol).

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