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Conversion time from smectite to illite. A preliminary study
Applied Clay Science, 7 (1992) 125-130
125
Elsevier Science Publishers B.V., Amsterdam
Conversion time from smectite to illite. A preliminary study J. Linares, F. Huertas and E. Barahona Estaci6n Experimental del Zaidin, CSIC. Prof. Albareda, 1. 18008 Granada, Spain (Received January 15, 1992; accepted after revision February 25, 1992)
ABSTRACT Linares, J., Huertas, F. and Barahona, E., 1992. Conversion time from smectite to illite. A preliminary study., In: A. Meunier (Editor), Clays and Hydrosilicate Gels in Nuclear Fields. Appl. Clay Sci., 7: 125-130. During the hydrothermal treatment of a bentonite in the presence of potassium so!utions a release of protons, silica and other elements was observed. It is suggested that these products arise as a consequence of the beginning of the transformation of smectite into illite. After the short time of the treatment, 21 days, it is not possible to detect any layer of neoformed mica, but this quantity can be indirectly determined from the stoiquiometric reaction of transformation smectite-to-illite through the number of moles of hydrogen ion released to the solution. Under the assumption that the transformation is a first order reaction, the time needed for the complete destruction of smectite was also calculated. In the range of 100 °C smectite resists more than eighty thousand years before being transformed into illite. These results are in agreement with other described in the literature.
INTRODUCTION
The conversion of smectite to illite is a subject of great interest for basic studies in natural environments: diagenesis and hydrothermal alteration (Srodon and Eberl, 1984; Velde, 1985; Chamley, 1989; Inoue et al., 1988; Elliot et al., 1991, among many others) and, specially, in the design of clay barriers for high level radioactive waste repositories (Pusch and Carlsson, 1985; Bucher and Mfiller-Vonmoos, 1989, etc.). In this case, smectites can transform into illite or other phyllosilicates by action of solutions percolating through the rocks of the repository and by the thermal environment surrounding the canister. The rate of this transformation is of paramount importance in the design and safety of these repositories. The longevity of smectites, or time of conversion of smectite into illite, is Correspondence to: Prof. Jos6 Linares, Estaci6n Experimental del Zaidin, C.S.I.C., 18008 Granada, Spain
0169-1317/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
126
J. LINARES ET AL.
an open question. The scarcity of data about this reaction may be due to the time-consuming experiences needed for kinetic studies. Besides, in the first stages of the reaction it is very difficult to detect mineralogical or chemical signals of the reaction progress. The methods normally used in mineralogical analyses do not permit to detect the presence of very small amounts of neoformed illite, and chemical studies on the evolution of the reaction are not well established. As a consequence, the development of indirect methods for illite determination would be of great interest. In a previous paper (Linares et al., 1991 ) we have described the results of the first stages of interaction between a smectite and potassium solutions at high temperatures, as a part of the Spanish Project on Radioactive Wastes promoted by the Spanish State Agency ENRESA. In this communication, a chemical approach to the smectite-to-illite transformation reaction is used, which permits a preliminary calculation of the conversion time of smectite into illite. METHODOLOGY
A green bentonite from the "Cortijo de Archidona" deposit (Serrata de Nijar, Almeria, SE Spain) was used. It contains 95% of a dioctahedral smectite with minor quantities of quartz, plagioclase, cristobalite and calcite. The bentonite has a surface area of 691 m2/g and a C.E.C. of 107 m e q / 1 0 0 g. The exchangeable cations are calcium, magnesium and sodium in similar amounts; potassium is negligible. The tetrahedral charge of smectite is over 0.2 per O2o(OH)4.
The bentonite was introduced in Teflon reactors, 25 cm 3 in capacity, shielded with a metal case. Two grams of bentonite were used with ten cm 3 of potassium chloride solution. The concentrations of KCI were 0.05, 0.1 and 0.5 M, but at 175 °C 0.025 and 0.3 M w e r e also used. The reactors were heated at 60, 120 and 175°C in temperature regulated ovens. The time of reaction was 21 days in all cases. The solid phase and the solution in equilibrium were analyzed at the end of each experience. EXPERIMENTAL RESULTS AND DISCUSSION
In the conditions of the experiment, the chemical composition of smectites did not change during the hydrothermal-potassium treatment, as described by Linares et al. ( 1991 ). Only a slight removal of octahedral iron was detected. Smectites exhibit a normal X-ray diffraction pattern when are solvated with ethylene-glycol, except in the case of samples subjected to the highest potassium treatment where potassium-smectites appear. In these cases the beginning of a slight inflexion at approximately 0.9 nm can be, also, observed
CONVERSIONTIME FROM SMECTITE TO ILLITE
127
in the diffractogram. All these facts indicate that a few layers of illite could have been formed during the hydrothermal treatment, but in quantities below the detection limits of X-ray diffraction and chemical analysis. On the other hand, the chemical composition of solutions at the end of each experience changes significatively depending on the temperature and of initial potassium concentration. Hydrogen and silica concentrations in the equilibrium solutions increase with temperature, as can be seen in Table 1. Similar changes in the chemical composition of equilibrium solutions have been described by other authors (see Giiven, 1990, for a review), although there are not good quantitative explanations for these changes. In Table 1, the variations of potassium contents are also included. All these facts could indicate that, in addition to the potassium adsorption process on smectite (Linares et al., 1991 ), a phase is being neoformed through a reaction in which hydrogen, silica and other elements are released. These elements could just come from the reaction of transformation of smectite into illite. In a simplified form, this reaction, applied to the smectite under consideration, would be as follows: Si7.8Alo.2A12.8Fe0.4Mgo.9Nao.802o(OH)4
+ K + =0.5
Si6AI6K202o(OH)4
+ 4.8SIO2 + 0.2F% O3 + 0 . 9 M g O + 0 . 4 N a 2 0 + 0 . 5 H 2 0 + H + According to this reaction, one mole of protons and different amounts of other compounds are produced per mole of smectite destroyed or per half mole of neoformed mica. Among the released elements, only protons, silica and iron were investigated at the end of the experiments. The amount of neoTABLEI Analytical data for equilibrium solution, SiO2: values in ppm. K ÷ in m e q / 1 0 0 g T
KClimt
pH eq.
SiO2
K + eq.
K + ads.
0.05 0.10 0.50 0.05 0.10 0.50 0.05 0.10 0.50 0.025 0.05 0.10 0.30 0.50
6.31 6.20 6.02 6.21 6.11 5.90 5.93 5.84 5.62 5.89 5.53 5.18 5.09 5.03
5.2 4.6 4.2 7.2 6.6 6.2 13.2 12.5 12.2 11.9 9.6
3.5 12.5 128.0 4.0 13.5 125.0 1.25 4.5 16.5 74.5 122.0
21.5 37.5 122.0 21.0 36.5 125.0 11.25 20.5 33.5 75.5 128.0
(°c) 20 20 20 60 60 60 120 120 120 175 175 175 175 175
128
J. LINARES ET AL.
formed mica (or destroyed smectite) can be inferred from the released quantities of any one of these elements. Besides, if the order of the reaction is known, the time needed for the conversion, to any conventional extent, of smectite into illite can also be calculated. Among the variables that could be used to calculate indirectly the amount of illite, the hydrogen concentration is the simplest to determine and was selected for this study. The amounts of protons were calculated from the difference between the hydrogen concentration at zero time (20 ° C values in Table 1 ) and those obtained at the end of each hydrothermal treatment. This quantity represents the moles of protons formed during 21 days of reaction, and from these data the stoichiometric amount of neoformed mica and destroyed smectite were inferred. The rate constant for a first order reaction (Eberl and Hower, 1976) is directly derived from the proportion of smectite destroyed during the experiment. Finally, the time needed for 99% destruction of smectite is calculated from the rate equation (Laidler, 1987 ). All the data derived from the experiment are given in Table 2. The results of these calculations for 0.05 and 0.50 M potassium concentrations are compared in Fig. 1 with other data from literature. References about the time required to convert smectite into illite were not found for natural systems. Moreover, experimental studies on clay mineral alteration do not include sufficient kinetic data. However, Eberl and Hower (1976) and Pytte and Reynolds (1989) give some results on this subject. Although the first authors do not include the kinetic equation, it was possible TABLE 2
Calculated parameters for the conversion of 99% of smectite to ullite for several temperatures and potassium concentrations KCli,i~
Moles of H + released Formed mica (g) Destr.smec./year ( m g ) Conversion time (years) Rate constant (mg/year) Moles o f H ÷ released Formed mica (g) Destr.smec./year ( m g ) Conversion time (years) Rate constant (mg/year) Moles of H ÷ released Formed mica (g) Destr.smec./year ( m g ) Conversion time (years) Rate constant (rag/year)
0.05 0.05 0.05 0.05 0.05 0.10 0.10 0.10 0.10 0.10 0.50 0.50 0.50 0.50 0.50
60°C 1.27" 10 -9 5.05" 10 - 7
0.0176 496,600 9.26.10 -6
1.45" 10 -9
120°C
176°C
0.68" 10 -8 0.27" 10 -5 0.094 92,986 4 . 9 5 . 1 0 -5 0.81" 10 -8
2.46" 10 -8 0.98" 10 -5 0.340 25,713
5.77" 10 - 7
3.24" 10 - 6
0.020 43,7000 1.05" 10 -5 0.30" 10 -8
0.112 78,036 5.89" 10 -5 1.44" 10 -8 5.75" 10 -6 0.200 43,685 1.05"10 -4
1.21" 10 - 6
0.042 208,145 2.21"10 -5
1.79.10 -4
5.98" 10 -8 2.38" 10 -5 0.818 10,685 4.30" 10 -4 837" 10 -8 3.33" 10 -5 1.158 7 547 6.10" l0 4
CONVERSION TIME FROM SMECTITE TO ILLITE
129
CONVERSION TIME (10 5 years) Eberl St Hower (1976)
t
o
Pytte & Reynolds
....--
This paper 0.0S M KCllatt 0.50 M KClhsjt
"°° 0 0
t
I'" 100
I
I
200
Temperstmre (tC) Fig. 1. Variation of the conversion time for 99% of smectite into illite versus temperature.
to make the calculation from the values of rate constants for the transformation of smectite into illite (20% swelling layers). The values derived from both papers are those included in Fig. 1 for comparison. As can be observed, the results given in the present paper are in fairly good agreement with them, specially between 60 and 120 ° C. This agreement suggests that the method used for determining the progress of the smectite-to-illite conversion is promising and plausibly useful. Moreover, the longevities calculated (more than eighty thousand years at about 100 °C) indicate that the investigated bentonite is, in a preliminary evaluation, adequate for its use as near field barrier material in radioactive waste repositories. Currently, a more detailed kinetic study of the smectite-to-illite transformation is being carried out with the aim of obtaining a more accurate estimation of the transformation rate, as well as other kinetic and activation parameters of interest.
130
J. L I N A R E S E T AL.
ACKNOWLEDGMENT This w o r k was c a r r i e d out with the financial s u p p o r t o f the S p a n i s h State A g e n c y E N R E S A . We are also grateful to M i n a s de G a d o r S.A. for the help t h e y gave us d u r i n g o u r visits to their b e n t o n i t e d e p o s i t s in Almeria.
REFERENCES Bucher, F. and Mfiller-Vonmoos, M., 1989. Bentonite as a containment barrier for the disposal of highly radioactive wastes. AppI.Clay Sci., 4:157-178. Chamley, H., 1989. Clay Sedimentology. Springer, Berlin, 623 pp. Eberl, D.D. and Hower, J., 1976. Kinetics of illite formation. Geol. Soc. Am. Bull., 87: 13261330. Elliot, W.C., Aronson, J.L., Matisoff, G. and Gautier, D.L., 1991. Kinetics of the smectite to illite transformation in the Denver Basin: Clay mineral, K-Ar data, and mathematical model results. Am. Assoc. Pet. Geol. Bulletin, 75: 436-462. Giiven, N., 1990. Longevity of bentonite as buffer material in a nuclear-waste repository. Eng. Geol., 28: 233-247. Inoue, A., Velde, B., Meunier, A. and Touchard, G., 1988. Mechanism ofillite formation during smectite-to-illite conversion in a hydrothermal system. Am. Mineral., 73:1325-1334. Laidler, K.J., 1987. Chemical Kinetics. Harper International, New York, 531 pp. Linares, J., Huertas, F. and Barahona, E., 1991. Stability ofsmectites. Potassium and temperature effects. Proc. Euroclay'91 Dresden, 2: 697-702. Pusch, R. and Carlssson, T., 1985. The physical state of Na-smectite used as barrier component. Eng. Geol., 21: 257-265. Pytte, A. M. and Reynolds, R.M., 1989. The thermal transformation of smectite to illite. In: N.D. Naeser and T.H. McCulloh (Editors), Thermal History of Sedimentary Basins. Springer, New York, pp. 132-140. Srodon, J. and Eberl, D.D., 1984. Illite. In: S.W. Bailey (Editor), Micas. Reviews in Mineralogy, 13: 495-544. Velde, B., 1985. Clay Minerals. A Physico-Chemical Explanation of their Occurrence. Elsevier, Amsterdam, 427 pp.
125
Elsevier Science Publishers B.V., Amsterdam
Conversion time from smectite to illite. A preliminary study J. Linares, F. Huertas and E. Barahona Estaci6n Experimental del Zaidin, CSIC. Prof. Albareda, 1. 18008 Granada, Spain (Received January 15, 1992; accepted after revision February 25, 1992)
ABSTRACT Linares, J., Huertas, F. and Barahona, E., 1992. Conversion time from smectite to illite. A preliminary study., In: A. Meunier (Editor), Clays and Hydrosilicate Gels in Nuclear Fields. Appl. Clay Sci., 7: 125-130. During the hydrothermal treatment of a bentonite in the presence of potassium so!utions a release of protons, silica and other elements was observed. It is suggested that these products arise as a consequence of the beginning of the transformation of smectite into illite. After the short time of the treatment, 21 days, it is not possible to detect any layer of neoformed mica, but this quantity can be indirectly determined from the stoiquiometric reaction of transformation smectite-to-illite through the number of moles of hydrogen ion released to the solution. Under the assumption that the transformation is a first order reaction, the time needed for the complete destruction of smectite was also calculated. In the range of 100 °C smectite resists more than eighty thousand years before being transformed into illite. These results are in agreement with other described in the literature.
INTRODUCTION
The conversion of smectite to illite is a subject of great interest for basic studies in natural environments: diagenesis and hydrothermal alteration (Srodon and Eberl, 1984; Velde, 1985; Chamley, 1989; Inoue et al., 1988; Elliot et al., 1991, among many others) and, specially, in the design of clay barriers for high level radioactive waste repositories (Pusch and Carlsson, 1985; Bucher and Mfiller-Vonmoos, 1989, etc.). In this case, smectites can transform into illite or other phyllosilicates by action of solutions percolating through the rocks of the repository and by the thermal environment surrounding the canister. The rate of this transformation is of paramount importance in the design and safety of these repositories. The longevity of smectites, or time of conversion of smectite into illite, is Correspondence to: Prof. Jos6 Linares, Estaci6n Experimental del Zaidin, C.S.I.C., 18008 Granada, Spain
0169-1317/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
126
J. LINARES ET AL.
an open question. The scarcity of data about this reaction may be due to the time-consuming experiences needed for kinetic studies. Besides, in the first stages of the reaction it is very difficult to detect mineralogical or chemical signals of the reaction progress. The methods normally used in mineralogical analyses do not permit to detect the presence of very small amounts of neoformed illite, and chemical studies on the evolution of the reaction are not well established. As a consequence, the development of indirect methods for illite determination would be of great interest. In a previous paper (Linares et al., 1991 ) we have described the results of the first stages of interaction between a smectite and potassium solutions at high temperatures, as a part of the Spanish Project on Radioactive Wastes promoted by the Spanish State Agency ENRESA. In this communication, a chemical approach to the smectite-to-illite transformation reaction is used, which permits a preliminary calculation of the conversion time of smectite into illite. METHODOLOGY
A green bentonite from the "Cortijo de Archidona" deposit (Serrata de Nijar, Almeria, SE Spain) was used. It contains 95% of a dioctahedral smectite with minor quantities of quartz, plagioclase, cristobalite and calcite. The bentonite has a surface area of 691 m2/g and a C.E.C. of 107 m e q / 1 0 0 g. The exchangeable cations are calcium, magnesium and sodium in similar amounts; potassium is negligible. The tetrahedral charge of smectite is over 0.2 per O2o(OH)4.
The bentonite was introduced in Teflon reactors, 25 cm 3 in capacity, shielded with a metal case. Two grams of bentonite were used with ten cm 3 of potassium chloride solution. The concentrations of KCI were 0.05, 0.1 and 0.5 M, but at 175 °C 0.025 and 0.3 M w e r e also used. The reactors were heated at 60, 120 and 175°C in temperature regulated ovens. The time of reaction was 21 days in all cases. The solid phase and the solution in equilibrium were analyzed at the end of each experience. EXPERIMENTAL RESULTS AND DISCUSSION
In the conditions of the experiment, the chemical composition of smectites did not change during the hydrothermal-potassium treatment, as described by Linares et al. ( 1991 ). Only a slight removal of octahedral iron was detected. Smectites exhibit a normal X-ray diffraction pattern when are solvated with ethylene-glycol, except in the case of samples subjected to the highest potassium treatment where potassium-smectites appear. In these cases the beginning of a slight inflexion at approximately 0.9 nm can be, also, observed
CONVERSIONTIME FROM SMECTITE TO ILLITE
127
in the diffractogram. All these facts indicate that a few layers of illite could have been formed during the hydrothermal treatment, but in quantities below the detection limits of X-ray diffraction and chemical analysis. On the other hand, the chemical composition of solutions at the end of each experience changes significatively depending on the temperature and of initial potassium concentration. Hydrogen and silica concentrations in the equilibrium solutions increase with temperature, as can be seen in Table 1. Similar changes in the chemical composition of equilibrium solutions have been described by other authors (see Giiven, 1990, for a review), although there are not good quantitative explanations for these changes. In Table 1, the variations of potassium contents are also included. All these facts could indicate that, in addition to the potassium adsorption process on smectite (Linares et al., 1991 ), a phase is being neoformed through a reaction in which hydrogen, silica and other elements are released. These elements could just come from the reaction of transformation of smectite into illite. In a simplified form, this reaction, applied to the smectite under consideration, would be as follows: Si7.8Alo.2A12.8Fe0.4Mgo.9Nao.802o(OH)4
+ K + =0.5
Si6AI6K202o(OH)4
+ 4.8SIO2 + 0.2F% O3 + 0 . 9 M g O + 0 . 4 N a 2 0 + 0 . 5 H 2 0 + H + According to this reaction, one mole of protons and different amounts of other compounds are produced per mole of smectite destroyed or per half mole of neoformed mica. Among the released elements, only protons, silica and iron were investigated at the end of the experiments. The amount of neoTABLEI Analytical data for equilibrium solution, SiO2: values in ppm. K ÷ in m e q / 1 0 0 g T
KClimt
pH eq.
SiO2
K + eq.
K + ads.
0.05 0.10 0.50 0.05 0.10 0.50 0.05 0.10 0.50 0.025 0.05 0.10 0.30 0.50
6.31 6.20 6.02 6.21 6.11 5.90 5.93 5.84 5.62 5.89 5.53 5.18 5.09 5.03
5.2 4.6 4.2 7.2 6.6 6.2 13.2 12.5 12.2 11.9 9.6
3.5 12.5 128.0 4.0 13.5 125.0 1.25 4.5 16.5 74.5 122.0
21.5 37.5 122.0 21.0 36.5 125.0 11.25 20.5 33.5 75.5 128.0
(°c) 20 20 20 60 60 60 120 120 120 175 175 175 175 175
128
J. LINARES ET AL.
formed mica (or destroyed smectite) can be inferred from the released quantities of any one of these elements. Besides, if the order of the reaction is known, the time needed for the conversion, to any conventional extent, of smectite into illite can also be calculated. Among the variables that could be used to calculate indirectly the amount of illite, the hydrogen concentration is the simplest to determine and was selected for this study. The amounts of protons were calculated from the difference between the hydrogen concentration at zero time (20 ° C values in Table 1 ) and those obtained at the end of each hydrothermal treatment. This quantity represents the moles of protons formed during 21 days of reaction, and from these data the stoichiometric amount of neoformed mica and destroyed smectite were inferred. The rate constant for a first order reaction (Eberl and Hower, 1976) is directly derived from the proportion of smectite destroyed during the experiment. Finally, the time needed for 99% destruction of smectite is calculated from the rate equation (Laidler, 1987 ). All the data derived from the experiment are given in Table 2. The results of these calculations for 0.05 and 0.50 M potassium concentrations are compared in Fig. 1 with other data from literature. References about the time required to convert smectite into illite were not found for natural systems. Moreover, experimental studies on clay mineral alteration do not include sufficient kinetic data. However, Eberl and Hower (1976) and Pytte and Reynolds (1989) give some results on this subject. Although the first authors do not include the kinetic equation, it was possible TABLE 2
Calculated parameters for the conversion of 99% of smectite to ullite for several temperatures and potassium concentrations KCli,i~
Moles of H + released Formed mica (g) Destr.smec./year ( m g ) Conversion time (years) Rate constant (mg/year) Moles o f H ÷ released Formed mica (g) Destr.smec./year ( m g ) Conversion time (years) Rate constant (mg/year) Moles of H ÷ released Formed mica (g) Destr.smec./year ( m g ) Conversion time (years) Rate constant (rag/year)
0.05 0.05 0.05 0.05 0.05 0.10 0.10 0.10 0.10 0.10 0.50 0.50 0.50 0.50 0.50
60°C 1.27" 10 -9 5.05" 10 - 7
0.0176 496,600 9.26.10 -6
1.45" 10 -9
120°C
176°C
0.68" 10 -8 0.27" 10 -5 0.094 92,986 4 . 9 5 . 1 0 -5 0.81" 10 -8
2.46" 10 -8 0.98" 10 -5 0.340 25,713
5.77" 10 - 7
3.24" 10 - 6
0.020 43,7000 1.05" 10 -5 0.30" 10 -8
0.112 78,036 5.89" 10 -5 1.44" 10 -8 5.75" 10 -6 0.200 43,685 1.05"10 -4
1.21" 10 - 6
0.042 208,145 2.21"10 -5
1.79.10 -4
5.98" 10 -8 2.38" 10 -5 0.818 10,685 4.30" 10 -4 837" 10 -8 3.33" 10 -5 1.158 7 547 6.10" l0 4
CONVERSION TIME FROM SMECTITE TO ILLITE
129
CONVERSION TIME (10 5 years) Eberl St Hower (1976)
t
o
Pytte & Reynolds
....--
This paper 0.0S M KCllatt 0.50 M KClhsjt
"°° 0 0
t
I'" 100
I
I
200
Temperstmre (tC) Fig. 1. Variation of the conversion time for 99% of smectite into illite versus temperature.
to make the calculation from the values of rate constants for the transformation of smectite into illite (20% swelling layers). The values derived from both papers are those included in Fig. 1 for comparison. As can be observed, the results given in the present paper are in fairly good agreement with them, specially between 60 and 120 ° C. This agreement suggests that the method used for determining the progress of the smectite-to-illite conversion is promising and plausibly useful. Moreover, the longevities calculated (more than eighty thousand years at about 100 °C) indicate that the investigated bentonite is, in a preliminary evaluation, adequate for its use as near field barrier material in radioactive waste repositories. Currently, a more detailed kinetic study of the smectite-to-illite transformation is being carried out with the aim of obtaining a more accurate estimation of the transformation rate, as well as other kinetic and activation parameters of interest.
130
J. L I N A R E S E T AL.
ACKNOWLEDGMENT This w o r k was c a r r i e d out with the financial s u p p o r t o f the S p a n i s h State A g e n c y E N R E S A . We are also grateful to M i n a s de G a d o r S.A. for the help t h e y gave us d u r i n g o u r visits to their b e n t o n i t e d e p o s i t s in Almeria.
REFERENCES Bucher, F. and Mfiller-Vonmoos, M., 1989. Bentonite as a containment barrier for the disposal of highly radioactive wastes. AppI.Clay Sci., 4:157-178. Chamley, H., 1989. Clay Sedimentology. Springer, Berlin, 623 pp. Eberl, D.D. and Hower, J., 1976. Kinetics of illite formation. Geol. Soc. Am. Bull., 87: 13261330. Elliot, W.C., Aronson, J.L., Matisoff, G. and Gautier, D.L., 1991. Kinetics of the smectite to illite transformation in the Denver Basin: Clay mineral, K-Ar data, and mathematical model results. Am. Assoc. Pet. Geol. Bulletin, 75: 436-462. Giiven, N., 1990. Longevity of bentonite as buffer material in a nuclear-waste repository. Eng. Geol., 28: 233-247. Inoue, A., Velde, B., Meunier, A. and Touchard, G., 1988. Mechanism ofillite formation during smectite-to-illite conversion in a hydrothermal system. Am. Mineral., 73:1325-1334. Laidler, K.J., 1987. Chemical Kinetics. Harper International, New York, 531 pp. Linares, J., Huertas, F. and Barahona, E., 1991. Stability ofsmectites. Potassium and temperature effects. Proc. Euroclay'91 Dresden, 2: 697-702. Pusch, R. and Carlssson, T., 1985. The physical state of Na-smectite used as barrier component. Eng. Geol., 21: 257-265. Pytte, A. M. and Reynolds, R.M., 1989. The thermal transformation of smectite to illite. In: N.D. Naeser and T.H. McCulloh (Editors), Thermal History of Sedimentary Basins. Springer, New York, pp. 132-140. Srodon, J. and Eberl, D.D., 1984. Illite. In: S.W. Bailey (Editor), Micas. Reviews in Mineralogy, 13: 495-544. Velde, B., 1985. Clay Minerals. A Physico-Chemical Explanation of their Occurrence. Elsevier, Amsterdam, 427 pp.