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Smectite-to-illite alteration in salt-bearing bentonites (The East Slovak Basin)

Published online by Cambridge University Press:  01 January 2024

M. Honty*
Affiliation:
Comenius University, Mlynská Dolina, 84215 Bratislava, Slovakia
P. Uhlík
Affiliation:
Comenius University, Mlynská Dolina, 84215 Bratislava, Slovakia
V. Šucha
Affiliation:
Comenius University, Mlynská Dolina, 84215 Bratislava, Slovakia
M. Čaplovičová
Affiliation:
Comenius University, Mlynská Dolina, 84215 Bratislava, Slovakia
J. Franců
Affiliation:
Czech Geological Survey, Leitnerova 22, 658 69 Brno, Czech Republic
N. Clauer
Affiliation:
Centre de Géochimie de la Surface (CNRS-ULP), Ecole et Observatoire des Sciences de la Terre, 67084-Strasbourg, France
A. Biroň
Affiliation:
Institute of Geology, Severná 5, 974 01 Banská Bystrica, Slovakia
*
*E-mail address of corresponding author: honty@fns.uniba.sk

Abstract

The effect of a saline environment on illitization in volcanoclastic rocks is examined in deep boreholes in the East Slovak Basin. Based on X-ray diffraction analysis, it is concluded that illite-smectite (I–S) expandability is always less in the salt-bearing bentonites (SBB) than in the salt-free bentonites (SFB) for a given depth interval. These two lithologies can be distinguished easily by water-leachate chemistry. Within the depth interval 2100–2500 m, the expandability in SBB varies within the range 25–10% expandable with R1 and R3 ordering in SBB and 68–35% expandable with R0 ordering in SFB. In two shallow SBB samples the expandability is close to that of SFB, suggesting that salinity alone does not enhance the illitization; but salinity may enhance it when combined with higher burial temperature. Vitrinite reflectance and Tmax of RockEval pyrolysis measured in adjacent shales confirm that the increased illitization in SBB is not due to heating and/or erosion. The model of burial and thermal history calibrated by organic maturity suggests that the same thermal history produces two different expandabilities in the two lithologies (SBB and SFB). Particle thickness measurements and K-Ar data were used to deduce the crystal growth mechanism of illitization in SBB. Whereas surface-controlled growth is typical for SFB, simultaneous nucleation and growth played a more important role in the case of SBB. The effect of a salty environment on the illitization is not yet fully understood and may have severe consequences for the utilization of bentonites as engineering barriers in radioactive waste disposal sites if salt formations used as host rocks are taken into account.

Information

Type
Research Article
Copyright
Copyright © 2004, The Clay Minerals Society

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