POTASSIUM FIXATION IN SMECTITES 261
POTASSIUM FIXATION IN SMECTITES BY WETTING
AND DRYING IN NaCl SOLUTION
, VLADIMÍR UCHA
and ¼UBICA PUKELOVÁ
Department of Mineral Deposits, Faculty of Natural Sciences, Comenius University, 842 15 Bratislava, Slovak Republic;
Institute of Geology, Slovak Academy of Sciences, Dúbravská cesta 9, P.O.Box 106, 840 05 Bratislava 45, Slovak Republic
(Manuscript received June 18, 2002; accepted in revised form December 12, 2002)
Abstract: Wetting and drying experiments performed at 60 °C were used to examine the effect of various concentrations
of NaCl solutions on the degree of K-fixation. Cheto and Texas montmorillonites were used as a starting material for
experiments. K-saturated samples were put through up to 100 cycles in 0.055 M NaCl solutions. The expandability
values, measured by XRD peak position method, indicate a higher degree of K-fixation in the most diluted NaCl solution
at a given number of cycles in comparison with smectites wet by distilled water. However, the effect of NaCl solutions
of higher molarity on the degree of K-fixation is not straightforward. Smectites wet by moderately concentrated NaCl
solutions approach the K-fixation pattern of water-wet smectites. K-fixation in the smectites in the most concentrated
NaCl environment was clearly retarded. The effect of NaCl on K-fixation was much less visible when a buried altered
volcanoclastic sample was used.
Key words: wetting and drying cycles, NaCl environment, K-fixation, smectites.
Many papers are devoted to the problem of potassium fixation
in smectites and to the related change of expandable layers
into non-expandable. This reaction, which occurs in a variety
of environments became one of the most extensively studied
clay mineral reactions. The most frequently studied reactions
are diagenetic K-fixation in shales, illite formation in sand-
stones as well as hydrothermal illitization. Significantly less
attention has been paid to potassium fixation (diagenetic and
non-diagenetic) in saline environments. Some studies focus on
potassium fixation within limited depth intervals in the sedi-
ments of saline lakes (Singer & Stoffers 1980; Deconinck et
al. 1988; Hay et al. 1991; Turner & Fishman 1991). The com-
mon conclusion of these studies is that a high degree of potas-
sium fixation is not caused by diagenetic change (i.e. burial re-
lated temperature), but is associated with specific chemical
compositions of lake water. The observed clays were affected
by alkaline-hypersaline environment and did not experience
deep burial. Moreover, ucha (unpublished data) observed a
higher degree of potassium fixation in the buried bentonites
from the saline environments compared to bentonites from
non-saline environments in the East Slovak Basin, even
though these bentonites experienced the same diagenetic tem-
In the laboratory, potassium fixation can be achieved by two
ways. K is rendered non-exchangeable by hydrothermal heat-
ing of K-smectites (Eberl & Hower 1976) and/or K-fixation
occurs when K-smectites are exposed to repeated wetting and
drying (WD) cycles (Gaultier & Mamy 1979; Eberl et al.
1986; ucha & iráòová 1991; Miklo & Èíèel 1993). Wet-
ting and drying lead to irreversible fixation of K and subse-
quent layer collapse. This transformation is not accompanied
by a significant change in the chemistry of the 2:1 layers as is
the case during hydrothermal smectite alteration. The number
of the layers collapsed at the end of the WD experiment is pro-
portional to the layer charge of the original sample (Eberl et al.
1986; ucha & iráòová 1991).
Most of the WD K-fixation experiments were performed in
distilled water. Eberl et al. (1986) observed that WD in KOH
solutions led to a higher proportion of collapsed layers than
WD cycles of K-saturated samples in distilled water. Later ex-
periments (Eberl et al. 1993) showed that heating smectites at
60 °C in 0.11 M KOH promoted layer collapse. The same
experiment with 3 M KOH initially led to a drastic reduction
in expanded layers finally leaving an amorphous product.
Heller-Kallai & Eberl (1997) performed WD experiments at
60 °C in the presence of K
and KCl. Samples cycled with K
more collapsed layers than those treated with acetate or chlo-
ride. Oxalate appeared to attack the clay layers. In addition,
smectites exposed to WD cycles at high pH using K
solutions underwent partial deprotonation.
The present study examines the effect of various concentra-
tions of NaCl solutions on the degree of potassium fixation by
WD at 60 °C. Whereas high alkalinities of saturated KOH so-
lutions are geologically rare, NaCl solutions of different con-
centrations may play a significant role in the diagenesis of
sediments taking place in the playas and sabkhas of arid re-
gions. In the case of the buried sediments, they may affect the
reaction as brines circulating along the faults and in the pore
Materials and methods
The starting material were two smectites from the Source
Clay Mineral Repository: Cheto (SAz-1), Texas (STx-1) and
smectite from the buried bentonite of borehole Bánovce 4
(Ban 4/1) in the East Slovak Basin having an expandability of
GEOLOGICA CARPATHICA, 54, 4, BRATISLAVA, AUGUST 2003
262 HONTY, UCHA and PUKELOVÁ
~90 %. The <2
m size fraction was obtained using
sedimentation for all samples. Sample Ban 4/1 was treated pri-
or to the size separation as described by Jackson (1975) to re-
move carbonates, Fe-Mn oxyhydroxides and organic matter.
Removal of the interference phases makes the identification of
I-S peaks easier. The smectites were converted into K-form
using 1 M KCl solution (the samples were treated 3 times
overnight and then washed with distilled water). Two grams
of clay were then put into 20 ml of sodium chloride solutions
of the following concentrations: 0.05, 0.1, 1, 2 and 5 M.
A gentle ultrasonic treatment, not exceeding 1 minute, was ap-
plied for better sample disintegration. The samples were dried
at 60 °C with occasional stirring. 20 ml of distilled water was
added to the dry sample for each subsequent WD cycle. When
a certain number of cycles (5, 20, 50 and 100) was completed,
aliquots of the samples were taken for XRD investigation. Ex-
changeable cations (K
) were replaced by treating the
samples with 0.05 M SrCl
solution overnight (Eberl et al.
1986), washed with deionized water and dialyzed to remove
the excess salts. Oriented specimens were prepared by settling
water suspensions on glass slides. The XRD of air-dried and
ethylene-glycolated specimens were carried out using a Phil-
ips 1075 diffractometer with Ni filter and Cu-K
Results and interpretations
The degree of the K-fixation, in other words the percentage
of smectite layers collapsed, was determined by the peak posi-
tion method described by rodoñ (1980, 1981). The experi-
mental XRD data were also compared to simulated patterns
using the NEWMOD computing program (Reynolds 1985).
The thickness of the ethylene-glycol complex for each sample
was determined to avoid the possible error in the expandabili-
ty measurement as documented by rodoñ (1980). The glycol
complex thickness of the studied samples varied between
1.641.73 nm. The percentages of smectite layers in mixed-
layer I-S (expandability) were determined in Sr exchanged,
glycolated samples after 5, 20, 50 and 100 WD cycles. The
XRD patterns of starting samples (K-smectites) are shown in
Fig. 1. The effect of K-fixation is also reflected in the thick-
ness of the coherent scattering domains. Fig. 2 shows how
samples with the highest degree of K-fixation affect the sharp-
ness of the XRD peaks. All measured data for NaCl environ-
ments of various concentrations are summarized in Table 1.
Generally, a trend of increasing the smectite layer collapse
with the number of WD cycles is observed, as has been docu-
mented in several papers (Eberl et al. 1986; ucha & iráòová
1991; Miklo & Èíèel 1993 etc.). The effect of NaCl concen-
tration on the number of collapsed layers is significant, and
shows consistent evolution (Figs. 3, 4, 5). Low starting NaCl
Fig. 3. The number of WD cycles with the percentage of smectite layers
in the variously concentrated NaCl solutions for the sample SAz-1.
Fig. 2. The evolution of XRD patterns of the sample SAz-1 in the
course of WD cycles in 0.05 M NaCl solution. The uppermost pat-
tern represents the original K-smectite, the lower patterns are Sr-sat-
urated and ethylene-glycolated samples after 5, 20, 50 and 100 WD
Fig. 1. XRD patterns of starting smectites (K-form, ethylene-gly-
Table 1: Percent of smectite layers in mixed layered I-S.
POTASSIUM FIXATION IN SMECTITES 263
concentrations (0.05 M, 0.1 M) lead to very fast collapse of
smectite layers and high concentrations (2 M, 5 M) prevent
collapse. Two bentonite samples, SAz-1 and STx-1, show
very similar trends for all samples that differ only in the num-
ber of collapsed layers. The number is constantly higher for
SAz-1 than for STx-1. This observation is most probably relat-
ed to the higher layer charge of SAz-1 (Eberl et al. 1986;
ucha & iráòová 1991). The maximum proportions of col-
lapsed layers after 100 WD cycles are 60 % for SAz-1 and
41 % for STx-1 (in the presence of 0.05 M NaCl solution). On
Fig. 6. The percentage of collapsed layers after 100 WD cycles in
the different NaCl solutions for the 3 studied specimens.
Fig. 5. The number of WD cycles with the percentage of smectite
layers in the variously concentrated NaCl solutions for the sample
Fig. 4. The number of WD cycles with the percentage of smectite
layers in the variously concentrated NaCl solutions for the sample
STx-1. Note the different scale of the ordinate.
the contrary, the minimum numbers are 15 % and 5 % for the
same samples when 5 M NaCl solution was added before WD
Sample Ban 4/1 (buried altered volcanoclastics) contains
more mineral phases (original illite-smectite, detrital illite and
chlorite), so it was much more difficult to determine the ratio
between expandable and collapsed layers. The sample shows
the same general trend, but behaves differently compared to
pure bentonite samples. Most of the changes take place within
the first 5 WD cycles, and the effect of NaCl is not the same as
for the pure samples. First of all, the different concentrations
have only a small impact on the collapsing process. Even so,
higher NaCl concentration slows down collapse and lower
NaCl concentrations enhance the process (Fig. 5), but the ef-
fect is subdued. Whereas the difference between the maximum
and minimum effect of NaCl reaches 45 % and 36 %, respec-
tively for the pure bentonite samples, it is only 12 % for the
Ban 4/1 sample (Fig. 6).
1. A clear effect of NaCl salt on the degree of K-fixation in
K-smectite during WD cycles was documented. The highest
NaCl concentrations slow down the K-fixation process, and,
on the contrary, low concentrations significantly enhance the
2. The first 5 to 20 cycles are the most important for the fix-
ation and most of the potassium is fixed during this time.
3. The NaCl effect was much less significant when the al-
ready slightly illitized sample with admixtures of other miner-
al phases was used.
4. The interactions between the K-smectite collapse during
the WD cycles and NaCl of different concentrations seems to
be a complex process difficult to explain. Further study is
needed to explain all these effects.
5. The fact of different effects of salt concentrations on the
K-fixation process may have significant consequences for the
use of illite-smectite as a paleotemperature indicator in geo-
logical processes as well for the use of bentonite barriers to
protect the environment.
Acknowledgments: This study was partly supported by Sci-
entific Grant Agency of Ministry of Education of SR, Project
No. 1/8204/01 and Comenius University Grant, Project No.
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