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Quantification of illite and smectite andtheir layer charges in sandstones and shales
from shallow burial depth
J. SRODON
Institute of Geological Sciences PAN, Senacka 1, 31002 Krakow, Poland
(Received 29 October 2008; revised 8 June 2009; Editor: Javier Cuadros)
ABSTRACT: Precise measurement of the content and the layer charge of illite and smectite is an
important aspect of the mineralogical calibration of geophysical well logs and of the evaluation of
mechanical and chemical properties of sedimentary rocks. A technique for obtaining such
measurements was developed during mineralogical studies of the Miocene clastic rocks from the
Carpathian Foredeep. X-ray diffraction (XRD), chemical, cation exchange capacity (CEC), H2O
sorption, and ethylene glycol monoethyl ether (EGME) sorption data were obtained for 65 samples of
sandstones, shales and carbonates. The illite + smectite sum, involving all detected 2:1 minerals
(smectite, illite-smectite, illite, glauconite and muscovite) was measured by XRD. The content of the
illitic component was evaluated separately using % K2O and accounting for % K2O in K-feldspar.
The content of the smectitic component was estimated from EGME retention using the reference data
for a smectite standard, and from CEC assuming the smectitic layer charge of 0.41/O10(OH)2 (Srodon
et al., in press). All these measurements produced very consistent results.
KEYWORDS: CEC, charge density, EGME sorption, illite, illite-smectite, layer charge, smectite, specific surfacearea, XRD, water sorption.
Total specific surface area (TSSA, m2/g) and cation
exchange capacity (CEC, mEq/100 g) are para-
meters of clastic rocks routinely measured by the
oil, construction and environmental industries, as
they are indicators of rock properties, such as
electrical conductivity, mechanical strength, swel-
ling in water, sorption, etc. In common clastic
rocks, the TSSA and CEC are controlled by the
dioctahedral 2:1 clay component (i.e. the sum of
smectite, illite-smectite, discrete illite, glauconite
and micas) which (1) is characterized by a large
number of charged surfaces when compared with
other common minerals including clay minerals
(kaolinite, chlorite; e.g. Tiller & Smith, 1990) and
(2) is the dominant clay component of such rocks
(e.g. Srodon et al., 2001). Thus the measurement of
TSSA and CEC of a common clastic rock is
effectively the measurement of the properties of its
dioctahedral 2:1 clay component. This rule does not
hold if other components with high TSSA, such as
trioctahedral expandable clays, zeolites, opal A, or
organic matter, are present in significant amounts.
CEC and TSSA are usually considered separately
(compare reviews in Bergaya et al., 2006) despite
the fact that these values are related, as CEC is the
amount of charge on TSSA. If smectitic surfaces are
dominant, the relationship between CEC and TSSA
is controlled by the smectitic layer charge. This
relationship has been studied recently for pure
smectites (Srodon and McCarty, 2008) and for
illite-smectites from K-bentonites (Srodon et al.,
2009). Different results have been obtained for the
two clay-mineral groups. There is no unique* E-mail: [email protected]: 10.1180/claymin.2009.044.4.421
ClayMinerals, (2009) 44, 421–434
# 2009 The Mineralogical Society
relationship for pure smectites, which display highly
variable charge density, reflected in the classification
of clay minerals (Guggenheim et al., 2006), while
illite-smectites seem to be characterized by a unique
value of the smectitic layer charge, Qs = �0.41/O10(OH)2, and by the illitic layer charge close to
mica, Qi = �0.95/O10(OH)2. It is not known if thischaracteristic can be extrapolated to regular clastic
rocks containing heterogeneous 2:1 clay assem-
blages. The answer to this question is crucial for
quantifying separately the illite and the smectite
contents of clastic rocks from their chemical
characteristics, a task which is very important for
many applications, but has proven extremely
difficult to perform precisely using X-ray diffraction
(XRD). Even the most accurate contemporary XRD
methods of quantitative mineral analysis of rocks are
capable of measuring only the sum of illite +
smectite, reported as the sum of 2:1 clays (e.g.
Srodon et al., 2001; Omotoso et al., 2006).
An attempt to solve these problems is presented
in the current study, which is part of a wider project
aimed at calibrating geophysical log responses from
Miocene clastic rocks of the Carpathian Foredeep
Basin, located in SE Poland (Zorski et al., 2000).
These rocks contain a clay fraction dominated by an
illite-smectite + illite assemblage (Srodon, 1984;
Ratajczak et al., 1993; Dudek, 2001). The symbols
used in this paper are defined in the appendix.
MATER IALS AND METHODS
Sixty five samples of clastic rocks, covering the
entire compositional range from clean quartz
sandstones (occasionally enriched in calcite) to
claystones, were taken from the 220�1040 m
deep sections of three boreholes in the Dzikow
gas field (Table 1). These rocks represent a
Miocene molasse that was deposited on a subsiding
platform as basin-floor fans and deltas (Dziadzio,
2000) in front of prograding flysch nappes (Fig. 1).
The sampled intervals did not experience burial
illitization, which starts in this basin at ~1700 m
(Dudek, 2001). Throughout the entire basin, the
clay fraction is dominated by detrital randomly
interstratified mixed-layer illite-smectite and
discrete illite, with minor admixtures of kaolinite
and chlorite (Srodon, 1984; Ratajczak et al., 1993;
Dudek, 2001). In the northern shallow part of the
basin, with the most condensed profiles, numerous
bentonitic horizons are present (Srodon, 1984) and
authigenic crystallization of smectite from volcanic
glass has been documented (Ratajczak et al., 1993).
In the study area (southern part of the basin,
FIG. 1. Location of the Dzikow wells analysed in this study, and other investigated wells, for which the findings
of this study apply.
422 J. Srodon
characterized by much higher sedimentation rates),
the volcanoclastic component is much more diluted,
though present as bentonites and tuffites
(Parachoniak, 1962). Glauconite was reported in a
study of Miocene sandstones from the area
(Kuberska et al., 2008).
The rocks were crushed in a hand mortar to pass
a 0.4 mm sieve, and were divided with a hand
splitter into portions for XRD and chemical
analyses. Quantitative XRD mineral compositions
were obtained for all samples using ZnO-spiked
random preparations and the QUANTA computer
program (Mystkowski et al., 2002; Omotoso et al.,
2006), based on the method of Srodon et al. (2001).
Only selected data were used in this paper.
Sixteen representative samples were treated
chemically to remove carbonates, organic matter
and Fe oxides (Jackson, 1975). After removing the
excess electrolyte by centrifugation followed by
dialysis, the <0.2 mm fractions were separated and
analysed by XRD as oriented preparations
(10 mg clay cm�2) solvated with ethylene glycol
in order to identify the clay assemblage.
A portion of the crushed bulk rock was Ca-
saturated by four exchanges with 1 N CaCl2. It was
purified by centrifugation followed by dialysis
(monitored by a conductometer) and was finally
freeze-dried for use in CEC, and H2O and EGME
sorption measurements following the method of
Srodon et al. (2009). Ca-exchanged samples were
equilibrated overnight at 47% RH over a saturated
solution of lithium nitrate. Then all samples were
weighed and replaced in the desiccator. They were
then placed in a programmable thermobalance,
where the weight of H2O, which was released
during heating to 200ºC and then isothermal heating
at 200ºC for 0.5 h, was measured. The hot sample
was immediately placed in the desiccator used for
the EGME determination. After equilibrating all
samples with EGME (three subsequent 24 h
sessions with intermediate control weighing), their
weights were remeasured. By using this approach,
the retention of both H2O and EGME (expressed in
mg liquid g�1 sample) can be referred to the same
weight of sample obtained at 200ºC, very close to
the absolutely dry weight, and thus independent of
changes related to relative humidity (Srodon &
McCarty, 2008). Finally, CEC was measured by the
Co-hexamine technique of Orsini & Remy (1976)
and Bardon et al. (1983) on the same sample split
and referred to the same weight at 200ºC. The
optimum Co-hexamine solution concentration was
estimated from the sorption data. Twelve samples
with the greatest organic carbon contents were
heated at 110ºC in order to test if the heating at
200ºC affects CEC due to thermal alteration of the
organic matter (A. Derkowski, pers. comm.). No
such effects were observed; CEC and EGME values
obtained after 110ºC heating were only slightly
lower, due to the higher reference weight resulting
from water remaining in the clay at 110ºC (see
Srodon & McCarty, 2008).
Major element analysis was carried out by the
WDXRF technique at SGS Minerals, Canada, on
glass discs obtained by lithium tetraborate/lithium
metaborate fusion. NIST 70a (potassium feldspar)
and 76a (burnt refractory) standards were used to
check the accuracy of these analyses. Only the K2O
data were used in this study (Table 1).
RESULTS
Mineral composition
Figure 2a presents representative XRD patterns
of the bulk rocks and Fig. 2b the patterns of
oriented glycolated preparations of <0.2 mm frac-
tions, both illustrating the compositional variability
encountered in the rock sequence under examina-
tion. The bulk rock analysis classified the rocks into
three major groups: shales, sandstones and carbo-
nates. All three contain the same suite of minerals,
present in different proportions; quartz, K-feldspar,
plagioclase, anatase, calcite, dolomite/ankerite,
siderite, pyrite, dioctahedral 2:1 minerals, chlorite
and kaolinite. The sandstones contain corundum
contamination from the grinding process.
The <0.2 mm fractions of all shales and most
sandstones present very stable characteristics and
proportions of minerals (sample 6733 in Fig. 2b);
randomly interstratified illite-smectite with ~55%
SXRD (measured by the method of Srodon, 1981),
fine-grained discrete illite, chlorite and kaolinite. In
one intermediate sandstone (sample 6748 in Fig. 2),
a similar illite-smectite was encountered, but the
remainder of the sample contained different
proportions and characteristics of other minerals,
including greater contents of quartz and coarse-
grained (narrow peaks) illite, chlorite and kaolinite.
The most quartz-rich sandstones and calcite-rich
carbonates from the bottom of the profiles contain a
different mineral suite � one that includes more
illitic illite-smectite (~46% SXRD, random, in
sample 6744 and ~25% SXRD, ordered, in sample
Quantification of illite and smectite 423
TABLE1.XRD(%
K-feldspar,and%dioctahedral2:1clays)andchemical(%
K2O,CEC,EGMEretention,andH2Oretention)datafortheDzikowsamplesusedinthis
study.
Sample
no.
Depth
below
surface
%Ksp
%2:1
clays
%K2O
CECmeas
EGME
retention
H2O
retention
CECcorrCECcorr2
TSSA
(EGME)
TSSA
(CEC)
TSSA
(mean)
f s(EGME)
f s(CEC)
f smean
BW
f i%S
(m)
(wt.%)
(wt.%)
(wt.%)
(mEq/
100g)
(mgg�1)(mgg�1)
(mEq/
100g)
(mEq/
100g)
(m2g�1)(m
2g�1)(m
2g�1)
(wt.%)
(%)
6727
475
336
2.41
22.62
61.80
42.50
23.19
23.26
151
160
155
0.20
0.21
0.21
3.54
0.15
57
6728
475
521
2.17
14.37
36.90
26.70
14.73
14.76
90
101
96
0.12
0.13
0.13
2.18
0.08
60
6729
738
336
2.85
23.38
67.40
45.60
23.96
24.05
164
165
165
0.22
0.22
0.22
3.75
0.14
60
6732A
817
332
2.23
12.76
35.80
25.90
13.08
13.10
87
90
89
0.12
0.12
0.12
2.02
0.20
37
6732B
817
510
1.69
5.29
15.40
15.70
5.42
5.43
38
37
37
0.05
0.05
0.05
0.85
0.05
49
6733
817
245
2.86
23.69
68.80
53.00
24.28
24.37
168
167
168
0.22
0.22
0.22
3.82
0.23
49
6734
817
413
1.66
5.60
13.40
12.70
5.74
5.74
33
39
36
0.04
0.05
0.05
0.82
0.08
37
6738
910
334
2.83
20.98
59.80
45.80
21.50
21.57
146
148
147
0.19
0.20
0.19
3.35
0.15
57
6740
945
0.4
50.41
1.00
3.70
9.80
1.03
1.03
97
80.01
0.01
0.01
0.18
0.04
21
6741
945
15
0.47
1.57
4.20
7.60
1.61
1.61
10
11
11
0.01
0.01
0.01
0.24
0.04
28
6742
945
15
0.42
1.58
4.70
7.70
1.62
1.62
11
11
11
0.02
0.01
0.01
0.26
0.04
30
6743
945
0.6
80.48
1.20
4.90
10.30
1.23
1.23
12
810
0.02
0.01
0.01
0.23
0.07
17
6744
945
16
0.49
1.63
6.60
9.40
1.67
1.67
16
11
14
0.02
0.02
0.02
0.31
0.04
30
6745
945
14
0.42
1.62
7.60
9.90
1.66
1.66
19
11
15
0.02
0.02
0.02
0.34
0.02
49
6746
945
29
0.83
2.78
8.80
12.00
2.85
2.85
21
20
21
0.03
0.03
0.03
0.47
0.06
30
6773
1046
05
0.31
2.05
10.60
11.00
2.10
2.10
26
14
20
0.03
0.02
0.03
0.46
0.02
53
6774
1046
0.3
30.34
2.43
12.40
16.40
2.49
2.49
30
17
24
0.04
0.02
0.03
0.54
0.00104
6775
1046
0.7
70.24
2.12
5.20
16.30
2.17
2.17
13
15
14
0.02
0.02
0.02
0.31
0.05
26
6776
1046
0.5
60.18
2.17
5.10
10.30
2.22
2.22
12
15
14
0.02
0.02
0.02
0.32
0.04
31
6702
555
236
2.39
18.03
52.30
42.70
18.48
18.53
128
127
127
0.17
0.17
0.17
2.90
0.19
47
6703
555
314
1.64
6.54
17.30
19.80
6.70
6.71
42
46
44
0.06
0.06
0.06
1.01
0.08
42
6705
555
332
2.33
15.20
42.90
34.40
15.58
15.62
105
107
106
0.14
0.14
0.14
2.41
0.18
44
6707
601
318
1.84
7.74
22.50
21.80
7.93
7.94
55
55
55
0.07
0.07
0.07
1.25
0.11
40
6708
601
422
2.11
11.19
33.50
27.90
11.47
11.49
82
79
80
0.11
0.10
0.11
1.83
0.11
48
6709
601
430
2.28
14.31
46.00
28.40
14.67
14.70
112
101
107
0.15
0.13
0.14
2.43
0.16
47
6710
601
325
2.1
11.46
34.70
28.40
11.75
11.77
85
81
83
0.11
0.11
0.11
1.88
0.14
44
6711
641
416
1.7
6.25
17.30
15.30
6.41
6.41
42
44
43
0.06
0.06
0.06
0.98
0.10
36
6712
641
241
2.7
23.39
65.40
44.80
23.97
24.06
160
165
162
0.21
0.22
0.21
3.70
0.20
52
6713
641
49
1.44
2.95
10.90
10.40
3.02
3.03
27
21
24
0.04
0.03
0.03
0.54
0.06
35
6714
641
424
2.25
13.93
40.50
37.70
14.28
14.31
99
98
99
0.13
0.13
0.13
2.24
0.11
54
6715
641
324
1.99
11.43
28.20
23.70
11.72
11.74
69
81
75
0.09
0.11
0.10
1.70
0.14
41
6716
641
332
2.37
17.52
43.70
33.50
17.96
18.01
107
124
115
0.14
0.16
0.15
2.62
0.17
48
6717
641
412
1.31
3.07
9.50
11.40
3.15
3.15
23
22
22
0.03
0.03
0.03
0.51
0.09
25
6718
641
412
1.65
6.00
16.90
17.70
6.15
6.16
41
42
42
0.05
0.06
0.06
0.95
0.06
46
6719
680
332
2.22
14.23
39.20
29.80
14.59
14.62
96
100
98
0.13
0.13
0.13
2.23
0.19
40
6720
680
322
2.07
7.11
21.80
21.20
7.29
7.30
53
50
52
0.07
0.07
0.07
1.18
0.15
31
6721
765
318
1.69
5.59
17.20
14.60
5.73
5.73
42
39
41
0.06
0.05
0.05
0.93
0.13
30
6722
765
234
2.3
13.67
40.20
41.60
14.01
14.04
98
96
97
0.13
0.13
0.13
2.21
0.21
38
6723
765
339
2.45
15.95
42.90
34.20
16.35
16.39
105
113
109
0.14
0.15
0.14
2.47
0.25
37
6724
765
326
2.41
17.18
49.70
33.60
17.61
17.65
121
121
121
0.16
0.16
0.16
2.76
0.10
62
6806
641
49
1.39
3.39
12.40
11.50
3.47
3.48
30
24
27
0.04
0.03
0.04
0.62
0.05
40
6807
680
328
2.06
11.48
34.20
25.60
11.77
11.79
83
81
82
0.11
0.11
0.11
1.87
0.17
39
6808
680
415
1.7
5.59
18.00
16.40
5.73
5.73
44
39
42
0.06
0.05
0.06
0.95
0.09
37
6809
680
329
2.15
13.46
38.30
28.90
13.80
13.82
93
95
94
0.12
0.13
0.12
2.14
0.17
43
6810
641
38
1.1
2.18
8.20
8.70
2.23
2.24
20
15
18
0.03
0.02
0.02
0.40
0.06
29
6748
225
319
1.75
6.89
21.20
18.20
7.06
7.07
52
49
50
0.07
0.06
0.07
1.14
0.12
35
6749
225
331
2.1
12.47
36.40
27.70
12.78
12.81
89
88
88
0.12
0.12
0.12
2.01
0.19
38
6750
225
325
2.13
11.79
31.50
30.00
12.08
12.11
77
83
80
0.10
0.11
0.11
1.82
0.14
42
6752
465
341
2.66
22.05
70.20
45.70
22.60
22.68
171
156
163
0.23
0.21
0.22
3.72
0.19
53
6753
465
346
2.62
19.97
61.30
53.00
20.47
20.53
150
141
145
0.20
0.19
0.19
3.31
0.27
42
6754
465
343
2.71
20.88
62.70
44.20
21.40
21.47
153
147
150
0.20
0.19
0.20
3.42
0.23
46
6755
735
325
2.06
9.11
26.10
20.80
9.34
9.35
64
64
64
0.08
0.08
0.08
1.46
0.17
34
6756
735
427
2.06
10.49
27.30
24.40
10.75
10.77
67
74
70
0.09
0.10
0.09
1.60
0.18
34
6757
735
222
1.87
7.12
23.20
20.30
7.30
7.31
57
50
53
0.07
0.07
0.07
1.22
0.15
32
6758
735
514
1.76
6.17
18.00
15.60
6.32
6.33
44
43
44
0.06
0.06
0.06
0.99
0.08
41
6759
735
311
1.26
3.07
9.80
8.80
3.15
3.15
24
22
23
0.03
0.03
0.03
0.52
0.08
27
6760
735
320
1.8
6.80
20.90
19.00
6.97
6.98
51
48
49
0.07
0.06
0.07
1.13
0.13
33
6761
735
319
1.81
8.41
24.00
16.60
8.62
8.63
59
59
59
0.08
0.08
0.08
1.34
0.11
41
6762
735
245
2.85
23.00
63.80
46.80
23.58
23.66
156
162
159
0.21
0.21
0.21
3.62
0.24
47
6764
835
331
2.27
10.75
33.90
26.10
11.02
11.04
83
76
79
0.11
0.10
0.10
1.80
0.21
34
6765
835
239
2.5
16.99
49.70
45.40
17.41
17.46
121
120
121
0.16
0.16
0.16
2.75
0.23
41
6766
835
242
2.89
24.49
67.60
51.60
25.10
25.19
165
173
169
0.22
0.23
0.22
3.85
0.20
53
6767
835
334
2.43
15.13
42.20
29.70
15.51
15.54
103
107
105
0.14
0.14
0.14
2.39
0.20
41
6768
835
335
2.7
17.79
53.50
40.80
18.23
18.28
130
126
128
0.17
0.17
0.17
2.92
0.18
48
6769
835
338
2.45
16.27
45.80
37.50
16.68
16.72
112
115
113
0.15
0.15
0.15
2.58
0.23
39
Keyparameterscalculatedfromthesedata:correctedvaluesofCEC(seetextfordetails),totalspecificsurfacearea(TSSA)andfractionofsmectiteintherock(fs),both
calculatedfromEGMEretention(equation1)andCEC(equation2),boundwaterasapercentageoftherock+watermass(BW),fractionofilliteintherock(fi)calculated
from%K2O,and%smectiteinthe2:1clay(%
S),calculatedfromthemeanf sandf i.
7774), much kaolinite and fine-grained Fe-rich
(weak 002 peak) discrete illite (glauconite?).
Table 1 lists mineral contents from QUANTA for
K-bearing minerals used in the interpretations
presented below; % K feldspar and % dioctahedral
2:1 clay fraction.
Smectitic layer charge and rock TSSA
calculated from CEC and sorption
measurements
The CEC and the retention data are presented in
Table 1. The measured CEC values were corrected
proportionally for the residual water left on clay at
200ºC (CECcorr in Table 1) using the mean value of
1.5% of the mass at 200ºC, established by Srodon
& McCarty (2008, tables 3 and 4) for pure smectite,
and additionally for the incomplete exchange of Ca
in the Co-hexamine procedure (CECcorr2 in
Table 1), according to the data of Srodon et al.
(2009) obtained for pure illite-smectites. A multi-
plication factor of 1.025, appropriate for the illite-
smectite composition of the studied clays, was used.
The three sets of data given in Table 1 (CEC,
H2O and EGME retention) correspond to the same
TSSA of the rock, dominated by the contribution
from the illite + smectite fraction. If we assume that
the contribution from other rock components is
negligible, and if the amount of charge per surface
area, i.e. smectitic layer charge Qs (per O10(OH)2),
and H2O and EGME coverages (the masses of
molecules per unit surface of the sample, in
mg m�2) do not change among samples of the
analysed sample set, the plots of CEC vs. retention
should be linear and should extrapolate to zero, as
they are linear functions of the same value (TSSA).
These two plot characteristics are satisfied for the
CEC-EGME pair (Fig. 3a) but not for the CEC-H2O
pair (Fig. 3b). Figure 3a indicates that in the case of
EGME both the coverage is stable and the
FIG. 2 (above and facing page). XRD patterns of the bulk rock (a; powder, side-loaded) and the <0.2 mm fractions
(b; Na-exchanged, oriented, glycolated), illustrating the mineralogical variability encountered in the studied
rocks. IS, mixed-layer illite-smectite; I, illite; K, kaolinite; Ch, chlorite; Q, quartz; Pl, plagioclase. Numbers are
sample labels (Table 1). Percent smectite in illite-smectite measured by XRD (% SXRD in Table 3) is listed along
with the sample number in part b. Cu-Ka radiation.
426 J. Srodon
contribution from non-charged surfaces is negli-
gible. If this contribution was significant, we should
observe that EGME retention >0 for CEC = 0. In
the case of H2O (Fig. 3b), the apparent coverage is
too large for low CEC values. These conclusions
are supported by Fig. 4, in which EGME retention/
CEC and H2O retention/CEC ratios are plotted as a
function of CEC. The data at CEC<5 mEq/100 g
are not reliable, as they are ratios of very small
numbers with large relative random errors. Above
CEC = 5 mEq/100 g the former ratio is stable,
while the latter increases for small CEC. The origin
of extra H2O observed in the small-CEC samples is
not apparent; this excess may indicate higher
coverage or may arise from non-charged surfaces
or from capillary porosity.
Another difference between H2O and EGME is
much better correlation of EGME retention with
CEC (Fig. 3a,b). These results indicate that both
CEC and EGME retention can be used to evaluate
the TSSA of the rock, while H2O retention is not
suitable for this purpose.
In Fig. 5 the data from Fig. 3a characterizing
rocks, i.e. mineral mixtures with discrete illite and
illite-smectite components, are plotted together with
the data of Srodon et al. (2009), obtained for
monomineral illite-smectites from K-bentonites.
The Dzikow rock values are much smaller
(EGME retention <80 mg/g) than the values for
monomineral illite-smectite. The randomly inter-
stratified illite-smectites from K-bentonites (CEC
>70 mEq/100 g) plot on the regression line
FIG. 3. Retention of EGME (a) and H2O (b) with respect to the CEC.
Quantification of illite and smectite 427
established for the Dzikow samples. The ordered
illite-smectites (18 < CEC < 70 mEq/100 g) plot
below, because they contain an excess of EGME
(see the detailed discussion of these data by Srodon
et al., 2009). The slope of EGME retention vs. CEC
plot is controlled by EGME coverage and Qs,
because these two parameters relate EGME
retention and CEC to the same surface (TSSA;
see explanations below). The equal slopes allow for
the hypothesis that the Qs and EGME coverage
values (0.41/O10(OH)2 and 0.41 mg m�2, respec-
tively), established for the pure randomly inter-
stratified illite-smectites by Srodon et al. (2009)
may apply to the Dzikow rock samples.
TSSA calculated from the EGME retention as the
ratio of retention to coverage is:
TSSA ðEGMEÞ ¼ EGME retention
EGME coverageð1Þ
To calculate TSSA from CEC, equation 11 of
Srodon & McCarty (2008) is applied:
TSSA ðCECÞ ¼ CEC� b2o � 3:477Qs
ð2Þ
where bo is the unit cell dimension of smectite (in
nm) and the number (3.477) results from the units
used. The typical value of smectitic unit cell, bo =
0.9 nm, was used in the calculations. Feasible
changes of this parameter have little effect on the
final results. TSSA (EGME) from equation 1 was
plotted against TSSA (CEC) from equation 2 in
Fig. 6. The correlation obtained demonstrates that
both calculations produce similar values of the rock
TSSA, confirming the validity of the assumption
based on Fig. 5.
Equations 1 and 2 can be combined to produce
the analytical equivalent of the experimental
regression from the data in Fig. 3a. If bo =
0.9 nm is assumed then:
CEC ¼
0:36� EGME retentionQs
EGME coverageð3Þ
Comparison of equation 3 and the regression
from Fig. 3a leads to the conclusion that the
experimental relationship of CEC and EGME
retention indicates not only that Qs and EGME
retention are stable values but also that their ratio is
close to one for the applied set of units. Thus the
values assumed by analogy to pure illite-smectites
(0.41/O10(OH)2 and 0.41 mg m�2) do not offer a
unique solution, and independent evidence is
required to check if they are correct. The XRD
FIG. 4. Ratios of EGME and of H2O retention to CEC
plotted vs. CEC, serving as an indicator of the mineral
composition of the rock. Note that the ratio is stable for
EGME (except for the clean sandstones (CEC <5 mEq/
100 g), where the relative errors of both CEC and
EGME measurements are large), and variable for H2O.
FIG. 5. CEC vs. EGME retention data for Dzikow rocks
(diamonds) plotted together with the analogous data
obtained for pure illite-smectites (squares) by Srodon
et al. (2009). Note that the most smectitic illite-
smectite samples (CEC > 70 mEq/100 g) plot on the
straight regression line established for the Dzikow
samples.
FIG. 6. Comparison of the total specific surface area
(TSSA) values of the bulk rock evaluated indepen-
dently from CEC and from EGME retention data by
assuming the values of smectitic layer charge and of
EGME coverage of Srodon et al. (2009).
428 J. Srodon
data are used as the test in the last section of
Results.
Equation 2 can be used also to calculate Qs from
CEC and from TSSA obtained from equation 1. The
resulting values converge to Qs = 0.41�0.42 for thelarge-CEC samples (Fig. 7). Greater errors are
observed for very small CEC and EGME retention
values, for which the relative errors of both
measurements are greater.
Fraction of smectite in the rock and water
bound to smectitic surfaces calculated from
CEC and EGME retention measurements
The fraction of smectite in the rock (fs) can be
obtained from the measured TSSA, and dividing it
by the TSSA of pure smectite. In order to carry out
this calculation accurately, the TSSA of smectite
with the same Fe content, which implies also the
same bo, has to be used. It was shown earlier that
the TSSA of smectite is linearly correlated with
these two values (e.g. Srodon & McCarty, 2008).
The value of 757 m2 g�1, corresponding to bo =
0.9 nm, was used. This calculation produces a slight
overestimation of TSSA because it assumes that the
rock grain density is equal to dry smectite density
(~2.76 g cm�3), while most often it will be smaller,
because of the lower density of quartz
(2.65 g cm�3). The TSSA obtained from both
EGME retention and from CEC were used in the
calculation, producing very similar results
(Table 1). The smectite defined by this calculation
is a measure of all charged rock surfaces, whichever
minerals they represent; discrete smectite and illite-
smectite crystals (both internal and external
surfaces) or fine illite or glauconite crystals
(external surfaces). In other words, the fraction of
smectite in the rock measured in this way (fs)
corresponds to the portion of the dioctahedral 2:1
fraction that has the ability to adsorb EGME and
exchange cations. The remaining portion of the
dioctahedral 2:1 fraction is the fraction of illite in
the rock (fi) which contains fixed cations and does
not adsorb EGME.
If water is present in smectite only as a
monomolecular layer on the surface, it accounts
for 17.24% of the mass of such hydrated smectite
(Srodon & McCarty, 2008), which can be
considered as the rock with fs = 1. Multiplying
17.24% by actual fs of the rock gives an estimate of
the mass of water bound to the rock surface (BW:
Table 1). This parameter, available from fs, is
interesting in particular for the interpretation of
geophysical log data, because it permits prediction
of the minimum fraction of porosity filled with
water, and thus unavailable for hydrocarbons.
Quantification of illite in the rock based on
CEC, EGME retention, XRD and % K2O
The bulk rock parameter fs calculated from CEC
and EGME retention can be applied to obtain an
improved characterization of the composition of a
dioctahedral 2:1 clay, which has been quantified by
XRD as one component (% 2:1 clays in Table 1).
By subtracting fs from % 2:1 clays/100, the fraction
of illite in the rock (fi in Table 1) is obtained. The
overall layer composition of the 2:1 fraction
(% smectite in 2:1 clays in Table 1, % S (CHEM
+ XRD) in Fig. 8) can be established from the two
numbers. Figure 8 demonstrates that % S (CHEM +
XRD) evolves systematically with lithology from
~30% for sandstones (low CEC) to ~50% for shales
(large CEC).
FIG. 7. Smectitic layer charge (Qs) evaluated using
CEC, TSSA and equation 1, and plotted vs. CEC.
FIG. 8. Percentage of smectite in 2:1 clays evaluated by
two techniques and plotted vs. rock composition
represented by CEC. % S (CHEM + XRD) from
QUANTA measurement of illite + smectite and EGME
measurement of smectite. % S (CHEM) from EGME
measurement of smectite and K2O measurement of
illite (see text for details).
Quantification of illite and smectite 429
The values of % S (CHEM + XRD) are strongly
dependent on the accuracy of the XRD measure-
ment of the % 2:1 clays. In order to evaluate their
accuracy, they were compared with the evaluation
based on the direct measurement of fundamental
particle thickness distribution in the <0.2 mmfraction by HRTEM and XRD. Such data were
available for shales from the same area (table 5 in
Dudek et al., 2002). The relationship % S = 100/
TI+S (Srodon et al., 1992) was utilized. The mean
number of layers of illite fundamental particles (TI)
measured by HRTEM and XRD (pvp technique of
Eberl et al., 1998) and the percent of monolayers
measured by HRTEM were used to calculate the
mean number of layers of all particles (TI+S). The
% S values obtained from this calculation for shales
(columns 7 and 10 in Table 2) are similar whether
TI measured by HRTEM or by XRD are used
(49�56% S), and fall in the same range as the
calculation for shales based on CHEM + XRD
(Fig. 8). Thus an attempt to use the latter data for
the evaluation of the illitic layer charge seems
justified.
In order to to calculate the illitic layer charge, the
contribution of K-feldspar to the K2O budget has to
be subtracted. K-feldspar with 0.1 Na per formula
(the common composition of K-feldspar) contains
15% K2O. This value and % K-feldspar from
QUANTA (Table 1) were used to subtract the
K-feldspar K2O from the total, and the remaining
K2O was assigned to illite. The K2O content of
illite calculated using fi from Table 1 is plotted in
Fig. 9 as a function of the rock composition,
expressed by % 2:1 clays measured by XRD. The
dispersion of results is large, but they converge to
values of about 11�12% K2O for the most clay-rich
samples, for which the relative errors of % 2:1
clays measurement are smallest. The value of
11.7% K2O corresponds to the muscovite interlayer
composition (1.0 K/O10(OH)2). Figure 9 offers an
indication that illite in the clastic rocks under
investigation has an interlayer composition close to
muscovite mica, similar to the illite in K-bentonites
(Srodon et al., 2009), and not the composition
markedly deficient in K, assumed in several earlier
studies reported in the introduction to this paper.
In further considerations, 11% K2O was assumed
for illite, allowing for Qi values slightly lower than
in mica and for some Na and NH4 substitution, as
established by Srodon et al. (2009). If this number
is fixed, then a reverse operation is possible; illite
K2O can be used to calculate the illite content of
the rock (fi). Adding it to fs produces the evaluation
of the 2:1 mineral content of the rock, based on
EGME retention, % K2O and % K-feldspar by
XRD, thus alternative to the direct XRD measure-
ment using QUANTA. There is no systematic shift
between the two data sets (Fig. 10). Such agreement
verifies the original assumptions based on Fig. 5
about the values of Qs and EGME coverage, which
allowed the calculation of fs.
TABLE 2. Calculation of % smectite in the 2:1 clay (% S) based on the HRTEM and XRD data from Table 5 of
Dudek et al. (2002).
Sample Depth(m)
% SXRD % monolayersHRTEM
TIHRTEM
TI+SHRTEM
% S TI pvp TI+S pvp % S
Zl-6 1704 67 62 3.7 2.026 49 3.6 1.988 50RW-1 119 86 63 3.1 1.798 56 3.5 1.95 51
% SXRD; percent smectite in mixed-layer illite-smectite measured by XRD: TI and TI+S; mean numbers of layersin illite fundamental particles and illite + smectite fundamental particles, respectively, measured by HRTEM andXRDpvp techniques.
FIG. 9. % K2O in illite evaluated from chemical and
XRD data and plotted vs. % 2:1 clays.
430 J. Srodon
The fi evaluated from the chemistry can be used
along with fs to obtain an estimate of % S, i.e. of
the layer composition of the illite-smectite fraction
of the rock (% S (CHEM) in Fig. 8), alternative to
the value based on % 2:1 clays from XRD and fs(% S (CHEM + XRD) in Fig. 8). Both calculations
produce the same trends of % S with respect to the
rock composition, represented in the figure by CEC.
The chemistry-based data are less scattered, which
probably reflect greater precision of the % K2O
measurement compared to the % 2:1 clays from
XRD. For very small CEC, both measurements
should be considered as unreliable because of large
relative errors.
The dioctahedral 2:1 clay was characterized
above by the percentage of its smectitic layers
(% S). This is a bulk characterization that regards
the 2:1 clay as one component of the rock. Such an
approach has the advantage of quantifying the
smectitic layers responsible for many important
rock characteristics, in particular CEC and TSSA.
On the other hand, such an approach ignores the
mineral heterogeneity of the 2:1 clay, i.e. the exact
mineralogical location of the smectitic layers.
According to the XRD studies, the 2:1 clay in
sedimentary rocks contains typically at least two
distinguishable minerals, mixed-layer illite-smectite
and discrete illite. This is also the case of the
studied Miocene rocks (Fig. 2b). For many applica-
tions (e.g. studies of diagenesis) quantification of
these components identified by XRD is a very
important issue and is difficult to achieve from the
oriented preparations used for their identification
(e.g. Moore & Reynolds, 1997). Such quantification
can be performed using % S and % SXRD, i.e. the
percent expandable layers in illite-smectite
measured by XRD. As the total number of smectitic
layers in the two-component mixture (2:1 clay) is
known (% S), fractions of the mineral components
of the mixture can be evaluated if their smectitic
contents are known.
In order to carry out such calculations, % SXRDhas to be corrected first for the smectite layers
underestimation that is inherent in the XRD
measurement (Srodon et al., 1992). Figure 10 of
Srodon et al. (2009) can be used to obtain the
corrected value (% SIS in Table 3 and in equation 4
below).
If pIS is defined as the fraction of mixed-layer
illite-smectite in the sum of dioctahedral 2:1
minerals then:
% S = % SIS6pIS + % SI6(1 � pIS)
thus
pIS ¼ % S�% SI% SIS �% SI
ð4Þ
where % SI is percent of smectitic crystal surfaces
of discrete illite (two basal surfaces of an illite
fundamental particle, equivalent to 1 smectite
interlayer per particle; cf. Srodon et al., 1992).
Based on the data of Srodon et al. (2009), the
possible range of % SI is zero (coarse-grained mica:
infinitely thick crystals) to 20 (diagenetic illite: five
layers thick crystals). Table 3 presents the composi-
tion of the 2:1 fraction of the samples for which
% SXRD was available, evaluated using this
approach. The calculation indicates that the clay-
rich rocks are enriched in illite-smectite with
respect to the discrete illite component. This
calculation could be carried out more precisely,
giving a unique number instead of the range of pIS,
if the actual crystal thickness of discrete illite in the
sample was measured directly.
D I SCUSS ION AND CONCLUS IONS
This study revealed characteristics of smectite and
illite from sandstones and shales identical to those
established recently for mixed-layer illite-smectite
from K-bentonites (Srodon et al., 2009), with the
smectite layer charge close to 0.41/O10(OH)2 and the
illite layer charge close to 1.00. This conclusion is
based on Fig. 3a and equation 3, which proves that
Qs and EGME coverages are stable and their ratio
equals one for all studied rocks; also on Fig. 10,
which proves that assuming the EGME coverage of
0.41 mg g�1 and the illite layer charge close to 1.00/
O10(OH)2 produces the contents of 2:1 fraction in
the rock very close to the values measured by XRD.
FIG. 10. Comparison of % 2:1 clays measured by XRD
and evaluated from chemical data.
Quantification of illite and smectite 431
The relationship presented in Fig. 3a was found
to hold for all wells marked in Fig. 1. Thus the
above findings apply at least to a large sector of the
studied Miocene basin. More data from different
basins are needed to check how widespread these
conclusions are. If they apply widely, CEC and
EGME retention can be used to calculate the
surface area of the rock (TSSA), the smectite
content of the rock (fs) and the amount of water
bound to the rock surface as a monomolecular layer
(BW), while % K2O can be used to evaluate the
illite content of the rock (fi) if the amounts and
compositions of other K-bearing phases are known
or negligible. Water coverage is not stable, with a
tendency to increase in rocks of low CEC. It is
much less suitable for the TSSA and related
measurements.
The illite content of the rock (fi) can also be
measured by two independent techniques; (1) from
% K2O in the rock and the XRD measurement of
the % K-feldspar, and (2) from fs and the % 2:1
clay fraction in the rock, measured directly by XRD
(QUANTA program). Both approaches produce
very close results and, when combined with the fsmeasurement, allow for the calculation of the bulk
layer composition (% S) of the 2:1 clay fraction.
Such a calculation accounts for all smectite-type
surfaces in the rock (charged surfaces adsorbing
polar molecules and exchange cations), i.e. both
internal and external surfaces of the mixed-layer
component, as well as the external surfaces of the
discrete illite.
The calculations of fs and fi developed in the
course of this study are new contributions to the
field of quantitative mineral analysis of sedimentary
rocks. In current practice, only the sum of illite and
smectite in the rock has been quantifiable with
acceptable accuracy, as shown by the rules and
results of the Reynolds Cup (e.g. Omotoso et al.,
2006). fs and fi combined with the XRD character-
istics of mixed-layer illite-smectite (% SXRD) allows
also quantification of the components of the 2:1
clay fraction, identifiable by XRD (illite-smectite
and discrete illite). This approach is easier and in
most cases probably more accurate than the
traditional quantification of these components
based on the XRD patterns of oriented preparations.
In the investigated Miocene sediments the bulk
layer composition of 2:1 fraction changes gradually
from ~50% S for shales to ~30% S for sandstones,
while the content of 2:1 clay in the rock decreases
from 45% to 5%. The change of % S is a combined
effect of a more illitic composition and a decreasing
proportion of illite-smectite in the 2:1 fraction of
the sandstones.
ACKNOWLEDGMENTS
This study was financed by the Polish Ministry of
Science and Higher Education within the scientific
network ‘Nuclear methods for borehole geophysics’.
Ms Dorota Bakowska and Małgorzata Zielinska are
thanked for their careful laboratory work, Dr Tadeusz
TABLE 3. Calculation of the proportion of mixed-layer illite-smectite in the 2:1 clays fraction of selected rocks
(pIS) by means of Equation 4. % 2:1 clay and % S are values from Table 1.
Sample % 2:1 clay % S % SXRD % SIS pIS (% SI = 0) pIS (% SI = 20)
6733 45 49 55 81 0.60 0.486729 36 60 49 78 0.77 0.696714 24 55 58 83 0.66 0.556715 24 54 59 84 0.64 0.536750 25 41 54 81 0.51 0.346748 19 35 55 81 0.44 0.256734 13 37 45 74 0.50 0.326744 6 30 46 75 0.40 0.18
% SXRD is the percentage of smectite in the mixed-layer component, measured by XRD, and % SIS is the samevalue after correction for smectitic edges of the mixed-layer crystals, using fig. 10 of Srodon et al. (2009). Twoextreme values of % smectitic edges of the discrete illite crystals are assumed: % SI = 0, corresponding toinfinitely thick crystals, and % SI = 20, corresponding to five-layer crystals. Ranges of feasible pIS are obtainedfrom this calculation.
432 J. Srodon
Kawiak for the XRD determinations and Dr Leszek
Chudzikiewicz for help with the figures. The permis-
sion of Chevron for use their proprietary QUANTA
program is highly appreciated. Thorough comments by
the editor Javier Cuadros, Peter Ryan and an
anonymous reviewer helped to improve the interpreta-
tions and clarify the presentation.
REFERENCES
Bardon C., Bieber M.T, Cuiec L., Jacquin C., Courbot
A., Deneuville G., Simon J.M., Voirin J.M., Espy
M. , Nec toux A. & Pe l l e r in A. (1983)
Recommandations pour la determination experimen-
tale de la capacite d’echange de cations des milieux
argileux. Revue de l’ Institut Francais du Petrole,
38, 621�626.Bergaya F., Theng B.K.G. & Lagaly G. (2006)
Handbook of Clay Science, Elsevier, Amsterdam,
1224 pp.
Dudek T. (2001) Diagenetic evolution of illite/smectite
in the Miocene shales from the Przemysl area
(Carpathian Foredeep). PhD thesis, Institute of
Geological Sciences PAN, Krakow, Poland.
Dudek T., Srodon J., Eberl D.D., Elsass F. & Uhlik P.
(2002) Thickness distribution of illite crystals in
shales. I: X-ray diffraction vs. high-resolution
transmission electron microscopy measurements.
Clays and Clay Minerals, 50, 562�577.Dziadzio P. (2000) Sekwencje depozycyjne w utworach
badenu i sarmatu w SE czesci zapadliska przedkar-
packiego. Przeglad Geologiczny, 48, 1124�1138 (inPolish).
Eberl D.D., Nuesch R., Sucha V. & Tsipursky S. (1998)
Measurement of fundamental particle thicknesses by
X-ray diffraction using PVP-10 intercalation. Clays
and Clay Minerals, 46, 89�97.Guggenheim S., Adams J.M., Bain D.C., Bergaya F.,
Brigatti M.F., Drits V.A., Formoso M.L.L., Galan E.,
Kogure T. & Stanjek H. (2006) Summary of
recommendations of nomenclature committees rele-
vant to clay mineralogy: report of the Association
Internationale pour l’Etude des Argiles (AIPEA)
Nomenclature Committee for 2006. Clay Minerals,
41, 863�877.Jackson M.L. (1975) Soil Chemical Analysis - Advanced
Course. Published by the author, Madison,
Wisconsin, USA.
Kuberska M., Kozłowska A. & Maliszewska A. (2008)
Spoiwa piaskowcow miocenu zapadliska przedkar-
packiego w jego polskiej i ukrainskiej czesci. I
Polish Geological Congress, Krakow, Poland,
Abstracts, p. 61 (in Polish).
Moore D. M. & Reynolds R. C. (1997) X-ray Diffraction
and the Identification and Analysis of Clay Minerals.
Oxford University Press, Oxford-New York, 378 pp.
Mystkowski K., Srodon J. & McCarty D.K. (2002)
Application of evolutionary programming to auto-
matic XRD quantitative analysis of clay-bearing
rocks. The Clay Minerals Society 39th Annual
Meeting, Boulder, Colorado, Abstracts with
Programs.
Omotoso O., McCarty D.K., Hillier S. & Kleeberg R.
(2006) Some successful approaches to quantitative
mineral analysis as revealed by the 3rd Reynolds Cup
contest. Clays and Clay Minerals, 54, 748�760.Orsini L. & Remy J-C. (1976) Utilisation du chlorure de
cobaltihexammine pour la determination simultanee
de la capacite d’echange et des bases echangeables
des sols. Science du Sol, 4, 269�275.Parachoniak W. (1962) Miocene pyroclastic horizons of
the Carpathian Foredeep in Poland. Prace
Geologiczne Komisji Nauk Geologicznych PAN,
Oddział w Krakowie, 11, 7�77 (in Polish).Ratajczak T., Gorniak K., Bahranowski K. & Szydlak T.
(1993) Clay minerals as evidence of volcanic activity
during the Miocene sedimentation in the NE part of
the Carpathian Foredeep (Poland). Geologica
Carpathica � Clays, 2, 81�92.Srodon J. (1981) X-ray identification of randomly
interstratified illite/smectite in mixtures with discrete
illite. Clay Minerals, 16, 297�304.Srodon J. (1984) Illite/smectite in low-temperature
diagenesis: data from the Miocene of the
Carpathian Foredeep. Clay Minerals, 19, 205�215.Srodon J. & McCarty D.K. (2008) Surface area and
layer charge of smectite from CEC and EGME/H2O
retention measurements. Clays and Clay Minerals,
56, 155�174.Srodon J., Elsass F., McHardy W.J. & Morgan D.J.
(1992) Chemistry of illite-smectite inferred from
TEM measurements of fundamental particles. Clay
Minerals, 27, 137�158.Srodon J., Drits V.A., McCarty D.K., Hsieh J.C.C. &
Eberl D.D. (2001) Quantitative XRD analysis of
clay-rich rocks from random preparations. Clays and
Clay Minerals, 49, 514�528.Srodon J., Zeelmaekers E. & Derkowski A. (2009) The
charge of component layers of illite-smectite in
bentonites and the nature of end-member illite. Clays
and Clay Minerals, 57, 649�671.Tiller K.G. & Smith L.H. (1990) Limitations of EGME
retention to estimate the surface area of soils.
Australian Journal of Soil Research, 28, 1�26.Zorski T., Pałka K. & Srodon J. (2000) Geofizyczne i
mineralogiczne aspekty identyfikacji składu miner-
alnego w cienkowarstwowych kompleksach piaszc-
zysto-ilastych na podstawie jadrowych profilowan
otworow. Materiały Konferencji GEOPETROL
2000, Zakopane 25�28.09.2000, Prace Instytutu
Gornictwa Naftowego i Gazownictwa, 110,
265�269 (in Polish).
Quantification of illite and smectite 433
APPENDIX: EXPLANATION OF SYMBOLS USED IN THIS STUDY
bo (nm): the unit-cell dimension of smectite.
BW (wt.%): mass of the monomolecular layer of water held on the sample TSSA, expressed as a
percentage of the hydrated sample mass.
CEC (mEq/100 g): cation exchange capacity of the sample.
CECcorr (mEq/100 g): CEC corrected for the residual water left on clay at 200ºC.
CECcorr2 (mEq/100 g): CECcorr corrected for the incomplete exchange of Ca in the Co-hexamine
procedure.
EGME retention (mg g�1): mass of adsorbed EGME referred to the mass of sample at 200ºC.
EGME coverage (mg m�2): the mass of EGME molecules per unit surface of the sample.
fs: the fraction of smectite in the sample (portion of the dioctahedral 2:1 fraction of the sample that
displays the ability to adsorb EGME and exchange cations).
fi: the fraction of illite in the sample (portion of the dioctahedral 2:1 fraction of the sample that holds fixed
cations in the interlayers).
H2O retention (mg g�1): mass of adsorbed H2O referred to the mass of sample at 200ºC.
pIS: the fraction of mixed-layer illite-smectite in 2:1 clay.
Qs: charge of smectitic layer.
Qi: charge of illitic layer.
TI+S: the mean number of layers in all fundamental particles of 2:1 clay.
TI: the mean number of layers in illite fundamental particles of 2:1 clay.
TSSA (m2 g�1): total specific surface area of the sample.
% S: percent smectite layers in 2:1 clay; (CHEM + XRD), established from EGME retention and % 2:1
clay measured by XRD; (CHEM), established from EGME retention and % K2O.
% SI: percent smectite of discrete illite particles (two basal surfaces of illite fundamental particle,
calculated as one smectite interlayer per particle).
% SIS: percent smectite in mixed-layer illite-smectite measured by XRD and corrected for smectitic crystal
edges.
% SXRD: percent smectite in mixed-layer illite-smectite measured by XRD.
% 2:1 clay (wt.%): content of the dioctahedral 2:1 clay in the sample (sum of smectite, illite-smectite,
discrete illite, glauconite and micas)
434 J. Srodon
