Clay Minerals (1985) 20, 291-300
DETERMINATION
OF LAYER-CHARGE
DENSITY
OF EXPANDABLE
2:1 C L A Y M I N E R A L S
IN SOILS
AND LOESS SEDIMENTS
USING THE
ALKYLAMMONIUM
METHOD
G. ROHLICKE
AND E. A . N I E D E R B U D D E
lnstitut ffir Bodenkunde, T. U. Miinchen- Weihenstephan, 8050 Fre&ing, FR G
(Received 10 January 1985; revised 21 February 1985)
A B S T R A C T : The alkylammonium adsorption method for the determination of layer-charge
density was slightly modified and applied to mixtures of expandable clay minerals (i.e.
predominantly 18 A minerals after glycerol sorption) from two loess samples and two soils
(Haplaquept and Aquentic Chromudert) having different K-adsorption properties. The
layer-charge density of the so-called 18 A minerals from these sediments and soil samples varied
between --0.23 and -0.85 per formula unit, which suggested the presence of different amounts
of vermiculite within the 18 A minerals. The amounts of these vermiculites were related to
K-fixing and K-buffering properties of the different samples. High-resolution electron
micrographs of vermiculites saturated with tetradecylammonium exhibited a basal spacing of 25
.A. It was confirmed, that the alkylammonium ions in the interlayers of the vermiculites formed a
paraffin-type structure. In the fine clay from the loess samples an interstratification of vermiculite
and illitic layers was observed.
In order to understand the cation exchange behaviour of soils containing smectites and
vermiculites it is important to quantify these two mineral species. The direct measurement
of the expansion of layer-silicates m a y help to explain the adsorption/desorption behaviour
of expandable clay minerals in sediments and their relation to clay mineral transformation
in soils. Knowledge of the layer charge is also essential to differentiate smectites from
vermiculites. Glycerol and ethylene glycol methods are probably not adequate for this
purpose, though they have been in c o m m o n use for some time (Brindley, 1966).
Adsorption of n-alkylammonium ions offers a way of measuring the layer-charge
density of smectites and vermiculites and also the charge heterogeneity of low-charge
vermiculites (Lagaly & Weiss, 1971; Lagaly et al., 1976; Lagaly, 1981, 1982).
The alkylammonium ions are arranged with their chains lying flat or tilted
(paraffin-type), the former being typical for low-charge silicates and the latter for
high-charge ones (Stul & Mortier, 1974; Lagaly & Weiss, 1975). Special embedding and
sectioning procedures (Vali, 1983) allow n-alkylammonium-saturated silicates to be cut
perpendicular to the a - b plane; because of the stability of expanded n-alkylammoniumsaturated layer-silicates, the layer distances are preserved. Electron micrographs show
clearly the layer-sequence in clay mineral mixtures from sediments and soils.
Although some information on the use of the n-alkylammonium procedure for the
determination of layer-charge heterogeneity of pure clay minerals is available, such
information on mixed clay fractions from soils and sediments is scanty. The purpose of the
present investigation was to determine the layer charge of the 'glycerol 18/~ clay mineral
9 1985 The Mineralogical Society
G. Riihlicke and E. A. Niederbudde
292
mixtures' of some soils and sediments, using a modified a l k y l a m m o n i u m procedure of
Lagaly et al. (1970, 1976), and to compare glycerol and a l k y l a m m o n i u m sorption
procedures. High-resolution electron micrographs ( H R E M ) of thin sections of tetradecyla m m o n i u m - s a t u r a t e d clay mixtures were also taken in order to make direct observations
of unit-layer distances.
MATERIALS
AND
METHODS
Samples
Samples of loess sediments (Rodheim and N t r d l i n g e n ) and soils (Freising) from
southern G e r m a n y were used for this investigation. Typical samples of the groups were
selected on the basis of multivariate discriminant analyses of some mineralogical and
chemical properties of the clay mixtures (Niederbudde, 1975, 1978). The 2 - 0 . 2 ,
0 . 2 - 0 . 1 and <0.1 prn size fractions were isolated. Some properties of the sediments and
soils investigated are given in Table 1, together with a s u m m a r y of the X R D
TABLE 1. Some chemical and mineralogicalproperties of materials investigated.
Location
Sample depth ( c m )
pH (0.01 N CaC12)
Org. matter (%)
CaCO 3 (%)
<2 gm (%)
K-fixation* mg/100 g <2 mm
K-fixation~"mg/100 g <2 gm
K/Ca exchange~ ARo~
properties
J BCKew
<2 gm CEC (mEq/100 g clay)
Mineralogicalcomposition (%)q
<2gm 10A
14/~
18 A
2-0.2 gm 10 ]~
14/~
18 A
0.2-0.1 gm 10 A
14/~
18 A
Rodheim
Sediment
loess
120-150
7.1
0.5
8.0
29
68
449
0.56
670
65
<0.1 g m l 0 A
14 A
18 A
31
16
46
43
14
34
32
18
45
21
tr.
78
Ntrdlingen
Sediment
loess
70-90
Freising I
Soil
Haplaquept
25-55
Freising II
Soil, Aquentic
Chromudert
60-95
7.6
1.3
13.7
25
58
371
0.67
750
56
7.2
4.6
1.5
45
99
786
0.05
1000
81
4.6
0.3
0
76
22
94
5.22
117
68
38
25
30
58
22
5
45
30
10
12
22
60
6
18
71
24
47
19
8
33
50
tr.
tr.
95
* 1 mg K added per g of <2 mm material.
"~ 10 mg K added per g of <2 gm material.
• 10 3 (M1/2).
wmEq K/100 g material/M1/2.
Based on glycerolmethod: 100 -- (10/~ + 14/~ + 18 A) = kaolinite + chlorite.
39
tr.
55
37
tr.
56
14
tr.
80
tr.
tr.
95
Layer-charge densities of 2 : l soil clay minerals
293
characteristics of the different size fractions. Analyses were performed according to the
procedures described by Black (1965).
Moosburg bentonite (Bavaria, FRG) and Llano vermiculite (Texas, USA) were used as
reference samples for smectite and vermiculite, respectively.
Determination of layer charge by n-alkylammonium chloride
The procedure used for preparing the n-alkylammonium chlorides was essentially that
of R/ihlicke & Kohler (1981). 350 mg of clay were equilibrated with 5 N NaC1 for 24 h and
then washed free of excess salt. The Na-clay was then dispersed in distilled water to give a
suspension concentration of ~25 mg clay/ml H20. One ml of this clay suspension was
poured into 4 ml graduated centrifuge tubes and the volume was made up to the 4 ml mark
with solutions of alkylammonium chlorides of different chain length (n c = 6-18). The
centrifuge tubes were stoppered, shaken vigorously and equilibrated at 65~ for two
weeks in an incubator with occasional shaking. The suspensions were then centrifuged and
the supernatant was discarded. The sediment was ultrasonically dispersed in ~2 ml ethanol
for 10-20 s, and the centrifuge tubes filled to the 4 ml mark with ethanol. The suspension
was centrifuged and the supernatant discarded, this procedure being repeated four times
for n c up to 12 and 6 times for n~ > 12.
The final sediment was dispersed in 1.5 ml ethanol and ~0.5 ml of this suspension was
uniformly spread on 2.5 x4 cm glass slides. After evaporation of the ethanol, the sample
was dried in a desiccator over P205 at 10-4 bar at 40~ for 12 h, and was X-rayed
immediately after removal from the desiccator to avoid any water adsorption. This
procedure is suited to the determination of charge density of expandable clay minerals in
soils and sediments; it can help to avoid the problems often arising during preparation of
samples for characterization of clays derived from soils (Lagaly & Weiss, 1970; Lagaly,
1981).
The interlayer charge density of the smectites was calculated by the method of Lagaly et
aL (1976). The critical chain used for calculation is the molecule with the alkylnumber (n c,
general formula CnH2nc+INH +) where the molecules attain their densest packing in the
interlayers of the silicate sheets. The spacings are mostly between 13 and 14 /k
(monolayer) and 17 and 18 /k (bilayer) (see Figs 4 and 5). The calculated charge
density ( ~ is the number of interlayer cations per (Si,A1)4014 (formula unit, f.u.) (Lagaly et
al., 1976). The transition range between mono- and bilayer structures was used for
determination of the charge distribution as described by Stul & Mortier (1974).
The interlayer charge density of vermiculites was determined from the mean increase of
d(001) with n~ (=Ad/An~). The relationship between the tilting angle trin a paraffin-type
structure and the layer charge per unit formula of some high-charged minerals (Lagaly &
Weiss, 1971) was used to determine the charge density. The basis of this calculation is the
perpendicular arrangement of the alkylammonium ions between the silicate layer with a
charge o f - I per formula unit (f.u.). The relationships are (where y -~ d(001) and x = n~):
y = 1.07x + 10.7 (Fig. 2, coarse clay); y = 1.11x + 10.3 (N6rdlingen, coarse clay);
y -- 1.10x + 10.5 (Fig. 4, coarse clay);y = 1.04x + 10.6 (Fig. 4, fine clay);y = 1.12x + 10.1
(Fig. 5). The arrangement of the ammonium hydrocarbon chains in the interlayer spaces
of vermiculite and smectite has been displayed graphically by Lagaly & Weiss (1970,
1971), Lagaly et al. (1976) and Lagaly (1981, 1982).
For glycerol solvation, 25 mg of Ca-clay was dispersed ultrasonically for 10-20 s in two
294
G. Riihlicke and E. A. Niederbudde
ml H20 and one drop glycerol. The suspension was prepared in the same way as for the
alkylammonium-saturated clay. The procedure for the determination of K-contractibility
of expandable layer-silicates and the measurements of the areas under the X R D peaks are
described elsewhere (Niederbudde, 1973; Niederbudde & Kul3maul, 1978).
RESULTS
AND
DISCUSSION
Mineralogy of clay fractions from loess sediments
Fig. 1 shows X R D traces of glycerol-saturated clays typical of the loess areas of
southern Germany. A rough estimation of mineral amounts was performed using the
method of Niederbudde & Kul3maul (1978). The weighted intensities so obtained were
normalized to 100% phyllosilicates to provide weight percentages in the clay fractions. It
was assumed that the degree of orientation for different clays did not vary substantially
because the samples under investigation derived from homogeneous series. Illite was the
main component of the coarse clay. 18 A minerals predominated over K-contractible 14 A
minerals (in soil clay mixtures occasionally called vermiculite). All fractions contained
traces of chlorite. 18 A minerals were enriched in the <0-1 pm fraction to ~80%. Amounts
of illites, K-contractible 14 A minerals and K-contractible 18 A minerals can be
recalculated to the <2 #m fraction (weight percentages); the results obtained from the
X R D traces in Fig. 1 were ~30, 15 and 45%, respectively.
The simultaneous occurrence of 18 A and 14 A peaks (Fig. 1) suggests that the coarse
clay contains a mixture of minerals with low and high charge. After alkylammonium
exchange the basal spacing of the expandable 2:1 clay minerals, however, was linearly
related to n c (Fig. 2), suggesting that high-charge minerals are present. Furthermore, the
very high correlation coefficient (r z = 0.998) inferred that the charge of these expandable
clay minerals is homogeneous. The regression coefficient gives a charge o f - 0 . 7 5 per f.u.
The charge of the expandable minerals of the coarse clay from N6rdlingen was --0.80
72~
183/~
Ioi/~ 14.#,
7.2/~
10.1/~ 1/.3A
I
21:1
lb
2b
2c
16 14 12 10 8
6
4. 2
~
16 14. 12 10 8
6
4. 2
FIG. 1. XRD traces of clay from loess Rodheim. (1) Glycerol-saturated, (2) K-treated at 20~
Clay fractions: (a) 2-0-2; (b) 0.2~3.1 ; (c) < 0-1 #m.
Layer-charge densities of 2:1 soil clay minerals
295
per f.u. with r 2 = 0.996. Hence, in the coarse clay fractions of these strongly K-fixing
sediments only high-charged expandable 2:1 minerals were determined; nevertheless a
considerable portion of these minerals expanded with glycerol to 18 ,~. The XRD peaks of
the high-charged layers of the n-alkylammonium-saturated clay fractions of all the soils
studied were sharper than those of the glycerol-saturated ones. The peaks became sharper
at n c > 10. As an example, the X-ray traces of the total clay fraction is presented in Fig. 3.
The relationship between d(001) and n c of the <0.1 gm fraction is depicted in Fig. 2. In
contrast to the linear relationship for the 2-0.2 #m fraction there appear to be three
sections with different slopes, namely for nc ranges of 6-8, 8-10, suggesting some charge
heterogeneity. This form of curve is typical for fine clay fractions of loess but its
interpretation remains uncertain. It is similar in some respects to the curves shown by
coarse-grained low-charged vermiculites (Beni Buxera, Young River) given by Lagaly
(1982). Comparison with these curves yields a charge range between --0.70 and - 0 . 5 5 eq.
per f.u., but since integral series of basal reflections could not be determined for this fine
clay sample, no further support for this range was obtained. Even more ambiguous is the
interpretation of the range with nr > 14. Non-equilibrium between the amines and the
small vermiculite particles is one means of explaining the shape of this part of the curve.
Nevertheless the charge density of the fine clay particles is higher than that expected from
glycerol solvation, which indicates the dominance of smectite.
Fig. 3 shows XRD traces of glycerol- and tetradecylammonium-saturated <2 pm
fractions of Moosburg bentonite, a soil clay and a loess. Sharp peaks between 25.3 and
25.5 A for tetradecylammonium-saturated specimens indicate the presence of some
well-crystallized minerals with paraffin structures (i.e. vermiculite) in all these samples, the
d I001
32-
[L•hOrizon
3oI
28~
261
/
24~
~
22~
d/
2oi
.=
2-0
.2~m
o : < 0.1pm
181
16
12J
,
6
,
,
8
,
,
,
,
I
,
,
,
10 12 lZ~ 16
C-ofoms/choin
,
,
18 n E
FIG. 2. Relationshipbetween chain lengths of the alkylammoniumions (no) and basal spacings
of expandable clay minerals after alkylammonium exchange (coarse and fine fractions of
Rodheim loess).
296
G. Rdhlicke and E. A. Niederbudde
GI.ycero[
Tebodecytornmonium
(nc=%}
I'72
1
<2pm
180
53
2S~
Bentonite
Moosburg
j
Aquentic
Chromudert
Sediment
Loess
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
25i
12 10 8 6 /+ 2 12 10 8 6 /* 2
~
I[oKo )
FIG. 3. XRD traces of glycerol-saturated and tetradecylammonium-treatedclays (<2 #m) of
reference bentonite, Chromudert soil and loess sediment.
amount being highest in the loess, and lowest in the bentonite. The weak but sharp peak at
25.3 /~ for the bentonite indicates that the alkylammonium procedure is capable of
identifying very small amounts of vermiculite in a smectite sample. The glycerol method,
on the other hand, indicated the absence of vermiculite in the bentonite as well as in the
soil. A very weak but distinctive peak at 14.3 /k for the alkylammonium-saturated clay
from the loess suggests trace amounts of primary chlorites.
Mineralogy of the soil clays
The relationship between d(001) and n c for the coarse and fine clay fraction from
Haplaquept, a strongly potassium-fixing clay soil, and the frequency distribution of layer
charge for the fine fraction are shown in Fig. 4. A straight-line relationship between d(001)
and nc for the coarse clay indicates the presence of minerals with homogeneous and high
charge. With n-alkylammonium ions there was no indication of smectite in the coarse clay
whereas glycerol solvation showed 20% of the layer-silicates to be of 18 A type (Table 1).
The layer-charge density of the silicates as deduced from the regression coefficient was
- 0 . 8 per formula unit.
The fine clay showed a stepwise relation between n~ = 6 and n c = 18 and a linear one
between nc = 8 and n~ = 12 (Fig. 4) suggesting smectite and a low-charged vermiculite.
The former with a charge distribution from - 0 . 5 1 to - 0 . 3 1 per f.u. is the main component
of this fine clay fraction. The smectite layers with layer charge > --0.40 per f.u. may
contribute to the high K-fixation of this s o i l in addition to the vermiculite of the
coarse-grained clay fraction. Moreover, small amounts of vermiculite were detected in the
Layer-charge densities of 2:1 soil clay minerals
Go-horizon,
9 30
Hoptoquept
2-02prn
/
Layer~horge'~: ,~
d(OO1)A
28
26
-081~
2~
./I
20I.
18
9 ,w" PF<O,1jJm
16:
/
$me~tite
9
12
I
& ' ~ ' 1'o'11'2' 1'~' 1'6' 1'8 nc
I E-atoms/chain
Smectite
Frequency
chQrge i001
lQyer
distribution
0J
,
. . . . .
,
,
,
.51 .~.2 36 .31 .28
neg.chorge/formu[n unit
FIG. 4. Relationship between chain lengths of alkylarnmonium ions (no) and basal spacings of
expandable clay minerals after alkylammonium-exchange (coarse and fine clay fractions) and
charge distribution of the smectite component of the fine fraction (Haplaquept).
Pg-horizon, Aquenfic Chromuderf
2JJm
Vermiculite
9
d(O01)A
,/ No 20% ~=_0.85
// No
200~C
30
28
26
/
24'
22
Smecfife
/ J ~ " Na 20%
201
/
io.
9
16'
~=-0.#1
.....--'-'-'j.------"
Li 2oooc
~"
~ =-0.31
/
I#i ~ . ~ / ~ /
12' 9 . . . . . . . . . . . .
6
8
t0 12 1# 16 18n C
[-ntoms/choin
FIG. 5. Relationship between chain lengths of n-alkylammonium ions (no) and basal spacings of
expandable clay minerals of clay from Chromudert soil, first treated with NaC1 at 20 ~C, or LiC1
and heated to 200~ and then exchanged with alkylammonium ions.
297
298
G. RffhHcke and E. A. Niederbudde
fine clay fraction. In this fraction of the soil clay the limit of detection coincided with n c =
8; peaks < 1 7 / k did not appear. The relatively small proportion of vermiculite in the fine
clay fraction was considered as a mineral with homogeneous charge. F r o m the regression
equation a charge o f - - 0 . 6 per Lu. was estimated between n c = 8 and n c = 12. Some
B
FIG. 6. Electron micrographs of (A) Llano vermiculite (<2 pm) expanded with tetradecylammonium (nc = 14), (B) fine clay fraction (<0.1 gm) from Rodheim loess sediment expanded
with tetradecylammonium (n~ = 14).
Layer-charge densities of 2 :l soil clay minerals
299
alteration of the spacing cannot be excluded (Lagaly, 1982). Along with the described
expandable forms, the low-charged vermiculite in fine clay fractions should be noted as a
K-fixing mineral; this soil and most of the strongly K-fixing soils contain high amounts of
fine clay material.
The relationships between d(001) and n~ for Na(20~ - and Li (100~
total
clay (<2 am) from a low K-fixing soil (Aquentic Chromudert) are shown in Fig. 5. The
near absence of 14 A minerals, (glycerol method, Table 1), and the non-appearance of <23
A peaks for n c = 6 to n c = 12 (Fig. 5) suggests that the proportion of high-charged
minerals is relatively low. Furthermore, a perfect straight line relationship for nc > 12
indicates that the small quantity of the high-charge mineral is mainly in the coarse clay.
The main mineral components of the <2 am fraction are smectites with a charge density
ranging between - 0 . 3 and - 0 - 5 with a mean value o f - 0 - 4 1 per formula unit (Na, 20~
which are similar to those reported by Lagaly & Weiss (1975) for a large number of
smectites from bentonites. After Li saturation at 200~ (Greene-Kelly, 1953) the charge
dropped to - 0 . 3 1 , which corresponds to the remaining tetrahedral charge. Because this
value is lower than the total charge it may not contribute much to the K-fixation capacity
of this clay. The K-fixation capacity is indeed relatively low (94 mg K per 100 g clay,
Table 1) and may be ascribed mostly to the presence of a small amount of high-charge
vermiculite (Fig. 5).
Electron micrographs
Lagaly & Weiss (1971) assumed that the tetradecylammonium molecules are arranged
as tilted chains (paraffin-type structure) between the layers of a vermiculite with a charge
density o f - - 0 . 7 to - 0 . 8 per formula unit, yielding an interlayer spacing of ~25 A. This
may be tested directly by observing thin sections with high-resolution electron microscopy
using the technique of Vali (1983). The teradecylammonium-saturated Llano vermiculite
(<2/an) and the fine clay of the loess are shown in Figs 6A and B. All layers of the Llano
vermiculite are expanded to 25 A, which indicates that the tetradecylammonium ions must
form a dense array of tilted chains, as suggested by Lagaly & Weiss (1971).
JOA
FIG. 7. Magnified sketch-section of fine clay from loess sediment expanded with tetradecyla m m o n i u m (n c = 14).
300
G. Rffhlicke and E. A. Niederbudde
The loess sample, however, does not show a uniform distance between the silicate
layers. This is because the clay is a mixture of different clay minerals, which behave
differently on tetradecylammonium saturation. It seems that vermiculite layers are
interstratified with illitic layers, making the electron micrographs more diffuse. For
clarification a sketch of the magnified micrograph is given in Fig. 7. This shows 4-5 25 A
layers alternating with 5-12 10 A layers. The crystals are 50-60 nm wide and some of the
10 A layers have wedge-shaped edges, which result obviously from the flexibility of the
layers. The crystals with 25 A spacing seem to be more extended than the illitic ones.
ACKNOWLEDGMENTS
The authors express their gratitude to Prof. U. Schwertmann and two anonymous referees for improvement
and revision of the manuscript, to Dr S. R. Poonia Department of Soils, University Hissar, India, now guest
soil scientist at Weihenstephan, for suggestions to the English text, to Dr H. Vali, Lehrstuhl Mineralogie T.U.
M/inchen-Garching, for advice on sample preparation of optical micrographs and to the Deutsche
Forschungsgemeinschaft, Bonn-Bad Godesberg, for financial support.
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