AGROKÉMIA ÉS TALAJTAN 55 (2006) 1 39–48
Correspondence to: TIBOR NÉMETH, Institute for Geochemical Research of the Hun-
garian Academy of Sciences, H-1112 Budapest, Budaörsi út 45. Hungary. E-mail:
ntibi@geochem.hu
Characterization of Clay Minerals in Brown Forest Soil Profiles
(Luvisols) of the Cserhát Mountains (North Hungary)
T. NÉMETH and P. SIPOS
Institute for Geochemical Research of the Hungarian Academy of Sciences,
Budapest
Introduction
The physico-chemical properties (structure, rheological properties, water reten-
tion etc.) of soils are greatly influenced by their clay mineral composition (S
ZEGI et
al., 2004). Due to their adsorption capacity clay minerals play an important role in
the distribution of trace elements within a soil profile. As the adsorption properties
of clay minerals depend not only on the amount and main type of clays, but on
characteristics like swelling and layer charge, a detailed investigation of clay min-
erals and their alteration processes are needed for trace element distribution studies.
Pedogenic clay minerals form dynamic systems. Based on their alteration from one
to another species, the direction of the weathering process is predictable (T
ERRITO
et al., 2005).
In earlier works the distribution of trace metals Cu, Ni, Pb and Zn in brown for-
est soil profiles (Luvisols) was studied by chemical (S
IPOS, 2003), sequential ex-
traction (S
IPOS, 2004) and complex adsorption–desorption methods (SIPOS et al.,
2005) within the frame of a regional environmental geochemical research project of
the Cserhát Mountains in North Hungary.
This paper presents the clay mineralogy findings of this study, including the
characterization of clay minerals in four brown forest soil profiles developed on
different bedrocks, but influenced by similar pedogenic processes (e.g. clay illuvia-
tion) and a suggestion on the possible genesis and evolution of clay minerals in
Luvisols of the Cserhát Mountains.
Materials and Methods
The clay fraction (< 2 µm) of soil samples was separated by sedimentation in
distilled water. Cation exchange capacity (CEC) values were determined by satura-
tion with Ba
2+
, based on the modified Mehlich method (RÉDLYNÉ, 1988).
NÉMETH – SIPOS
40
X-ray diffraction (XRD) measurements were carried out using a Philips PW
1710 diffractometer with CuKα radiation at 45 kV and 35 mA. Mineral composition
of the bulk soil was determined on random-powdered samples by semi-quantitative
phase analysis. Clay minerals were identified by XRD diagrams obtained from
parallel-oriented specimens. The following diagnostic treatments were carried out
for all of the samples: ethylene glycol solvation at 60 ºC overnight, Mg saturation
followed by glycerol solvation at 95 ºC overnight, K saturation, heating at 350 and
550
o
C for 2 hours. The GREEN-KELLY (1953) test (Li saturation) was used to dis-
tinguish between montmorillonite and beidellite, as recommended by L
IM and
JACKSON (1986). The layer charge of smectite was determined by the alkyl-
ammonium method of L
AGALY (1994).
For TEM and AEM analyses a few milligrams of metal-ion saturated sample
was dispersed mechanically in ethanol and dropped onto 3 mm collodion-carbon
coated gold grids. Morphological and structural observations by TEM and chemical
analyses by AEM were performed with a Philips CM20 transmission electron mi-
croscope used at 200 kV accelerating voltage and equipped with a Noran energy
dispersive system (EDS).
Description of the soil profiles. – The studied forest soil profiles are from differ-
ent sites of the Cserhát Mountains in near vicinity of each other. The area has a
temperate subcontinental climate, with a mean annual temperature of 8.9 ºC, a mean
annual precipitation of 600–620 mm, reaching 650 mm at higher topography (P09
profile). The profile names refer to those used by S
IPOS (2004). Profile P09 is situ-
ated near to Karancslapujtő village in an oak–hornbeam forest on Oligocene carbo-
naceous siltstone bedrock dominated by chloritic clay mineralogy. Profile P131 has
developed on a similar Miocene siltstone near Kisbárkány in a small oak forest. The
parent rock of Profile P151 is a Miocene limestone, sandy limestone. The profile
itself can be found in an oak forest 2 km north of Mátraverebély–Szentkút. Profile
P161 has developed on Miocene pyroxene-phyric andesite and is located 2 km
south of the village Alsótold in an oak forest.
The main chemical and physical properties of the studied soil samples are sum-
marized in Table 1. Further and more detailed description of the topography, geol-
ogy, pedology, and chemical composition of the profiles can be found in S
IPOS
(2004), and on brown forest soils in Hungary in general in S
TEFANOVITS (1971).
Results and Discussion
Profile P09
The mineral composition of the bulk soil is 60–80% quartz with decreasing
amounts downward, 5–10% feldspar, 10% and 25% calcite in the BC and C hori-
zons, respectively. The amount of phyllosilicate clay minerals reaches its maximum
in the B horizon (15%), elsewhere it is around 10%.
XRD patterns of the clay fraction after the different diagnostic treatments are
shown in Fig. 1. The greatest peak at 14.5 Ǻ shifted to 15.5 Ǻ upon ethylene-glycol
Characterization of Clay Minerals in Brown Forest Soil Profiles
41
Table 1
Some physico-chemical characteristics of the studied brown forest soil profiles
(Cserhát Mountains, North Hungary)
TOC Clay Carb Al
2
O
3
Fe
2
O
3
Parent
rock
Genetic
horizon
Depth,
cm
Munsell
color
pH
(H
2
O)
%
Profile P09
A 0–5 10YR3/4 6.39 6.74 8 12.33 4.66
E 5–15 7.5YR5/8 5.66 0.92 12 16.45 6.39
B 15– 10YR5/8 6.85 0.39 11 16.30 6.76
BC 45– 10YR6/8 8.08 9 10 13.72 5.03
Silt-
stone
C 55– 2.5YR6/4 8.41 7 25 10.09 3.40
Profile P131
A 0–45 10YR4/2 5.23 2.31 10 10.87 3.31
E 45– 2.5YR5/2 5.05 0.25 13 14.03 5.57
B 60– 7.5YR5/2 5.09 0.27 16 14.87 6.59
Silt-
stone
C 150– 2.5YR6/2 5.51 0.14 15 15.07 6.39
Profile P151
A 0–30 10YR3/3 5.27 3.48 11 13.15 4.84
E 30– 7.5YR4/4 5.27 1.06 14 14.72 5.77
B 40– 10YR5/4 5.80 0.78 17 15.47 6.14
Lime-
stone
C 100– 10YR6/4 7.84 9 50 10.40 3.09
Profile P161
A 0–40 10YR3/3 5.84 2.29 13 12.05 5.10
E 40– 7.5YR4/2 6.3 1.05 9 13.98 5.86
B 60– 10YR5/6 6.05 0.6 13 15.83 7.44
Andes-
ite
C 130– 2.5YR5/4 6.12 0.35 12 15.05 7.38
Remarks: TOC = total organic carbon, Carb = carbonate minerals, Clay = clay sized fraction
solvation in the A, E and B horizons. The degree of expansion is less in the BC, and
the least in the C horizon. When Mg saturated and glycerol solvated, the basal peak
remained at 14.5 Ǻ in all cases, showing the vermiculitic character of this swelling
clay mineral. Potassium saturation partly caused the full collapse of high charge
vermiculite to 10 Ǻ, and partly the partial collapse of the chloritic phase to 14.1–
13.6 Ǻ. The partial instability of the chlorite structure against heating at 550 ºC
refers to the presence of vermiculitic component, which is indicated by the appear-
ance of the 12 Ǻ peak in the A, E and B horizons. In deeper horizons this phase
behaved like a chlorite and remained stable at 550 ºC heat treatment. To sum up,
two major clay mineral phases were distinguishable in the P09 profile: a vermiculite
and a randomly interstratified chlorite/vermiculite (soil chlorite), where interstratifi-
cation of the layers could be present along the c axis and also lateral. The degree of
structural collapse due to K saturation and heating of soil chlorite – similarly to the
expansion capacity – increases upward in the profile. The chloritic character is the
strongest in the C horizon, and the vermiculitic one in the A horizon. This suggests
the gradual alteration of chlorite to vermiculite during the weathering processes.
NÉMETH – SIPOS
42
Fig. 1
XRD patterns of the clay fractions in the genetic horizons of Profile P09 after the
diagnostic treatments. Remarks: Numbers indicate d values in Ǻ. Qtz = quartz, Cal = calcite,
Goe = goethite
Based on the 1.50 Ǻ value of 060 reflections both vermiculite and soil chlorite are
dioctahedral. Hydroxy-interlayered vermiculite is characteristic in fairly acidic
(pH=4.9–5.4) forest soils in Hungary (S
IMON et al., 2002). As the soil pH is higher
(near to neutral), the presence of OH-interlayered species – which exhibits similar
XRD features – is improbable in the P09 profile.
The 10 Ǻ reflection belongs to a minor amount of illite in the whole profile. In
the clay fraction the highest proportion of illite is found in the A horizon.
Fig. 2 shows the TEM image of vermiculite in the B and chlorite in the C hori-
zon. Chlorite forms larger (0.5–1 µm), 200 nm thick platy crystals, while vermicu-
lite is thinner and smaller (0.2–0.3 µm). Both vermiculite and chlorite have high
iron contents (6–10 wt%). TEM revealed the presence of goethite and other less
crystallized iron-phases in the B horizon, which is in good agreement with the
chemical data.
Characterization of Clay Minerals in Brown Forest Soil Profiles
43
Fig. 2
TEM micrographs and referring TEM-EDS spectra of chlorite in the C horizon (A) and
vermiculite in the B horizon (B) (Profile P09)
Profile P131
The bulk soil mineralogy is characterized by 40–70% quartz and 5–15% feld-
spars decreasing with depth. This profile has the highest clay mineral content, its
quantity increases from 15 to 40% downward. Carbonate minerals are not present.
The predominating clay mineral in the whole profile is smectite, which swelled
to 17 Ǻ due to ethylene-glycol and to 18 Ǻ as a result of Mg-saturation and glycerol
solvation (Fig. 3). 10 Ǻ and 7 Ǻ reflections refer to minor amounts of illite and
kaolinite, respectively. The proportion of these clay minerals is constant in the pro-
file, and they are most likely inherited phases from the parent rock. K-saturation
caused 12.5 Ǻ collapse of the smectite structure, indicating low layer charge. In-
crease of the 10 Ǻ peak in the upper horizons, however, suggests the presence of
high charged smectite layers too. The heterogeneous layer charge distribution of
smectite at 100 cm depth (top of the B horizon) is evidenced by alkyl-ammonium
exchange (L
AGALY et al., 1976). The layer charge ranges between 0.26 and 0.39,
with a mean of 0.325 charge per half unit cell. The CEC of the same clay fraction is
56 cmol/kg. Based on d
060
= 1.50 Ǻ the smectite is dioctahedral. After Li saturation
and heating at 250 ºC the smectite lost its expansion capacity, demonstrating the
octahedral character of the layer charge, i.e. the smectite is montmorillonite. A
small proportion of tetrahedral charge can be detected only in the E and A horizons,
