Reprinted from
GEODEW
Geoderma 86 (1998) 99-122
Weathering and soil forming processes under
semi-arid conditions in two Mexican volcanic ash
.soils
Didier Dubroeucq a, * , Daniel Geissert
b
b,
Paul Quantin a
QRSTQM, 32 avenue H.Varagnat, 93143 Bondy Cedex, France
Instituto de Ecologia A.C., apartado postal 63, 91000 Xalapa, Veracruz, Mexico
Received 14 May 1996; accepted 5 March 1998
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GEODERMJ
ELSEVIER
Geoderma 86 (1998) 99-122
Weathering and soil forming processes under
serni-arid conditions in two Mexican volcanic ash
soils
Didier Dubroeucq ai* , Daniel Geissert b, Paul Quantin a
I
a ORSTOM, 32 avenue H.Varagnat, 93143 Bondy Cedex, France
Instituto de Ecologia A.C., apartado postal 63, 91000 Xalapa, Veracruz, Mexico
Received 14 May 1996; accepted 5 March 1998
Abstract
Weathering and neoformation of allophane, imogolite and halloysite in volcanic ash soils have
been studied extensively in humid climates but little data are available on these soils in arid and
semi-arid conditions. Weathering and soil formation under semi-arid conditions on acid pyroclastics have been investigated in two profiles, both at the micro-site scale and on bulk samples. X R
microdiffractions and SEM-EDX microanalysis were performed on soil thin sections. Chemical,
particle-size, XRD and mineral identification analysis were performed on conventional soil
samples. Results show the interplay of various sub-processes: (a) intense division of the coarse
minerals into small fragments 50-200 p m in size; (b) diagenesis of noncrystalline products in the
zone of contact with the parent minerals; (c) transformation of noncrystalline minerals into
halloysite in the compact soil microstructures and preservation of the noncrystalline minerals in
the topsoil; (d) desiccation and condensation of the crystalline and noncrystalline minerals into
microaggregates in the topsoil. The results of these interacting weathering processes are silty-loam
soils with no cohesion and high susceptibility to wind erosion. Differences appearing between
different analytical methods at different sampling scales need special precautions in explaining the
results. O 1998 Elsevier Science B.V. All rights reserved.
Keywords: weathering; soil formation; volcanic ash; Andosols; semi-arid environment
1. Introduction
Volcanic ash is a unique parent material for soils. It is composed of
non-welded glass particles &d fine-sized minerals which are more susceptible to
Fonds Documentaire ORSTOM
* Corresponding author. E-mail: dubroeucq@[email protected] 4
0 EX : 5
0016-7061/98/$
- see front matter O 1998 Elsevier Science B.V. All rights reserved.
PII: SOO 16-706 1(98)00033-O
1O0
D.Ditbroeucq et nl./Geodemza 86 (1998) 99-122
weathering than minerals found in crystalline and sedimentary rocks. The
coloured glass of basaltic andesite composition, richer in cations than noncoloured glass of rhyolitic composition, is the most rapidly altered (Kobayashi et
al., 1976; Shoji et al., 1993b). The fine particle size and the predominance of
glass favour preferential formation of noncrystalline products, the most common
includes allophane, imogolite, hisingerite and ferrihydrite, and a specific type of
clay mineral, halloysite. Two causes are given to this process. The rapid
weathering releases over-saturated soil solutions which precipitate into noncrystalline hygroscopic phases (Shoji et al., 1993a). The second cause is the lack of
layered minerals such as mica and chlorite in the parent ash that may serve as
template for further crystallization. This probably impedes the formation of
secondary layer silicates (Dahlgren, 1994) and could explain that, in Andisols,
silicates are mostly in tubular and spherical structures rather than layered
structures.
The weathering environment's influence, which regulates Al and Si activities
in the soil solution, is determinant on the formation of either noncrystalline
components or crystallized clay minerals, or both (Ugolini and Dahlgren, 1991).
The effect of climatic conditions on mineral formation in Andisols has been
largely studied in different regions of the world and particularly in Japan
(Aomine and Wada, 1962; Saigusa et al., 1978; Shoji et al., 1993b), in
New Zealand (Parfitt and Wilson, 1985; Parfitt and Kimble, 19891, in Italy
(Quantin et al., 1985), on the islands of Martinique (Quantin et al., 1991) and
Hawaii (Chadwick et al., 1994). These soils were studied under precipitations
> 1000 mm, where andic characteristics have developed in situ and cannot be
disputed. In these conditions the age of the soil and the presence or absence of a
marked dry season are the most determinant factors on the mineral evolution of
the Andisols. With the ageing of the soil and with climatic changes from cold
plus' wet to warmer plus dryer climates, three colloidal assemblages tend to
prevail in soils deriving from volcanic ashes (Shoji et al., 1993b; Mizota and
Van Reeuwijk, 1989; Dahlgren, 1994): (a) Al-humus complexes together with
hydroxy-Al polymers and/or hydroxy-Al interlayered 2: 1 layer silicates, (b)
allophane and imogolite and/or ferrihydrite, (c) halloysite.
In comparison with Andisols from humid regions, very little information is
available on volcanic soils of semi-arid climates. Carbonate accumulation,
formation of 2:l layered clay minerals and surficial imports of Ca, Mg and
quartz from aeolian dust and seaspray are active processes on volcanic ash soils
from Lanzarote, Canary Islands (Fernandez Caldas et al., 1981; Jahn et al.,
1987; Jahn and Stahr, 1994). In Kenya, siliceous iron oxides such as hisingerite
are the predominant amorphous component formed in volcanic ash soils under
ustic moisture regime and isohyperthermic conditions (Wakatsuki and Wielemaker, 1985). In the semi-arid interandine plateaux of Central America, the
weathering products from volcanic ashes are quite different from those in humid
climates and soil formation in this environment is not yet entirely understood.
D.Dubroeihcq et al./Geodenna 86 (1998) 99-122
101
Most of these soils are compact in the middle part of the profile and, in the
upper horizons, they crumble into very fine particles consisting of halloysite and
smectite clay minerals mixed with glass fragments (Colmet-Daage et al., 1969).
In the Mexican Altiplano, these soils are open to wind erosion and constitute the
most important source of the aeolian dust which obscures the sky during the dry
season.
We have studied the weathering processes and the pedogenic products of two
different ash-flow deposits under semi-arid conditions in the Mexican Altiplano.
One is of rhyolitic composition and the other of dacitic composition. The aims
of this study are: (1) to establish the morphology, chemistry and mineralogy of
the weathering products, (2) to establish their genesis, and (3) to propose
explanations for the very uncohesive and fine structure of these soils.
2. Materials and methods
2.1.Location and general irzfomation
The study area belongs geologically to the complex andesitic strato-volcano
Cofre de Perote (4250 m) in the eastern Transmexican Volcanic Belt (Fig. 1).
The climate of the western leeside of the mountain range is cold temperate
(midslope, above 3000 m) to temperate (footslope at 2600 m), with an average
annual temperature between 11°C and 14°C (isomesic soil temperature regime).
Enlargedarea
Fig. 1. Location map.
L
D. Dubroeucq et al./ Geoderrna 86 (1998) 99-122
102
From the mountain side to the altiplano, the total annual rainfall decreases from
800 to 400 mm, and the dry period increases from 4 to 8 months (ustic to aridic
moisture regime). Parent soil materials are generally andesitic or dacitic ashes of
late Pleistocene age, with underlying lavas or ash-and-blocks flows of the same
mineral composition, or thick rhyodacitic pumice deposits of middle Pleistocene
age.
2.2. Field sites and soils
Two volcanic ash soils were selected as representative soil types of the
region. Their major macromorphological characteristics are given in Table 1;
their physical and chemical properties are presented in Tables 2 and 3.
Profile A is SW from Perote, Veracruz State (19’28’30”N, 97’15’20”W) at an
altitude of 2500 m and on a 5% slope. Annual precipitation is 400 mm, with 8
months of dry season, and the evaporation is 1640 mm. Land-use is maize
associated with pine reforestation. The volcanic substratum is composed of a
1-2 m thick pumice fall deposit overlain by less than 1 m of fine ash. These
materials are discordant upon a rhyolitic ash-and-pumice flow deposit, about 10
m thick, unconsolidated, and which shows a subhorizontal banded pattern of
slightly undulated lamellae, each 2 to 5 cm thick, that tends to disappear deep
down (Fig. 2). According to Ferriz and Mahood (1984), the ash-and-pumice
flow corresponds to the Xaltipan ignimbrite dated 460,000 yr BP, and the
pumice fall to the Toba Faby formation dated 240,000 yr BP, both originating
Table 1
Major macromorphological characteristics of profiles A and B
Horizon Depth (cm) Colour
Texture
Structure
Profile A
AP
AB
B
CRb
Bkxb
BCb
Cb
20
60
110
170
220
310
400
10YR3/6
IOYR5/3
10YR5/4
10YR6/2
10YR6/6
7.5YR5/6
5YR5/4
sand
sandy loam
sandy loam
loamy sand
loamy sand
sand
sand
loose
w. subang. bl.
w. ang. bl.
loose
abundant
massive
loose
common
nd.
many
40
1O0
150
230
300
7.5YR5/4
7.5YR5/6
10YR5/4
7.5YR5/4
nd.
sandy loam
sandy loam
loam
silt loam
silt loam
loose
massive
m. subang. bl.
w. ang. bl.
m. subang. bl.
Profile B
AP
B
Bb
BCb
Cb
Coarse
NaF test HCl test
Gravel
W.
-
Stones
few
m.
few
m.
few
W.
common many
-
subang. bl. = subangular blocky; ang. bl. = angular blocky; w. = weak, m. = moderate.
-
103
D.Dubroeucq et al./ Geoderrna 86 (1998) 99-122
Table 2
Chemical and physical characteristics of profiles A and B
Profile A
Profile B
Depth (cm)
Depth (cm)
20
60
110
170
220
40
B
CRb
Bkxb
Ap
2.3
2.5
1.33
5.1
8.2
7.5
9.5
2.5
1.14
3.9
6.3
5.5
10.4
3.45
4.0
2.3
1.4
Horizons
Ap
Solid density
Bulk density
Moisturecontent
pH, HZO
pH, KC1
PH,
% allophane
% Org. matter
%C
%N
Available P
CECcmol(+)/kg
Na cmol(-t)/kg
Kcmol(+)/kg
Cacmol(+)/kg
Mgcmol(+)/kg
Extr.Hcmol(+)/kg
Extr. Al cmol( +)/kg
2.4
1.03
1.3
6.6
6.0
9.3
0.04
1.2
0.7
-
t
4.6
0.24
0.49
3.54
0.84
0.14
-
150
230
B
Bb
BCb
2.6
1.54
5.4
6.3
5.5
10.3
2.42
0.7
0.4
0.3
2.6
1.95
4.2
6.1
5.2
9.2
0.31
0.2
0.1
0.2
2.7
2.61
3.7
5.6
9.1
7.6
0.24
0.1
0.07
0.1
100
Horizons
AB
2.5
1.12
2.9
6.5
5.9
8.8
0.03
0.7
0.4
-
t
6.0
0.46
0.78
3.54
1.15
0.28
-
2.4
0.82
3.3
7.3
6.6
9.3
0.20
0.6
0.3
-
1.3
8.3
7.6
9.9
-
0.1
0.08
-
-
t
t
8.2
0.46
0.45
2.08
0.29
0.14
-
3.6
-
-
0.28
-
-
0.2
0.1
-
t
9.4
-
0.28
-
t
10.8
0.28
0.64
6.66
0.47
0.28
-
t
8.0
0.36
0.92
4.47
1.03
0.14
-
t
t
7.4
0.24
1.02
4.47
1.03
0.14
-
'
7.6
0.50
0.60
1.59
0.90
0.14
-
from the Los Humeros caldera, 20 km to the NW of the site. This profile may be
allocated to the Soil Taxonomy class Vitrandic Ustorthent (Soil Survey Staff,
1996).
Profile B is near Los Altos, district of Perote, Veracruz State (19"27'40"N,
97"ll'lO"W) at the altitude of 3100 m and on a 20% slope. The annual
precipitation is 700 mm, with six months of dry season and the evaporation is
1300 mm. Land-use is potato crop in a partly cleared pine forest. The volcanic
substratum is a 5-6 m thick rhyodacitic ash-and-blocks flow originated from the
Cofre de Perote volcano. The ash has a sandy-loam texture and is brown
coloured. The blocks are generally 0.5-1 m across, some are fragmented with
radiating shearing cracks and others subangular, probably parts of the collapsed
walls of the volcano (Fig. 2). Since the ash-and-blocks flow overlaps the Toba
Faby formation downslope, it is younger than 240,000 yr BP, but postdates the
last major explosive event of the Cofre de Perote volcano dated 38,800 yr BP in
this study (sample OBDY 1011, ORSTOM, Bondy, France). This profile may be
allocated in Soil Taxonomy as Typic Ustivitrand over Andic Eutropept (Soil
Survey Staff, 1996).
104
D. Diibroeiicq et al. / Geodernia 86 (1998) 99-122
Table 3
Particle size distribution in profiles A and B
0-2
2-20
20-50
50-200
200-2000
Pm
Pm
Pm
Pm
Pm
2.10
2.81
3.48
9.38
6.63
7.89
3.36
2.45
2
13
15
14.5
5
17.5
23.5
11
18.5
3.2
24.5
27
27.2
39
24.1
15
15
2.8
4.2
28.5
11.7
9.3
4.5
6.5
2.1
14.2
1.3
2.1
18
27.8
26
2.8
25.9
26.2
25.3
30.6
31.7
16
18.5
23
48.7
26
30.2
34.5
46.8
58.8
3.4
6.24
7.34
6.9
10.5
17
22.5
22
24.5
28.5
35
36
13
5.5
8
7.5
35.5
37.5
19
17
16.5
28.5
15.5
17.5
Depth
(cm)
MD
(pm>
So
Profile A
AP
AB
B
Lapilli
Bkxb
Lamella
Matrix
Lamella
Matrix
10
40
80
130
190
240
240
300
300
49
37
45
100
62
97
182
185
250
Profile B
AP
B
Bb
BCb
20
60
120
190
Horizon
48
41
11
12.8
MD is the medium size of the particles. So is the Trask' sorting index, equal to 1for a perfect
sorting and to higher values for nonsorted particle size distributions.
2.3. Laboratory analyses
The samples of fine-earth (30 g of < 2 mm soil fraction) were previously
treated with H,O, and NaOAc buffer solution (pH 5) to remove organic matter
and mechanically agitated for 8 h with 2 cc Na-hexametaphosphate N/loOO.
The particle-size analysis was carried out in two steps: (a) conventional sieving
for the 50 to 2000 p m fractions, (b) using a Sedigraph 5000 in automatic
procedure for clay and silt fractions.
Chemical analyses were carried out on the fine-earth from each horizon.
pHHZ0,pH,,,
pH,,
were determined potentiometrically, CEC by ammonium
acetate saturation and removal by NaC1, Na and K by flame photometry, Ca and
Mg by atomic absorption spectrometry. Exchangeable H+ and Al3+ were
determined titrimetrically and available P by the Bray 1 method. Allophane
content was calculated from the oxalate extractable Si, estimating the overall
composition as (SiO,, Alzo,, 2.5H20) with SiO,/A1,0, = 1.
Heavy and light minerals of the sand fraction (> 50 pm) were separated by
bromoform (density 2.89 at 20°C) and observed on glass slides with an optical
microscope.
Undisturbed samples from each horizon were resin impregnated to obtain soil
thin sections. Micromorphological observations were performed with a standard
petrographic microscope, using the terminology of Bullock et al. (1985) and
D. Dubroeucq et al./ Geodenna 86 (1998) 99-122
AP
105
AP
AB
B
B
Bb
CRb
Bkb
BCb
BCb
Cb
PROFILE B
Cb
PROFILE A
Fig. 2. Profiles A and B, with horizon designations from Soil Taxonomy and showing sampling
sites of soil thin sections.
Mackenzie and Guilford (1988). The soil thin sections were carbon-coated and
examined with a scanning electron microscope (S.E.M.) Cambridge Stereoscan
200 equipped with an energy dispersive X-ray analyser (EDXRA) Link System
AN10000 calibrated with a Co sample.
Conventional X-ray analyses were carried out on a portion of fine-earth,
using a Siemens diffractometer with Cu anticathode and applying several sample
treatments: (a) non-oriented clay fraction ( < 2 ,um), (b) heating to 110°C and
480°C, (c) Glycerol-treated clay sample, (d) fully oriented clay sample by a drop
of clay suspension on a platelet and subsequent air drying.
X-ray microdiffractions were performed on soil thin sections using a Siemens
microdiffractometer with Cu anticathode, linear localisation detector and
0.15 mm2 collimator producing irradiated areas of about 0.5 m2.X-ray
intensity and timing were identical for each sample. In order to cut down the
small angles spectrum baseline variations, a blank with a glass slide was made
and the final spectra are the difference between the samples and the blank.
In the case of crystalline material, XR microdiffraction offers a good definition of the diffraction peaks, in conformity with the conventional band intensities of the mineral (Rassineux et al., 1987). In the case of paracrystalline
minerals, clear reflections are absent, and diagrams are not interpretable. When
both species are mixed, the relative peak height comparison is no longer valid.
106
D.Dubroeucq et al./Geodermn 86 (1998)99-122
One can detect the absence or presence of noncrystalline material by the flat or
bulging baseline between 0.5 and 0.3 nm, but not the abundance.
3. Results
3.1. Profile A
3.1.1. The coatings on the glass shards ia the
C horizons
In the rhyolitic ash flow, at a depth of 330 cm, more than 70% of the mineral
components were glass shards, the rest being pumice lapilli, small plagioclase
crystals and dacite lava clasts. A majority of the glass shards were larger than
500 pm, had a smooth surface and were partially coated with a discontinuous,
pale grey coloured, isotropic, fine-textured material. Under S.E.M. observation,
traces of incipient weathering were evident on their surface, such as dissolution
pits, alveoles and fine indents. The pale grey material is spongy and rich in very
small mineral fragments (Plate 1). The spongy structure has been formed by
strong dehydration, presumably in the vacuum chamber during the S.E.M.
observation, suggesting that the pale grey material was initially a hydrated gel.
Semi-quantitative EDXRA analyses show that the coatings are mainly siliceous
and not very different in their composition from that of the rhyolitic glass, with
a Si/Al molar ratio of 6.1 (Table 4).
Plate 1. SEM image of the parent glassy ash in soil A. The glass shards show discontinuous
coatings rich in small mineral fragments.
107
D.Dubroeucq et al./Geoderina 86 (1998) 99-122
d
Y
-2
s
8U
LI
8
2
111
7x
.v)
3
O
s
1
Y
8
+
I
O
U
E
rcI
O
d
u
2
3
%I3
E-.w
108
D. Dubroeiicq et al./ Geodernta 86 (1998) 99-122
b
a
3231
O
O 5mm
Fig. 3. Microstructure of the parent rhyolitic ash of profile A. (a) In the glassy matrix, the glass
shards are bridged by colorless isotropic coatings. (b) In the lamellae, the corroded glass shards
are embedded in a brown halloysitic groundmass.
At a depth of 250 cm and between the lamellae, the glass shards, rarely
bigger than 500 ,um, show a rough etched surface and are bridged and welded
together by fine-textured coatings. These are colourless, isotropic, non-laminated
and limpid in PPL, indicating a neoformed amorphous material (Fig. 3a).
Semi-quantitative EDXRA analyses show a higher content in Fe and Al in these
colourless coatings than in the former pale grey coatings and a Si/A1 molar
ratio of 1.5 (Table 4). X-ray diffractograms of the clay fraction from the same
glassy material (Fig. 4, samples 3231 and 3362) show a 1.0 nm halloysite plus
plagioclases (andesine, labrador) and traces of calcite. The bulge of the baseline
between 0.5 and 0.3 nm is attributed to noncrystalline products. After heating to
480°C a small 1.0 nm peak remained, pointing to traces of illite clay mineral
(Fig. 5, sample 3362). No peaks were detected in the X-ray microdiffraction
diagrams performed directly on the colourless coatings, except for two bulges
which are attributed to noncrystalline material (Fig. 6, sample 3241).
3.1.2. The coatings on the pumice lapilli
Unweathered pumice lapilli are subangular with a rough surface. Their
structure varies from being fibrous to vesicular. Weathered lapilli were gradually
round and smooth and coated with an isotropic, pale yellow, fine material which
partly infills the voids and the vesicles in the pumice. Needle-shaped calcite
crystals were commonly found on the surface of the coatings, attesting to a
secondary crystal growth from evaporation of Ca-saturated solutions in the
macrovoids (Fig. 7a). The pale yellow coatings are isotropic, laminated and
translucent. Observed under S.E.M., the surface of the coating is very spongy
with a botryoidal surface and internal microvoids, suggesting an initial hydrated
state of gel which, subsequently, was dehydrated in the field, and probably also
in the vacuum chamber of the microscope. Similar structures of amorphous
D. Dubroeucq et al. / Geodernia 81 ‘I998) 9 -122
109
Profile A XRD-Graph
3500
3000
2500
P
z
2000
3
O
1500
1O00
500
O
O
10
20
30
40
50
2 THETA
Fig. 4. X-ray diffractogramsof clay fractions from profile A. Near the soil surface the small peaks
at 0.74 and 1.0 nm refer to poorly crystalline and dehydrated halloysite and the bulge of the
baseline indicates noncrystalline products.
alumino-silicate coatings have been described in Andisols of humid climates
(Jongmans et al., 1995). The chemical composition given by the EDXRA
corresponds to a silica gel with a low content of Ca, Fe and AI and an Si/&
molar ratio of 20.5 (Table 4). X-ray diffractograms display small reflection
peaks of plagioclases, calcite and cristobalite, but no clay was detected. The
broad bulge of the baseline centred on 0.4 nm indicates abundant noncrystalline
products.
3.1.3. The fine groundmass in the B horizons
Between depths of 300 to 220 cm, the lamellae showed a dense microstructure compared with the loose bridged grain structure of the surrounding glassy
D. Dubroeiicq et al./ Geodernin 86 (1998) 99-122
110
Profile A, sample 3362 XRD-Graph
2500
u) 2000
I-
z
3
8 1500
1000
500
O
I
O
10
I
20
2 THETA
I
30
I
50
40
Fig. 5. X-ray diffractograms of the clay fraction from the glassy material between the lamellae in
soil A. Diagrams show weak peaks of halloysite and a bulge of the baseline indicating
noncrystalline products. The 1.0 nm peak remaining after heating refer to small quantities of 2:l
clay minerals.
material. The glass shards were 0.4 to 0.6 mm in size and embedded in a
brown-coloured, isotropic, fine groundmass (Fig. 3b). This material is richer in
Si and Fe than the colourless coatings, and its composition is close to that of 2:l
clay minerals, with Si/Al molar ratio = 2 (Table 4). As illuviation features and
laminated structures are absent in the groundmass, the clay minerals are
supposed to be formed in situ from noncrystalline products. X-ray diffractograms of the < 2 ,um fraction from a lamellae showed clear peaks at 1.0 and
Profile A XRmicroD Graph
l
I
I
O
I
10
I
20
2 THETA
l
30
I
40
50
Fig. 6. X-ray microdiffractograms from selected areas of fine material between the glass shards, in
the ash and in a lamella of the parent material from profile A. Contrary to the diffractograms from
clay fraction samples, clear peaks of clay are not detected in the ash and in the lamella.
111
D. Dubroeucq et al./ Geoderrna 86 (1998) 99-122
a
b
O
0.25”
O
C
lmm
O
glass and plagioclase fragments
Imm
brown fine groundmass
dense calcitic infilling O p a l e yellow coating =voids
nodules
pumice
Fig. 7. Microstructure of profile A. (a) Pale yellow amorphous coatings on the corroded pumice
lapilli and secondary growth of acicular calcite crystals. (b) Compact microstructure of the buried
B horizon with infillings of fine calcite crystals in root channels. (c) The continuity of the massive
structure of the €3 horizon is broken by irregular vughs connected to fine planar voids developed
generally around the coarser grains.
0.445 nm related to 1.0 nm-halloysite, small peaks of plagioclases and a bulge in
the baseline corresponding to noncrystalline products (Fig. 4, samples 3232 and
3361). Diagrams of X-ray microdiffraction on the brown fine groundmass
displayed only two bulges with a small peak at 0.445 nm (Fig. 6, sample 3231)
which was interpreted as poorly crystallized halloysite mixed with noncrystalline
products.
Between depths of 220 to 170 cm, the same dense microstructure as in the
lamellae, was observed in the whole horizon. Abundant, corroded glass fragments, 0.1 to 0.3 mm in size, were embedded in an isotropic, brown-coloured,
fine groundmass. XR diffractograms of the < 2 p m fraction displayed clear
peaks of 1.0 nm-halloysite and a shoulder at 0.74 nm which was interpreted as a
partly dehydrated halloysite (Fig. 4, samples 335 and 335s). Dense infillings of
fine calcite crystals were observed in elongated voids and channels of about 0.4
to 1 mm in diameter (Fig. 7b). Several of them showed a radiating orientation of
the crystals pseudomorphosing plant roots. Iron oxide nodules and coarse
112
D. Dirbroeucq et al./ Geodenna 86 (1998)99-122
pumice lapilli were also observed. This material has been interpreted as the B
horizon of a buried soil, whose maximum age corresponds to the overlying
pumice deposit, i.e., 24,000 yr BP (Ferriz and Mahood, 1984).
3.1.4. The fine groundmass in the topsoil
Between 110 cm and 60 cm, the soil texture is loamy sand with only 14 to
15% clay. This material corresponds to an ash-fall composed of abundant coarse
grains of pumice lapilli, glass, plagioclase and dacite microliths. The groundmass consists of fine particles of plagioclase, pyroxene and glass and of an
undifferentiated pale brown fine material. An average of five microanalyses
from the pale brown groundmass indicates an Si-rich product, with Fe and Na as
predominant cations and Si/A1 molar ratio = 5.6 (Table 4). Compared with the
B horizon, which has a Si/A1 molar ratio = 2, the weathering of the upper part
of the soil is not so advanced, and the B horizon has been probably buried under
more recent deposits. X-ray diffractograms of the clay fraction indicate a partly
dehydrated halloysite with clear peaks at 1.0 and 0.74 nm and a little of
plagioclase. The smooth bulge of the baseline between 0.5 and 0.3 nm was
interpreted as having small quantities of noncrystalline products. At 30 cm
below the surface (Fig. 7, sample 3321, the X-ray diffractograms show a more
accentuated bulge of the baseline and a weak peak at 0.74 nm, indicating only
small quantities of dehydrated halloysite together with noncrystalline minerals,
presumably allophane.
The mineral evolution of the fine groundmass is accompanied by an evolution
of the microstructure. Compact structures were observed within depths of 110 to
60 cm. Above 60 cm, loose structures were observed to be developing gradually
(Fig. 7c). They are mainly vughy and crumb microstructures in which the fine
material is dominantly consisting of dehydrated halloysite and noncrystalline
products.
3.2. Profile B
3.2.1. The coatings and infillings in the weathered rock boulders
Both the ash and the blocks of the pyroclastic flow are of dacitic composition.
The blocks are composed of 30 to 40% fenocrysts of plagioclase (essentially
oligoclase, labrador and andesine), K-feldspar, pyroxene and amphibole grains;
20% smaller crystals, 0.5 to 1 mm in size, mainly of plagioclase and pyroxene;
40 to 50% of microlitic matrix, essentially plagioclases and glass.
In the weathered blocks, a yellow, isotropic and limpid fine mass infills the
voids of the microlitic matrix (Fig. 8a). Since no laminated structure was
noticed, the yellow fine mass is presumed to be formed in situ and is considered
as the product of an initial stage of weathering, resulting from the co-precipitation of Al and Si released from the weathering of overlying and in situ primary
minerals (Jongmans et al., 1995).
113
D.Dubroeucq et al./ Geodernia 86 (1998) 99-122
a
b
34501
34401
34402
O
b
Imm
0.5 mm
d
C
!
yellow coatings and infillings
811 pyroxene
-
imm
O
O
feldspar
microlitic matrix
biologic microaggregates
Imm
Glass shards and nodules
brown groundmass
0Plagioclse
0voids
Fig. 8. Microstructure of profile B. (a) Isotropic yellow infillings in the microlitic matrix of the
dacite blocks. (b) Centrifugal alteration of a feldspar phenocryst and yellow isotropic weathering
products. (c) Thick yellow coatings and brown groundmass in a sample from the bottom of the B
horizon. (d) In the topsoil are two types of microaggregates, one consisting of clusters of mineral
particles and the other consisting of rounded bodies containing organic matter.
Between depths of 150 to 230 cm, yellow coatings of the same characteristics
were found around and inside the weathered feldspar and plagioclase fenocrysts,
and more particularly, along the cleavage planes and within the cavities of the
crystals (Fig. 8b). Close to the parent minerals (Fig. 8b, sample 34401), the
yellow coatings are optically isotropic and limpid, and the EDXRA analyses
gave the following composition: SiO, = 59.7%, Alzo, = 24.6%, Fe,O, = 0.8%,
Na,O = 5.7%, Ca0 = 7.4%, K,O = 0.9%; Si/A1 mol = 2.05. In the external
zone of the minerals (Fig. 8b, sample 34402), the fine mass has a denser yellow
colour and its chemical composition shows an increase in Fe, a decrease in bases
and Si/Al molar ratio = 1.2 (Table 4). This latter composition is close to that of
halloysite. Therefore, various grades of differentiation of the yellow coatings
may coexist near the same crystal, from siliceous products rich in bases, to
D. Dubroeucq et al./ Geodermn 86 (1998) 99-122
114
aluminous products depleted in bases. X-ray microdiffractions on a yellow
coating in a rock fragment (Fig. 8a) shows clear peaks of plagioclase, a bulge of
the baseline at the small angles indicating abundant amorphous products and a
hump at 0.445 nm indicating traces of poorly crystallized 1:l clay minerals
(Fig. 9, sample 34501). X-ray microdiffractions of yellow coatings inside and
around a weathered plagioclase (Fig. 8b) show plagioclase, sharp and high peak
of cristobalite at 0.403 nm and small peaks of halloysite at 0.74 and 0.445 nm
Profile B XRmicroD Graph
4000
3000
u)
t-
z
3
8
2000
iooa
t
I
O
'IO
l
20
I
30
I
40
50
2 THETA
Fig. 9. X-ray microdiffractograms of selected areas in thin sections from profile B. Diagrams
show contrasted differences between the yellow and the brown fine materials: sharp peaks of
cristobalite and bulges of the baseline for the yellow fine material (samples 34501, 34402, 3401,
34321), clear peaks of clay, plagioclase and hornblende for the brown groundmass (samples
34322, 34301). In the topsoil (sample 34101) the peaks of clay are not discernible and the baseline
bulges are accentuated, indicating noncrystalline products.
D.Dubroeucq et al./ Geodeniia 86 (1998) 99-122
115
(Fig. 9, samples 34402 and 34401). X-ray microdiffractions of thick yellow
coatings on the surface of a dacite block at a depth of 150 cm (Fig. Sc), indicate
a mixture of allophane-like products with halloysite and cristobalite (Fig. 9,
sample 34321).
3.2.2. The yellow coatings in the B horizon
Between depths of 150 to 250 cm, the yellow coatings were very abundant,
isotropic, and 20 to 600 p m thick. They were covering the surface of the dacite
blocks and pebbles, the walls of the planar voids and the coarse minerals and
infilled the porosity, resulting in a relatively dense microstructure. Both structureless and microlaminated features were observed in the same samples,
indicating that neoformed and translocated coatings were coexisting in this part
of the profile. At a depth of 150 cm (Fig. Sc), semi-quantitative EDXRA
analyses show that a thick yellow coating consists predominantly of Si, Al and
Fe with Si/A1 molar ratio = 1.3 and with an excess of Fe and Si in comparison
with halloysite (Table 4). In accordance with the X-ray microdiffraction diagrams of this sample (Fig. 9, sample 34321), the yellow coating composition is
interpreted as a mixture of halloysite with minor amounts of allophane and
substantial contents of Fe-oxides and silica minerals such as cristobalite. Above
a depth of 120 cm, the yellow coatings were fragmented into rounded or
amiboidal clay features and incorporated within the groundmass. Faunal activity
is thought to be the cause of fragmentation.
3.2.3. The brown fine groundniass in the B horizon
At a depth of 150 cm, only small areas of brown-coloured fine groundmass
were discernible among the yellow coatings (Fig. Sc). Higher in the profile, they
were more abundant. The brown fine groundmass is reddish-brown in PPL, with
a speckled b-fabric in XPL and a preferred orientation around the coarse grains.
Its chemical composition from EDXRA analyses is dominantly of Si and Al
with a substantial Fe content and %/A1 molar ratio = 4.1. At a depth of 120 cm
in the profile, the brown fine material was denser, embedding the coarse
minerals and forming a channel microstructure. X-ray diffractograms of the
< 2 p m fraction have given evidence of a partly dehydrated halloysite by a
clear peak at 1.0 nm and a shoulder at 0.72 nm, and of plagioclase (Fig. 10,
sample 343). X-ray microdiffractions have shown a small peak of halloysite at
0.74 nm and peaks of plagioclase at 0.403, 0.322 and 0.252 nm, without
evidence of noncrystalline products (Fig. 9, sample 34322). Semi-quantitative
EDXRA analyses of the brown groundmass at a depth of 100 cm have indicated
a silica-rich calcoalkaline product with substantial Fe content and Si/A1 molar
ratio = 1.85 (Table 4). The X-ray microdiffractions on the same sample have
shown halloysite by a small peak at 0.74 nm, hornblende at 0.843 nm and
0.270 nm, and plagioclase at 0.318 nm and 0.322 nm (Fig. 9, sample 34301).
These results confirm that, in the B horizons, the brown coloured fine material is
D. Dubroeiicq et al./ Geoderma 86 (1998) 99-122
116
Profile B XRD Graph
4000
In
O
x
M
3
3000
2000
u)
I-
z
3
O
o
1O00
O
O
I
I
10
20
2 THETA
I
30
l
40
50
Fig. 10. X-ray diffractograms of clay fractions from profile B. In the upper part of the soil
(samples 342 and 341), small peaks at 0.74 nm and a bulge in the baseline between 0.5 and
0.3 nm indicate dehydrated and poorly crystallized halloysite clay with noncrystalline products.
mainly composed of 1.0 nm-halloysite accompanied by fine particles of plagioclase and hornblende.
3.2.4. The fine groundmass in the topsoil
At a depth of 100 cm, areas of microaggregated structures were observed to
be developing at the expense of dense microstructures, and above 80 cm, the
microaggregation occupied the whole soil material. Each microaggregate is
irregular-to-subrounded, 0.05 to 0.15 mm across, composed of coarse grains
embedded in a brown fine material. The brown groundmass composition, from
semi-quantitative EDXRA analyses, corresponds to a silica-rich product, with
higher amounts of Fe and bases than in the groundmass from the underlying
horizons and with Si/Al mol = 1.7. X-ray diffractograms of the < 2 p m soil
D.Dubroeucq et al./Geodertna 86 (1998) 99-122
117
fraction at a depth of 60 cm show a decrease of the 1.0 nm peak of halloysite
and a bulge of the baseline between 0.6 nm and 0.3 nm, indicating allophane-like
products, mixed with a significant amount of fine plagioclase grains (Fig. 10,
sample 342). Near the soil surface, changes in the mineral composition of the
clay fraction were even more pronounced. At a depth of 20 cm, X-ray
diffractograms showed a wide hump of the diffraction line centred on 0.44 nm,
indicating allophane-like products, and low peaks at 0.445 nm and 0.74 nm,
indicating small quantity of dehydrated halloysite together with some plagioclase grains (Fig. 10, sample 341). X-ray microdiffraction diagrams have shown
the same results: halloysite peaks are not discernible and two bulges of the
baseline indicate significant contents of noncrystalline products (Fig. 9, sample
34101).
Near the soil surface, other aggregates probably of faunal origin were
observed. They were roundish, 0.03 to 0.08 mm across and dark-brown-coloured
due to the presence of organic matter (Fig. 8d).
3.2.5. The division of the coar-se rninerals
To a depth of 300 cm and above, plagioclase and feldspar fenocrysts were
intensely weathered. Alteration begins from the centre and extends to the
periphery of the minerals by way of radiating and irregular anastomosed fingers
of amorphous material. Upon ongoing weathering, increasingly small crystal
clumps of 0.5 to 1 mm in size were isolated (Fig. 8b). In the middle part of the
profile, between depths of 100 to 150 cm, micromorphological observations
have shown that all the grains were fissured and smaller than 0.6 mm, with an
average size of 0.2 mm. From the surface to a depth of 80 cm, plagioclase and
feldspar grains were of about 0.5 mm and slightly weathered, attesting to the
presence of another material, presumably an ashfall.
From the bottom to the middle part of the profile, the pyroxene grains became
progressively smaller, round-shaped and deeply fissured, the amphibole grains
were divided into small angular fragments along the cleavage planes. In the
middle part of the profile, the pyroxene grains were impregnated with thick
Fe-oxide coatings such as iron boxwork crystal pseudomorphs (Nhon, 1991)
and gradually transformed into nodules. Between depths of 40 to 100 cm, small
Fe-oxide nodules, relics of pyroxenes from the underlying material were observed together with sharp pyroxene fragments from the fresh volcanic deposits
on the topsoil.
4. Interpretation and discussion
4.I. The first stages of weathering
The first stages of weathering in these soils are not very different from those
described in wetter climates (Jongmans et al., 1995; Quantin et al., 1991).
118
D. Dubroeucq et al. / Geoderma 86 (I998) 99-122
Yellow isotropic products were observed as coatings on glass shards, fenocrysts
and rock fragments and also as infillings in the microlitic matrix. At the
microsite scale, these coatings appeared mainly as noncrystalline products in
X-ray microdiffractions. But the chemical extractions have revealed only low
allophane contents (Table 2), and the X-ray diffractions on the < 2 ,um fraction
have evidenced more halloysite clay than noncrystalline minerals. This has
probably two causes. The first is the fine-sized particle separation, which has
excluded the coarse minerals and the rock fragments in which features of initial
weathering were abundant. The second could be a significant content of these
products in amorphous siliceous iron oxides or proto-ferrihydrite (Parfitt and
Childs, 19831, which are more easily extractable in DCB than in acid oxalate,
and were found to be formed in ustic or drier moisture regimes rather than udic
moisture regime (Wakatsuki and Wielemaker, 1985).
4.2. The origin of the fine groundnznss of the soils
In profile A, the colourless and noncrystalline coatings appeared as direct
products of the weathering of the rhyolitic glass shards. In profile B, micromorphological observations have shown that the yellow coatings develop from the
coarse minerals in a centrifugal way, retaining the outline of the mineral
Table 5
Estimation of the mineralogical composition of the sand sized grains in profiles A and B, in
percent of the total number of grains
Profile A
Profile B
Horizon
Horizon
BCb
Ap
140
240
20
70
130
180
.o
0.8
-
-
-
-
33.6
23.0
0.5
-
-
7.9
57.9
76.9
79.7
55.1
13.8
-
91.6
-
10.3
62.4
-
26.2
4.4
11.5
15.1
7.2
0.8
85.5
0.3
1.0
13.0
28.6
37.0
21.7
6.6
6.0
10.3
50.1
34.5
2.0
3.2
25.7
42.7
17.5
3.3
8.9
21.4
38.2
25.3
6.1
8.9
24.7
39.9
14.4
6.6
14.4
15.6
24.5
18.2
22.4
19.3
10.2
31.0
2.3
21.4
35.0
AB
CRb
Depth (cm)
40
Light mineral
Quartz
Feldspar
Biotite
Rhyolitic glass
Dacitic glass
Rhyolitic pumice
Heavy mineral
Augite
Hypersthene
Green hornblende
Brown hornblende
Opaque
1
-
B
Bb
BCb
Depth (cm)
-
-
-
-
20.2
-
D. Dubroeucq et al./ Geodenna 86 (1998) 99-122
119
(Nahon, 1991). This yellow, optically amorphous fine mass is thus the product
of an in situ weathering.
The genesis of the brown fine groundmass depends on the evolution of the
yellow coatings. In the soil lamellae of profile A, a replacement of the
colourless isotropic coatings by a brown fine groundmass has been observed,
correlative with the formation of 1.O- nm halloysite and interstratified halloysite-smectite clay minerals and with increasing Fe contents. In profile B, a
gradual fragmentation of the yellow coatings with progressive admixtures of
brown fine groundmass have also been observed. These two materials have
different compositions. The yellow fine groundmass is composed of 1.0-nm
halloysite accompanied by Fe oxides and cristobalite. The brown fine groundmass is composed of 1.0-nm halloysite with small particles of hornblende and
plagioclase. This difference reveals certainly different origins. The yellow
coatings are a weathering product of the dacitic ash-and-blocks flow, whereas
the brown groundmass results presumably from the weathering of the rhyodacitic ash of the topsoil which contains more rhyodacitic glass, green homblende and quartz (Table 5).
4.3. The existence of allophane in the topsoil
Both profiles have small amounts of allophane in the topsoil. This mineral is
the product of the initial weathering of fresh minerals. Since the ultimate
explosive event of the volcano is 38,800 yr BP, another newer source of
wind-transported minerals is probable.
4.4. The sandy-loanz texture of the soils
Micromorphological observations showed that, as weathering proceeds,
feldspar and hornblende fenocrysts were progressively divided into small grains,
pyroxenes were transformed into fine nodules and glass shards were etched and
considerably reduced in size. This evolution could explain the vertical particle
size distribution of the profiles which shows a decrease of the 200-2000 p m
fraction and an increase of the 0-2 p m and 50-200 p m fractions between
unweathered and weathered parent materials. This general evolution is modified
in the topsoil where an increase of the 50-200 p m and 20-50 p m fractions,
and a medium size of the particles of about 50 p m is noticed (Table 3). The
decrease of the 0-2 p m fractions and increase of the 20-200 p m fractions in
the surface horizon could be interpreted as an input of fine sand from aerial dust.
But this does not correspond to the particle size of deposited dust registered in
West Africa (Drees et al., 1993). This could be also interpreted as the deflation
of the 0-20 p m fraction and the relative increase of the remaining 20-200 p m
fraction, more in accordance with the local observations of wind erosion effects
on cultivated soils.
120
D. Dubroeiicq et al. / Geoderma 86 (1998)99-122
4.5. The microaggregate structure of the topsoil
Another observation was the progressive dehydration of halloysite and concomitant development of microaggregates near the soil surface. These microaggregates are different from biological features. The presence of dehydrated
halloysite clay together with allophane is an undisputed indicator of climatic
changes towards dryer conditions. Desiccation may cause dramatic changes in
volcanic soils and, in particular, the irreversible dehydration of crystallized and
noncrystalline clay minerals (Shoji et al., 1993b). Since biological activity was
not reported as the factor of microaggregation, desiccation, probably consecutive
to cultivation, is thought to be the main cause of the loose structure of these
soils in the upper horizons.
4.6. Susceptibility to wind erosion
Wind erodibility depends not only on low contents of clay and organic matter
(Kemper et al., 1987) but also on the quantity of erodible particles (Chepil,
1950, 19511, the size of which has been experimentally established at 840 p m
for aggregates (Gillette, 1978) and 85-95 p m for simple grains (CoudBGaussen, 1994). The surfaces of the two soils studied, being predominantly composed
of microaggregates and grains of lesser values, respectively, and about 500 p m
and 50 pm, are therefore very susceptible to be wind eroded.
5. Conclusion
The two studied soils are formed of superimposed volcanic materials of
different composition and ages, spanning from recent surficial aeolian deposits,
subsurface layers at most of 38,800 yr old and underlying materials undoubtedly
much older. In the zone of contact with the primary minerals, the initial products
of weathering are amorphous coatings in which silica predominates. In the outer
zone of the minerals, the noncrystalline products are transformed into halloysite
clay, accompanied by a notable depletion in bases, an enrichment in.Fe and a
progressive division of the coarse minerals into small grains 50-200 p m in size.
The result is the formation of a groundmass embedding small-sized grains.
Under semi-arid conditions the small grains are preserved even in the subsurface
horizons and contribute to form sandy-loam textured soils.
In the topsoil, the fine mineral grains are slightly weathered. This fine
material is presumably a surficial import of fresh volcanic ash from aerial dust.
The weathering products are Si-rich allophane and dehydrated halloysite. These
clay minerals are assembled in a loose microaggregate structure interpreted as a
result of their desiccation.
D.Dubroeucq et al./ Geodernta 86 (1998) 99-122
121
All these sub-processes of weathering lead to the formation of soils without
any cohesion and very exposed to wind erosion.
Acknowledgements
This research was undertaken in ORSTOM laboratories, Bondy, France. It
was supported in part by 'the Mexican National Council of Science and
Technology (CONACYT) and by ORSTOM. We are grateful to A. Bouleau for
assistance in scanning electron microscopy, to G. Millot for help in X-ray
diffraction analysis, and to M. Delaune for carrying out heavy mineral determination and automatic particle-size analysis. Special thanks are extended to N.
Portilla (Instituto de Ecologia, Mexico) for helpful technical assistance in soil
analysis.
References
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Soil Thin Section Description. Waine Research Publ., England, 153 pp.
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Koukoui, M., 1969. Caractéristiques de quelques sols d'Equateur dérivés de cendres volcaniques: Part II. comparaison de I'évolution de quelques sols des régions tropicales chaudes et
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Volcanic Ash Soils
Genesis, Properties and Utilization
by S. Shoj M. Nanzyo and R.A. Dahlgren
Developments in Soil
Science Volume 21
Jolcanic eruptions are
pnerally viewed as.
agents of destruction,
ret they provide the
)arent materials from
which some of the most
xoductive soils in the
world are formed. The
iigh productivity results
rom a combination of
inique physical,,
:hemical and
n ineralog¡cal
xoperties. The
mportance and
iniqueness of volcanic
ish soils are
?xemplifiedby the
ecent establishment of
he Andisol soil order in
;oil Taxonomy. This
mok providesthe first
:omprehensive
tynthesis of all aspects
)f volcanic ash soils in
I single volume. It
:ontains in-depth
:overage of important
opics including
erminology,
norphology, genesis,
:lassification,
nineralogy, chemistry,
,hysical properties,
)roductivity and
tilization. A wealth of
ata (37tables, 81
gures, and Appendix)
iainly from the Tohoku
hiversity Andisol Data
Base is used to
illustrate major
concepts. Twelve color
plates provide a
valuable visual-aid and
complement the text
description of the
world-widedistribution
for volcanic ash soils.
This volume will serve
as a valuable reference
for soil scientists, plant
scientists, ecologists
and geochemists
interested in
biogeochemical
processes occurring in
soils derived form
volcanic ejecta.
Short Contents:
1.Terminology,
Concepts and
Geographic Distribution
of Volcanic Ash Soils.
2. Morphology of
Volcanic Ash Soils.
3. Genesis of Volcanic
Ash Soils.
4. Classification of
Volcanic Ash Soils.
ELSEVIER
SCIENCES
5. Mineralogical
characteristicsof
Volcanic Ash Soils.
6. Chemical
Characteristics of
Volcanic Ash Soils.
7.Physical
Characteristics of
Volcanic Ash Soils.
8.Productivity and
Utilizationof Volcanic
Ash Soils. Appendices.
References Index.
Subject Index.
1993 312pages
Dfl. 290.00
(US $165.75)
ISBN 0-444-89799-2
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