Millot, G.

Sieffermann, Gaston

Horizon / Pleins textes

\ 
EQUATORIAL AND TROPICAL WEATHERING OF RECENT 
BASALTS FROM CAMEROON : ALLOPHANES, HALLOYSITE, 
METAHALLOYSITE, KAOLINITE AND GIBBSITE 
GASTON SIEWERMANN and GEORGES MILLOT 
Office de la Recherche Scientifique et Technique Outre-Mer 
Institut de Géologie de l'université Strasbourg 
ABSTRACT 
The evolution of allophanes in young soils in Cameroon, developed on basalts under different tropical 
conditions, were studied: 
Under a mean annual rainfall of 10 in, without a dry season, allophanes change into gibbsite and well- 
crystallized kaolinite. 
Under a mean annual rainfall of 3 to 6 m, with a dry season, allophanes give rise to halloysite and 
metahalloy site. 
Under a mean annual rainfall of 1.5 m, with a long dry season, the allophane stage is very restricted 
and metahalloysite appears rapidly. 
In the two climatic zones with heavy rainfall, the upper horizons of the soils with allophane contain 
many diatoms, the silica of which plays a role in the neoformation of clay minerals. 
The distribution of rainfall controls the rate of desaturation of soils and the seasonal concentration 
of solutions, which are the two principal factors governing neoformation. 
INTRODUCTION 
Many areas of the Federal Republic of Cameroon are covered with volcanic and 
pyroclastic rocks, scoria, tuffs and ash of basaltic composition. The genesis of soils 
and the weathering of these rocks were studied in three different climatic zones 
(Fig. 1). 
Zone I-Equatorial climate, always humid, on the western flank of the Cameroon 
Mountain. 
417 
mm.. I I 
r 
Z O N E  I 
I/ 3 F n A n :
CAMEROON 
FIG. 1. Localization of the three climatic zones and annual rainfall distribution 
Zone II -Equatorial climate, more moderate, with a dry season, on the south- 
Zone III -Tropical climate, with a well-marked dry season, on the Central 
eastern and eastern part of the Cameroon Mountain. 
Adamawa Plateau of Cameroon. 
Nomenclature 
The term allophane here designates amorphous silica-alumina products with 
a Si0,/A1,03 molecular ratio between 0.9 and 1.9. The term halloysite designates 
the hydrated halloysite or hydrohalloysite of 10 A spacing. The term metahalloysite 
designates the dehydrated halloysite of 7.4 A spacing (Caillère and Hénin, 1963 ; 
Brindley et al., 1966). 
ZONE I. ANDOSOLS DEVELOPED FROM RECENT BASALTIC ROCKS 
ON THE WESTERN PART O F  THE CAMEROON MOUNTAIN 
The climate is equatorial, with a mean annual precipitation of about 10 m (400”) 
and without a dry season. Mean annual temperature is 29°C. Drainage is good. 
Six soil profiles were studied, varying in thickness from 30 to 140 cm. They are 
“weakly developed soils formed on volcanic deposits” (oxy-hydrandepts), rich in 
organic matter, with A(B)C profile. The sum of exchangeable bases, measured on 
the clay fraction, varies between 3 and 10 meq.1100 g at the bottom and between 1 
and 4 meq./100 g at the top. Base exchange capacity is between 20 and 32 meq./100 g. 
Base saturation (ratio of the sum of exchangeable bases to base exchange capacity) 
is low: 5%. The pH varies between 6.4 and 5 from bottom to top. 
The clay fraction (< 2 p) is characterized by a SiOz/A1,03 molecular ratio 
between 1-1.3. Thermal analysis and X-ray diffraction reveal predominance of allo- 
phane with a small admixture of gibbsite and well-crystallized kaolinite. The com- 
position of the amorphous fraction, analysed by the method of Segalen (1968), 
is as follows: SiO,, 23%; Alzo3, 32%; iron hydroxides, 27%; HzO, 18%. The 
specific surface varies between 120-400 m2/g. 
All these soils contain’ numerous diatoms in their upper part. The fragments of 
their tests frequently constitute more than 10% of the < 2 p fraction, and this occurs 
down to 50 cm in depth (Plate I). 
Electron micrographs show fuzzy flakes which can be considered to be allophanes 
(Plate II), as shown or described by many authors: Birrel and Fieldes (1952); Aomine 
and Yoshinaga (1955); Sudo and Takahashi (1956); Fieldes et al. (1955, 1966); 
Yoshinaga and Aomine (1962a); Alonso et al. (1963); Chukhrov et al. (1963, 1964); 
Guiseppetti et al. (1963) ; Robertson (1963) ; Kirkman, Mitchell and Mackenzie (1966) ; 
Colmet-Daage et al. (1967); Sieffermann, Yehl and Millot (1968). But one must 
- i r  
1. Test of diatom (d) coated with allophane (a)- x 14,000 
PLA? 'E I. Samples 
2. Fragment of diatom < 0 . 2 ~  (d) in an allophanic mass (a) 
- x 100,000 
from Zone I 
3. Unperforated fragment of diatom (d) coated with allophane 
(a)- x 70,000 
4. Fragment of diatom in process of dissolution (d) coated 
with allophane (a) - x 45,000 
PLATE I. Samples from i o n e  I 
5. Felt-like facies of allophane (a)- x 100,000 6 .  Fuzzy flakes of allophane (a) - x 100,000 
4 
PLATE II. Facies of allophanes of Zone I 
~ ---- * -._-ll-__-_l - -.--I--_..-l__.-.-l_-_.__I_____.-._____ _.I.__.__I _ _  . . __ 
7 
P 
7. Allophane (a) and unidentified mass in process of organiza- 
tion- x 100,000 
8. Allophane (a) and kaolinite (k)- x 100,000 
PLATE II. Facies of allophanes of Zone I 
. 1 
. :s 
note in addition the presence of well-crystallized kaolinite (Plate II, 8 and Plate 
III, 9) and gibbsite. Neither tubular- nor globular-shaped halloysite, nor fibrous 
allophane were observed. 
ZONE II. ANDOSOLS DEVELOPED FROM RECENT BASALTIC ROCKS 
ON THE EASTERN AND SOUTHEASTERN PART OF 
THE CAMEROON MOUNTAIN 
The climate here is equatorial, with a mean annual precipitation between 2.5 and 
6 m (90-250"). A moderate dry season occurs three months of the year. The mean 
annual temperature is between 27-29°C. Drainage is good. 
Eleven profiles, ranging from "weakly developed soils formed on volcanic de- 
posits" to "eutropic brown soils of tropical lands" were studied. They are generally 
A(B)C profiles ranging from 20 to 100 cm in thickness. The sum of exchangeable 
bases, measured on the clay fraction, is between 2-20 meq./100 g at the bottom 
and between 8-30 meq./100 g at the top. Base exchange capacity ranges from 
20-60 meq./100 g at the bottom to 40-75 meq./100 g at the top. Base saturation 
vanes between 4-20% at the bottom, and 20-50% at the top. The pH ranges from 
6 at the bottom to 5 at the top. 
The < 2p fraction is characterized by a SiO2/AI2O3 molecular ratio between 
0.9-2 at the bottom and between 1.6-2 at the top. The amorphous fraction analysed 
by the method of Segalen shows, as previously, a Si0,/A1203 molecular ratio 
close to 1, but with a smaller percentage of iron hydroxides than in Zone I. The 
specific surface varies from 80-400 m2/g. Thermal analysis and X-ray diffraction 
reveal a mixture of allophane, halloysite (10 A), metahalloysite (7.4 A) and disordered 
kaolinite, with traces of gibbsite at the bottom. 
Electron micrographs show allophanes in the form of either amorphous fuzzy 
flakes or exceedingly fine, fibrous and thread-like particles (Plate III, 10) [Yoshinaga 
and Aomine (1962b); Aomine and Wada (1962); Aomine and Miyauchi (1965); 
Miyauchi and Aomine (1966); Jaritz (1967); Wada (1967); Yoshinaga et al. (1968)l; 
tubular- or globular-shaped halloysite (Plate III, 11 and 12) [Kinoshita and Muchi 
(1954); Sudo and Takahashi (1956); Kurabayashi and Tsuchiya (1960); Sieffermann 
and Millot (1968)l; and the outlines of hexagonal flakes. 
At the bottom of the profiles allophane is the dominant phase, whereas at the top 
halloysite and metahalloysite predominate, which indicates a replacement of allo- 
phane by halloysite. 
I 1 
426 International Clay Conterence, i Y b Y  
ZONE III. SOILS DEVELOPED FROM RECENT BASALTIC ROCKS 
ON THE CENTRAL ADAMAWA PLATEAU OF CAMEROON 
The climate is the tropical mountain climate of Adamawa. Mean annual-irecip 
itation is between 1.5 and 1.6 m, with a well-marked dry season lasting five months 
The mean annual temperature is 23°C and drainage is good. These soils range fron 
“weakly developed soils, formed on volcanic deposition” to “humic weakly de 
saturated ferrallitic soils.” 
.Six soil profiles were studied, ranging in thickness from 40 cm-2 m. The sum o 
exchangeable bases varies from 9-15 meq./100 g at the bottom, and from 15-21 
meq./100 g at the top. Base exchange capacity is between 15-30 meq./100 g a 
the bottom and between 30-50 meq/ lOOg at the top. Base saturation is betweei 
50-80%, and the pH varies between 7.3-6 from bottom to top. 
The clay fraction is characterized by a Si0,/A1,03 molecular ratio of 2-1 
for the youngest soils, reaching 2 for the most advanced ones, and contains onl! 
a small amount of allophane. The specific surface of the clay fraction varies fron 
74-150 m2/g. 
Thermal analysis and X-ray diffraction show that the mixture contains littlc 
or no allophane and that it is composed of halloysite (abundant at the bottom 
and of metahalloysite (abundant at the top) (Sieffermann and Millot, 1968). 
Electron micrographs show amorphous masses corresponding to iron oxides 
tubular forms of halloysite or metahalloysite, and abundant globular-shaped ha1 
loysite. These globules either have a distinct helical or concentric structure and arc 
ovoid or polyhedral in form. In many cases the winding consists of rectilinea 
fragments with curved ends. These elements sometimes seem to separate from thc 
globules, whose crystalline nature can be discerned by electron microdiffraction 
If disaggregated by ultrasonic energy, they yield the X-ray diffraction peaks of ha1 
lo ysite. 
In some profiles the only mineral occurring at the bottom is halloysite, while il 
all the profiles metahalloysite predominates at the top. This indicates a replace 
ment of halloysite by metahalloysite. Soils from the earlier Quaternary are simila1 
but gibbsite is even more highly developed at the top of these profiles. 
INTERPRETATION 
Allophanes 
It is difficult to purify allophanes. In particular the presence of diatoms, consti 
tuting up to 20 % of the clay fraction (Plate I, 1,2,3 and 4), makes the interpretatio] 
of the Si0,/A1,03 molecular ratio (0.9-1.5) difficult. Moreover, the silica of th 
428 International Clay Conference, 1969 
diatoms in contact with allophane having a high dumina content may be incor- 
porated into newly formed clay minerals. (Plate I, 3 and 4). 
Under an equatorial climate the genesis of allophane by weathering of basaltic 
material is very important. We think that humidity and continuous leaching favor 
the preservation of allophane. 
Kaolinite and Gibbsite in Andosols 
In Zone I, which is equatorial and permanently humid, allophanes are always 
present in the form of amorphous fuzzy flakes; never as imogolites (Plate II). Hal- 
loysite or metahalloysite in the form of tubes or globules never occur. But the clay 
fraction of these andosols may contain up to 10% of very beautiful plates of well- 
crystallized kaolinite (Plate III, 9) and up to 20% gibbsite, especially at the bottom 
of the profiles. 
Local desaturation permits direct crystallization of kaolinite, particularly using 
the silica of diatoms. 
More important is the direct genesis of gibbsite immediately upon onset of weather- 
ing. This occurs in a desaturated, always humid environment, without any dry season 
and without transition through halloysite. 
Halloysite and Metahalloysite 
Halloysite can form in two habits: 1) as tubes only. In profiles, the exclusive oc- 
currence of tubes often coincides with the presence of amorphous products rich 
in thread-like, fine, fibrous particles (Plate III, 10 and 11) ; 2) as globules with defined 
form and crystallinity. They occur in soils in which allophanes as amorphous flakes 
predominate (Plate III, 12). Subsequently, these globules give rise to tubes (Plate III, 
The genesis of halloysite takes place only in Zones II and III, where an annual 
dry season occurs. In Zone III, where the dry season is very intense, allophanes are 
characterized by a high SiO2/AI2O3 molecular ratio (2 to 3), and the neoformation 
of halloysite is fast enough to keep the allophane content down to a few percent. 
At the top of the profiles of Zone II, and particularly of Zone III, metahalloysite 
replaces halloysite and in older soils metahalloysite becomes predominant. In very 
ancient soils gibbsite is formed at the top of the profiles by degradation of metahal- 
loysite (Sieffermann et al., 1968). 
12). 1 
Evolution According to Climate and Enze 
Two weathering sequences can be discerned: 
a) Parent rock + allophane + kaolinite seizm stricto + gibbsite (Zone I) 
b) Parent rock .-+ allophane --f halloysite -+ metahalloysite + gibbsite (Zones II 
and III) 
I 
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I 
Weathering of Recent Basalt from Cameroon 429 
The first sequence is typical of an equatorial environment which is always humid. 
Well-crystallized kaolinite and gibbsite form early in andosols, from basaltic ma- 
terials. 
The second sequence and gibbsite formation under different conditions were 
reported in part or entirely by several authors: Sudo (1953); Fieldes et al. (1955, 
1966); Sudo and Takahashi (1956); Jackson (1959); Aomine and Wada (1962); 
Gastuche, Fripiat and De Kimpe (1962); Keller (1963); Wada (1967); Pedro and 
Lubin (1968); Sieffermann and Millot (1968); Trichet and Svoronos (1968). Here 
it is described, beginning with basalts in intertropical regions : 
in soil profiles (vertical variation); 
in the transition from a humid equatorial climate to intermittent tropical climate; 
in the course of time. 
Geoclzernical Evolution of Silicates 
In intertropical regions basaltic material of fine grain size is rapidly hydrolyzed, 
giving rise to allophanes. 
Gibbsite and well-crystallized kaolinite may occur in the initial stages of weather- 
ing in permanently humid regions, which lack annual dry seasons. 
In the less leached zones, which have a dry season, halloysite occurs. The seasonal 
concentration of solutions makes crystallization easier, but the weaker desaturation 
gives rise to disordered and hydrated crystalline lattices. 
In the upper horizons, which are preferentially exposed to dehydration, meta- 
halloysite replaces halloysite. 
In the course of time a second-generation gibbsite can occur, but this requires 
both desaturation and removal of silica by seasonal desiccation. 
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sols Ci hydroxydes, 29 p., Publ. O.R.S.T.O.M. 
Gaston Siefferman and Georges Millot 
ISRAEL UNIVERSITIES PRESS 
Jerusalem, 1969 
P E D O L O G I E  
EQUATORIAL AND TROPICAL WEATHERING 
OF RECENT BASALTS FROM CAMEROON: 
ALLOPHANES, HALLOYSITE, METAHALLOYSITE, 
KAOLINITE AND GIBBSITE 
REPRINTED FROM THE 
PROCEEDINGS OF THE INTERNATIONAL CLAY CONFERENCE 
TOKYO, 1969, VOL. I 
JAN, 4638 
O. R.S.T.O.M. Fonds Documentaire 


Equatorial and tropical weathering of recent basalts from Cameroon : Allophanes, halloysite, metahalloysite, kaolinite and gibbsite

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