Variability of Clays from Gounioube Deposit (Ivory Coast)
A. Abba Toure,
The aim of this study is to estimate, from the statistics (PCA), the variability of the clays from the deposit of Gounioube, in order to choose the different grades convenient for a given application and eventually correct the defective ones. Indeed, most of industrial minerals are used in fields where benefit is taken from variable properties that, directly or not, govern their applications. So, fourteen samples of kaolin clays from Gounioubé deposit (Ivory Coast), have been studied by chemical, crystallographic (X-Ray Diffraction (XRD)), spectroscopic (IR, UV, EPR), particle size and textural analysis.
Kaolin is a major industrial mineral used in very diversified applications
(papers, polymers, ceramics, cosmetics, drugs). Though the production countries
are significantly changing these last years (mainly Brazil) and paying account
of ambiguous definitions, the annual world output of based on kaolinite materials
is stable and estimated to 45 millions of metric tons (USGS,
The beneficiation of these materials in each domain needs to know and understand
the links between intrinsic physicochemical properties of the material and their
macroscopic behaviours. In addition, any one that try to transform a raw material
needs to know as early as possible in the time and in the upstream of the process,
what are the best raw materials and the most convenient process leading to optimal
service aptitudes of the final grades. Initially, this approach does not consider
how the mechanisms by which the properties of components or the processing conditions
rule the final efficiency of the composite, but it defines a set of ranking
criteria that lead to evaluate what quality of mineral can enter under satisfying
conditions in what compound, paying account of specific conditions (Yvon
et al., 2002). At last, the efficiency of developing trials in countries
where the applications of the mineral industry are not still integrated into
the economic activity need to avoid a strict copy of products used in northern
markets, but for being successful have to develop products adapted to local
This study aims at evaluating the variability of the Gounioubé deposit based upon the knowledge of the specific mineralogy, crystal-chemistry and physical-chemistry of kaolins from this deposit, in connection with their possible uses.
MATERIALS AND METHODS
Location and geology of the deposit: Kaolins samples have been collected
on the Gounioubé deposit (City of Anyama, Ivory Coast). The field is
mainly out of a detrital sandy-clay plateau from the terminal continental level
(miocene-pliocene), resulting from a ferallitic alteration of the basal rocks
under humid tropical climate (Dorthe, 1964).
Methods: Fourteen samples have been collected in different pits, at
different depth of the deposit of Gounioube, a city located at 30 km from Abidjan
in the south of Côte d’Ivoire (Andji, 1998).
The samples have been 5 sec ground at 1400 rpm in a cylinder mill until obtaining
less than 3 mm fractions. After that, they have been homogenised and quartered
into 50 g specimens. The specimens were dispersed in 1.5 L of ultra pure water
using a 520 rpm rotative stirrer until homogenisation of the suspension. The
initial pH, close to 5, was increased at 9 with ammonia. The suspension was
then decanted and sieved at 30 μm. The fine fractions were submitted to chemical,
crystallographic and spectroscopic, Infra Red (IR) and Ultra Violet (UV), Electron
Paramagnetic Resonance (EPR), analysis; microscopic determinations were carried
out using Transmission Electron Microscopy (TEM); particle size determination
and measurement of specific surfaces were also determined.
Chemical analysis were obtained at the Centre de Recherches Pétrographiques
et Géochimiques (Nancy, France) by Inductively Coupled Plasma-Atomic
Emission Spectroscopy (ICP-AES) for major elements and Inductively Coupled Plasma-Mass
Spectroscopy (ICP-MS) for trace elements, after fusion with LiBO2
and dissolution in HNO3. Analytical conditions and limits of detection
are found by Carignan et al. (2001).
Sulphur is oxidized into sulphuric acid in an induction furnace and then titrated by impulse coulometry using an Hermann-Moritz device. The relative preciseness reaches 5% around 0.3% S and 20% around 0.01% S.
Structure water H2O+ and hydration water H2O–
were titrated according to Penfield’s method.
X-Ray Diffraction (XRD) analyses were carried out using the following devices
||Disoriented powder diffraction diagrams were obtained using a Jobin-Yvon
Sigma 2080 diffractometer working by reflection using the copper Kα1
radiation. Oriented preparations were analysed by reflection with a Bruker
D8 device using the cobalt Kα1 radiation
Disoriented total rock diffractometry: The diffractograms show the whole
set of (hkl) reflections of kaolinite and associated minerals, mainly quartz,
micas and anatase (Fig. 1). The crystal defects lead to variations
of the (hkl) profile lines. Here, R1 and R2 indexes (Table 1)
according to Lietard (1977) have been used, they measure
the amount of random defects in the (a, b) plane.
Diffractograms on oriented preparations: The X-ray diffractometry on
oriented preparations allowed us to measure the angular width at half intensity
of the (00l) line of kaolinite, in order to deduce the coherent scattering thickness
(Table 1) along the c axis that measures the packing order
according to Scherrer (1918).
The absorption spectra are carried out in the field of the mean infrared, that is to say for numbers of wave ranging between 400 and 4000 cm-1 (Fig. 2).
The half cell of kaolinite contains four hydroxyl groups the O-H stretching
vibration of which are revealed by four absorption bands at 3655, 3670, 3695
cm-1 for external OH and at 3620 cm-1, for internal OH
(Farmer, 1964). The bending vibrations of the Al-OH group
are revealed by a large band centred around 915 cm-1 with a shoulder
at 938 cm-1 on its high frequency side.
||Example of diffractograms obtained (sample G14)
||Example of infrared spectra obtained (sample G9)
For evaluating the crystal order, the two index P1 and P2 defined by Lietard
(1977) and Cases et al. (1982), have been
used (Table 1). P1 is the slope of the segment joining the
band at 938 cm-1 to the band at 915 cm-1; a positive slope
reveals a well defined band, a negative one reveals a simple shoulder. P2 is
the ratio between the 3672 and the 3655 cm-1 bands; this ratio is
slightly lower to 1 for well ordered kaolinites and growths with the number
of defects. The spectra have been recorded in transmission or diffuse reflectance
mode, using an IFS55 Bruker device with a usual spectral resolution of 2 cm-1.
A double beam spectrometer equipped with an integration sphere has been used
(Shimadsu UV 2100) working in visible and ultra violet (800-200 nm), on slightly
pressed powders. The spectroscopic data allow to calculate the conventional
parameters L*, a* and b*. These data respectively correspond to white-black,
red-green and blue-yellow axis that allows to place the samples in the trichromatic
space, the calculation are carried out according to the usual wave lengths 458,
570 and 770 nm. Their formulas are as follows:
L*=116 (Green filter absorbance/100)1/3
a*=500 [(Red filter absorbance/100)1/3–(green
filter absorbance/100 )1/3]
b*=200 [(Green filter absorbance/100 )1/3–
(blue filter absorbance/100)1/3]
IB, IJ and Eab* parameters, respectively represent brightness, yellow index
and the difference between the pure white (L* = 100, a* = 0, b* = 0) they have
been calculated using the usual following formulas:
IB=(4×Blue filter aborption) – (3xGreen filter
Eab*=[(L*–100 )2 + (a*–100)2
+ (b*–100)2 ]
where, X, Y and Z are three chromatic coordinates, respectively measuring the
red, green and blue colour, of the materials.
The specific surfaces (Table 2) have been deduced of the
77 K nitrogen adsorption isotherms, according to the usual BET method (Brunauer
et al., 1938). The adsorbometer is a conventional step by step volumetric
device built by LEM (Nguetnkam et al., 2005).
The samples were outgased under a residual pressure of 10-4 Pa for
18 h at 110°C.
The particle size analysis (Table 3) has been determined
by laser diffraction using a Malven Mastersizer MS 20 device operating from
0.1 to 600 μm, the particle size distributions lead to the passing sizes
at 90, 75, 50 and 25%, respectively corresponding to d90, d75,
d50 and d25 values.
||Specific surfaces (BET) and Cation Exchange Capacity (CEC)
||Particle size analysis (μm)
The cation exchange capacity of a clay material (Table 2)
is the number of equivalent positive charge for neutralizing its negative charge
at pH 7. It has been determined by deplacing the compensating cations using
cobalt-hexammonium (Co[(NH3)6]3+) cation according
to the method developed by Mantin and Glaeser (1960)
and Remy and Orsini (1976). The exchanged species have
also been titrated in the exchange solution.
The different phases of the materials were determined by Energy Dispersive Spectroscopy (EDS) using a Philips SM 20 transmission microscope working at 200 kV and equipped with a PGT model EDS with ultra thin windows. Samples have been sonicated in ethanol, sampled as a drop and spread on a copper supported carbon grid.
The fundamentals of calculation used here for determining the quantitative
mineralogy of specimens are completely described by Yvon et
al. (1990). The vector of the elementary composition (CHIMe) of a mixture
of minerals can be described as the product of a nominal amount matrix (STOEC)
by a vector of the phase amounts (X), then:
This equation admits a less square solution as:
where, STOECT is the STOEC transposed matrix. One can appreciate
the quality of the determination by calculating the residues based on :
With the absolute residue
If the residue coordinates are positive or close to zero and if the mineralogical composition is in qualitative agreement with the other analyses, (DRX, DRIFTS,), then the determination is considered as valid.
The whole set of data has been processed through normalized Principal Component
Analysis according to Lebart et al. (1979).
RESULTS AND DISCUSSION
Nature of phases and associated elements: A preliminary evaluation of
the Gounioubé deposit (Andji et al., 2001),
using conventional methods revealed that the coarser fractions of the crude
material are out of kaolinite associated to quartz, potassic feldspars, maghemite,
gœthite, rutile, illite, gibbsite and small quantities of phosphoferrite.
For evaluating the variability of beneficiable fractions, the crude ore was
sieved at 30 μm the chemical analysis (Table 4, 5)
and mineralogical determination (Table 6) of this fraction
were carried on. In addition, the illite composition was determined by TEM/EDS
analysis that also, revealed the presence of swelling clays (Fig.
Quantitative mineralogy: Kaolinite, quartz, anatase and/ or rutile and
iron oxy-hydroxydes, are revealed by XRD and DRITF spectroscopy. Both these
techniques give also a semi-quantitative evaluation of the mixture and identify
small amounts of gibbsite.
CBD treatment (Table 7), shows that most of the iron contained
in Gounioubé materials is included in iron minerals, intimately associated
with kaolinite what is a usual occurrence (Hogg et al.,
1975; Delineau, 1994; Delineau et
al., 1994). Then we shall consider the octahedral iron evaluated through
ESR after CBD treatment for defining the unit cell composition of kaolinite,
what leads to the following formula :
Si4 (Al3.98 Fe0.02)
||Transmission electron microscopy/energy dispersive spectrometry,
(a) of a cristal of illite in G9 and (b) of a smectite clay in G13
||Chemical analysis of fraction at 30 μm: percentages of major elements
||Chemical analysis of fraction at 30 μm
|Minor elements express in ppm
Qz: Quartz, Ana: Anatase, Rut: Rutile, Hém.: Hematite,
Gibb: Gibbsite, Go: Goethite, Lép: Lepidocrocite, Sid: Siderite,
Py: Pyrite, Carb: Carbonate
According to the EDS data (Fig. 3) illite and smectites
were defined upon the following principles:
|(Al2-x FeIIIx) (Si4-y Aly)
O10 (OH)2 Ky
|(Al2-x-y FeIIIx Mgy)
(Si4-z Alz) O10 (OH)2 Nay+z-t-u
What leads to
|(Al1.85 Fe0.15) (Si 3.49 Al0.51)
O10 (OH)2 K0.51
|(Al1.7 Fe0.1 Mg0.2) (Si3.7
Al0.3) O10 (OH)2 Na0.06 Al0.13
||Gibbsite is free of iron
||Titanium oxides are grouped under the name Anatase
||The fraction of iron which does not belong to major silicates (kaolinite,
micas and smectites) and non expressed under the form of gœthite is
grouped under the label residual iron, the same for magnesium residual MgO
The stoichiometry matrix (STOECH) that contains the nominal amounts of oxides
in minerals is as follows:
Results of quantitative mineralogy: Numerical results are shown in Table
As expected, kaolinite is the major mineral (48.19-81.81%). The amounts in
smectites (4.4-11.69%); gibbsite (4.85-11.21%), illite (2.7-6.03%) and gœthite
(1.13-19.69%), are not so low that the X-ray diffractometry would suggest; that
can result from the fact that quartz disorient the X-ray preparation what leads
to minimize the response of basal reflections of minor lamellar species. The
residual iron is always positive or equal to zero and the residual H2O+
after calculation is always positive what is in agreement with the fact that
XRD reveals iron oxyhydroxides identified as goethite (FeOOH).
Statistics: The whole set of numerical data (67 parameters, including:
chemical analysis of trace elements, mineralogical composition, order indexes,
colour parameters, CEC, particle size and textural characteristics) was processed
through Normalized Principal Component Analysis (PCA) (Fig. 4A),
this PCA analysis reveals that the variability of the system depends on a large
number of parameters, since the four first factors only explain 42.8% of the
||Along the first axis (F1), in the (F1,
F2) diagram, it can be seen on the West side, that Light Rare
Earth Element (LREE) are associated to R1, R2 and P2 (Lietard,
1977; Cases et al., 1982) order indexes
||Iron amount before and after DCB process
||Numerical results of quantitative mineralogy
Principal component analysis realised with physico-chemistry
parameters on the 14 samples of Gounioubé. (A) projection of variables
in the (F1, F2) plane and (B) projection of samples
in the (F1, F2) plane
Toward the North West side of the diagram, the kaolinite amount is in normal
dependence according to Loss On Ignition, the variation of which is correlated
to chromium amount. This relation derives from a double effect: in this mineralogical
association, kaolinite is a major mineral containing the highest equivalent
of structure water and in addition, chromium traces are statistically correlated
to structure disorders (Cases et al., 1982) that
increase the fineness (Cases et al., 1986) and then the adsorbed water.
Brightness and whiteness are also associated to this pole.
||On the Eastern side, the particle size characteristics (D25,
D50, D75, D90), the gœthite amount
and the yellow index normally vary together. The correlation between the
goethite amount and the yellow index results from the presence of gœthite
in the coarser fractions which bring them a yellow coloration. The variations
of molybdenium, vanadium and arsenic are, in part, correlated to those of
||On the southern side, the variations of mica are associated
to the variations of quartz. This point is usual in secondary kaolins since
during the transportation and the sedimentation process, both quartz and
micas are mechanically sorted in the coarser fractions and then sediment
in beds or levels where kaolinite, usually finer, is less abundant
To the South-West side, the smectite amount is correlated to the amount in
gibbsite and the BET specific surface. This relation probably results from the
fact that the external surface of smectites as described by nitrogen adsorption
(Cases et al., 1992) is usually greater than the
total specific surface of kaolinite (Cases et al.,
1982). However the relation between gibbsite and smectites would be a little
unusual considering a standard in situ lateritic alteration profile that
concentrate gibbsite in the surface horizons and smectites in the deeper (Nguetnkam
et al., 2008). Then this point may be the trace of a paleo-alteration
before sedimentation, without posterior alteration.
On the Southern pole, The illite amount is associated with, rubidium and caesium
What shows that both the elements are mainly associated to the mica-like fraction
(the feldspar amount is very low) of the materials as suggested by the high
statistical correlations between potassium on one hand and rubidium (Fig.
5) and caesium (Fig. 6) on the other hand. It must be
also noticed that these statistical relations show a low dispersion and pass
close to zero, what means that mica-like phase is the only significant vector
of rubidium and strontium with constant Rb/K and Cs/K ratio. It must be also
noticed that the character to be rich in kaolinite is opposed to the BET surface.
At last, the usual relation between niobium and tantalum is observed (Fig. 7). The low dispersion of the distribution and the unique ratio Nb/Ta=12 reveal that the sediments derive from the same parent rocks or from rocks with the same niobiotantalites.
Typology: The projection of samples in the (F1F2)
plane, (Fig. 4B) leads to a typology of Gounioubé
samples. Then, it will be considered that the Gounioubé clays are out
of three main types.
||Correlation between rubidium and potassium
||Correlation between Caesium and potassium
||Correlation between niobium and tantalum
||Type I is represented by samples G2, G4, G7, G8, G9 and G14 the main characteristic
of which is the richness in kaolinite; G9 and G14 are the richest. They
are white, very disordered, with specific surfaces lower than those of the
||Samples G3 and G13 define the type II. They are coarse and their yellow
colour results from the presence of iron under the form of gœthite
||The type III is represented by samples: G1, G5, G6, G10, G11 and G12.
They are characterised by the presence of smectites and 10 Å d-spacing
sheet-silicates, with the highest specific surfaces, compared to other samples
This typology reveals that the clays from Gounioubé deposit cannot be
used as coating pigments for paper industry, due to their low cristallinity,
the presence of smectites and their colour (type II) (Delon
et al., 1982). To the contrary, types I an II, could be used in semi-reinforced
rubber in applications where the colour is not a concern (Yvon
et al., 1991, 2002). Great volume applications
are certainly possible in cast or pressed ceramics for types I and III. The
type II could be specifically dedicated to applications in terra cotta products
(floor tiles, roof tiles, bricks, pottery) (Balavoine, 2002;
Balavoine et al., 2003).
This study was done in the Laboratoire Environnement and Mineralurgie (LEM) of Nancy. We are grateful to all of the members of the LEM for their contribution of this work. We also take this opportunity to thank the french government for financial support.
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