Characterization of El-Tih kaolin quality using mineralogical, geochemical and geostatistical analyses

Characterization of El-Tih kaolin qualityusing mineralogical, geochemical and

geostatistical analyses

A. A. MASOUD1 , * , G . CHRISTIDIS2AND K. KOIKE3

1 Geology Department, Faculty of Science, Tanta University, Tanta 31527, Egypt, 2 Department of Mineral Resources

Engineering, Technical University of Crete, Chania 73100, Greece, and 3 Graduate School of Engineering, Kyoto

University, Kyoto 615-8540, Japan

(Received 30 May 2012; revised 5 September 2012; Editor: John Adams)

ABSTRACT: Detailed multi-scale characterization of the kaolin quality and the controlling

depositional environment is crucial for optimal quality upgrading and for prioritizing potential

exploitation areas. In the present work, the quality of El-Tih kaolin, Egypt, was investigated using the

chemical/mineralogical characteristics as well as the field observations of the clay. Chemical analysis

of major oxides was carried out using energy dispersive X-ray fluorescence (EDS-XRF)

spectrometry. Mineralogical analyses were carried out using X-ray diffraction (XRD) and scanning

electron microscopy coupled with wavelength-dispersive X-ray spectroscopy (SEM-WDS). Spatial

heterogeneity of the quality was evaluated applying kriging geostatistical techniques and potential

zones were identified.

Results clarified an upward gradual deterioration of the quality via a decrease in the Al2O3 content

and thickness of the clay layers, and an increase in the TiO2 content. According to the kriging maps,

areas of high potentiality indices (PI) characterized by high Al2O3 and low SiO2 content and

maximum thickness of the kaolin are located to the west and east, and decrease toward the central

part of the study area. The high PI zones are dominated by pseudo-hexagonal platy kaolinite, often

forming accordion- and book-like aggregates with subordinate quartz and traces of Fe and Ti oxides,

yielding minimal TiO2 and Fe2O3 contents. These zones of high PI are considered optimal for

exploitation. Kaolinite was formed as a result of intensive weathering of rhyolite/granite and basalt in

the source area, and subsequent erosion, transportation and deposition of the weathering mantles in a

flood environment with marked depositional energy variations. Results allowed comparison with

worldwide kaolin occurrences, and suggested the suitability of the studied kaolins for use in paper

coating and filling and in higher-grade ceramics, after removal of free Fe- and Ti-oxide impurities.

KEYWORDS: kaolin, potentiality modelling, spatial variability, depositional environment, oxide minerals, Sinai,Egypt.

Due to its significant physical and chemical

properties and inertness, kaolin is considered a

valuable, versatile and widely used industrial clay

with numerous applications in high-quality paper,

ceramics, plastics, rubber, paint, pharmaceuticals

and cosmetics and many others (Murray, 2007;

Gomes & Silva, 2007, and references therein;

Carretero & Pozo, 2009; Christidis, 2011). Smaller

quantities are used for pollution prevention through

removal of metal ions from wastewater (Oladoja &

Asia, 2005; Ma & Wang, 2006; Quintelas et al.,

2009; Li et al., 2011), remediation of polluted

environments (e.g. rivers, lakes, lagoons) and lining* E-mail: [email protected]: 10.1180/claymin.2013.048.1.01

ClayMinerals, (2013) 48, 1–20

# 2013 The Mineralogical Society

landfills to impede migration of inorganic and

organic pollutants into neighbouring soil, ground-

water and surface water (Vimonses et al., 2009;

Naganathan et al., 2010; Brown et al., 2011. The

variability of physical properties of sedimentary

kaolins and, hence, of its industrial uses depends on

the depositional environment and post-depositional

modifications (Hurst & Pickering, 1997; Ekosse,

2000; Nyakairu et al., 2001; Sousa et al. 2007;

Christidis, 2011) which affect the mineralogical and

chemical composition, the kaolinite crystal size and

order, the colour (brightness/whiteness) and the

firing characteristics (Grim, 1962; Pinheiro et al.,

2005; Siddiqui et al., 2005; Murray, 2007;

Christidis, 2011).

Egypt produces 295,000 tons (USGS Minerals

Yearbook, 2009) of the 33 million tons of kaolin

produced worldwide per annum, according to 2009

estimates by USGS-MCS (2011). The high-quality

El-Tih kaolin deposits in West Central Sinai, Egypt

(Fig. 1) belong to the Lower Cretaceous Malha

Formation. The total reserves were estimated to be

~88 million tons (Qusa, 1986), with kaolinite content

exceeding 97 wt.% in places (Abdel-Khalek, 1999,

and references therein). Although the clay is of

relatively high quality compared with most world-

wide deposits (Nour & Awad, 2008), its application

is restricted to the fabrication of tiles, bricks and

sanitary ware (El-Shishtawy et al., 2008).

So far, previous geological mineralogical and

geochemical studies of El-Tih kaolin deposits have

been based on a limited number of samples and

they have not considered the spatial variability of

the kaolin characteristics when attempting to make

a comprehensive assessment of the clay properties

(e.g. Salem, 1990; Morsy & Shata, 1992; Aly,

2005). The purpose of this work was, therefore, to

characterize El-Tih kaolin deposits from a field

survey based on detailed mineralogical and

geochemical analysis and determination of the

physical properties, across the horizontal and

vertical extent of the clay layers. The prime

objective of this approach was to identify the

spatial variation and the possible controlling factors

of the clay characteristics, and the depositional

environment of the kaolins. The results will

contribute to setting priorities for potential mining

areas and exploration of the kaolin resources.

GEOLOGICAL SETT ING

El-Tih kaolin resources are confined to the El-Tih

scarp which dips 60º to the south and extends

across the study area at an altitude of 650�1180 m

(Fig. 1). Kaolin occurs in Cretaceous clastic beds

with Tertiary basaltic dykes cutting through the

middle part of the sequence. The scarp is dissected

by major and minor faults ranging in length from a

few metres to 5 km with a dominant NW�SE strike

parallel to the Gulf of Suez, and a subordinate

NE�SW strike. The kaolins are sedimentary and

are interbedded with trough cross-bedding and, in

places, gravelly and pebbly claystone and fluvial

sandstone beds. These beds constitute the Early

FIG. 1. Surface geology of the study area and distribution of the sampling points.

2 A.A. Masoud et al.

Cretaceous Malha Formation (Abdallah et al., 1963)

which dips gently to the north at an angle less than

10º. The maximum thickness of the Malha

Formation is 330 m in the southern cliffs of the

El-Tih Plateau (Alsharhan & Salah, 1997). The

presence of palynomorphs associated with active

fluvio-deltaic settings confirms a proximal deltaic

environment for deposition of the Malha Formation

(El Beialy et al., 2010).

El-Tih kaolin deposit consists of five layers

exposed at limited outcrops along the scarp. This

exposure is due to the presence of local bedrock

highs combined with extensive erosion at the base

of the succession, where lower layers are covered

by Holocene sand, gravel and till. Due to the

steepness of the topography, the upper layers are

accessible only through low-relief gullies. The scarp

defines the southern limit of kaolin resources and

the thick overburden on the plateau renders the

kaolin accessible to mining only along the

escarpment. The kaolin layers were assigned

chrono-stratigraphically with alphabet letters from

the oldest to the youngest, i.e. A�E. All kaolin

layers are well preserved at the normal fault block

outcrops. Layer E is limited to two occurrences

close to the fault block in the central part of the

field area, adjacent to sample 9 (Fig. 1). Layers A

to D extend across the scarp (Fig. 2) with varying

thicknesses and, in places, wedge out laterally.

Sandstone lenses, iron-rich surfaces, channel

deposi t ional surfaces and plant remains

(Phlebopteris sp.) in carbonaceous kaolin are

common in the thicker layers A and B (Fig. 2b�d).

MATER IALS AND METHODS

Sixty six (66) kaolin samples were collected from

23 locations from the five layers: A (13 samples), B

FIG. 2. Field photographs showing (a) kaolin layers (A to D) alternating with sandstone located between samples

19 and 20; (b) sandy kaolin layers (B and C) located at sample 15; (c) sandstone lens within layer B at a location

50 m east of sample 19; and (d) channel surface within kaolin located 20 m west of sample 19.

Characterization of El-Tih kaolin quality 3

(17 samples), C (17 samples), D (17 samples), and

E (2 samples). The kaolin spatial occurrence with

respect to the surrounding rocks, thickness, colour

variation from Fe and Ti-oxide impurities and coal

content, particle size distribution, and the sedimen-

tary textures were recorded at the sample locations

along the clay deposit. The mineralogical composi-

tion was determined by X-ray diffraction (XRD)

with a Philips PW 1710 diffractometer equipped

with a graphite monochromator, using Cu-Karadiation, 40 kV and 30 mA, 1º divergence and

detector slits, 0.02º (2y) step size and counting time

FIG. 3. Field description of the kaolin succession, from base to top: (a) massive light grey to white kaolin;

(b) massive reddish kaolin; (c) carbonaceous kaolin with plant remains; and (d) highly fractured, listric, lobed

kaolin.


of 1 s/step. The diffraction maxima were compared

with the database of the International Centre for

Diffraction Data (ICDD, 2009).

Scanning electron microscopy (SEM), supported

by wavelength-dispersive spectrometry (JAXA-

7200 SEM-WDS) with four channel detectors,

was used to evaluate the kaolinite morphology

and to conduct X-ray mapping of the iron, titanium

and carbonaceous impurities, as well as to

determine the spatial distribution of the impurities

at the micrometre scale. Three cubes (1 cm3) of the

bulk samples, representative for each studied

FIG. 4. XRD patterns of the bulk samples under V = 40 kV, I = 40 mA, and l (Cu-Ka) = 1.5418 A, showing

(a) the exclusive composition of kaolinite (K) with subordinate quartz (Q) in pure white kaolin, (b) creamy quartz

sandy variety, and impurities of (c) iron concretions of goethite (G), and (d) carbonaceous coal-rich variety.


kaolin variety, were polished, gold coated and

examined.

Fused beads of the bulk samples were used to

obtain major element analyses by energy-dispersive

X-ray fluorescence (EDS-XRF) spectrometry

(Rigaku XRF spectrometer RIX3000) using rock

standards supplied by the MBH Reference Material

and Breithlander companies. The fused glass beads

were prepared from powdered freeze-dried samples

mixed with Flux No. 100B SPECTROFLUX at a

1:2 sample:flux ratio and LiNO3; these were melted

at ~1200ºC using a TK-4100 Bead & Fuse-Sampler.

FIG. 5. SEM images showing pseudo-hexagonal platy, accordion- and book-like kaolinite forms (a�d). Angularand euhedral quartz are abundant in western occurrences (e and f).


Major element chemistry was used to compare the

El-Tih kaolin with well known world class kaolin

deposits using ternary diagrams (Ligas et al., 1997).

A statistical study was conducted to identify

significant compositional relationships of the kaolin

layers using linear regression and Pearson’s correla-

tion coefficient matrices. Quality parameters control-

ling the suitability of the kaolin, i.e. Al2O3/SiO2

ratio, thickness of the clay layer, potentiality index

(PI; product of multiplication of the Al2O3/SiO2 ratio

and thickness) and TiO2 and Fe2O3 contents, were

modelled by the ordinary kriging implemented in the

geostatistical analyst of the ArcGIS9.3 package.

Ordinary kriging uses a weighted average of

neighbouring samples to estimate the ‘unknown’

value at a given location. Weights are optimized

using the semi-variogram model, the location of the

samples, and all the relevant inter-relationships

between known and unknown values (Goovaerts,

1997; Siska, et al., 2005). The technique provides

two-dimensional surfaces representing approximated

regional trends rather than the exact values. It also

provides a ‘‘standard error’’ which may be used to

quantify confidence levels. The experimental semi-

variograms and the best-fitted theoretical models for

all variables were built based on trial and error

parameter selection. Models attained the best good-

ness of fit resulted in minimum mean error (ME),

root mean error (RME), and mean squared error

(MSE), and attained root mean squared error

(RMSE) close to unity are considered the best-fit

models (ESRI, 2009) and were selected for further

analysis among which the spherical model was of

major use.

The chemical and mineralogical indices of

alteration (CIA after Nesbitt & Young, 1982, and

TABLE 1. Summary statistics of the chemical composition (wt.%) and the quality indexes of the kaolin layers

(A�E).

Thickness(m)

Al2O3 SiO2 Fe2O3 TiO2 MgO CaO Na2O K2O LOI Al2O3/SiO2

A Mean 2.0 30.4 51.3 1.4 1.7 0.19 0.58 0.86 0.13 10.5 0.6StdDev.

0.6 4.1 5.0 0.4 0.7 0.27 0.63 2.76 0.20 4.9 0.1

Min. 1.0 24.1 45.1 0.9 0.1 0.03 0 0 0 0.0 0.4Max. 3.0 34.8 60.3 2.2 2.9 0.81 1.95 10.03 0.76 14.1 0.8

B Mean 1.8 30.2 51.5 1.4 1.8 0.16 0.67 0.06 0.11 12.5 0.6StdDev.

1.0 4.0 4.8 0.3 0.5 0.15 0.60 0.11 0.12 1.3 0.1

Min. 0.8 21.8 44.5 0.9 1.1 0.05 0.05 0 0 9.9 0.4Max. 5.0 36.1 60.8 2.2 2.7 0.64 2.07 0.4 0.47 14.4 0.8

C Mean 1.5 30.1 51.8 1.3 2.0 0.17 0.36 0.05 0.06 12.2 0.6StdDev.

0.5 4.6 6.7 0.4 0.6 0.15 0.28 0.10 0.05 2.1 0.2

Min. 0.8 19.4 43.9 0.7 1.1 0.06 0.04 0 0 8.0 0.3Max. 2.5 36.3 67.1 2.2 3.3 0.6 0.84 0.27 0.15 16.0 0.8

D Mean 1.4 30.0 51.3 1.1 2.2 0.13 0.49 0.14 0.06 12.7 0.6StdDev.

0.4 4.0 5.5 0.3 0.8 0.10 0.55 0.20 0.06 1.6 0.1

Min. 1.0 20.1 45.3 0.6 0.9 0.03 0 0 0 8.7 0.3Max. 2.5 34.8 65.1 1.7 4.1 0.39 1.8 0.57 0.15 15.5 0.8

E Mean 1.75 26.23 55.28 1.43 2.37 0.07 0.94 0.36 0.08 9.46 0.47StdDev.

0.4 1.97 0.75 0.52 0.90 0.04 1.10 0.50 0.11 0.22 0.53

Min. 1.5 24.83 54.75 1.06 1.73 0.04 0.16 0.00 0.00 9.30 0.80Max. 2.0 27.62 55.81 1.79 3.00 0.10 1.71 0.71 0.16 9.61


FIG.6.(a)SpatialdistributionofAl 2O3/SiO

2,thickness(m

),and[(Al 2O3/SiO

2)6

thickness]

byordinarykriging,forkaolinlayersA,B,C

andD.


MIA after Voicu & Bardoux, 2002) were used to

estimate weathering intensity in the source area. As

the source areas are not known with certainty,

estimation of the CIA and MIA indices was based

on the assumption that the kaolins were derived

from a single source.

RESULTS

Field description

A typical outcrop of a kaolin succession located

close to sample 18 (Fig. 3) consisted of thick-

bedded kaolin, massive, light grey to white at the

base (Fig. 3a), overlain by a reddish kaolin layer

that wedges out laterally (Fig. 3b). Layers A & B

were interbedded with a lens of dark grey

carbonaceous kaolin with plant remains (Fig. 3c),

and a highly fractured lobed-shaped kaolin

(Fig. 3d). The latter was listric close to the

channel surface and graded into a mottled zone

with red- and pink iron-rich spots. The mottled zone

terminated upwards to iron concretions and

fragmented kaolin mixed with quartz pebbles

close to the contact with the overlying cross-

bedded sandstones.

The massive light grey to white kaolin commonly

extends across the lower, relatively thicker clay

layers (A and B) to the east, which in turn grade

westward to the grey kaolin with increasing

abundance of the associated silt-size and then to

the sand-size quartz grains. Iron oxides imparting

the red-brown colour to the clay form concretions

or laminations commonly close to depositional

surfaces. The carbonaceous matter was finely

disseminated or occurred as coal fragments, lignite

and coal seams, or as plant remains (Phlebopteris

sp.). The kaolin layers do not display gradual

transition to a fresh precursor, in accordance with a

secondary origin of the deposit.

FIG. 6. (b) Spatial distribution of TiO2 and Fe2O3 for kaolin layers A, B, C and D by ordinary kriging.


Kaolin mineralogy and kaolinite morphology

Kaolinite was the main phase present in the El-

Tih kaolin, with quartz being subordinate (Fig. 4).

Kaolinite was well ordered with a Hinckley index

exceeding 1.0 (average 1.18) in all XRD patterns.

Traces of TiO2 determined by XRF analyses (see

below) were rarely identified as anatase because the

main diffraction maxima of anatase are masked by

kaolinite and quartz (Maynard et al., 1969; Hutton

1977). Goethite and hematite were accessory

phases. Observation under the SEM showed the

presence of pseudo-hexagonal platy kaolinite

crystals, which often form accordion- and book-

like aggregates (Fig. 5). Platy kaolinite crystals

dominated in the eastern part of the study area,

sometimes mixed with tubular aggregates (Fig. 5a).

Accordion- and book-like aggregates predominated

in the west, and were frequently associated with

angular and/or euhedral quartz crystals (Fig. 5b, d).

The abundance of the quartz crystals and grains,

together with the existence of minor muscovite

(Fig. 5e, f) in the western part of the study area,

with also the highest PI values, suggests proximity

to source rocks and short transportation time on

steeper slopes. The formation of kaolinite vermi-

form booklets indicates diagenetic modification

processes (Hurst & Pickering, 1997) which may

be associated with weathering of pre-existing micas

(Psyrillos et al., 1999).

Chemical composition

According to the average chemical composition

of the studied samples (Table 1), they are classified

as sensu stricto kaolin (Grim, 1968). The samples

had higher SiO2 content, due to the notable

presence of quartz, and thus a lower Al2O3

content and LOI than the ideal kaolinite. The

average Fe2O3 and TiO2 contents were less than

1.5% and 2.4%, respectively, in all layers. The

median Al2O3/SiO2 ratio is 0.8, i.e. close to the

value of 0.85 assigned for ideal kaolin (Schroeder et

al., 2004; Gamiz et al., 2005).

Layer E displayed the lowest quality criteria of

all the layers, and it was marked by the highest

SiO2, Fe2O3, TiO2 and lowest Al2O3 contents. The

average SiO2 and TiO2 contents tended to increase

TABLE 2. Pearson’s correlation coefficients between various quality parameters of the studied kaolin layers.

Layer Thickness Al2O3 Fe2O3 SiO2 TiO2

A Thickness 1Al2O3 0.40 1Fe2O3 0.33 –0.29 1SiO2 –0.24 –0.93 0.43 1TiO2 0.24 –0.17 0.22 0.16 1

B Thickness 1Al2O3 0.08 1Fe2O3 –0.34 –0.65 1SiO2 –0.12 –0.98 0.61 1TiO2 0.48 –0.22 –0.14 0.13 1

C Thickness 1Al2O3 0.31 1Fe2O3 0.22 –0.22 1SiO2 –0.29 –0.98 0.26 1TiO2 0.08 –0.12 –0.07 0.09 1

D Thickness 1Al2O3 0.42 1Fe2O3 0.05 –0.18 1SiO2 –0.39 –0.95 0.03 1TiO2 0.08 –0.31 –0.11 0.30 1

NB: Values in bold correspond to significant coefficients.


from A to D layers, whereas Al2O3 and Fe2O3

contents followed an opposite trend. These

geochemical variations corresponded to the

upward decrease of layer thickness, which mark

the deterioration of quality from layer A to layer D.

Correlation coefficients among the major

chemical elements and kaolin layer thickness

(Table 2) clarified their form of occurrence. The

Al2O3 was negatively correlated with SiO2 (ave. R2

= 0.96) and to a lesser degree with Fe2O3 (ave. R2

= 0.5) and TiO2 (ave. R2 = 0.4), whereas it was

positively correlated with thickness (ave. R2 =

0.42). The negative correlation of Al2O3 with Fe2O3

and TiO2 suggests that the latter may be partly

associated with the detrital sand/silt fraction or they

might occur in different later phases. A positive

correlation between Fe2O3 and SiO2 also was noted

for all layers except for layer D (ave. R2 = 0.5)

suggesting the possibility of Fe-oxide coatings on

quartz grains and kaolinite. The Al2O3/SiO2 ratio

had a strong positive correlation with LOI in all

layers (R2 = 0.92 in A, 0.87 in B, 0.9 in C, and 0.75

in D).

Spatial quality modelling

The spatial modelling approach was important in

understanding the variations of quality parameters

FIG. 7. (A) SEM-WDS of the white kaolin with Fe and Ti elemental maps and their corresponding qualitative/

quantitative analyses. (B) SEM-WDS of the concretional iron-rich kaolin with Fe and Ti elemental maps and

their corresponding qualitative/quantitative analyses. (C) SEM-WDS of the carbonaceous kaolin with C, Fe, and

Ti elemental maps and their corresponding qualitative/quantitative analyses.


related to the chemical and physical properties of the

kaolin deposit in three dimensions. The Al2O3/SiO2

ratio, thickness, PI, TiO2, and Fe2O3 were selected

as the quality parameters, and their spatial distribu-

tions were modelled using ordinary kriging (Fig. 6).

The experimental semivariograms and the best-fit

theoretical models from the criteria of Mean Error

(ME), Root Mean Error (RME), Mean Squared Error

(MSE), and Root Mean Squared Error (RMSE) had

nugget effects for all variables. The selected models

were spherical, Gaussian, and rational quadratic

models, depending on the variable.

Figure 6a, b shows some interesting trends for

the studied parameters. The Al2O3/SiO2 ratio

gradually decreased in layers A and C, and

increased in layers B and D, from the central to

the eastern and western parts of the study area

(Fig. 6a). The layer thickness was a minimum at the

centre and increased towards the west and the east

in all layers. Also, generally the thickness decreased

upwards from layers A to D. As the kaolin with

highest Al2O3 content and, lowest SiO2 content and

the greatest thickness had the highest PI value,

prospective zones of mining for high-quality kaolin

can be established from the PI map. In all four

layers, the PI values were generally lowest in the

central part and increased towards the west and east

ends of the kaolin. Some local maxima of PI occur

in layers A and C. High Fe2O3 and TiO2 contents

were observed in general in the western sectors and


decreased towards the east (Fig. 6b). Marked local

enrichment of these impurities occurred close to the

faults and to the Tertiary basaltic dykes. It can be

concluded that the eastern occurrences of the four

layers have higher quality with grades increasing

upwards from layer A to layer D.

Elemental mapping of impurities

The main contaminant elements in El-Tih kaolin

were Fe, Ti and C, which imparted red to yellow,

brown to tan, and grey, colourations respectively to

the clay. These impurities decreased the whiteness

of the kaolin and adversely affected its quality that

limited its applications. The influence on colour and

the beneficiation method to remove the impurities

depend on the type, the content, and the distribution

of the contaminants. Thus, revealing the type of

impurities by WDS was of prime importance for

optimum utilization of the kaolin.

WDS analyses and X-ray maps of Fe, Ti and C

of representative samples, of the pure white to grey

(Fig. 7A), iron-rich (Fig. 7B) and carbonaceous

(Fig. 7C) kaolin showed variation in the relative

contents of elements in the probed areas, while the

spatial distribution patterns of elements were

similar in spite of differences in the layers and

the PI zones. Iron occurred in trace and finely

disseminated particles in the pure white to grey

kaolin, and reached 8 wt.% in the iron concretions.

The high boron content (8.7 wt.%) detected in the

white kaolin (Fig. 7A), is most probably an artifact,



FIG. 8. (a) (facing page) SEM-WDS photographs of iron-rich kaolin with Fe-Ti rhombs showing the

backscattering image (IMG1), Al, Si, Ti, Fe, RGB colour composite (CC) using Ti as red, IMG1 as green, and Fe

as blue, as well as their qualitative EDS analysis. (b) SEM-WDS photographs of the Quartz-rich kaolin with Fe-

stained titania showing the backscattering image (IMG1), Al, Si, Ti, Fe, RGB colour composite (CC) using Ti as

red, IMG1 as green, and Fe as blue, as well as their qualitative EDS analysis.


because (a) B-minerals were not detected by XRD

analysis and (b) the intensity of the B-spectrum was

comparable to that of the carbonaceous and the

Fe-oxide bearing kaolins, in which the detected

B-content was 1%. Ti was low (Fig. 7B) in the iron

concretions (1%). Nevertheless most samples

contain detectable boron. Carbon, ranging from

micron- to millimeter- size particles (Fig. 7C),

tends to be disseminated in the carbonaceous

kaolin layer. WDS quantitative spectra indicated

high contents of Fe (4%), Ti (24%) and C (11%) in

the carbonaceous kaolin. The high Ti and Fe

contents relative to the bulk analyses (Table 1)

indicate local enrichment and micro-environmental

chemical heterogeneity of the kaolin. The high Ca

content (13%) may also be associated with gypsum

(2% S content in Fig. 7C) and calcite, as part of the

C content may be linked to carbonates.

Nevertheless neither calcite nor gypsum was

detected in the XRD traces of the bulk samples,

suggesting that this enrichment of Ca may reflect

micro-environmental heterogeneity of the kaolin.

The distribution of Ti and Fe in the iron-rich

concretions strongly suggests that these elements

are present in discrete phases and that they do not

participate in the kaolinite structure (Fig. 8a, b). In

fact, Fe-Ti oxide phases occur as rhombs and as

Fe-stained Ti phases in the pure white to gray

kaolin (Fig. 8a) and in the quartz-rich samples

(Fig. 8b). This occurrence of Fe and Ti suggest

that their removal may be plausible by bleaching,

magnetic separation or flotation (Murray, 2007),

thereby enhancing the whiteness and hence

upgrading the quality for use in paper coating

and filling, and in higher grade ceramics (Abdel-

Khalek et al., 1998; Abdel-Khalek, 1999, and

references therein). Indeed, carrier and column

flotation techniques yielded promising results by

reducing the TiO2 to 0.61% (whiteness 90%) and

0.38% (whiteness 91.5), respectively (Abdel-

Khalek et al., 1998; Abdel-Khalek, 1999, and

references therein).

D I SCUSS ION

Results integrating field geological data, spatial PI

modelling, XRD and SEM-WDS analyses

confirmed the secondary sedimentary origin of El-

Tih kaolin. The source rocks of the kaolins are

considered to be the weathering products of

Precambrian rocks located to the south of the

study area that were transported north to the

sedimentary basin. The depositional conditions

during kaolin deposition reflected the operating,

morpho-hydrodynamic regime, which was most

likely related to climatic/tectonic conditions. A

plot of TiO2 vs. Al2O3 for the kaolin studied

(Fig. 9a) indicated precursor rocks predominately of

acidic (rhyolitic/granitic) affinities possibly with

minor basaltic admixtures. Such source rocks were

readily weathered and altered to kaolin minerals

and quartz under high rainfall on gentle slopes with

a fluctuating water table and rapid water percolation

(Murray & Keller, 1993). These conditions

prevailed during the Cretaceous in the fluvio-

lacustrine and floodplain environments of the

intra- and epicontinental basins of NE Africa

(German et al., 1994).

The chemical alteration indices (CIA) and

mineral alteration indices (MIA) indicated intensive

weathering of the source rocks. Such weathering

conditions suggest a relatively warm and humid

palaeoclimate with fluctuation of the water table

and ready water percolation. The CIA values of

unweathered rocks are generally about 50, whereas

intensely weathered rocks may reach 100 (Nesbitt

& Young, 1982). MIA values of 60�100 typically

indicated intense to extreme weathering (Voicu &

Bardoux, 2002). El-Tih kaolin samples yielded CIA

and MIA values greater than 87 and 85, respec-

tively. It is noteworthy that average CIA values

increased gradually upward from layers A to D, and

they had strong negative correlations with average

Al2O3 content (R2 = 0.8). This trend implies that

the Al2O3 content was controlled not only by the

intensity of weathering, but also by contributions

from different source rocks.

The samples of El-Tih kaolin are characterized

by low colouring oxide content and alkali contents

and geochemically are very similar to the kaolins of

Latium, Italy (Fig. 9b; Ligas et al., 1997; Nyakairu

et al., 2001). Also the El-Tih kaolins plot very close

to the Georgia sedimentary kaolin, but are richer in

SiO2. Similarly they are richer in SiO2 than the

kaolins of Buwambo (Uganda), Saxony and Bavaria

(Germany), Devon and St. Austell (UK), and

Bretagne (France). The kaolins of Latium are the

largest deposits of volcanic origin in central Italy

(Veglio et al., 1996), whereas the soft kaolins of

Georgia, USA, formed from weathering of gneiss

and granite source rocks (Dombrowski, 1993).

Quartz grains in the sandy kaolin, dominant at

the western extent of the resource, showed high

angularity and poor sorting. This feature suggests


shallow water sediments formed on steeper terrain

slopes with heavy rainfall (Soman, 1997), and such

conditions suggest either more intense weathering

in the west than the eastern part of the area or the

kaolin sediments were transported from streams

flowing from a western direction. Kaolin layers are

intercalated with sandstone and clay beds and, in

places, confine wedge-shaped sandstone bodies.

FIG. 9. Diagrams showing (a) possible source rocks of the studied kaolin, and (b) El-Tih kaolin compared to

world kaolin occurrences.


These structures were probably formed by deposi-

tional energy variation that favoured a flood

environment where the local regime changed from

a kaolin-forming, quiet water environment on flats,

to high local gradients and high-energy water

currents in stream channels as part of a highly

constructive (anastomosed) river system.

Deterioration of quality from layers A to D was

likely induced by two factors. First, deposition

occurred more rapidly with time, probably due to

gradual increase of transportation velocity i.e. more

dynamic conditions. Second, there is an increase in

the TiO2 content initially present as ilmenite.

Although the relatively low Fe2O3 content of the

El-Tih kaolin is in accordance with more acidic

precursors (cf. Fig. 9a), part of the Ti of the

samples collected higher in the sequence may be

related to proximity to younger Tertiary basaltic

dykes. This was supported by the spatial modelling

of elemental oxides (Fig. 6). Typically, kaolin clays

forming from mafic parent materials have high

TiO2 contents because large amounts of alkali ions

and silica are removed during weathering, resulting

in considerable TiO2 concentration (Melo et al.,

2001). However, weathering of basaltic rocks also

yields smectite, which was not determined in this

study. In any case, the low Fe2O3 content of the El-

Tih kaolin suggests that any possible contribution

of the basic precursor should be minor.

Correlation of bed thickness and Al2O3 content

was influenced by the hydrodynamics and the

associated sand input because thinner beds of

kaolin were deposited in more dynamic conditions

with higher sand input, and vice versa. The negative

correlation of the bed thickness with Al2O3 and the

positive correlation with TiO2 in layer B is an

exception that might be attributed, at least partially,

to local thrusts and Tertiary basaltic dykes,

increasing the bed thickness and the Ti input in

the form of ilmenite, respectively.

CONCLUS IONS

The sedimentary El-Tih kaolin deposit of

Cretaceous age consisted mainly of kaolinite often

forming accordion or book-like aggregates with

subordinate quartz and traces of Fe and Ti oxides.

The Fe-oxides locally formed iron concretions.

Kaolinite was formed from deep weathering of

granitic-basaltic rocks and subsequent transportation

and deposition of the weathering products in a

lacustrine-swampy environment with variable

depositional energy from a low energy environment

on flats to high local gradients and high-energy

water currents in stream channels. The deposit had

undergone post-depositional modifications, as was

indicated by the formation of kaolinite aggregates.

The deposit consists of five main kaolin layers

and its chemical composition was comparable to

that of the Latium kaolins, Italy. The kaolin quality

deteriorates gradually upwards with layer E at the

top having the highest SiO2, iron, TiO2, and the

lowest Al2O3 contents, due to a combination of

morpho-hydrodynamics, local structural implica-

tions, and composition of the source rocks. This

trend was supported by the upward gradual decrease

in thicknesses of kaolin layers towards the top of

the sequence. The higher potentiality indices (PIs)

of the kaolin, i.e. those attaining the highest Al2O3,

lowest SiO2, and greatest thickness, were spatially

concentrated to the west and east of the studied

area, whereas low PI values marked the central part

of the deposit. The PI values can be used to specify

areas suitable for optimal exploitation of the clay.

As the Fe and Ti were in free oxide phases, their

removal is a viable approach to produce high-

quality grades used in paper coating and filling and

in higher grade ceramics (Abdel-Khalek et al.,

1998; Abdel-Khalek, 1999, and references therein).

ACKNOWLEDGMENTS

The authors wish to thank the anonymous referees and

the editor for their constructive comments that

significantly improved the manuscript.

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Characterization of El-Tih kaolin quality using mineralogical, geochemical and geostatistical analyses

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