Composition, origin and industrial suitability of the Aswan ball clays, Egypt

CLAY-03189; No of Pages 11 Applied Clay Science xxx (2014) xxx–xxx

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Composition, origin and industrial suitability of the Aswan ball clays, Egypt Hassan M. Baioumy a,⁎, Ismael S. Ismael b a b

School of Physics, Univesriti Sains Malaysia, 11800 USM, Penang, Malaysia Faculty of Science, Suez Canal University, Suez, Egypt

a r t i c l e

i n f o

Article history: Received 29 June 2013 Received in revised form 7 August 2014 Accepted 25 September 2014 Available online xxxx Keywords: Aswan ball clays Egypt Mineralogy Geochemistry Source Industrial applications

a b s t r a c t Although Aswan ball clays occur in relatively large reserves, information on their geochemistry and source is still lacking. This paper presents detailed petrographic, mineralogical, and geochemical investigations on these clays to examine their source as well as their possible use as raw materials for ceramic and refractory industries. Aswan ball clays occur as gray, yellowish gray, reddish to brownish gray, massive to faint laminated, and moderately hard clays. Grain-size distributions indicate the dominance of clay fractions (45–57 wt.%). Low-ordered kaolinite is the main constituent (39–60 wt.%) along with quartz (24–46 wt.%) and low crystalline illite (10–19 wt.%). Fine-grained anatase is reported as a minor constituent (~2 wt.%). Aswan clays are typically ball clays (SiO2/Al2O3 = 2.3–4.1) with relatively high Fe2O3 and TiO2 contents. The trace elements occur in two assemblages. Elements associated with the Fe-bearing phases include Cu, Ni, Co, Zn, V, and Pb as indicated from the positive correlations with the Fe2O3 contents. Elements occur as silicate minerals but not in clay minerals such as Nb, Zr, Y, Hf, Ta, and U as revealed from the positive correlations of these elements with the SiO2 and the negative correlations with the Al2O3 contents. The sum of rare earth elements (REEs) ranges from 291 to 335 ppm with negative correlations with the Al2O3 and positive correlations with the SiO2 suggesting the occurrence of REE as silicates but not in clay minerals. Chondrite-normalized REE patterns exhibit light rare earth element enrichment relative to heavy rare earth elements ((La/Yb)N = 9.2–11.7) and slightly negative Eu anomalies (Eu/Eu* = 0.74–0.83) without Ce anomalies (Ce/Ce* = 0.96–1.03). Major, trace, and rare earth elements geochemistry of the clay fractions indicates a mixture of more than rock types as a source of the Aswan ball clays. The high Zr and Y contents and La/Yb ratios suggest a contribution of granitic rock, while the relatively high contents of TiO2 and Ti are suggestive for a contribution of mafic source rock. Plot of the study clays in the Rb-K2O and Hf–La/Th binary plots supports mixed felsic and mafic source rocks of the study clays. In addition, high Nb contents indicate a contribution of alkaline source rock. Plot of the study clays in the phyllite and schist field in the Co–Th binary diagram also indicates contribution of metamorphic source rock to the source of the clays. The abundance of clay size fractions (b 2 μm), low-order kaolinite and illite, absence of I/S minerals, low fluxing agents such as alkali oxides (Na2O and K2O) and alkaline earth oxides (CaO and MgO), low S and Cl contents, and low contents of toxic elements (As, Cd, Hg, and Pb) reveal the suitability of the Aswan ball clays as a good quality and environment-friendly raw material for ceramic and refractory industries. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The term “ball clay” originated from an early English mining practice of rolling the highly plastic clay into balls weighing 30–50 lb. Ball clay is used primarily in the ceramic industry for making such items as pottery, dinnerware, stoneware, and sanitary ware (e.g. Andreola et al., 2009). Ball clay is composed of poorly crystalline kaolinite with small amounts of illite, and (or) smectite. Quartz sand or silt and iron oxide minerals ⁎ Corresponding author at: School of Physics, Universiti Sains Malaysia 11800 USM, Penang, Malaysia. Tel.: +60(0)465335315; fax: +60(0)46579150 E-mail address: [email protected] (H.M. Baioumy).

are virtually absent from the best-grade ball clays, but carbonaceous material may be abundant. The color of ball clay is nearly white, but some colors range from pink to brown through shades of gray to black. After firing, it is usually almost white. Ball clays require between 40 and 65 percent water of plasticity to become workable. Its plasticity, toughness, high green strength, and adhesion are the outstanding characteristics of ball clay. When fired, ball clay becomes dense and vitreous and its temperature of deformation (melting) is between 1670 °C and 1765 °C (Hosterman, 1984; Wilson, 1998). Ball clays in Egypt occur in relatively large reserves (10 million metric tons for sure) mainly northeast of Aswan City (Fig. 1A). They are produced by several companies for domestic ceramic

http://dx.doi.org/10.1016/j.clay.2014.09.041 0169-1317/© 2014 Elsevier B.V. All rights reserved.

Please cite this article as: Baioumy, H.M., Ismael, I.S., Composition, origin and industrial suitability of the Aswan ball clays, Egypt, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.09.041

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H.M. Baioumy, I.S. Ismael / Applied Clay Science xxx (2014) xxx–xxx

Fig. 1. Stratigraphic column shows the stratigraphic position of the Aswan ball clays (A) with field photo of the studied section (B). (C and D) field photos show the lithological characteristics of the Aswan ball clays. (C) Yellowish to brownish gray faint laminated clays. (D) Gray massive clays with fractures filled with Fe-rich brownish clays. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

and tile industries mainly from Wadi Abu Sobeira and Wadi Abu Agag areas (Fig. 1B). The production rate is approximately 5000 metric tons per month. Due to the industrial importance of the Aswan ball clays, they were subjected to some technical studies to examine their physical properties as the main raw materials for ceramic and tile industries or as a mixture with other raw materials (e.g. Amin et al., 2011; Nour and Awad, 2008). However, to the best knowledge of the authors, the studies on the geochemistry of these clays particularly trace and rare earth elements and their source and origin are lacking in the previous investigations. This paper shows the mineralogy of both bulk and clay fractions as well as the major, trace and rare earth element geochemistry of the clay fractions of the Aswan ball clays. The results of these analyses address the possible source rock(s) of these clays. The industrial suitability as well as the environmental impact of these ball clays is also examined based on these results. 2. Geological setting of the studied area The studied area comprised Precambrian metamorphic and igneous rocks as well as the Upper Cretaceous sandstones and clays of the Nubian Sandstones Series (e.g. Attia, 1955; Germann et al., 1987; Khedr, 1984; Mesaed, 1995). The metamorphic rocks are mainly schists with subordinate gneisses. Mica- and hornblende-schist occurs either as huge masses covering extensive areas or as small masses in the granite. Mica schist is grayish, brownish or buff in color and composed of mica (biotite, sericite, and muscovite), quartz, and feldspars. The hornblende schist is greenish or grayish and composed of hornblende, quartz, and feldspars. Hornblende schist is distributed through the southern main wadis. The area of Wadi Abu Agage is formed mainly of metamorphic rock including quartz–biotite–schist, muscovite schist and hornblende schist. The schist is grayish or greenish and intruded by thin (15 cm)

milky-white quartz veins. Gneisses occur as dark gray and finegrained, and are composed of plagioclase feldspar and quartz with minor biotite, hornblende, and muscovite. The igneous rocks in the study area are comprised of diorites, granodiorites and granites. The diorites are dark-green massive and composed of amphibole and feldspar. They represent the oldest intrusive rocks in the area and intruded into the gneisses and schists. Granodiorites only exposed in the east of the River Nile between Aswan and El-Shellal district as well as on the islands scattered in the river. The most extensive variety, which forms the main mass and represents the normal type of granodiorite, is gray with up to 3 cm white and pinkish porphyroblasts. They are composed of quartz, feldspars, biotite, and hornblende (Attia, 1955). The red coarse-grained granite of Aswan is the most abundant rock in the area between the River Nile and the Paleolithic Nile Channel. It forms most of the hills between Aswan and El-Shallal and underlies the Nubian Sandstone in many parts. This granite generally forms low hills composed of huge rounded masses owing to its spheroidal weathering. The coarse-grained Aswan granite is essentially composed of feldspars, quartz, biotite, and hornblende. Pegmatite occurs in the form of dyke-like bodies and irregular masses attain a length of about 1 km and a width of 2–3 m cutting the metamorphic and igneous rocks. They are pink, very coarsegrained and composed of feldspar and quartz with occasional flakes of mica. The sedimentary sequence in the study area is represented by the late Cretaceous sedimentary rocks that were subdivided by Klitzsch (1986) into three units: the basal Abu Agag Formation, the Timsha Formation and the uppermost Um Barmil Formation. The Timsha Formation, which is of Coniacian to Santonian age, has a thickness between 10 and 35 m and consists of three coarsening-upward sequences, which contain at least four horizons of ooidal ironstone.

Please cite this article as: Baioumy, H.M., Ismael, I.S., Composition, origin and industrial suitability of the Aswan ball clays, Egypt, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.09.041

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3. Materials and methods Eight ball clay samples were collected from the Wadi Abu Sobeira (2 samples) and Wadi Abu Agag (6 samples) areas, northeast of the Aswan City. Detailed geology as well as geologic map of the studied area was reported in Mucke (2000). The samples were subjected to detailed grain-size, petrographic, mineralogical, and geochemical analyses. Grain-size distributions of a number of these samples were determined by sedimentation from aqueous suspensions after complete disaggregation of the bulk clays. A 10-g sample of the bulk clays was soaked in distilled water for ~2 weeks. During this period, the samples were ultrasonically treated using a cleaning ultrasonic instrument and then washed several times using distilled water until a complete suspension. To minimize the effect of sonication on the grain size distribution of the original bulk clays, the samples were treated with low ultrasonic intensity and for short periods of time. No dispersant chemicals were used to avoid any contamination of the clay fractions. The sand fractions were obtained using the 63 μm sieve, while the silt (63–2 mm) and clay (b 2 μm) fractions were determined using pipette method (e.g. Gee and Bauder, 1986). The clay fractions (b 2 μm) were obtained through sedimentation after ~7 h to ensure their purity and then precipitated on glass slides to investigate the clay mineralogy of the study ball clays. A number of thin sections were prepared and investigated under the optical microscope to investigate the texture of the study ball clays. Morphology and chemistry of Fe- and Mn-bearing phases were investigated with SEM-EDX JEOL-JSM 5410. Approximately 10 spots were analyzed for each sample. Mineralogical and petrographic investigations were conducted at the Central Metallurgical R & D Institute (CMRDI), Egypt. Both bulk (8 samples) and clay fractions (air-dried, glycolated, and heated at 550 °C) (5 samples) were analyzed for their mineralogical composition by the X-ray diffraction (XRD) using a Philips PW1800 instrument (Co-Kα radiation, 30 kV, 20 mA). The quantitative mineralogical compositions were determined using the Rietveld program BGMN® (Kleeberg and Bergmann, 1998). Operating conditions were 40 kV and 25 mA. The degree of structural disorder of the kaolin was evaluated using the Hinckley Index (HI) (Galan et al., 1994; Hinckley, 1963) of the clay fractions (b2 μm), while the crystallinity of illite was examined using the Kübler Index (KI) (Kübler, 1967). Two representative samples were thermally analyzed using a Netzsch simultaneous thermal analysis apparatus type STA 409 at the Central Metallurgical R & D Institute (CMRDI), Egypt. 200 mg of each sample was heated from the room temperature up to 1100 °C using a heating rate of 10 °C/min, platinum 10% radium–platinum thermocouple and alumna as a reference material. Fused discs prepared from eight bulk ball clay samples from the study area were analyzed for their major oxides using Philips PW 2400 X-ray fluorescence (XRF) spectrometer at Tohoku University, Japan. Tube voltage and current for W target were 40 kV and 60 mA, respectively. Loss on ignition (L.O.I.) was obtained by heating sample powders to 1000 °C for 6 h. Trace and rare earth elements of the clay fractions of five samples were determined by inductively coupled plasma mass spectrometry (ICP-MS) at the ACME Lab., Canada. The sample powders were digested with 2 mol/L concentrated HF in capped teflon bombs on an electric hot plate (~ 150 °C) for 24 h. The solution was evaporated to near dryness, and re-dissolved in 2 mol/L 6 N HNO3 in capped teflon at 150 °C for two days. The samples were then evaporated near to dryness, then 1 mol/L of 6 N HNO3 was added, and the solutions were further diluted for analysis. 4. Results 4.1. Occurrence of ball clays in the studied areas Aswan ball clays occur and are produced for industrial applications from Wadi Abu Sobeira and Wadi Abu Agag areas, northeast of Aswan (Doering, 1990). From bottom to top, the sedimentary sequence at

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Wadi Abu Sobeira and Wadi Abu Agag areas (Fig. 1) is composed of gray, yellowish gray, reddish gray and brownish gray clays of what is known as the Aswan ball clays. Aswan clays are overlain by approximately 1 m thick of oolitic iron ore, which occasionally has brownish gray very thin (5–15 cm thick) clayey intercalations. The Fe-bearing horizon is overlain by a thick (more than 30 m) of shale, clay, and sandstone sequences. Iron ore in these areas occurs as hard and massive beds with metallic luster and red strike. The ooids of the ore are coarse enough to be seen in the hand specimen. 4.2. Grain-size analysis The grain-size distributions of the Aswan ball clays (Table 1 and Fig. 2) show that the clay fractions (b2 μm) represent the major fractions in the majority of the analyzed samples varying between 45.4 and 57.4 wt.%. Sand fractions occur in very low contents ranging between 0.1 and 1.8 wt.%. The silt fractions range between 41 and 52 wt.%. Majority of the analyzed samples are fine-grained and located in the fine (15–8 μm) to very fine silt (8–4 μm) categories. There was no difference in the grain-size distributions between the ball clays from Wadi Abu Sobeira and Wadi Abu Agag areas. Exception is sample S1 from the Wadi Abu Sobeira area which shows relatively higher sand (1.8 wt.%) and silt (52 wt.%) fractions compared to other samples. 4.3. Petrography The Aswan ball clays occur as gray, yellowish gray and reddish to brownish gray clays. The yellowish, reddish, and brownish colors characterize the iron-rich horizons of these clays (Fig. 1C and D). They are mainly massive but sometimes faint laminated and moderately hard. Fissures, which occasionally field with iron-rich clays, are common within the clay beds (Fig. 1C and D). Under the petrographic microscopy, the Aswan ball clays appear as very fine pale gray to yellowish gray clayey materials. Few relatively coarse grains (fine to very fine silt size) of detrital quartz, iron oxide minerals, and heavy minerals are scattered in the clayey matrix (Fig. 3A). Fe-rich samples are characterized by the brownish tarnish of the clayey matrix (Fig. 3B). Under the SEM, kaolinite from the Aswan ball clays occurs as crystals of variable sizes (1 μm up to 20 μm) of undefined outlines and edges (Fig. 4A) with EDX analysis of typical kaolinite (Fig. 4B) that dominated by Si, Al, and O. Very weak and broad peaks of Fe and Ti can also be observed in the EDX pattern of kaolinite (Fig. 4B) probably due to the staining of kaolinite grains with some iron oxide and occurrence of very fine-grained anatase in the analyzed area. Illite plates of relatively coarse grain size (Fig. 4C) were identified with EDX analysis of typical illite (Fig. 4D) that is dominated by Si, Al, K, and O. SEM observation of bulk clays also showed the occurrence of anatase as clusters of very fine, uniform, rounded crystals between kaolinite flakes (Fig. 4E). EDX analysis (Fig. 4B) showed the abundance of Ti of these clusters. EDX also shows the occurrence of Fe in considerable concentrations in the anatase structure. The presence of Fe in the anatase structure was reported in Schroeder et al. (2004). X-ray diffraction analysis confirms the occurrence of anatase as one of the main constituents of the ball Table 1 Grain-size distribution (wt.%) in representative samples from the Aswan ball clays. Size (μm)

N63 63–31 31–15 15–8 8–4 b2 Total

Wadi Abu Sobeira

Wadi Abu Agag

S1

S2

A1

A2

A3

A4

A5

1.8 1.3 7.1 20.1 24.3 45.4 100

0.9 1.2 5.1 9.1 26.3 57.4 100

0.4 1.9 6.6 14.8 19.6 56.7 100

0.1 0.5 5 18 25.9 50.5 100

0.5 1.6 6.1 15.1 26.8 49.9 100

0.5 0.9 3.9 16.7 30.2 47.8 100

0.3 1.5 4.8 10.1 30.3 53 100

Please cite this article as: Baioumy, H.M., Ismael, I.S., Composition, origin and industrial suitability of the Aswan ball clays, Egypt, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.09.041

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the semi-quantitative mineralogical analysis of the bulk samples. In both Wadi Abu Sobeira and Abu Wadi Aagag areas, bulk ball clays are composed of kaolinite (39–60 wt.%), quartz (24–46 wt.%), and illite (10–19 wt.%). The clay fractions are composed mainly of kaolinite and illite with traces of anatase that can be identified only in the heated samples (Fig. 5B) after the breakdown of kaolinite and disappearance of kaolinite peak at 3.55 Å that overlaps the main anatase diffraction peak at 3.51 Å. Sample S1 from Wadi Abu Sobeira area shows relatively lower kaolinite and higher quartz contents compared to other samples due to the relatively high sand and silt contents in this sample compared to other samples. The values of the Hinckley Index (HI) range from 0.55 to 0.70 (Table 2) indicating low to medium crystallinity of the kaolinite in the Aswan ball clays. The low crystallinity of illite in the Aswan ball clays was confirmed by broad 002 and 004 reflections and high Kübler Index (KI N 1°).

4.5. Thermal analysis Fig. 2. Cumulative grain-size distribution of the Aswan ball clays.

clays as identified by its characteristic peak at 3.51 Å especially in the heated samples since the main anatase peak in many cases is overlapped with the kaolinite peak. 4.4. Mineralogy and crystallinity index Fig. 5A and B shows an example of X-ray patterns of the bulk samples and clay fractions of the Aswan ball clays, while Table 2 shows

(A)

(B)

Fig. 3. Photomicrographs of the Aswan ball clays that appear as fine-grained clayey matrix with some fine silt-size quartz (solid arrows) and heavy minerals (doted arrow). (A) Photomicrographs of the Fe-poor gray clays and (B) shows the brownish Fe-rich clays.

The differential thermal (DTA) curves of two representative samples from the Wadi Abu Sobeira (sample S1) and Wadi Abu Agag (sample A1) ball clays are shown in Fig. 6. The DTA patterns are similar although minor variation can be observed. The two curves show relatively strong endothermic peaks at approximately 100 °C that represent the dehydration of ball clays. The strong endothermic peaks at approximately 529 °C are due to the kaolinite and illite dehydroxylation. Small and broad exothermic effects at ca. 970 °C can be attributed to the transition into metakaolinite. The minor variations between the analyzed ball clays are represented by the existence of a very weak and broad endothermic peak at approximately 297 °C in sample S1, which does not exist in sample A1 and the presence of a weak and broad endothermic peak at approximately 397 °C in sample A1, which does not exist in sample S1. According to Rowland (1955), clays with monovalent cations exhibit one endothermic loop at about 150 °C; most clays with divalent cations have a second loop or a shoulder on a loop similar to the monovalent loop at a higher temperature (220 °C). Various organic compounds, particularly those which blanket the space between the layers of the lattice, also have their particular effect on the hydration loop. The occurrence of a second loop or a shoulder at approximately 297 °C in sample S1 could be due to the relatively high abundance of bivalent cations in the structure of clay minerals in this sample compared to sample A1 and/or occurrence of some organic matter in this sample (S1) as well. As the endothermic peak at approximately 397 °C was not reported in the typical kaolinite and illite DTA curves (e.g. Carthew, 1955; Grim and Rowland, 1944), it can be attributed to the occurrence of hydrous iron oxide phase (e.g. Kulp and Trites, 1951) especially in sample A1 that has the highest iron oxide content among the analyzed samples (Table 3). In addition, the endothermic peak at approximately 529 °C is stronger in sample A1 when compared with that in sample S1. Speil (1944) has found that the area of the endothermic peak at about 600 °C in the differential thermal curve of kaolinite decreases with decreasing particle size of the kaolinite. Grim (1947) has found that variations in size and perfection of crystallinity of particles of kaolinite appear to be reflected in variations in the intensity of the thermal reactions of the mineral. According to Carthew (1955), the amplitude of the endothermic peak at 600 °C is practically proportional to the weight of kaolinite and increases as both the particle size and the degree of crystallinity of kaolinite decrease. Grain-size distribution and mineralogical analysis showed that sample A1 has higher proportion of clay fraction as well as kaolinite contents (57 and 58 wt.%, respectively) compared to the sample S1 (45 and 39 wt.%, respectively). Therefore, the higher amplitude of endothermic peak at 529 °C in sample A1 can be attributed to the higher proportion of clay fraction and/or the kaolinite contents rather than to the crystallinity index of the kaolinite since the former is almost the same in both samples (HI values are 0.65 and 0.68 for sample A1 and S1, respectively).

Please cite this article as: Baioumy, H.M., Ismael, I.S., Composition, origin and industrial suitability of the Aswan ball clays, Egypt, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.09.041

H.M. Baioumy, I.S. Ismael / Applied Clay Science xxx (2014) xxx–xxx

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(A)

(B)

(C)

(D)

(E)

(F)

Fig. 4. (A) SEM photo of the kaolinite in the Aswan ball clays that occurs as ill-defined grains of variable sizes. (B) EDX pattern of the kaolinite with strong Al, Si, and O peaks and weak broad Fe and Ti peaks. (C) SEM photo of the illite (arrow) in the Aswan ball clays that occurs as relatively large plates within the finer kaolinite grains. (D) EDX pattern of the kaolinite with strong Al, Si, O, and K that characterize the illite composition. (E) SEM photo of fine-grained, very fine, uniform, rounded anatase crystals as pockets (arrow) between kaolinite flakes. (F) EDX pattern of the anatase crystal with strong Ti, O, and Fe.

4.6. Geochemistry 4.6.1. Major elements Major element distributions among the Aswan ball clays were shown in Table 3. The Wadi Abu Sobeira sample S1 that is characterized by higher sand and silt fractions shows relatively lower Al 2 O 3 and higher SiO2 contents compared to other samples. Although no

Fe-bearing minerals were identified in the X-ray diffraction patterns of the studied samples, all samples show relatively high Fe2O3 contents ranging from 2.3 wt.% to 11.3 wt.%. This indicates that the iron oxides occur mainly as oxides and/or hydroxides coatings and adsorbed on the surface of the clay particles and/or as amorphous grains embedded in the clay matrix. Petrographic investigations indicate the presence of such grains. The fine-grained nature (mostly

Please cite this article as: Baioumy, H.M., Ismael, I.S., Composition, origin and industrial suitability of the Aswan ball clays, Egypt, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.09.041

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H.M. Baioumy, I.S. Ismael / Applied Clay Science xxx (2014) xxx–xxx

Kln

Alteration (CIA) is calculated by the following formula (Nesbitt and Young, 1982): CIA = [Al2O3 / (Al2O3 + CaO + Na2O + K2O)] · 100. CIA values range from 91 to 95 suggesting the strong weathering conditions that prevailed during the formation of the Aswan ball clays.

Qz

(A)

Qz= Quartz Kln= Kaolinite Ilt= Illite Ant= Anatase

Kln

Qz Kln

Ilt

Kln Qz QzQz Qz Wadi Abu Agag (A4) Ant

Wadi Abu Sobeita (S1) 0

10

20

30

40

50

2θ° Ilt

(B) Ant Kln Kln

Heated Ilt

Ilt

Glycolated

Air-dried 0

10

20

30

40

2θ° Fig. 5. Representative X-ray patterns of the bulk sample (A) and clay fractions (B) of the Aswan ball clays.

clay and silt sizes) of Aswan ball clays can explain the relatively high amounts of adsorbed and stained of iron oxides. TiO2 contents that occur mainly as very fine anatase, range between 1.4 and 2.2 wt.%. K2O contents range between 0.9 wt.% and 1.3 wt.% and occur mainly as an illite. Na 2O and MgO occur in relatively low concentrations (0.5–0.8 wt.%). Other oxides such as MnO, CaO, P 2O 5 , SO3 and Cl occur in very low concentrations. Shale-normalized patterns of the Aswan ball clays (Fig. 7) exhibit higher TiO2, and Al2O 3 and lower Fe2O3 (except sample A1), MgO, CaO, K2O, and P2O5 contents compared to the average shale values (Condie, 1993). They have almost the same SiO2 contents as the average shales. Chemical Index of Table 2 Semi-quantitative mineralogical analysis (wt.%), Hinckley Index, and Kübler Index of representative samples from the Aswan ball clays. Minerals

Illite Kaolinite Quartz Anatase HI KI

Wadi Abu Sobeira

Wadi Abu Agag

S1

S2

A1

A2

A3

A4

A5

A6

13 39 46 2 0.68 1.22

16 58 25 2 0.70 1.28

16 54 28 2 0.65 1.20

15 59 24 2 0.55 1.30

10 60 27 2 0.62 1.32

11 46 41 2 0.58 1.25

15 52 30 2 0.70 1.23

19 55 25 2 0.67 1.20

HI = Hinckley Index of kaolinite crystallinity. KI = Kübler Index of illite crystallinity.

4.6.2. Trace elements Trace element distributions in the clay fractions of representative samples from the Aswan clays are listed in Table 4. Zirconium shows the highest contents among the analyzed samples followed by Sr, Ba, and V. Elements such as Cu, Zn, Co, Ni, Ga, Nb, Rb, Th and Y occur in relatively high concentrations, while elements such as Pb, Hf, Ta and U occur in relatively low concentrations. Another group of elements occurs in concentrations below the detection limit (b 0.1 ppm) of the analytical tool such as Mo, As, Cd, and Hg. Some elements which include Cu, Ni, Co, Zn, and V show strong positive correlations with Fe2O3 contents (r2 ranges from 0.6 to 0.9) suggesting the association of these elements with iron-bearing phases. Other elements such as Nb, Zr, and Y have negative correlations with the Al2O3 contents but positive correlations with the SiO2 contents (r2 ranges from 0.6 to 0.9) indicating the occurrence of these elements as silicates but not clay minerals. Elements such as Hf, Ta, and U show positive correlations with SiO2 (r2 ranges from 0.8 to 0.9) suggesting the occurrence of these elements as silicate minerals. Pb exhibits positive correlation with the Fe2O3 contents (r2 = 0.9) but shows negative correlation with the SiO2 suggesting its association with the Fe-bearing phases. Although Baioumy et al. (2012) indicated that anatase holds some trace elements such as Zr, Nb, and Cr in the Carboniferous and Cretaceous sedimentary kaolin deposits in Egypt, no correlations were observed between TiO2 that occurs mainly as anatase, and any of the analyzed trace elements. The mode of occurrence of the trace elements in the clay fractions of the Aswan ball clays can be subdivided into two categories. Elements that associate the Fe-bearing mineral phases include Cu, Ni, Co, Zn, V, and Pb as indicated from the positive correlations of these elements with the Fe2O3 contents. Elements that occur as silicate but not clay minerals include Nb, Zr, Y, Hf, Ta, and U as indicated from the positive correlations of these elements and the SiO2 and at the same time from the negative correlations with these elements with the Al2O3 contents. Shale-normalized patterns of the Aswan ball clays (Fig. 7) exhibit higher Co, Hf, Nb, Sr, Ta, Th, U, V, Y, and Zr and lower Ba, Ni, Pb and Rb contents compared to the average shale values (Condie, 1993).

4.6.3. Rare earth elements Rare earth element distributions in the clay fractions of representative ball clay samples are shown in Table 4. The ΣREE ranges from 291 ppm to 335 ppm. Like many of other trace elements, ΣREE shows negative correlations with the Al2O3 and positive correlations with the SiO2 (Fig. 8A) suggesting the occurrence of these elements mainly as silicates but not as clay minerals. No correlations have been observed between the rare earth elements and Fe2O3 and TiO2 (Fig. 8A). The REE patterns were chondrite-normalized using the REE of chondrite provided by Taylor and McClennan (1985). The Eu anomaly is calculated as E/E* = EuN / (SmN · GdN) and Ce anomaly is calculated as Ce/Ce* = (3Ce / CeN) / (2La/LaN + Nd/NdN). Chondrite-normalized REE patterns for the clay fractions of the study ball clays are shown in Fig. 8B. Samples from both areas have higher ΣREE contents compared to the REE concentrations of the chondrite. The REE patterns exhibit LREE enrichment relative to HREE as shown by (La/Yb)N ratios, that vary from 9.2 to 11.7 and slightly negative Eu anomalies are pronounced with Eu/Eu* from 0.74 to 0.83. Almost no Ce anomalies were observed in the study clays (Ce/Ce* = 0.96–1.03). Shale-normalized patterns of the Aswan ball clays (Fig. 7) exhibit enrichments of all REE especially LREE compared to the average shale values (Condie, 1993).

Please cite this article as: Baioumy, H.M., Ismael, I.S., Composition, origin and industrial suitability of the Aswan ball clays, Egypt, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.09.041

H.M. Baioumy, I.S. Ismael / Applied Clay Science xxx (2014) xxx–xxx

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Fig. 6. Differential thermal analysis (DTA) curves of two samples of the Aswan clays represent the Wadi Abu Sobeira (S1) and Wadi Abu Agag (A1) areas.

5. Discussion 5.1. Source of Aswan ball clays Several attempts have been made to determine the source area composition of detrital sediments due to its importance to reconstruct the paleogeography of a sedimentary basin using their major, trace, and rare earth element geochemistry (e.g. Floyd and Leveridge, 1987; Floyd et al., 1989; Honty et al., 2008; Ohta and Arai, 2007; Ryan and Williams, 2007). Binary or ternary scatter plots of selected element abundances were also used as “geochemical discrimination diagrams” for inferring the character or tectonic setting of source rocks to a clastic sedimentary basin and the amount of weathering that the detrital sediments have undergone (e.g. Lopez et al., 2005; Willan, 2003; Yan et al., 2006; Zimmermann and Bahlburg, 2003). Although there are several constrains in applying these proxies and geochemical discrimination diagrams, they have proved useful in discriminating the gross character of source rocks in complex ancient orogens (Pe-Piper et al., 2008).

According to Cullers and Graf (1983), high La/Yb ratios reflect the high abundance of LREE enriched granitic rocks, while low La/Yb ratios probably reflect the high abundance of mafic rock types. The high La/Yb ratios in the Aswan ball clays suggest a contribution of LREE enriched granitic rocks to the source of these deposits. The relatively high Zr and Y contents in the studied ball clays compared to the average shales could support this interpretation. On the other hand, Floyd et al. (1989) proposed that high Ti and Ni contents of sediments indicated a mafic rather than a felsic source rock, and Ryan and Williams (2007) found that Ti is a useful discriminator of tectonic setting. However, Pe-Piper et al. (2005) and Pe-Piper et al. (2008) indicated that the unusually high TiO2 content of Scotian Basin sediments is not related to mafic sources, but rather to derivation of ilmenite from granites and pelitic metasediments based on the positive correlations between Zr (occurring mainly in the granite) and both the Ti and Cr. In the current study, clay fractions from the Aswan ball clays exhibit relatively high TiO2 and Ni contents. The lack of correlation between the Zr and both Ti and Ni (Fig. 9) excludes the possibility of granite as a possible source of Ti and Ni. Thus, the relatively high TiO2 and Ni contents can be used to

Table 3 Major element distribution (wt.%) in representative bulk samples from the Aswan ball clays. Oxides

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 SO3 Cl L.O.I. Sum SiO2/Al2O3 CIA

Wadi Abu Sobeira

Wadi Abu Agag

S1

S2

A1

A2

A3

A4

A5

A6

66.64 1.62 16.41 4.81 0.04 0.45 0.05 0.69 0.87 0.03 0.03 0.13 7.97 99.74 4.1 91.1

55.64 1.41 23.36 5.03 0.02 0.55 0.06 0.73 1.11 0.02 0.03 0.12 11.03 99.11 2.4 92.5

53.04 1.59 19.7 11.28 0.11 0.62 0.06 0.84 1.0 0.01 0.05 0.18 11.5 99.98 2.7 91.2

56.05 1.78 24.43 3.54 0.02 0.64 0.14 0.57 1.0 0.03 0.05 0.26 11.23 99.74 2.3 93.5

57.86 2.07 24.48 3.39 0.02 0.45 0.12 0.5 0.72 0.02 0.03 0.18 10.09 99.93 2.4 94.8

65.91 2.21 19.35 2.3 0.02 0.47 0.08 0.69 0.76 0.01 0.03 0.14 7.63 99.6 3.4 92.7

58.76 1.84 21.74 3.69 0.04 0.56 0.14 0.69 1.02 0.03 0.03 0.27 10.65 99.46 2.7 92.2

55.2 1.63 22.63 5.37 0.04 0.76 0.11 0.72 1.25 0.02 0.04 0.26 11.51 99.54 2.4 91.6

Average shales (Condie, 1993) 63.6 0.82 17.8 5.89 2.3 1.3 1.1 3.84 0.14

Please cite this article as: Baioumy, H.M., Ismael, I.S., Composition, origin and industrial suitability of the Aswan ball clays, Egypt, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.09.041

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H.M. Baioumy, I.S. Ismael / Applied Clay Science xxx (2014) xxx–xxx

Fig. 7. Spider diagram shows comparison between the composition of the ball clays and the average shales (Condie, 1993).

indicate a mafic source rock. In the Rb-K2O binary plot (Floyd and Leveridge, 1987), Aswan ball clays are plotted at the boundary between the acidic and mafic fields, which supports the possible mixed acidic

and mafic source rocks of these clays. This is also supported from the plot of these clays in or close to the mixed felsic and mafic field in the Hf–La/Th binary plot of Floyd and Leveridge (1987). In addition, high

Table 4 Trace and rare earth element distribution (ppm) in the clay fractions of representative samples from the Aswan ball clays. Elements

As Ba Cd Co Cs Cu Ga Hf Hg Mo Nb Ni Pb Rb Sr Ta Th U V Y Zn Zr La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Sum (La/Yb)N Eu/Eu* Ce/Ce*

Wadi Abu Sobeira

Wadi Abu Agag

Average

S1

S2

A1

A2

A4

bdl 215 bdl 23 8.3 15.7 32.9 6.9 bdl bdl 40.2 17.9 1.7 54.8 166.5 2.4 13.2 4 171 47 13 326.9 68.0 148.9 15.5 58.6 10.1 2.3 7.8 1.4 8.1 1.8 5.2 0.8 5.0 0.8 334.2 9.2 0.79 1.03

bdl 149 bdl 21.7 7.2 19.4 35.2 4.4 bdl bdl 29.4 8 2.3 60 144.3 2 14.7 3.2 172 38.4 6 220.1 64.1 130.4 13.6 49.7 8.0 1.6 5.5 1.1 6.5 1.5 4.3 0.6 3.9 0.6 291.5 11.1 0.74 0.98

2.7 121 bdl 41.8 6.6 38.9 36.3 3.8 bdl bdl 26.6 26.9 4.4 57 170.9 1.7 14.2 3.1 194 36.6 25 193.3 61.9 127.4 14.1 54.3 9.0 2.1 6.7 1.1 6.4 1.5 4.1 0.6 4.1 0.6 293.7 10.2 0.83 0.97

2.3 132 bdl 21.2 5.8 30.3 35.5 4.6 bdl bdl 27.4 18.2 2.7 52.6 179.8 2 15.5 3.3 170 32.4 16 204.2 63.8 131.5 14.3 48.8 9.0 2.0 6.7 1.1 6.2 1.3 3.8 0.6 3.7 0.6 293.3 11.7 0.79 0.99

bdl 178 bdl 18.2 3.7 13.1 34 6.6 bdl bdl 38.2 3.8 1.1 54 223.9 2.3 13.1 4.7 174 40.9 2 285.4 68.7 142.4 15.7 63.6 11.6 2.7 9.1 1.4 8.0 1.6 4.3 0.7 4.6 0.7 335.2 10.1 0.80 0.96

2.5 159.0 bdl 25.2 6.3 23.5 34.8 5.3 bdl bdl 32.4 15.0 2.4 55.7 177.1 2.1 14.1 3.7 176.2 39.1 12.4 246.0 65.3 136.1 14.6 55.0 9.5 2.1 7.2 1.2 7.0 1.5 4.3 0.7 4.3 0.7 309.6 10.3 0.78 0.99

Average shales (Condie, 1993)

551 20

4.6

15.4 54 22 163 136 1.4 13.5 2.9 117 33 201 38.8 82 6.1 32.3 5.75 1.14 5.22 0.81

0.5 2.95 0.74

bdl = below the detection limit.

Please cite this article as: Baioumy, H.M., Ismael, I.S., Composition, origin and industrial suitability of the Aswan ball clays, Egypt, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.09.041

H.M. Baioumy, I.S. Ismael / Applied Clay Science xxx (2014) xxx–xxx

Fig. 8. (A) ΣREE shows negative correlations with the Al2O3 and positive correlations with the SiO2. (B) Chondrite-normalized REE patterns of the clay fractions from the Wadi Abu Sobeira and Wadi Abu Agag ball clays.

Nb contents are used to indicate alkaline nature of igneous rocks (e.g. Pik et al., 1999). The studied clays show relatively high Nb contents (average of 33 ppm) compared with the average shales (11 ppm). This, in turn, indicates a contribution of a source rock of alkaline nature to the source of the studied ball clays. The plot of the study clays in the phyllite and schist fields of the Co–Th binary diagram (e.g. Dombrowski, 1982; Dombrowski and Murray, 1984) can also suggest a possible contribution from metamorphic rocks to their source. Crystalline rocks in the Aswan area that could be the possible source of the study ball clays are composed of variety of igneous rocks including basic and alkaline (basalts of the Tertiary volcanics), and acidic (granites) as well as the metamorphic rocks (schists and gneisses) (e.g. Ali et al., 2009; Attia, 1955; Germann et al., 1987; Khedr, 1984; Mesaed, 1995). 5.2. Suitability of Aswan ball clays for industrial applications Although the ceramic properties of the Aswan ball clay were not measured in the current study, the suitability of these clays for ceramic industrial applications can be examined through their mineralogical and geochemical properties that are addressed in detail in this paper. The ceramic properties of plastic ball clays depend on the grain size, the content of the clay minerals (especially I/S minerals), and crystallinity of kaolinite and illite (e.g. Abadir et al., 2002; Andreola et al., 2002; Esposito et al., 2005; Galos, 2011a,b; Leonelli et al., 2001). These features

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Fig. 9. Lack of correlations between Zr and both TiO2 (A) and Ni (B) contents in the Aswan ball clays.

influence the grain size distribution and specific BET surface area of the clays. Plastic ball clays contain commonly large amounts of grains b1 μm (50–90 wt.%). The grain size of clay is, in general, inversely proportional to the specific BET surface area and plasticity. Levels of crystallinity of kaolinite and illite are the other factors influencing the plasticity of clay (Dondi et al., 2003; Stoch, 1964). Therefore, optimal ball clays should contain higher amounts of low-ordered kaolinite and illite and some amounts of I/S minerals. Aswan ball clays are characterized by abundance of clay fractions (b 2 μm) as well as low-order kaolinite and illite and no I/S minerals were detected. These properties suggest the suitability of the Aswan ball clays as good quality raw materials for ceramic industrial applications. As to the chemical compositions, the SiO2/Al2O3 molar ratio usually ranges between 1.6 and 2.6 in plastic fireclay, 2.4 and 4.0 in ball clay, and 3.5 and 6.7 in siliceous fireclay (Ogbukagu, 1980; Plummer and Romary, 1967). According to this classification, the Aswan clays are typically ball clays (SiO2/Al2O3 = 2.3–4.1). In general fluxing agents such as alkali oxides (Na2O and K2O) and alkaline earth oxides (CaO, MgO) are low, suggesting that densification of these clays will occur at high temperatures, and therefore confirming the aforementioned refractory behavior. However, the coloring oxides TiO2 and Fe2O3 are relatively high indicating that the fired products of these clays are expected to be colored. The low S and Cl contents in the Aswan ball clays guarantee limited SO2 and HCl emissions to the atmosphere during the ignition of these clays. In addition, the concentrations of toxic elements such as As, Cd, Hg, and Pb are very low and sometimes below the detection limits.

Please cite this article as: Baioumy, H.M., Ismael, I.S., Composition, origin and industrial suitability of the Aswan ball clays, Egypt, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.09.041

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H.M. Baioumy, I.S. Ismael / Applied Clay Science xxx (2014) xxx–xxx

These characteristics in turn add more advantages to the industrial applications of the Aswan ball clays and are considered as environmentally friendly.

6. Conclusions Aswan ball clays occur as gray, yellowish gray, reddish to brownish gray, massive to faint laminated and moderately hard clays dominated by clay-size fractions. They are composed of low-ordered kaolinite, quartz, low crystalline illite, and fine-grained anatase. Geochemical analysis indicated that the Aswan clays are typically ball clays with relatively high Fe2O3 and TiO2 contents. Trace elements occur in two modes. Elements associated with the Fe-bearing mineral phases include Cu, Ni, Co, Zn, V, and Pb and elements occurring in silicates but not in clay minerals include Nb, Zr, Y, Hf, Ta, and U. The REEs occur mainly as silicate minerals but not in clay minerals with chondrite-normalized patterns exhibiting LREE enrichment relative to HREE and slightly negative Eu anomalies. Geochemical data suggested a mixture of more than rock types as a source of the Aswan ball clays. The high Zr and Y contents and La/Yb ratios suggest a contribution of granitic rock, while the relatively high contents of TiO2 and Ni in the clay fractions are suggestive for a contribution of mafic source rock. Plot of the study clays in the Rb-K2O and Hf–La/Th binary plots supports mixed felsic and mafic source rocks. In addition, high Nb contents indicate a contribution of alkaline source rock. Plot of the study clays in the phyllite and schist field in the Co–Th binary diagram also indicates contribution of metamorphic source rock. The abundance of clay-size fractions (b 2 μm), low-order kaolinite and illite, absence of I/S minerals, low fluxing agents such as alkali oxides (Na2O and K2O) and alkaline earth oxides (CaO and MgO), low S and Cl contents and low contents of toxic elements (As, Cd, Hg, and Pb) revealed the suitability of the Aswan ball clays as good quality and environment friendly raw materials for ceramic and refractory industries.

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Please cite this article as: Baioumy, H.M., Ismael, I.S., Composition, origin and industrial suitability of the Aswan ball clays, Egypt, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.09.041