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Materials Chemistry and Physics 114 (2009) 313–318

Contents lists available at ScienceDirect

Materials Chemistry and Physics

journa l homepage: www.e lsev ier .com/ locate /matchemphys

lectrical properties and mineralogical investigation ofgyptian iron ore deposits

.M. Gomaaa,∗, A.A. Shaltoutb, M. Boshtab

National Research Centre, Geophysical Sciences Dep., El Behooth St., 12622 Dokki, Cairo, EgyptNational Research Center, Physics Division, El Behooth St., 12622 Dokki, Cairo, Egypt

r t i c l e i n f o

rticle history:eceived 23 June 2008eceived in revised form 6 August 2008ccepted 14 September 2008

a b s t r a c t

Electrical properties and X-ray diffraction of 20 hematite sandstone samples of the most economicallyinteresting Egyptian iron ore deposits have been investigated. Samples were collected from two differentareas in Egypt (Aswan and Bahariya). Complex impedance measurements in the frequency range from10 Hz to 100 kHz were performed at room temperature (∼20 ◦C). The observed dielectric behavior was

eywords:-ray diffractionielectric propertieslectrical conductivityron ores

characterized by Maxwell–Wagner interfacial polarization at low frequencies and bulk polarization atrelatively higher frequencies. The frequency dependence on conductivity shows a classical relaxationbehavior followed Jonscher’s universal law. The measured electrical properties vary strongly with thefrequency and sample composition. The difference in the electrical properties may be attributed to thefluctuations in the concentration of the sample constituents and to the degree of heterogeneity of thegrains. The XRD-patterns of Egyptian iron ore deposits prove that the main phases are hematite and

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quartz.

. Introduction

The electrical properties of materials consisting of a mixture ofetals and dielectrics are extremely sensitive function of the rela-

ive concentrations of the components. Furthermore, the electricalroperties of the material depend on the components in which theyre distributed throughout the volume of the material which can bexplained by X-ray diffraction. In addition, the electrical response ofulticomponent system depends basically upon the volume frac-

ion and electrical properties of each individual component [1,2].ue to the presence in a mixture, the individual components will

nteract, leading to the development of a region of distinct elec-rical and physical properties. AC measurements yield informationhich can be used to determine the conduction process applicable.

hree main conduction models are applicable to materials, the hop-ing model, the variable hopping model and the band theory [3,4].ecently, the DC electrical properties of various materials have beenxtensively studied, but relatively little work has been carried out

n their AC electrical behavior [5,6]. The effective electrical prop-rties of mixtures depend basically on the particle size [7], particlehape [8], the effective conductivity and dielectric constant of thenterstitial constituents [9], heterogeneity and randomness of the

ixture, and the frequency of the applied field [10].

∗ Corresponding author. Tel.: +20 115985275; fax: +20 233370931.E-mail address: mmmsgomaa@yahoo.com (M.M. Gomaa).

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254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2008.09.012

© 2008 Elsevier B.V. All rights reserved.

The composite properties of rocks vary appreciably with fre-uency [11]. At low frequency range, the effective conductivitys gradually increasing function of frequency, while the effectiveielectric constant is much more strongly dependent on frequency12,13]. Electrical characteristics have been interpreted as beingaused by geometric or textural heterogeneities of the rock system12,14], or were related to electrical and electrochemical processeseveloped at the interfaces between rock grains [1,15]. Interactionsetween charged particles give rise to double layer around thesearticles. Polarization of such layer by an applied electric field haseen pointed out as the main mechanism for the anomalous behav-

or observed in rocks [16,17].Knight and Abad [18] observed a power law dependence of

he dielectric constant on the frequency and that was related tohe texture of the sandstone samples. The power law response ishought to be due to the random nature of the constituents withinhe samples [19,20]. Dry sandstone samples without a metallicomponent at room temperatures are good dielectrics with con-uctivity value of the order of 10−10 S m−1 and a relative dielectriconstant of the order of several units. For a combination of sandnd hematite, the interface gives rise to large values of dielectriconstant with strong frequency dependence [21,22], which can be

xplained by surface conductivity and polarization processes. Inhe absence of an applied electric field, ions are free to move. Whenn oscillating electric field is applied, the ions polarized aroundhe rock grains give rise to large dipoles and to large apparentielectric constants [21–23]. As the frequency increases, ions have

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14 M.M. Gomaa et al. / Materials Che

ess time to polarize and contribute more to the conductivity, sincehey are more in phase with the applied oscillating electric field.

The addition of a small amount of conductor to the sample mayncrease the conductivity and decreases the dielectric constant.he air is one of the important factors in representing the dielec-ric properties of the mixture. These effects come from physicaleactions of the grain interface and can be considered as surfaceontributions to the complex electrical conductivity of a porousystem [21,22]. The most probable polarization mechanism is therientation polarization of molecules or hopping of ions along theurface [21,22,24]. Chelidze et al. [22] concluded that the polar-zation of non-conducting particle coated with a conducting onend embedded in an AC field leads to high dielectric values at lowrequencies. This explains why the addition of small quantities ofonductor to a porous rock causes a strong dielectric values at lowrequencies.

Hematite as a rock occurs in many types of igneous, metamor-hic and sedimentary rocks. The largest and most economically

mportant hematite deposits are mainly of sedimentary origin,orming from the weathering of iron bearing minerals. In theseedimentary deposits, hematite is thought to have precipitatedrom lakes or seas by organic and/or chemical processes. Theematite often occurs with intermixed layers of quartz or chert.his study has thrown the light upon the nature of Egyptianron ores, its group minerals, its crystal structure and their rela-ion with the electrical properties. The iron ore deposits of theseocalities vary greatly in their mineralogical and chemical compo-ition.

In the present work, the electrical properties and the X-rayiffraction of 20 iron ore deposits from East Aswan (Easternesert) and Bahariya Oasis (Western Desert) were demonstrated.he electrical measurements have been measured in a dry con-ition in the frequency range from 10 Hz to 100 kHz at roomemperature (∼20 ◦C). Mechanisms of conduction and polarizationsere presented under different constituent concentrations of theresent samples. XRD patterns were also demonstrated in order to

nvestigate the different phases of the different constituents andnderstand the crystal structure of the different natural pattern ofhese samples.

. Samples

.1. Aswan iron ore deposits

Aswan iron ore deposits in the eastern desert of Egypt have beenhosen for the present study in order to understand the composi-ion and texture of this sediment. The heavy minerals in the Nubianandstone in Aswan have been found to be mainly iron ores, zircon,ourmaline and futile [10]. Attia [25] considered that the Nubianandstone is of marine origin. Lithologically, Aswan sandstone isssentially composed of conglomerates, sandstones, sandy shales,lays and quartzitic bands [10,25]. The iron ore bands are oftenssociated with ferruginous sandstones and clays. Nakhla and She-ata [26] supposed that the iron ores of east Aswan are composedf cryptocrystalline hydrated hematite (Fe2O3–nH2O), microcrys-alline hematite, clay minerals, cryptocrystalline and amorphousilica, quartz possesses a sub angular to sub-rounded form andome other ingredients. X-ray diffraction proved the presence ofematite and quartz with the occasional presence of chamosite inome ore specimens. According to pervious semi-quantitative spec-

rographic analysis [26] of iron ore samples there is a wide rangef variation in chemical composition. A brief outline on the geo-hemical nature of the most important elements present in Aswanron ore deposits are Fe, Si, Ca, Mg, Mn, P, S, Cu, Ti, Ni, Cr andr.

4

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and Physics 114 (2009) 313–318

.2. Bahariya iron ore deposits

Bahariya iron ores deposits in the western desert of Egypt haveeen also chosen for the present study and it represents the sec-nd source of iron ore in Egypt. The Bahariya iron ore deposits areenerally capped [25] by an alluvial cover and/or quartzite. Thepper part of the ore body is usually composed of hard goethiteith some pockets of manganiferous hematite or conglomerate.

he lower part is composed of manganiferous hematite with someockets of hard goethite, and may be changed, especially at its lowerarts, into pisolitic and oolitic goethite without manganese pockets.he main minerals associated with the ore include halite, gypsum,arite, quartz and clayey material.

. Experimental work

.1. X-ray diffraction

The X-ray diffraction measurements have been carried out by using Siemens00 instruments. The applied current in the X-ray tube was 36 mA and the appliedoltage was 45 kV. Cu target was used in Siemens 500 instruments. X-ray diffrac-ion measurements were carried out overnight at interval of the diffraction angle�� = 0.01). The peaks of these XRD patterns correspond to those of the theoreticalatterns from the ASTM data file to determine the crystal structures, lattice param-ters, and crystal planes (h k l) for all phases found on Egyptian iron ore samples.

.2. Electrical measurements

Electrical and dielectric properties of 20 hematitic sandstone samples of Egyp-ian iron ore deposits in Aswan and Bahariya have been investigated. Complexmpedance measurements were carried out at room temperature (∼20 ◦C). Data

ere performed in the frequency range from 10 Hz up to 100 kHz using Hioki522-50 LCR Hitester Impedance Analyzer. The measurement system was discussedlsewhere [27–29]. In order to measure the electrical response of the sample undernvestigation, the effect of heterogeneity and randomness of the mixture was elim-nated by grinding the samples. The samples were grinded for 3 min (600 rpm) by

iniMill 2, PANalytical, Netherlands (particle size ∼1 �m), Afterwards, the samplesere pressed at 120 kN for 1 min by pressing machine (Herzog hydraulic HTP40, UK).

he samples become like a homogenous pellets produced by the mentioned pressool. Samples dimensions were in the order of 3-mm thickness and 40-mm diameter.he homogenous pellet samples were measured electrically at a relative atmo-pheric humidity (∼50%). The samples were initially evacuated and measured in ansolated chamber. A voltage of 1 V was applied and the current density in the sampleas ∼=4 × 10−6 (�A cm−2). The complex relative dielectric constant is given by,

∗ = ε′ − iε′′; ε′ = Cp

C0& ε′′ = Gp

� · C0& C0 = ε0 ×

(A

d

)(1)

here A, d, Cp, Gp, �, ε′ , and ε0 are the cross sectional area of the sample, samplehickness, the parallel capacitance, the parallel conductance, the angular frequency,elative dielectric constant and the permittivity of free space (8.85 × 10−12 F m−1),espectively. The measured parameters are both series and parallel capacitance andesistance at different frequencies. In the series mode, the complex impedance Z isiven by,

= Rs − iXs; Xs = 1�Cs

(2)

here Rs is the series resistance (real impedance), Xs is the reactance and Cs is theeries capacitance. The complex resistivity �* is given by,

∗ = Z ×(

A

d

); � = 1

�∗ (3)

For the parallel model,

= Rp ×(

A

d

); � = 1

�(4)

here Rp, � are the parallel resistance and conductivity.

. Results and discussions

.1. Results of electrical properties

Generally, the increasing of conductor constituents increases theonductivity and decreases the dielectric constant. The conductiv-ty increases due to the increase of conduction paths between the

M.M. Gomaa et al. / Materials Chemistry

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atstthis is why the sample (AS4) has the highest dielectric constant.When the grains touches each others there is no air gap betweenthem and the dielectric constant comes to its lowest value, this iswhy the sample (AS10, and BA7) has the lowest dielectric constant

ig. 1. Variation of the conductivity with frequency for Aswan (a) and Bahariya (b)ron ore deposits.

wo electrodes, while the dielectric constant decreases due to theecrease of air gap between the grains. Also, frequency, pressurend chemical reaction may lead to the same results as the con-uctor concentration [4,5]. The samples are mainly hematite as aonductor and sand as insulator and the measured conductivityf the samples ranges from insulator to semiconductor. Lima andharma [7] have shown that the effective electrical conductivitynd dielectric permittivity of shaly sands depend basically on theonductor volume content in the sand and the frequency of thepplied electrical field.

.1.1. Relation between conductivity and frequencyFig. 1a and b depicts the variation of the conductivity versus

requency for the twenty samples of Aswan and Bahariya ironre deposits. As concentration increases, more continuous pathsre formed between electrodes and more conductor paths (con-uctivity) from the conductor grains are added to the currentnd consequently increase the conductivity. There is supposedo find a critical concentration and a critical frequency. Criticaloncentration is found when the conductor grains (hematite) begino contact with each other, forming the first continuous path ofonductor between the two electrodes. This critical concentrations not clear at the Aswan area samples whereas nearly all theamples did not reach that concentration, but it can be seen clearly

n, Fig. 1b, for samples BA1 and BA2. Also, other samples show theeginning of this behavior (samples BA6 and BA10). The same isone for the critical frequency. Critical frequency is found whenhe curve changes its slope with the increase of frequency dueo the change in the mechanisms of conduction in the sample.

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and Physics 114 (2009) 313–318 315

here is no change in the conductivity at low frequencies foramples BA1 and BA2. It is observed that the conductivity shows arequency dependent behavior for all the samples under study. Atelatively high frequencies, curves tend to merge with each otherith a constant slope (Fig. 1). Generally the conductivity increasesith increasing frequency with a characteristic wn dependence,ith n ∼ 0.75, n is the power law exponent which generally varies

etween 0 and 1 depending on many parameters (grain size, grainhape, texture, etc.). The value of n decreases with increase ofnteraction between the grains and the surrounding surfaces. Sam-les BA1 and BA2 show a flat response with wn dependence. Thehenomenon of the conductivity dispersion in solids is generallynalyzed using Jonscher’s law [19]. The exponent n represents theegree of interaction between mobile ions with the grains aroundhem. According to Jonscher models [19], the interaction betweenll dipoles participating in the polarization process is characterizedy the parameter n. A unit value of n implies a pure Debye case,here the interaction between the neighboring dipoles is almostegligible and the only conductive element is the DC resistance.

.1.2. Relation between dielectric constant and frequencyFig. 2a and b shows the variation of the dielectric constant

gainst frequency for Aswan and Bahariya iron ores deposits. Withhe increase of the conductor concentration the air gaps in theample decrease and thus generally increase the dielectric constanto a certain limit before the grains just begin to touch each other,

ig. 2. Variation of the dielectric constant with frequency for Aswan (a) and Bahariyab) iron ore deposits.

316 M.M. Gomaa et al. / Materials Chemistry

Fi

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rdarc in the impedance plane. With the increase of concentrationthe angel between that arc and the Im Z (imaginary impedance)increases.

For higher concentrations this arc is expanded to be a part ofsemicircle or depressed semicircle. In Fig. 4a the samples shows

ig. 3. Variation of the loss tangents with frequency for Aswan (a) and Bahariya (b)ron ore deposits.

Fig. 2a and b). Increasing the frequency decreases the dielectriconstant until the sample reaches the value of the insulatoromponent (quartz) [6]. The samples BA1 and BA2 (Fig. 2b) haveigh dielectric values, especially at high frequencies. The dielectriconstant (ε′) is frequency dependent (ε′ ∝ wn−1; n ∼ −0.25) nearlyor all the samples under study. The dispersive nature of theelaxation may be attributed to the broad distribution of relaxationimes of the dipoles. The low-frequency dielectric dispersion isidely found in many systems in which less mobile charge carriersominate the behavior [19]. Generally, the dielectric constant haswo regions with different slopes with the variation of frequency.he low frequency slope is steep (≈−0.25) and it decreases untilt reaches a slope of ≈−0.1 (sample BA7). The anomalous dielectricroperties of the present samples can also be interpreted usingercolation theory. This theory predicts that, when the conductiveraction (hematite) increases, clustering of conductive inclusionsevelops and the thickness of insulating gaps between conductivelusters decreases, causing a large increment in the capacitance ofhe sample. Further increases in the conductive component causehe shunting of insulating capacitive gaps.

.1.3. Relation between loss tangents with frequencyFig. 3a and b shows the loss tangent (tan ı = ε′′/ε′) versus fre-

uency. Samples AS1 and AS4 give their maximum at 200 Hz. Theiagram of the loss tangent versus frequency (Fig. 3a) is naturallyubdivided into two segments, while it can be subdivided intohree segments for Fig. 3b (sample BA2). The decreasing tan ı atow frequencies (Fig. 3b, below 20 Hz) indicates that the fraction of

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and Physics 114 (2009) 313–318

onducting particles in the material is increasing. Only the samplesith no dispersion (BA3 and BA10 as an example) with frequency

bey Jonscher’s universal law [30].

.1.4. Relation between real and imaginary parts of complexmpedance (Z)

Fig. 4a–c shows the variation of the imaginary part versus theeal part of complex impedance for Aswan and Bahariya iron oreeposits. It is clear from the figure that conduction paths show an

ig. 4. Variation of the real with imaginary parts of the complex impedance (Z) forswan (a) and Bahariya (b and c) iron ore deposits.

M.M. Gomaa et al. / Materials Chemistry and Physics 114 (2009) 313–318 317

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dtT4(tAf(

Fig. 6. XRD-patterns of Bahariya (BA) iron ore deposits. 1 represents the peaks ofFe2(SiO4), 2 to SiO2, 3 to Fe2O3, 4 to Fe3O4 and 5 to Mn3O4.

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ig. 5. XRD-patterns of Aswan (AS) iron ore deposits. 1 represent the peaks ofe2(SiO4), 2 to SiO2, 3 to Fe2O3, 4 to Fe3O4 and 5 to Mn3O4.

nly the arc with different angels (different concentrations). Thateans that there is not any conduction paths between the elec-

rodes and/or the concentration of the samples is below the criticaloncentration. Fig. 4b shows the arc with different angels from them Z (samples BA3 to BA10). That means that all these samples haveo conduction paths between the electrodes or have concentrationselow the critical concentration. Samples BA1 and BA2 (Fig. 4b)hows a semicircle. The increase of the slope of the arc and itsovement towards a semicircle is an indicator of the increase of

he continuous paths of the conducting medium. Fig. 4c illustrateshe behavior of sample BA1 indicating a highest conduction pathsith highest concentration.

.2. X-ray diffraction results

The X-ray diffraction measurements have been carried out bysing Siemens 500 instruments. The X-ray powder diffraction pat-erns of a representative Aswan iron ore deposits (AS) are shownn Fig. 5. As can be seen in the pattern, reflections representativef Fe2O3, Fe3O4, Fe2(SiO4), Mn3O4 and SiO2. The reflection peaksf Fe2O3 (hematite) at 2� = 24.2, 33.2, 35.65, 40.88, 54.2 are theain peaks which corresponding to the plans (0 1 2), (1 0 4), (1 1 0),

1 1 3), and (1 1 6), respectively. These reflection peaks are slightlyhanged its intensity with position changing. Also, the XRD pat-erns of Aswan (AS) iron ore deposits show a few reflection peaksf Fe3O4 at 2� and (h k l) are 34.1 (0 2 3), 34.66 (1 1 1), 47.5 (1 3 2),8.87 (1 3 4). Furthermore, Fe2(SiO4) peaks at 2� and (h k l) are 57.351 5 2), 69.1 (3 4 0) and 82.68 (3 5 2) which are relatively low inten-ity comparing with that of Fe2O3 peaks. While it shows a threeery low intensity reflection peaks for Mn3O4 at 2� (h k l) 36.452 0 2), 77.5 (4 0 4), 80.6 (3 1 6) and only one reflection peak for SiO2t 2� = 21.1 (1 1 1). The diffraction patterns of Aswan samples havepproximately the same phases with slightly change in the peakntensity. Therefore, the electrical properties of these samples showhe same behavior as mentioned above (Figs. 1a, 2a and 3a).

X-ray powder diffraction patterns of Bahariya (BA) iron oreeposits are shown in Fig. 6. As can be seen in the pattern, reflec-ions representative of Fe2O3, Fe3O4, Fe2(SiO4), Mn3O4 and SiO2.he reflection peaks of Fe2O3 (hematite) at 2� = 24.2, 33.2, 35.65,0.88, 54.2 are the main peaks which corresponding to the plans

0 1 2), (1 0 4), (1 1 0), (1 1 3), and (1 1 6), respectively. These reflec-ion peaks are slightly changed its intensity with position changing.lso, the XRD-pattern of Bahariya (BA) iron ore deposits show a

ew reflection peaks of Fe3O4 at 2� and (h k l) are 34.1 (0 2 3), 34.661 1 1), 47.5 (1 3 2) and 58.87 (1 3 4). In the case of Fe2(SiO4), the

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ig. 7. XRD-patterns of Bahariya (BA) iron ore deposits where the peaks 2 for SiO2

nd 3 for Fe2O3.

� and (h k l) are 57.35 (1 5 2), 69.1 (3 4 0) and 82.68 (3 5 2) whichre relatively low intensity comparing with Fe2O3 peaks. While ithows five very low intensity reflection peaks for Mn3O4 of 2� andh k l) at 36.45 (2 0 2), 65.4 (3 2 3), 71.7 (1 0 7), 77.5 (4 0 4) and 80.63 1 6). For some samples, one reflection peak of SiO2 has appearedt 2� = 21.1 corresponding to (h k l) = (1 1 1).

The diffraction patterns of Bahariya (BA) samples have approxi-ately the same phases with slightly change in the peak intensity

or BA3, BA4, BA5, BA7, BA8 and BA9. Therefore, the electrical prop-rties of these samples show the same behavior as mentioned aboveFigs. 1b, 2b and 3b). As shown in Fig. 7, the XRD patterns of SiO2t 2� = 21.1 disappear for samples BA1, BA2, BA6 and BA10 due tohe slightly content of SiO2 in these samples comparing with otheramples (sample BA7). The enhancement of the electrical proper-ies of these samples may be due to the absence of SiO2 (quartz)hase which illustrated in Fig. (1b, 2b, 3b and 4c).

. Conclusion

The electrical properties and X-ray diffraction of 20 Egyp-

ian iron ores samples from two different areas in Egypt (Aswannd Bahariya) have been investigated. XRD measurements provehat, the samples are mainly hematite (Fe2O3) as a conductornd quartz (SiO2) as insulator. At frequency range from 10 Hz to00 kHz, Complex impedance measurements were performed at

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18 M.M. Gomaa et al. / Materials Che

oom temperature (∼20 ◦C). The frequency dependence on con-uctivity shows a classical relaxation behavior followed Jonscher’sniversal law. The measured electrical properties vary stronglyith the frequency and sample composition. The difference in the

lectrical properties may be attributed to the fluctuations in theoncentration of the sample constituents and to the degree of het-rogeneity of the grains.

eferences

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[[[

[[

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