الفهرس 
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Abstract
The talc rocks are considered as important resources for the magnesium oxide ceramics. The talc rocks are mainly quarried at the Eastern Desert, therefore, selected areas were the main target to study the petrography, geochemistry and industrial applications of the outcropped serpentinites and their derivatives. The study target areas are Wadi Abu Fannani, Wadi Mubarak, Gabal Hijllij, Gabal Um Salatit, Wadi Barramiya, Gabal Mudargag, Wadi Antr, Wadi Nukharia and Wadi Allaqi.
The rocks of the ten areas are distinguished with different grades of talc rocks. Petrographically, the rocks of Wadi Allaqi are considered as pure talc whereas Wadi Mubarak is dominated by talc-tremolite rocks. Geochemically, the serpentinites and talc rocks represent the following geochemical features: (a) the serpentinites have considerable values of SiO2 (36.29 – 42.28 wt.%) and MgO (29.06 – 41.27 wt.%) which might refer to a developed magma of mantle composition; (b) talc rocks show high percentages of SiO2 (up to 74.25 wt.%) and depleted values of MgO (3.79 – 32.54 wt.%) due mainly to alteration processes (such as silicification and carbonation).
Seven areas are considered promising for cordierite-based ceramic production. These areas are Abu Fannani, Wadi Mubarak, Gabal Hijllij, Gabal Um Salatit, Wadi Antr, Wadi Nukharia and Wadi Allaqi. The talc samples of these areas were mixed with high grade kaolin selected from Southern Sinai. This mix design for all nine representative talc samples is based on the stoichiometric cordierite composition. The synthesized cordierite ceramics contain crystalline and amorphous phases with mutual relations to affect their characteristics.
The chemical composition of the whole nine batches encourages their solid-state reaction upon sintering in the temperature range between 1000 and 1375
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Summary and conclusion
ºC. Megascopically, the sintered discs show remarkable variation in their color due to the different content of coloring oxides (such as Fe2O3) and phase composition at each sintering temperature. The mineral constituents of the sintered discs are composed of cordierite (Mg2Al4Si5O18), cristobalite (SiO2), enstatite (MgSiO3), diopside (MgCaSi2O6), mullite (Al6Si2O13) and Mg-Al spinel (MgAl2O4) at variable sintering temperatures with different concentrations. Cordierite starts to crystallize at 1200 ºC and increases with the applied sintering temperatures up to 1375 ºC. Enstatite, diopside, Mg-Al spinel and mullite started to appear at 1000 ºC and decrease in contents with increasing the sintering temperatures on the expense of cordierite crystallization. Cristobalite shows its maximum ratio at 1200 ºC.
Microstructurally, Antr, Nukharia and Allaqi sintered samples show massive cordierite crystals that grow-up into large sub-hedral cordierite with compact structure at the temperature range (1300-1350 ºC). However, Nukharia sintered sample show neogenic cordierite holohedral crystals at 1350 ºC. On the other hand, Mubarak and Fannani sintered samples show the embryonic and neogenic subhedral prismatic cordierite crystals with compact structure at 1200 ºC. These cordierite crystals grow-up into neogenic well-developed holohedral cordierite crystals with bloating structure (foamed structure) at 1300 ºC due to their enrichment with Fe2O3 (5.49 – 5.63 wt.) and CaO (3.05 – 4.48 wt.%) contents.
The sintered ceramic samples show increase in the average bulk density (1.95
– 2.09 g/cm3) and average linear shrinkage (2.05 – 3.58 %) with the increase of temperature (1000 – 1200 ºC). This could be related to the samples pore filling with silicate liquid phase during sintering. The latter liquid phase vitrifies and crystallize in the pores into phases with high densities (e.g., enstatite, mullite
and Mg-Al spinel). The average bulk density (2.00 – 2.08 gm/cm3) and average linear shrinkage (2.91 – 3.22 %) show general decreasing trends when the
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temperature increase due to the increase of cordierite on the expense of enstatite, mullite, Mg-Al spinel and cristobalite phases (1300 – 1375 ºC).
The sintered ceramic samples of Antr, Nukharia, Salatit and Allaqi at 1300-1375 ºC show low dielectric loss measurements. Particularly, Nukharia sample showed the lowest loss tangent values (" ~ "0.0016 at 1MHz) comparing to Salatit and Antr for all sintered temperatures. This could be attributed to the presence of Ca-carbonate which facilitates crystallization of low loss hexagonal phase of cordierite (indialite). These characteristics make Nukharia sintered samples promising to be used in many applications such as electrical capacitors, microwave devices and wireless communications. In addition, Salatit at 1350 ºC and 1375 ºC shows a higher loss tangent value (" ~ "0.0077) compared to that given for Nukharia (" ~ "0.0016). The highest permittivity and loss tangent is measured in Allaqi sample sintered at 1350 ºC because of the presence of Mg-Al spinel and crystallization of high loss orthorhombic cordierite phase. The higher the crystallite size and the lower the crystallinity is the higher conductivity of all sintered samples. Nukharia samples sintered at 1375 ºC show high conductivity value (" ~ "6.7×10-7S/m at 106 Hz) due to the lowest crystallinity (57.90 %) and high crystallite size (1.24 µm).
The interesting characteristics of Nukharia ceramics make them technologically significant and competitive to other alternative materials, not only due to their electrical behavior but also because of their cost-effectiveness.