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

Abstract

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 (45–57wt.%). Low-ordered kaolinite is the main constituent (39–60wt.%) along with quartz (24–46wt.%) and low crystalline illite (10–19wt.%). Fine-grained anatase is reported as a minor constituent (~2wt.%). 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 335ppm 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 (<2μm), 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.