Ti-bearing minerals in sedimentary kaolin deposits of Egypt

Titanium (Ti) is a major impurity in the economic Carboniferous and Cretaceous sedimentary kaolin deposits in Egypt. The current study clarifies the distribution of Ti and its modes of occurrence in these deposits through petrographic, mineralogical, and geochemical investigations. It is found that Ti is present as an independent mineral phase rather than in the crystal structure of kaolinite. Ti occurs mainly as uniform very fine-grained and rounded anatase crystals in pockets within the kaolin mass of all deposits. Traces of rutile, leucoxene and ilmenite are present in the sand fractions. Average anatase content ranges between 1.4% and 3%. Cretaceous kaolin deposits show a homogeneous distribution of anatase regardless of the location (1.8 and 2.8%), while Carboniferous deposits are heterogeneous with the highest anatase content in the Abu Natash deposit (3%) and the lowest (1.4%) in the Khaboba deposit. The positive correlations between TiO2 and some trace elements such Cr, Nb and Zr in the clay fractions, where anatase is the only Ti-bearing mineral, suggest the occurrence of most of such elements in the anatase structure. currence of anatase as uniform very fine-grained crystals as pockets within the kaolin mass, together with the presence of ilmenite and/or leucoxene in the sand fractions of the studied kaolin samples suggests an authigenic origin for the anatase as a result of post-depositional alteration of ilmenite precursor to leucoxene. The positive correlations between TiO2 and trace element contents probably support this assumption. The higher trace element contents in the anatase from different deposits compared to the associated leucoxene and ilmenite are considered to result from relative enrichment of the immobile trace elements during the alteration of ilmenite and leucoxene to anatase.