Study of the Mineralogical and Chemical Compositions of the Weakly Magnetic Fractions of the Egyptian Black Sand Altered Ilmenite

Abstract

One of the most extensively studied topics in dozens of studies is the alteration process of ilmenite, the formation of leucoxene, and the presence of some impurity oxides: SiO2 and Al2O3. The altered Egyptian black sand ilmenite grains of relatively lower magnetic characters are studied using the binocular microscope and the Cameca SX-100 microprobe instrument. Both individual brown- and black-altered grains separated at 0.5 and 1 ampere values are investigated. The detection of the various alteration phases, their molecular formulas and limits, and the role of SiO2 and Al2O3 in alteration mechanisms are detected. The alteration phases include pseudorutile (psr) and leached pseudorutile (lpsr) of different phases in addition to rutile. Few analyzed spots are detected to be leached ilmenite (lilm). Several Excel software are adopted to calculate the chemical formulas of each alteration phase. The contents of TiO2 and Fe2O3 of all the investigated psr/lpsr in the study are in the range of 59.16–86.56% and 37.3–6.68%, respectively. The Ti/(Ti + Fe) ratio for these formulas ranges between 0.60 and 0.88. The psr/lpsr chemical formulas of all the investigated grains range as follows: Fe2.01-0.50Ti3O8.97-4.50(OH)0.03-4.50. The concluded lowest cationic iron content of the well-defined accepted lpsr phase is 0.5 with a corresponding molecular formula of Fe0.50Ti3O4.5(OH)4.5. The results revealed that in the region of 68–70 TiO2%, the mechanism of ilmenite alteration may be changed where neither all the analyzed TiO2 of the spot nor all the calculated structural water are contained within the molecular formula of lpsr. There are other associated mineral phases containing some TiO2 and also some structural water which most probably are removed from the lpsr phase. As the analyzed spots are located at highly fissured locations, the alteration process is relatively faster and the lpsr phase can be broken into rutile and hematite. Additionally, as the analyzed TiO2 and structural and/or molecular water contents increase, the darkness of the BSE image areas of the grain increases; this may reflect the existence of an individual TiO2 phase, most probably rutile, mixed in homogeneity with the existed lpsr component. As the content of TiO2 increases, within a definite TiO2 range (80–85%), the associated contents of Al2O3 and SiO2 increase. When the contents of the structural and/or molecular water contained within the lpsr phases decreases, the total oxides sum is more than 98%, the contents of Al2O3 are highly depleted. In the late alteration stages, the lpsr structure does not suddenly collapse but gradually produces other associated mineral phases. The relatively enriched contents of SiO2 and/or Al2O3 in some secondary rutile grains can be explained as most of the SiO2% is associated with mol water or bearing for mol and/or str water necessary for the leachability of Fe3+ from the psr structure. The XRD patterns of the investigated grains before and after roasting at 1100 °C for one hour are detected and interpreted.