Authigenic halloysite from El-Gideda iron ore, Bahria Oasis, Egypt: characterization and origin

Halloysite in El-Gideda iron mine occurs as very soft, light and white-to-pinkish white pockets and lenses ranging in diameter from 50 cm to I m within the iron ore. Highly hydrated halloysite is the main constituent of these pockets beside some kaolinite and alunite. The diffraction pattern of the clay fraction (<2 &mu;m) shows a rather broad and diffuse 001 reflection spread between 10.3 and 13.6&DEG;2&theta;. Upon treatment, the 001 reflection of halloysite expands up to 10.94 &ANGS; and 11.9 &ANGS; corresponding to ethylene glycol and dimethyl formamide treatment, respectively. After these treatments, kaolinite appeared with its characteristic basal spacing (&SIM;7 &ANGS;). The percentage of halloysite in halloysite-intercalated kaolinite ranged between 80 and 90%. Heating to 350&DEG;C, produces a kaolinite-like structure (&SIM;7-1 &ANGS;) that developed to a metakaolinitc-structure when heated to 550&DEG;C. Morphologically, halloysite appears as well developed tubes composed entirely Of SiO2 and Al2O3, while kaolinite is characterized by very fine platelets arranged in book-like or rosette-like shapes. A differential thermal analysis curve of the studied halloysite showed an endothermic peak at &SIM;138&DEG;C due to the dehydration of interlayer water of halloysite. The small shoulder at &SIM;540&DEG;C and the endothermic peak at &SIM;593&DEG;C is attributed to the dehydroxylation of halloysite, kaolinite and alunite. On the other hand the exothermic peak that appeared at 995&DEG;C is due to the formation of new phases such as mullite and/or spinel. The infrared vibrational spectrum is typical of highly disordered halloysite and kaolinite. Halloysite was formed as a result of alteration of the overlying glauconite suggesting intensive chemical alteration during a humid wet period that prevailed in the Bahria Oasis during the late Eocene. Glauconite alteration releases K, Fe, silica and alumina. Iron forms at least part of the iron ore in the El-Gideda mine while alumina forms halloysite as well as alunite when interacted with silica in an acidic environment.