Spectroscopic characterization of iron ores formed in different geological environments using FTIR, XPS, Mössbauer spectroscopy and thermoanalyses

Abstract

Application of thermoanalyses, FTIR, XPS and Mössbauer spectroscopic methods can differentiate between iron ores formed in different geological environments. Two types of iron ore are formed in shallow marine environments in the Bahariya Depression, Egypt, yellowish brown ooidal ironstones (type 1) and black mud and fossiliferous ironstones (type 2). Both types were subjected to subaerial weathering, producing a dark brown lateritic (pedogenic) iron ore (type 3). Microscopic investigation indicates goethite is the main mineral in types 1 and 3, while hematite is the main mineral in type 2 and also occurs in type 3. Thermoanalyses indicated the dehydroxylation endothermic peak of goethite of type 1 occurs between 329 and 345°C, while in type 3 occurs between 284 and 330°C. This variation can be attributed to the nanocrystalline nature of the pedogenic goethite. The presence of an exothermic peak at 754°C in type 3 is probably attributed to goethite-hematite phase transformation. FTIR spectroscopy indicated that goethite of type 1 is characterized by the presence of the δ-OH band between 799 and 802cm−1, the γ-OH between 898 and 904cm−1 and the bulk hydroxyl stretch between 3124 and 3133cm−1. Goethite of type 3 is characterized by the absence of the bulk hydroxyl stretch band and the δ-OH and γ-OH are shifted to higher Wavenumbers that can attributed to a relative Al-for Fe-substitution. Hematite is identified by two IR bands; the first is between 464 and 475cm−1 and at the second is between 540 and 557cm−1. Quartz is identified in all iron ore types, nitrates are identified in types 1 and 2, but absent in type 3 and Kaolinite is identified in type 2. The Mössbauer spectrum of type 1 is fitted with one magnetic sextet corresponding to goethite with an isomer shift (IS)=0.374mms−1, a quadruple splitting (QS)=−0.27mms−1 and a hyperfine magnetic field (BHF)=∼37. The Mössbauer spectrum of type 2 is fitted with one magnetic sextet corresponding to hematite with IS=0.363mms−1, QS=−0.23mms−1 and BHF=∼50. The Mössbauer spectrum of type 3 is best fitted with a single doublet corresponding to ferrihydrite and one sextet corresponding to hematite. The XPS survey scans and the high resolution of the Fe 2p3/2 can differentiate between the yellowish-brown and green ooidal laminae of type 1. The XPS survey scans indicate the presence of Fe, O, C, N, Na, Cl, Ca and Si in all laminae, while S, Zn, Ti and P are only restricted to the green laminae. The high resolution of the Fe 2p3/2 indicates that Fe is linked to OH− ligand in the yellowish-brown laminae that correspond to goethite, while Fe is linked to SO42− ligand in the green laminae. The XPS survey scans of types 2 and 3 indicate that Fe is linked to O2− ligand that corresponds to hematite.