Incorporation of W6+ into hematite (α-Fe2O3) in the form of ferberite nanolamellae

Abstract

The industrial demand for tungsten (W) is steadily rising, leading to increase in mining and subsequent release into the environment. In this work, we investigated the uptake of W into the crystal structure of hematite (alpha-Fe2O3) as a potential immobilization mechanism. Natural zoned crystals from the low-sulfidation epithermal deposit Bansk & aacute; Hodru & scaron;a, with up to 5.0 wt % WO3, were used. The 57Fe M & ouml;ssbauer spectroscopy at room temperature showed that the hematite studied consists of two magnetic structures, one of them below and one of them above the Morin transition. The two magnetic structures are caused by the presence of W-rich and W-poor regions in hematite. The W-rich regions must be intimately intergrown with the hematite host to influence the magnetic ordering in substantial volumes of the hematite structure. X-ray absorption spectra in the W-rich regions of hematite were described very well by a structural model of ferberite (FeWO4). Transmission electron microscopy (TEM) identified nanolamellae of ferberite and magnetite that were epitaxially intergrown with the hematite host. Electron energy loss spectroscopy (EELS) confirmed that iron occurs mostly as Fe2+ in the W-rich lamellae. M & ouml;ssbauer spectroscopy, X-ray absorption spectroscopy (XAS), and TEM converge, at different spatial scales, to a model where W6+ is not taken up by hematite itself but by ferberite nanodomains intergrown with hematite.