Properties of hematites formed by the thermal transformation of ferrihydrites containing Si

Abstract

The ubiquitous formation of iron (Fe) oxides in natural, near surface environments does not proceed in pure systems such as those often used in laboratory syntheses. In the natural world, "impurities" such as soluble silicate (probably the most frequent), organics and phosphate are invariably present. As silicate has a high affinity for the Fe-OH functional group of Fe oxides, it is likely that Si will interfere with the growth of Fe oxide crystals. The high Si content of up to 100 g SiO₂ kg⁻¹ of many natural ferrihydrites (Carlson and Schwertmann, 1981; Childs et al., 1994) may contribute to their formation and poor crystallinity and hinder the subsequent rearrangement of Fe octahedra to form hematite (Glasauer et al., 2000). Galvez and his co-workers (1999) have recently shown that phosphate may enter the hematite structure. The aim of this investigation is to study how Si interferes with hematite formation and thus obtain more information on the mechanism(s) of ferrihydrite to hematite transformations in the presence and absence of Si and in systems of varying water activities. This information is needed to better explain the properties of hematite in surface environments and to understand its conditions of formation. The parameter(s) needed for the formation of nano-sized natural hematite, such as that detected in the Atlantis II Deep sediment (Schwertmann et al., 1998) remain unknown.