The influence of silicate on transformation pathways of synthetic 2-line ferrihydrite

Abstract

In this study, a series of synthetic ferrihydrite samples of Si/Fe molar ratios ranging from zero to 1.5 were heated up to 1000 °C using simultaneous TG–DTA equipment. The XRD, FTIR, SEM–EDS and magnetic susceptibility measurements were carried out prior to and after heating. It has been found that silicate retards ferrihydrite transformation to hematite and affects crystallinity of the product. Low Si admixture in the precursor reduces hematite crystal size severely, but the increase in average crystal dimensions with increasing Si/Fe molar ratio was observed. High Si content results in the formation of hematite which exhibits a wide range of crystal habits. The conversion of pure ferrihydrite to hematite proceeds without any intermediate phase, whereas the increasing silicate content in the pristine oxyhydroxide strongly affects the transformation pathway. During annealing of high-Si ferrihydrites, the presence of two or three intermediate Fe2O3 polymorphs (gamma, epsilon and beta) was demonstrated prior to the crystallization of final α-Fe2O3. The conditions favoring crystallization of intermediate phases result from progressive silica polymerization which forms separate matrix-type phase and impedes the aggregation of iron oxide nanoparticles.