Formation of iron-rich phyllosilicates in the FeO–SiO 2 –H 2 O system during hydrothermal synthesis as a function of pH

Abstract

The formation of iron-rich phyllosilicates can occur at different natural or engineered settings. In this study, the influence of pH in the hydrothermal synthesis of iron-rich phyllosilicates was investigated in the pH range 8.50–12.10 after the ageing of the precursor. The synthesized samples were characterized by powder X-ray diffraction, Raman and Mössbauer spectroscopies and transmission electron microscopy. Three domains of pH were identified, and these correlated with silica availability and its speciation in the solution. The formation of 1:1-type Fe III /Fe II phyllosilicate was observed between pH 9.67 and 10.75. Above pH 10.75, two types of phyllosilicate-like mineral phases were observed. In addition to 1:1-type Fe III /Fe II phyllosilicate, 2:1-type Fe III /Fe II phyllosilicate was observed. Below pH 9.67, mainly amorphous silica and iron oxides were observed. The findings show that pH governed the crystallinity and nature of the obtained phyllosilicate-like phases.