Nucleation of hematite : A non-classical mechanism

Abstract

Hematite (α-Fe2O3) is thermodynamically stable at ambient conditions, of vast geological importance, and widely used in applications. It forms at elevated temperatures, while room temperature reactions typically yield metastable akaganéite or ferrihydrite. The mechanistic key changes underlying this observation were explored in the present study. The entropic contribution to the pre-nucleation hydrolysis reaction categorically implies the presence of pre-nucleation clusters (PNCs) as fundamental precursors. The formation of hematite is then due to a change in the reaction mechanism above ~50 °C, where the reaction limitation towards oxolation within phase separated clusters is overcome. We propose a model that rationalizes the occurrence of hematite, akagenéite and ferrihydrite based upon the chemistry of olation PNCs. Supersaturation, or the temperature dependencies of olation and oxolation rates from monomeric precursors are irrelevant in this non-classical mechanism.