Synthesis of nanometer-sized fayalite and magnesium-iron(II) mixture olivines

Olivines are divalent orthosilicates with important geologic, biological, and industrial significance and are typically comprised of mixtures of Mg²⁺ and Fe²⁺ ranging from forsterite (Mg₂SiO₄) to fayalite (Fe₂SiO₄). Investigating the role of Fe(II) in olivine reactivity requires the ability to synthesize olivines that are nanometer-sized, have different percentages of Mg²⁺ and Fe²⁺, and have good bulk and surface purity. This article demonstrates a new method for synthesizing nanosized fayalite and Mg-Fe mixture olivines.First, carbonaceous precursors are generated from sucrose, PVA, colloidal silica, Mg²⁺, and Fe³⁺. Second, these precursors are calcined in air to burn carbon and create mixtures of Fe(III)-oxides, forsterite, and SiO₂. Finally, calcination in reducing CO-CO₂ gas buffer leads to Fe(II)-rich olivines. XRD, Mössbauer, and IR analyses verify good bulk purity and composition. XPS indicates that surface iron is in its reduced Fe(II) form, and surface Si is consistent with olivine. SEM shows particle sizes predominately between 50 and 450 nm, and BET surface areas are 2.8-4.2 m²/g. STEM HAADF analysis demonstrates even distributions of Mg and Fe among the available M1 and M2 sites of the olivine crystals. These nanosized Fe(II)-rich olivines are suitable for laboratory studies with in situ probes that require mineral samples with high reactivity at short timescales.