Particle-Size Effect in Oxidation of Natural Magnetite

Abstract

Significant oxidation to γFe2O3 of natural magnetite (Fe3O4) crystals ground in water to <1 μ was produced by heating in air; the proportion converted at 250°C exceeded 75% as measured by specific magnetization, chemical, and x‐ray diffraction analysis. This resembles the behavior of synthetic magnetite. The same natural material ground to ≥25 μ, similarly heated and characterized, converted to αFe2O3 only, with less than 45% conversion at temperatures below 400°C. This striking effect of small particle size is attributed to the stabilizing effect of water adsorbed or hydrated on the fine‐particle surface. Differential thermal analysis measurements in various atmospheres and thermogravimetric analysis tend to support this view. In a corroboratory experiment, natural magnetite was ground in dry acetone to <1 μ. Oxidation in dry oxygen produced only αFe2O3, but in an atmosphere of water vapor and oxygen oxidation proceeded exclusively toward γFe2O3. The importance of particle size and water on the oxidation of both natural and synthetic magnetite helps resolve reported differences between the two, since typical samples of natural magnetite differ from synthetic material in being of larger particle size and unhydrated.