Photostimulated oxidation of magnetite: 1. Kinetics and alteration phase identification

It has been demonstrated that magnetite (Fe3O4) becomes oxidized upon exposure to ultraviolet and visible radiation in O2-bearing atmospheres. X ray diffraction and diffuse reflectance studies of the oxidation product indicate that hematite (α-Fe2O3) is the principal reaction phase, although some maghemite (γ-Fe2O3) may also be produced. A set of kinetic constraints on the photostimulated oxidation mechanism has been determined by measuring the response of the reaction rate to variations in atmospheric composition and pressure, incident spectral radiation intensity distribution, temperature, and the amounts and types of adsorbed species present. The reaction rate is proportional to (pO2)1/2, for O2 partial pressures between 1.0 × 10−4 torr and 1.0 × 102 torr; to (PT)−2 for total atmospheric pressures between 6 and 800 torr; to (Φ0.350)3/2 for the incident photon flux at λ = 0.350 μ ranging between 6.6 × 1014 and 2.8 × 1019 photons cm−2 sec−1 A−1; and the rate responds in a complex way to variations in the quantity of H2O adsorbed to the grain surfaces. The reaction rate is apparently not sensitive to variations in the partial pressures of CO2, CO, N2, Ar, or H2O, to the temperature, or to the amounts of Ar, N2, CO2, or CO adsorbed to the grain surfaces. An attempt to determine the effect of particle size on the reaction rate was unsuccessful. The results indicate that recent experimental evidence for ‘lunar bleaching’ may be spurious. Photostimulated oxidation may occur naturally on Mars, however.