Magnetic changes accompanying the thermal decomposition of nontronite (in air) and its relevance to Martian mineralogy

Abstract

Thermal treatment of nontronite in air, for long periods at 700°C or short periods at 900°C, results in destruction of the nontronite structure, a distinct reddening in color, and a spectacular increase in magnetic susceptibility and saturation magnetization (up to 4.4 Am2 /Kg). Magnetic property measurements and calculations suggest that the magnetism is due to the presence of ultrafine particles of α or γFe2O3; the precise identity has not yet been resolved. The highly magnetic thermally treated nontronite is amorphous to X rays consistent with an ultrafine grain size. Prolonged heating results in the growth of αFe2O3, as suggested by X ray, IR, and magnetic measurements. Reflectivity spectra of a sample heated for 1 hour at 900°C indicates, in addition to αFe2O3, the presence of an opaque, magnetite‐like phase. Given the composition of the starting material, and the experimental conditions, the presence of Fe3O4, is unlikely; the cause of this opacity is as yet unknown. Thermally treated nontronite has chemical, color and magnetic properties akin to those found by Viking on Mars. These results favor an origin for the fine grained Martian surface material by repeated impacts into an Fe‐rich smectite‐charged regolith (Weldon et al., 1980).