Electronic absorption spectra of lunar minerals

Abstract

Transition metals play an important rôle in the lunar rock-forming silicates, occurring particularly in pigeonite, augite, olivine and pyroxferroite. Measurements of the absorption spectra of lunar silicates can provide information on the oxidation states and co-ordinations of the transition metal ions. Such measurements lead-us to conclude that Ti3+ as well as Fe2+ and Ti4+ ions are present in lunar silicates. Fe3+ ions do not occur in concentrations greater than a few parts per million, and the spectral evidence for Cr2+ ions in olivines requires further substantiation - whereas occurrence of Cr3+ is very likely. The occurrence of low oxidation states in the transition elements of the lunar rocks suggests that low oxygen fugacities prevailed during the formation of the lunar crust. However, other reduction mechanisms postulated include release of solar-wind gas, and pressure-induced reduction on meteorite impact or during magma genesis at depth. Correlations of laboratory measurements with remote telescopic measurements confirm that major features in remote reflectance profiles arise from absorption by pyroxenes, enabling average pyroxene compositions to be determined for areas of the lunar surface. Intense absorption towards the ultraviolet-blue end of the spectrum corresponds to high titanium contents, as confirmed by molecular orbital calculations which support the use of remote spectral measurements to map titanium-rich areas of the Moon.