The cause and significance of luminescence in lunar plagioclase

Abstract

Most lunar samples luminesce under proton or electron excitation, and most of the emission comes from the plagioclase present. The cause of this luminescence has been found by investigating the emission and excitation spectra of lunar, terrestrial and synthesized plagioclases. Emission spectra show three broad peaks: a weak one around 450 nm which is common to most silicates; a dominant one around 560 nm for which the activator is found to be Mn 2+ in Ca sites; and a very weak one between 700 and 780 nm for which we conclude the activator to be Fe3+ in A1 sites. However, this near-infrared peak is usually the dominant one for terrestrial plagioclases; its weakness in lunar samples is attributed to reducing conditions when the lunar surface materials were formed causing more of the iron to be present as Fe2+. Luminescence photography of lunar rock chips is found to be a simple method of surveying plagioclase crystal forms in rough samples.