Frequency distributions (normalized number of intensity ratios observed) of Al/Si, Mg/Si, and Mg/Al after mapping. Left: distributions without correction of Apollo 16 data. Right: distributions with Apollo 16 data corrected for softening of Solar spectrum. The corrections result in additional peaks appearing in the Mg/Al and Mg/Si distributions, which is due to the relatively low values measured for Mg/Si and Mg/Al of the highland area between ~ 100–150 degrees longitude by Apollo 16. In both figures, the intensity ratio bin width is approximately 0.02.