Volatility inversion of silicon and magnesium oxides during the evaporation of HASP glasses on the moon

An inversion of SiO2 and MgO volatility occurs during high-temperature melt evaporation in the CaO–MgO–Al2O3–SiO2 (CMAS) system. This results in that SiO2, which is usually more volatile than MgO, becomes less volatile during the evaporation of melts enriched in the refractory oxides CaO and Al2O3. The volatility inversion is adequately explained within the theory of acid–base interaction of silicate melt components developed by D.S. Korzhinskii. The compositions of high-Al2O3 and SiO2-poor glasses (known as HASP glasses) from the lunar regolith show a systematic decrease in MgO/SiO2 with increasing CaO content, which is a direct consequence of the influence of acid–base effects.