The effect of oxygen fugacity on the evaporation of boron from aluminoborosilicate melt

Abstract

Abstract. We present the results of B2O3 evaporation experiments from Ca- and Mg-bearing aluminoborosilicate melts. Our experiments were conducted at 1245 to 1249 ∘C and 1350 to 1361 ∘C for different run times (60–1020 min), and at oxygen fugacities (logfO2) relative to the fayalite–magnetite–quartz (FMQ) buffer of FMQ−6 to FMQ+1.5, and in air. Our results show that with increasing fO2, evaporation of B from the melt increases by a factor of 5 compared to reducing conditions. Using Gibbs free energy minimization calculations, we suggest two possible evaporation reactions for B2O3 which constrain its speciation in the gas phase to be either 3+ or 4+ (B2O3(g) and BO2(g)). The measured B2O3 contents of the B evaporated residual glasses were used to calculate evaporation rate constants (ki) for B2O3 in oxidizing conditions (air, ki=2.09×10-4 cm min−1 at 1350 ∘C) and reducing conditions (FMQ−4, ki=4.46×10-5 cm min−1 at 1350 ∘C). The absence of diffusion profiles in the experimental glasses suggests that the evaporation rates are slower than B2O3 diffusion rates and therefore the rate-limiting process. Overall, the rate of B evaporation in air is approximately a factor of 5 higher compared to reducing conditions at FMQ−4.