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

Flemetakis, S; Renggli, CJ; Pangritz, P; Berndt, J; Klemme, S

Abstract

We present the results of B2O3 evaporation experiments from Ca- and Mg-bearing aluminoborosil- icate melts. Our experiments were conducted at 1245 to 1249◦C and 1350 to 1361◦C for different run times (60–1020min), 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 f O2, evaporation of B from the melt in- creases 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 tocalculateevaporationrateconstants(ki)forB2O3 inoxidizingconditions(air,ki =2.09×10−4cmmin−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.