The effect of titanium and phosphorus on ferric/ferrous ratio in silicate melts: an experimental study.

The effect of TiO 2 and P 2O 5 on the ferric/ferrous ratio in silicate melts was investigated in model silicate melts at air conditions in the temperature range 1,400–1,550 °C at 1-atm total pressure. The base composition of the anorthite–diopside eutectic composition was modified with 10 wt % Fe 2O 3 and variable amounts of TiO 2 (up to 30 wt %) or P 2O 5 (up to 20 wt %). Some compositions also contained higher SiO 2 concentrations to compare the role of SiO 2, TiO 2, and P 2O 5 on the Fe 3+/Fe 2+ ratio. The ferric/ferrous ratio in experimental glasses was analyzed using a wet chemical technique with colorimetric detection of ferrous iron. It is shown that at constant temperature, an increase in SiO 2, TiO 2, and P 2O 5 content results in a decrease in the ferric/ferrous ratio. The effects of TiO 2 and SiO 2 on the Fe 3+/Fe 2+ ratio was found to be almost identical. In contrast, adding P 2O 5 was found to decrease ferric/ferrous ratio much more effectively than adding silica. The results were compared with the predictions from the published empirical equations forecasting Fe 3+/Fe 2+ ratio. It was demonstrated that the effects of TiO 2 are minor but that the effects of P 2O 5 should be included in models to better describe ferric/ferrous ratio in phosphorus-bearing silicate melts. Based on our observations, the determination of the prevailing fO 2 in magmas from the Fe 3+/Fe 2+ ratio in natural glasses using empirical equations published so far is discussed critically. [ABSTRACT FROM AUTHOR]