Excel spreadsheet. Tool for calculating the ferric/ferrous iron ratio in a melt for any given fO2 and vice versa. Also calculates new wt% and mole fraction values for FeO and Fe2O3 in your melt. Useful for determining theoretical iron speciation when total is given as FeOtotal or Fe2O3total and fO2 is known. Can also calculate or recalculate FeO and Fe2O3 wt% from given Fe3+/FeT and FeOT (or existing FeO and Fe2O3 wt%).References: Kress & Carmichael (1991); B. R. Frost in Mineralogical Society of America “Reviews in Mineralogy” Volume 25. Version 3.3 Updated all molecular weights to match those from the NIST Chemistry WebBook, SRD 69