Calculate fO2 Buffer

Excel spreadsheet. Tool to calculate oxygen fugacity in terms of the common buffers when logfO2 is known or for translating between fO2 values expressed in terms of various buffers. This is useful for converting calculated oxygen fugacity values into and between delta buffer values (e.g. ΔNNO, ΔQFM…).

References:

• B. R. Frost in Mineralogical Society of America “Reviews in Mineralogy” Volume 25.
• Campbell, A. J. et al. High pressure effects on the iron–iron oxide and nickel–nickel oxide oxygen fugacity buffers. Earth Planet Sc Lett 286, 556–564 (2009).
• French, B. M. & Eugster, H. P. Experimental control of oxygen fugacities by graphite‐gas equilibriums. J Geophys Res 70, 1529–1539 (1965).
• Pownceby, M. I. & O’Neill, H. St. C. Thermodynamic data from redox reactions at high temperatures. IV. Calibration of the Re-ReO2 oxygen buffer from EMF and NiO+Ni-Pd redox sensor measurements. Contr. Mineral. And Petrol. 118, 130–137 (1994).
• Holzheid, A. & O’Neill, H. St. C. The Cr-Cr203 oxygen buffer and the free energy of formation of Cr203 from high-temperature electrochemical measurements. Geochim Cosmochim Ac 59, 475–479 (1995).
• Bygden, J., Du Sichen, and S. Seetharaman. "A thermodynamic study of the molybdenum-oxygen system." Metallurgical and Materials Transactions B 25.6 (1994): 885-891.

Version 1.6 23 Feb 2026 Fixed Graphite-CO buffer (previously converted pressure to GPa when it should have been in bars). Credit Michael Anenburg for the fix!