An internally-consistent set of data for the oxygen-fugacity buffers magnetite-hematite (MH), manganosite-hausmannite (MnO–Mn3O4) nickel-nickel oxide (NNO), fayalite-mag-netite-quartz (FMQ), and cobalt-cobalt oxide (Co–CoO) at 1 atm has been obtained by using the hydrogen-fugacity sensor technique of Chou and Eugster (1976a) at 2 and 4 kbar total pressures between 600° and 800°C.

Using thefO2 values given by Huebner and Sato (1970) for NNO buffer as a reference, the data can be represented by the following equations (at 1 atm) with T in K:

For MH buffer:
log fO2(±0.12)=0.0260+0.4381(104/T)−0.1572(104/T)2 for 1073≥T≥873,and log fO2(±0.12)=(−26629/T)+15.288 for extrapolation to T≥1073;
for MnO–Mn3O4 buffer:
log fO2(±0.12)=(−25793/T)+13.714;
for FMQ buffer:
log fO2(±0.08)=(−23973/T)+7.985 for T≤873;
and for Co-CoO buffer:
log fO2(±0.08)=(−24391/T)+7.382.

The present fO2 values for MnO–Mn3O4 and Co–CoO buffers are in good agreement with those of Huebner and Sato (1970) and Robie and Waldbaum (1968) respectively. However, for MH and FMQ buffers, the present fO2 values are consistently lower and higher respectively than those reported previously. The difference can be minimized if the values of Gibbs free energy of formation for magnetite are 0.68, 0.35, and 0.17 kcal/mole less negative than those reported by Haas and Robie (1973) at 900, 1100, and 1300 K respectively.

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