We present a new oxybarometer for magnesiowustite-bearing systems, which is easily applied using widely available techniques. A scale relating the proportion of Fe (super 3+) [alpha = Fe (super 3+) /(Fe (super 3+) +Fe (super 2+) )] to the position of the (220) reflection and total Fe (y) in magnesiowustites (Mg (sub 1-y) .Fe y ) (sub 1-x) O has been derived from measured values in samples equilibrated at various oxygen fugacities: alpha = 13.0047 - 39.4829f + 40.0540f 2 -13.5701f 3 (+ or -0.007+0.09alpha ), f = (d 220 -1.4890)/(0.0510y + 0.0206y 2 )(+ or -0.0001). Previously established partition coefficients can then be used to relate estimated Fe (super 3+) content to equilibrium oxygen fugacity (f O2 ) in the composition range 0<y<0.2. Equilibrium oxygen fugacities of log f O2 < or =-1.7 can be estimated to + or -0.5 log units using just X-ray powder diffraction and electron microprobe analytical techniques.

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