THE EQUATION F STATE OF Mg 0 . 6 Fe 0 . 40 TO 200 GPa

Abstrac,t. New Hugoniot data on polycrystalline (avg.porosity 6.9%) samples of the magnesiowustite Mgo.6Fe0.40 are presented, covering the pressure range up to 200 GPa. When our data are fit by a single 3rd order Eulerian Hugoniot with K 0 constrained to its ultrasonic value of •61.5 GPa, the required isentropic pressure derivative K 0' is 4.37 +/0.37. This value is significantly lower than the ultrasonic one of 6.18; existing isothermal compression data, however, are in agreement with our value rather than the ultrasonic one. Our data are adequately explained without phase transitions. There is some marginal evidence for a possible phase transition around 120 GPa. If such a transition indeed occurs it i• probably of Small volum• change compared to the transition observed in FeO; we place ans..extreme upper bound of 3% on the density change such a transformation could involve and still be consistent