Normal and inverse ringwoodite at high pressures

We present a first-principles computational study of the structural propert ies of ringwoodite and the influence of cation exchange on these properties as well as the: enthalpy differences between ringwoodite and inverse ringw oodite. Our results agree with low-temperature experiments, in that cubic r ingwoodite is the stable structure up to 25 GPa. This pressure range encomp asses the lower part of the mantle transition zone where ringwoodite is tho ught to be the most abundant phase. The equation of state as derived from e xperiment and theory are in good agreement. In contrast to normal ringwoodi te. inverse ringwoodite compresses highly anisotropically and the compressi on mechanisms differ considerably for the two structures. The predicted ent halpy difference between normal and inverse ringwoodite is significantly sm aller than that obtained from a simple ionic model (O'Neill and Navrotsky 1 983), emphasizing the importance of including structural relaxation in mode ls that address the energetics of order-disorder reactions, at least in the case of ringwoodite.