THE nature of the post-spinel phase of forsterite (the olivine structure of Mg2SiO4) and its importance to our understanding of the mineralogy, petrology and seismological discontinuities in the transition zone and the lower mantle of the Earth has attracted much recent attention1,2. Kumazawa et al.1 reported that the post-spinel phase of forsterite recovered from 3.3 × 1010 Pa and 1,000 °C was a component oxide mixture of stishovite (SiO2) plus periclase (MgO). In their studies of Mg2SiO4–Fe2SiO4 solid solutions at pressures up to 2.5 × 1010 Pa, Ming and Bassett2 agreed with the conclusions of Kumazawa et al. Consequently, the oxide mixture seems to be widely accepted as the post-spinel phase of forsterite3. In this paper, however, I present results that disagree with this.
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