Isotopic composition of lead and strontium in lavas and coarse-grained blocks from Ascension Island,South Atlantic—an addendum

Natural marokite (CaMn₂O₄) has been studied at high pressures and temperatures using a diamond-anvil press coupled with laser heating in the pressure range 100–250 kbar. A mixture of marokite, CaMnO₃ (perovskite) and MnO (rocksalt) has been observed in all runs in the above pressure range by X-ray diffraction study of the quenched samples. It was interpreted that marokite disproportionates into the mixture CaMnO₃ (perovskite) + MnO (rocksalt) at pressures below 100 kbar. A general comparison of the molar volume for all known compounds having the marokite-related structures (including CaFe₂O₄ and CaTi₂O₄) with those for a mixture of perovskite plus rocksalt structures suggested that the mixture is more stable than the marokite-related structures at high pressures, as confirmed by the present experimental result. The CaFe₂O₄-modification of common nepheline (NaAlSiO₄) is also suggested to be unstable relative to the component oxides of α-NaAlO₂ + SiO₂ (stishovite) at high pressures.