The crystal structure of a high-pressure Fe 3 O 4 phase was determined by in situ X-ray diffraction measurements at high pressure and temperature, using an imaging plate detector and monochromatic synchrotron X-radiation. The high-pressure phase has the Pbcm space group (CaMn 2 O 4 -type structure) with cell parameters a = 2.7992(3) Aa, b = 9.4097(15) Aa, and c = 9.4832(9) Aa at 23.96 GPa and 823 K. Fe (super 3+) occupies an octahedral site and Fe (super 2+) is in an eightfold-coordinated site described as a bicapped trigonal prism. The high-pressure CaMn 2 O 4 -type Fe 3 O 4 phase is about 6.5% more dense than the spinel form at 24 GPa.

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