High-pressure polymorphism of the iron oxides

Summary. High-pressure shock-wave data for hematite (Fe,CI,) and magnetite (Fe,O,) are remarkable for the very large density increases (- 10 per cent at 100 GPa) associated with shock-induced transformation from the relatively close-packed corundum and spinel structures. The recently reported data of Jeanloz & Ahrens for wustite (Fe0,940) also reveal a major shock-induced phase change which they interpret as B1 * B2 polymorphism. Reassessment of the wustite data, however. indicates that the zero-pressure density increase, associated with shock-induced polymorphism, is at least 10-16 per cent and very possibly as great as 16-28 per cent. The density increases for all three iron oxides thus appear to be too large to be explained in terms of geometric rearrangement (e.g. B1 + B2) of the usual Fez’, Fe 3 + and 02- ions, unless it is assumed that these ions possess substantially smaller effective radii in the high-pressure phases. In this connection, it is proposed that the more covalently bonded nickel arsenide (NiAs) and derivative structures might accommodate very dense iro