Why olivine transforms to spinel at high pressure

Abstract

Because of its importance in understanding the static and dynamic properties of the Earth's mantle, the olivine to spinel structure transformation is one of the most frequently studied crystal structure changes. The volume change is considerable (∼7–10%, depending on the compound involved) but the reason for this is unclear. In most high-pressure transformations, the volume contraction is accompanied by an increase in the primary coordination number of the atoms. However, there is no such increase in the case of the transformation from the olivine to the spinel structure. Both are A₂BX₄ compounds (for example, Mg₂SiO₄ olivine and Al₂MgO₄ spinel) with cation coordinations AX₆ (octahedral) and BX₄ (tetrahedral) and anion coordinations XA₃B (tetrahedral). Indeed the conventional description of both structures starts with approximately ‘close-packed’ (‘eutactic’¹) anion arrays (hexagonal for olivine and cubic for spinel) with cations in one half of the octahedral interstices and one eighth of the tetrahedral interstices, suggesting that the volumes of each structure should be very similar for a given compound. Here we show that a less conventional, but more appropriate description of the structures resolves the volume problem and also sheds light on the crystal chemistry of these and related structures.