Post-spinel Transition in AB2O4

Abstract

AbstractSpinel-structured minerals with AB2O4 stoichiometry occur in various rocks in the crust and the upper mantle. In this chapter, high-pressure phase transitions of various AB2O4 compounds are discussed. The high-pressure phases of spinel-type AB2O4, called “post-spinel phases”, have generally CaFe2O4-, CaTi2O4- and CaMn2O4-type structures. All the structures consist of double chains of edge-sharing BO6 octahedra, and the four double chains form tunnel-like spaces in which relatively large A cations are accommodated. Stability of CaFe2O4- and CaTi2O4-type AB2O4 compounds are discussed in terms of cation radii. Hollandite-type phases and hexagonal aluminous (NAL) phases have structures related with the post-spinel structures. Aluminosilicates with the hollandite-type and NAL phases can accommodate relatively large cations such as Na+ and K+, and are known as major phases in basaltic crust and continental crust materials subducted into the lower mantle. The post-spinel type AB2O4 compounds are also interesting in materials science, because the pseudo-one-dimensional structure of double chains of octahedra containing magnetic cations may induce interesting magnetic and electrical properties and the post-spinel type compounds may have possibly high-ionic conductivity though the tunnel-like spaces.KeywordsPost-spinelSpinelCalcium ferriteCalcium titanateNAL phaseHollandite