Polymorphism in the Earth's mantle

Abstract

Polymorphism has been proposed to explain seismic discontinuities without chemical change. Topological and thermodynamic classifications of polymorphism indicate a much wider range of possible types than previously described, with the further possibility of seismic discontinuities independent of density changes. Polymorphic transformations are topologically classified as: (1). open packing to cp (close‐packed), (2) rearrangement of interstitial atoms in cp structures, (3) coordination change, (4) electronic transformation, and (5) rearrangement of cp atoms. Calculation of repulsion energy for coordination change indicates an increase of cation coordination number with high pressure, but the effect of temperature cannot be generalized.Transformations of types (1), (3), and (4) are first order, with a volume change, latent beat, and change of elastic constants. Types (2) and (5) are second order in an ideal case, with an abrupt change of elastic constants at a critical point, but with volume and heat changes spread over a wide range.With increasing depth, a single composition might undergo a series of transformations with consequent effect on the seismic observations: a low‐pressure open structure collapses to cp; the cations move from tetrahedral to octahedral holes, then into high coordination positions where they are an essential part of the packing; and ions release their electrons to the conducting state (starting with the oxygen ion). While interstitial atoms are still present, they may rearrange in response to temperature or pressure changes. At some undetermined stage cp atoms may rearrange, as from cubic cp to hexagonal cp.