Lattice energies, phase transformations and volatiles in the mantle

Abstract

While an exact calculation of the conditions of a phase transition is yet impossible in most chemically complex materials, it may be possible to show that a given transition is at least likely at some position in the mantle. The necessary conditions for a transition are that the volume change be negative and that the free energy barrier does not exceed 36 kcal for each cm 3 of negative volume change. It is this rather large energy change ( ΔP. ΔV) involved over the range of mantle pressures that allows any reasonable prediction. With simple substances with relatively small lattice energies predictions can be made with some degree of certainty; e.g. AB, AB 2 compounds such as MgO, SiO 2 etc. It is shown that none of the common types of hydrates minerals, amphiboles and micas, can carry water into the deep mantle. If water is trapped, it is more likely to be found in phases such as hydroxy silicates where (OH) n species replace SiO groups and are partially stabilized by dilution in solid solutions.