pressure atmospheric gradational perovskite Bi2O3 polymorphs gradient PbO2 monoclinic metastable Adiabatic baddeleyite PaQfs humidity solutes MgSiO3 GPa GQdgform austenite olivine crystalline facies crystal gaseous liquid isobaric membrane semiconductive MgO HQdgform solute displacive fluid metamorphism molar exsolution Nb blueschist