Properties of the (Na,K)-Hollandite I-II transition

The continental crust is generally accepted as being silica rich and relatively light, such that it mostly “floats” at the surface over geological time. However, recent geochemical models require that some continental material has been recycled to the core-mantle boundary to explain the presence of enriched mantle signatures in plume derived ocean island basalts. Whether such crustal recycling is feasible depends on its density, with existing data implying that crust becomes negatively buoyant (i.e. it will sink) once beyond 300 km depth, partly due to the presence of (Na,K)-Hollandite in the phase assemblage. Once formed at ~ 10 GPa, (Na,K)-Hollandite I undergoes a composition and temperature dependent transition to (Na,K)-Hollandite II starting at 15-19 GPa. Here we are proposing to study the crystallography of this transition at high PT conditions using ID06-LVP.