Knowledge Bank

Potassium and Rubidium are minor and trace elements of the Earth’s mantle, both of which have long-lived radioactive isotopes. 40K is a significant energy source that contributes to the convection of the mantle and outer core, while 87Rb is a geochemical tracer for long-lived mantle processes. In the Earth’s crust, K and Rb are stored in feldspar, KAlSi3O8, the most abundant mineral in the crust. The mineral host of K and Rb in the mantle is uncertain, with implications on reactivity of alkali metals with the Earth’s core. Sample preparation consisted of compressing RbAlSi3O8, Rubidium microcline (rubicline), under pressures of ~15 GPa (gigapascals) in a laser-heated diamond anvil cell (LHDAC). Synchrotron-based x-ray diffraction gives the structure and density at high pressures. The high-pressure structure of Rubidium microcline is similar to that of the high-pressure structure of feldspar, transforming to the hollandite structure at ~15 GPa, then to the hollandite II structure at ~25 GPa. Preliminary results yield the bulk modulus of Rubidium hollandite to be 210 (±10) GPa.