Hollandite II phase in KAlSi₃O₈ as a potential host mineral of potassium in the Earth's lower mantle

High-pressure and high-temperature experiments on the KAlSi₃O₈ composition were conducted in a laser-heated diamond-anvil cell at pressures up to 128 GPa, which correspond to the lowermost mantle conditions. In situ synchrotron X-ray diffraction measurements revealed that the hollandite II phase in KAlSi₃O₈ with a monoclinic symmetry of I2/m was stable over the entire range of mantle conditions, and the tunnel structure formed by the double chains of edge-sharing (Si,Al)O₆ octahedra, which could accommodate a larger cation such as potassium, was sustained. The (Si,Al)O₆ octahedra in the KAlSi₃O₈ hollandite II phase showed a similar compression behavior to those in high-pressure silicate structures, such as rutile-type and perovskite-type phases, and were found to be less compressible than the KO₈ polyhedra. The KAlSi₃O₈ hollandite II phase is a potential host mineral for potassium under lower mantle conditions and, therefore, may have a significant influence on geochemistry if potassium feldspar KAlSi₃O₈ in the Earth's crust is transported into the Earth's mantle through subduction.