A crystalline-to-crystalline phase transition in Ca(OH)2 at 8 GPa and room temperature

X-ray diffraction and Raman spectroscopy measurements reveal that Ca(OH)₂-portlandite transforms to a crystalline phase with an ordered OH sublattice between 8 and 14 GPa at room temperature under quasi-hydrostatic stress conditions in an Ar pressure medium. The amorphization previously reported at 11 GPa under no pressure medium was not observed up to 26 GPa. The width and separation of the fluorescence peaks from the ruby chips embedded in Ca(OH)₂ without a medium suggest that deviatoric stress is responsible for the previously observed amorphization. Our study demonstrates that the behavior of hydrous phases in the subducting slab could be sensitive to local deviatoric stresses. Together with similar reports on quartz, our observation on Ca(OH)₂ indicates that deviatoric stress is an important factor to consider for P-induced amorphization reported in a wide range of materials.