Lawrence Berkeley National Laboratory

Calcium carbonate (CaCO₃) significantly affects the properties of upper mantle and plays a key role in deep carbon recycling. However, its phase relations above 3 GPa and 1000 K are controversial. Here we report a reversible temperature-induced aragonite-amorphization transition in CaCO₃ at 3.9-7.5 GPa and temperature above 1000 K. Amorphous CaCO₃ shares a similar structure as liquid CaCO₃ but with much larger C-O and Ca-Ca bond lengths, indicating a lower density and a mechanism of lattice collapse for the temperature-induced amorphous phase. The less dense amorphous phase compared with the liquid provides an explanation for the observed CaCO₃ melting curve overturn at about 6 GPa. Amorphous CaCO₃ is stable at subduction zone conditions and could aid the recycling of carbon to the surface.