Temperature-induced amorphization in CaCO3 at high pressure and implications for recycled CaCO3 in subduction zones

Calcium carbonate (CaCO 3 ) 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 CaCO 3 at 3.9–7.5 GPa and temperature above 1000 K. Amorphous CaCO 3 shares a similar structure as liquid CaCO 3 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 CaCO 3 melting curve overturn at about 6 GPa. Amorphous CaCO 3 is stable at subduction zone conditions and could aid the recycling of carbon to the surface. Subduction of oceanic crust introduces huge amounts of carbonates into Earth’s mantle, contributing to the global carbon cycle. Here, based on high-pressure-temperature experiments, the authors present a reversible temperature-induced transition from aragonite to amorphous CaCO 3 .