The temperature – induced amorphous CaCO3 may help carbon cycling – Dr. Mingqiang Hou

APRIL 30, 2019

New work from a team led by HPSTAR's Dr. Mingqiang Hou confirmed that aragonite — a high-pressure mineral form of CaCO₃, will turn to a stable amorphous phase at the subduction zone conditions.  Their findings  published in a paper in the journal Nature Communications,  proposed that the amorphous calcium carbonate may help cycle carbon to the surface.

Calcium carbonate (CaCO₃) plays an import role in carbon cycling, so any new discovering on it is potentially important. For its phases, there have been extensive studies. However, for the conditions of above 3 GPa and 1000K, previous experiments show controversial results about its crystal structures.

To clarify this mysterious, Hou and his colleagues set out to probe these condition transitions in CaCO₃ using a pressure vessel called large-volume press combined with synchrotron x-ray diffraction. They found that the aragonite CaCO₃ becomes amorphous upon heating above 3 GPa and 1000K and the amorphous state turns back to aragonite with temperature decreasing. These indicate that the transition is reversible.

"We conducted seven runs of experiments and the repeated results were consistent with the features of an amorphous phase instead of the previously reported disordered or crystallized phases”, explained Dr. Hou. “The temperature-induced amorphization in CaCO₃ might be caused by the collapse of the aragonite crystal structure during heating”.

They also estimated the density of the amorphous phase from the diffraction data and found that the amorphous CaCO₃ might be one of the lightest components in subduction zone. So it would likely escape from the earth interior into the surface via volcano eruption, serving as an important carbon source in the atmosphere.

Caption: Phases of CaCO₃ at upper mantle conditions and the schematic for deep carbon recycle.