High-Pressure Stability Of Carbonates - Quenching Of Calcite-Ii, High-Pressure Polymorph Of Caco3

We present the results of a series of experiments on calcite I at around 45 GPa and temperatures between 300 and 2000 K, in a YAG-laser-heated diamond-anvil cell. Quenched samples have been examined by transmission electron microscopy. In the unheated part of the samples, calcite has been partially transformed into aragonite and preserved during quenching. This partial transformation must be related to the high deviatoric stresses present in the diamond anvil cell. In the heated parts of the samples aragonite, calcite-I and also calcite-II are observed. It is the first time that calcite-II has been quenched and characterized at ambient conditions. The sequence of formation, preservation or back-transformation during pressure and temperature quenching of the high-pressure polymorphs of CaCO3 in this kind of experiment is yet not completely elucidated, but several explanations are proposed to account for these observations.