Mechanisms behind the ikaite-to-calcite phase transformation from molecular dynamics calculations

Abstract

Since the discovery of ikaite (CaCO3·6H2O) in Antarctic marine sediments and east Arctic polar oceans, studying the ikaite-to-calcite phase transformation above 100K has attracted a lot of interest, both experimentally and theoretically. Despite intensive efforts, details about the thermal behavior and the mechanisms of the phase transformation of ikaite at ambient temperature are still ambiguous. To unlock the secrets of this rare and unstable mineral, we provide here a clear picture about the structural phase changes as well as the thermodynamic behavior of ikaite from 2K to 400K. The obtained results demonstrate the presence of an anisotropic thermal expansion character more pronounced above T=80K. Our X-ray diffraction patterns show that the structure of ikaite breaks down gradually during thermal expansion from 2K to 200K. At 200K, it transforms into a vaterite-like structure, at short range, then further heating up to 300K, leads the structure to be in the vaterite phase. However, at 338K, the latest phase changes quickly to the calcite structure. Our data show that the thermal expansion coefficients of ikaite obtained from 2K to 80K increase with temperature, and are greater than those of ice (Ih). However, above T=100K, the situation is inverse and the relative expansion of ikaite is found to be smaller than that of ice. A larger volume thermal expansion is noticed from T=300K to 400K, due to the contribution of extensive H bonds. These bonds allow greater flexibility of ikaite during expansion.