Calcium-43 NMR Studies of Polymorphic Transition of Calcite to Aragonite
Journal
Journal of Physical Chemistry B
Journal Volume
116
Journal Issue
49
Pages
14295-14301
Date Issued
2012
Author(s)
Abstract
Phase transformation between calcite and aragonite is an important issue in biomineralization. To shed more light on the mechanism of this process at the molecular level, we employ solid-state 43Ca NMR to study the phase transformation from calcite to aragonite as regulated by magnesium ions, with 43Ca enrichment at a level of 6%. Using the gas diffusion approach, the phase of Mg-calcite is formed initially and the system subsequently transforms to aragonite as the reaction time proceeds. Our 43Ca solid-state NMR data support the dissolution-recrystallization mechanism for the calcite to aragonite transition. We find that the 43Ca NMR parameters of Mg-calcite are very similar to those of pure calcite. Under the high-resolution condition provided by magic-angle spinning at 4 kHz, we can monitor the variation of the 43Ca NMR parameters of the aragonite signals for the samples obtained at different reaction times. Our data suggest that in the presence of a significant amount of Mg2+ ions, aragonite is the most stable polymorph of calcium carbonate. The initial precipitated crystallites of aragonite have spine-like morphology, for which the 43Ca spin-lattice relaxation data indicate that the ions in the lattice have considerable motional dynamics. As the crystallinity of aragonite improves further, the 43Ca T1 parameter of the aragonite phase changes considerably and becomes very similar to that obtained for pure aragonite. For the first time, the difference in crystal morphologies and crystallinity of the aragonite phase has been traced down to the subtle difference in the motional dynamics at the molecular level. © 2012 American Chemical Society.
Type
journal article