WEB THE PSEUDOMORPHIC TRANSFORMATION OF DOPED AMORPHOUS CALCIUM CARBONATE AND ITS INFLUENCE ON ELEMENT PARTITIONING
Additional to the stable anhydrous calcium carbonate polymorphs (vaterite, aragonite, and calcite) an unstable form exists, namely amorphous calcium carbonate (ACC). This phase is often found in biominerals as a biogenic precursor of crystalline calcium carbonate, as shown by Addadi et al. (2003).
A detailed understanding of the crystallization pathway via ACC is of importance especially for paleoclimate reconstructions where calcium carbonates of marine organisms are important proxy archives. However, the main question which needs to be answered is: Which conditions do calcite and aragonite partition coefficients reflect when ACC is – respectively was – present? Are the partition coefficients reflecting the environmental conditions during the ACC formation, or are they modified during the crystallization?
In general, two different crystallization pathways are described in the literature: Firstly, solid-state transformation, and secondly, dissolution-reprecipitation crystallization. Upon crystallization of ACC via solid-state transformation partition coefficients remain unchanged; therefore the environmental conditions during the ACC formation are reflected. Instead, a dissolution-reprecipitation pathway modifies the partition coefficients during crystallization. Liu et al. (2020) described a shape-preserving crystallization pathway for calcium carbonate, the so-called pseudomorphic transformation, which is induced by the addition of trace amounts of magnesium.
In this work, preservation of the partition coefficients for strontium was revealed for the pseudomorphic transformation. Additionally, we found that the pseudomorphic transformation can be induced by poly(acrylic acid), polyaspartic acid, and phosphate. As the latter additives are widely present in seawater, the pseudomorphic transformation in calcium carbonate may have significant impacts on paleoclimate reconstruction.
Our first results suggest that the pseudomorphic transformation is facilitated by a thin film of the organic additive covering calcium carbonate surface, which shields the crystallizing surface against water. In the work of Demmert et al. (2019) it was already shown that it is possible to cover calcium carbonate with an organic additive.