Hydro-gel environment and solution additives modify calcite growth mechanism to an accretion process of amorphous nanospheres
Abstract
Various biominerals form via the transformation of a transient amorphous precursor phase into a mature crystalline phase. The mature biominerals usually exhibit morphology reminiscent of aggregated nanoparticles. Although these observations suggest an accretion-based growth process consisting on nanoparticles, the key factors that control the accretion process are unknown. We investigated the transformation of solid amorphous calcium carbonate (ACC) into calcite. When plant cystoliths, a biogenic stable ACC phase, are transformed into calcite in vitro by immersion in water, calcite crystals grow in two distinct steps (Gal et al., Angewandte Chemie, 2013). First, rhombohedral crystals grow that show flat facets as expected from ion-by-ion growth. These crystals then grow by the aggregation and crystallization of the original ACC nanospheres leading to a surface morphology dominated by aggregated spheres. The transformation process occurs within an organic hydro-gel that originates from inside the cystoliths. We tested the importance of the gel phase to the transformation process by transforming synthetic ACC into calcite inside various gels. In all the investigated systems: in gelatin, agarose, and pectin gels, calcite crystals grew that showed the nanosphere aggregation morphology. In additional experiments we demonstrated that also other additives, such as phosphate ions and biogenic macromolecules, that slow down ACC dissolution and calcite precipitation from ions can induce the accretion process dominance (see figure attached). These experiments show that although in solution the dominant process is dissolution to ions of the ACC and crystal growth by ion-by-ion mechanism, the presence of an additive that slows the ion-mediated processes makes the ACC nanospheres stable long enough to interact with the crystal surface. As a result, the metastable ACC nanospheres undergo secondary nucleation on the crystal surface without dissolving. These experiments highlight two factors that may underlie many biomineralization processes in nature: the first formed amorphous mineral phase can transform to a crystalline phase without dissolving if the solution properties of the environment are altered by an additive. And, accretion-based crystal-growth may become dominant when the amorphous precursor is abundant and the competing ion-based process is slowed down. SEM images of: (A) calcite crystal that grew from the transformation of ACC in DDW by ion-by-ion growth mechanism; (B) calcite crystal that grew from the transformation of ACC in 10mM phosphate solution by nanosphere accretion mechanism. Scale bars are 100 nm.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.V33G..04G
- Keywords:
-
- 0419 BIOGEOSCIENCES Biomineralization;
- 1042 GEOCHEMISTRY Mineral and crystal chemistry