On the mechanism of calcium carbonate polymorph selection via confinement

Organisms deposit various biominerals in the course of their biomineralisation. The most abundant of these is calcium carbonate, which manifests itself in several polymorphs. While organisms possess the ability to...

Crystallisation and polymorph selection in active Brownian particles

We explore crystallisation and polymorph selection in active Brownian particles with numerical simulation. In agreement with previous work (Wysocki et al. in Europhys Lett 105:48004, 2014), we find that crystallisation is suppressed by activity and occurs at higher densities with increasing Péclet number ($${ Pe }$$ Pe ). While the nucleation rate decreases with increasing activity, the crystal growth rate increases due to the accelerated dynamics in the melt. As a result of this competition, we observe the transition from a nucleation and growth regime at high $${ Pe }$$ Pe to “spinodal nucleation” at low $${ Pe }$$ Pe . Unlike the case of passive hard spheres, where preference for FCC over HCP polymorphs is weak, activity causes the annealing of HCP stacking faults, thus strongly favouring the FCC symmetry at high $${ Pe }$$ Pe . When freezing occurs more slowly, in the nucleation and growth regime, this tendency is much reduced and we see a trend towards the passive case of little preference for either polymorph.

Mechanisms of bastnasite formation: replacement of calcite by rare earth carbonates.

<div> <p>The interaction of rare earth bearing (La, Nd, Dy) aqueous solutions with calcite crystals at was studied at ambient and hydrothermal conditions (25-220 °C) and resulted in the solvent-mediated surface precipitation and subsequent pseudomorphic mineral replacement of calcite by rare earth carbonates. Calcite grains were replaced from their periphery inwards, and the newly formed REE-bearing carbonates follow the crystallisation sequence lanthanite [REE<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>·8H<sub>2</sub>O] → kozoite [orthorhombic REECO<sub>3</sub>(OH)] → hydroxylbastnasite [hexagonal REECO<sub>3</sub>(OH)]. The specific rare earth involved in these processes and the temperature have a significant role in the polymorph selection, crystallisation pathways and kinetics of mineral replacement. La- and Nd-bearing kozoite, grows oriented onto the calcite surface, forming an epitaxy, due to their structural similarities. This phase forms elongated crystals on [100], with the {011} and {0-11} as major forms. The epitaxial relationship is (104) [010]<sub>cc </sub>║(001) [100]<sub>koz</sub> and is strongly dependent on the ionic radius of the rare earth in the structure of kozoite. These results have strong implications for the understanding of mineralisation reactions occurring in REE-bearing carbonatite deposits, the most important resources of rare earths in the world.</p> </div>

Polymorph selection during melt crystallization of the isotactic polybutene-1 homopolymer depending on the melt state and crystallization pressure

This work investigated the crystalline forms obtained from melt crystallization in the isotactic polybutene-1 (iPB-1) homopolymer via manipulation of the temperature at which samples were melted (Tmelt) and crystallization pressure (Pcry).