Abstract
The crystallization of calcium carbonate is shown to be dictated by the Ostwald rule of stages (ORS), for high relative initial supersaturations (), under sweet (carbon dioxide saturated) and anoxic (oxygen depleted) solution conditions. Rhombohedral calcite crystals emerge after the sequential crystallization and dissolution of the metastable polymorphs: vaterite (snowflake-shaped) and aragonite (needle-shaped). However, the presence of certain cations, which can form trigonal carbonates (e.g. Fe2+ and Ni2+), in concentrations as low as 1.5 mM, triggers the emergence of calcite crystals, with a star-shaped crystal habit, first. These star-shaped crystals dissolve to yield needle-shaped aragonite crystals, which in turn dissolve to give the rhombohedral calcite crystals. The star-shaped crystals, formed at high SCaCO3, possess higher surface free energy (therefore higher apparent solubility) than their rhombohedral counterparts. This sequence of dissolution and recrystallization demonstrates that the ORS does not only drive the crystal towards its thermodynamically most stable polymorph but also towards its most stable crystal habit.
Highlights
In 1897 [1,2], the Nobel Laureate Wilhelm Ostwald wrote in relation to the crystallization of salts: 2022 The Authors
The X-ray powder diffraction (XRPD) measurements were conducted at a 2θ range from 5° to 80°, with a step size of 0.01° (2θ ) and a count time of one second
The most metastable polymorph, vaterite, emerged first as snowflake-shaped crystals. This unusual crystal habit has only been previously obtained through double diffusion experiments [31]
Summary
In 1897 [1,2], the Nobel Laureate Wilhelm Ostwald wrote in relation to the crystallization of salts: 2022 The Authors. This phenomenon and experimental evidence establishing this relationship between crystal habit and the ORS are further elucidated This is achieved using a model system: calcium carbonate (CaCO3), under anoxic (oxygen depleted) and sweet (carbon dioxide saturated) solution conditions, with cationic additives. With the use of cationic additives, it is possible to achieve a range of crystal habits within individual polymorphs In this context, the ‘prematureearly’ nucleation of calcite, in the presence of certain additves, is demonstrated as the trigger for a cascade of transformations, commencing with a star-shaped calcite crystal, moving to aragonite and subsequently to rhombohedral calcite. The study was expanded to investigate the influence of other cations: Ba2+, Fe3+, Li+, Ni2+ and Zn2+
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