Abstract
Crystal growth experiments carried out in aqueous solutions, supersaturated with respect to calcite (CaCO3) at room temperature and pressure, show that the lithium ion added to growth solutions not only acts as a habit modifier of calcite, but promotes the nucleation and growth of the two other CaCO3 polymorphs: aragonite and vaterite. This behavior is interpreted on the grounds of foreign adsorption going beyond Langmuir’s isotherm model: two-dimensional (2D)-adsorbed nuclei of Li2CO3 (mineral zabuyelite) can change, under varying Li+ concentrations in solution, the growth habit of calcite, promoting as well both nucleation and growth of aragonite and vaterite, through the mechanism of 2D-epitaxy. Calculation proves how the epitaxy aragonite/2D-zabuyelite reduces the average value of the specific surface energy of aragonite, allowing the nucleation of aragonite and then the coexistence of calcite and aragonite in the same batch. Finally, it is outlined that the 2D lattices fulfilling the epitaxy constraints for the couples calcite/zabuyelite and aragonite/zabuyelite show, as a common feature, a remarkable pseudohexagonality.
Highlights
A crystal can change either its equilibrium (ES) or growth shape (GS), according to whether the surrounding mother phase has, or not, the same composition of the crystal
It is unsaturated with respect to 3D-Li2CO3: we cannot prove, at this time, this statement through the evaluation of the chemical equilibria ruling the mother phase; we can say that zabuyelite cannot nucleate and grow as a 3D-crystal phase since we never observed, in our experiments, zabuyelite crystals neither in optical nor in electron microscopy
Lithium added to the aqueous growth solutions of Ca-carbonate has been proven to modify the growth habit of calcite.[52]
Summary
A crystal can change either its equilibrium (ES) or growth shape (GS), according to whether the surrounding mother phase has, or not, the same composition of the crystal. The simplest mother phases are represented either by pure vapor or pure melt (pure systems). In both cases, the ES changes with equilibrium temperature (Teq), which determines, in turn, the specific surface energies (γhkl) of the different {hkl} forms of the crystal. A more complex situation arises when foreign substances (solvent and/or impurities) are present in the mother phase, as for the growth from solution, not pure vapor, not pure melt. In all these cases, adsorption is the new complex phenomenon to be taken into account
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