Heterogeneous nucleation and growth of crystalline micro-bubbles around gas cavities formed in solution

Perfect single three dimensional crystals with the shape of bubbles can nucleate and grow encompassing gas filled cavities previously formed on solid substrates in aqueous solutions. Bubbles are double walled, both internal and external faces belonging to the cleavage calcite rhombohedron which separate the growth solution from the enclosed gas cavity. The bubbles thickness amounting to a few nanometers, while the size of the bubble is strictly related to that of the cavity (of the order of a few micrometers), the surface-to-volume ratio is the highest obtained till now for 3D single crystals. A tentative model of nucleation and growth mechanisms of these peculiar objects is proposed. Growth morphology modifications due to the effect of impurities (Li⁺and acetate ions) or to foreign gas bubbling in the growth solution (methane) are also described. A peculiar attention is paid to both mechanical and thermodynamic conditions to be fulfilled in order to nucleate crystals around gas cavities from solution and melt. By analysing growth and patterning of Ca-carbonate hollow shells in reverse microemulsions along with the growth from melt of sodium disilicate around gas microcavities, we found the criterion ruling the nucleation and growth of poly- or single crystals around a gas cavity and outlined the major role played by the nature of the substrate. Finally, is proposed a new interpretation of hollow shaped crystals, recently found in methane-derived carbonate rocks (Western Alps-Italy) and in late Cenozoic dolomites of the Bahamas.