Particle-mediated nucleation pathways are imprinted in the internal structure of calcium sulfate single crystals

Calcium sulfate minerals are found under the form of three crystalline phases: gypsum (CaSO4.2H2O), bassanite (CaSO4.0.5H2O), and anhydrite (CaSO4). Due to its relevance in natural and industrial processes, the formation pathways of these calcium sulphate phases from aqueous solution have been the subject of intensive research. There is a growing body of literature, that calcium sulfate forms essentially through a non-classical nanoparticle- or cluster-mediated crystallisation process. At the early stages of precipitation calcium sulfate crystals grow through the reorganization and coalescence of aggregates rather than through classical unit addition. Here, we show by using low-dose dark field (DF) transmission electron microscopy (TEM) and electron diffraction, that these re-structuring processes by no means continue until a near-perfectly homogeneous single crystal is obtained. Instead the growth process yields a final imperfect mesocrystal of the overall morphology resembling that of a single crystal, yet composed of smaller nano-domains. Hence, our data indicate that organic-free mesocrystal grown by a particle mediated-pathway may preserve a 'memory or 'imprint' of the non-classical growth process in the final crystal structure, something that has been largely overlooked until now. Furthermore, the nano-scale misalignment of the structural sub-units within crystals might propagate through the length-scales, and potentially is expressed macroscopically as misaligned zones/domains in large single crystals, e.g. as those observed in some of the giant crystals from the Naica Mine, Chihuahua, Mexico.