Particle-Mediated Nucleation Pathways Are Imprinted in the Internal Structure of Calcium Sulfate Single Crystals

Calcium sulfate minerals are found in nature as three hydrates: gypsum (CaSO4 center dot 2H(2)O), bassanite (CaSO4 center dot 0.5H(2)O), and anhydrite (CaSO4). Due to their relevance in natural and industrial processes, the formation pathways of calcium sulfates from aqueous solution have been the subject of intensive research, and there is a growing body of literature, suggesting that calcium sulfates form through nonclassical nanoparticle-mediated crystallization processes. We showed earlier (Stawski et al. Nat. Commun. 2016, 7, 11177) that at the early stages in the precipitation reaction, calcium sulfate nanocrystals nucleate through the reorganization and coalescence of aggregates rather than through classical unit addition. Here, we used low-dose dark field (DF) transmission electron microscopy (TEM) and electron diffraction and document that these restructuring processes do not continue until a final near-perfectly homogeneous single crystal is obtained. Instead, we show that the growth process yields a final imperfect mesocrystal with an overall morphology resembling that of a single crystal, yet composed of smaller nanodomains. Our data reveal that organic-free calcium sulfate mesocrystals grown by a particle mediated-pathway may preserve in the final crystal structure a memory or imprint of their nonclassical nucleation process, something that has been overlooked until now. Furthermore, the nanoscale misalignment of the structural subunits within these crystals might propagate through the length-scales, which is potentially expressed macroscopically as misaligned zones/domains in large single crystals. This is akin to observations in some of the giant crystals from the Naica Mine, Chihuahua, Mexico.