Seeds of imperfection rule the mesocrystalline disorder in natural anhydrite single crystals

Abstract

In recent years, we have come to appreciate the astounding intricacies associated with the formation of minerals from ions in aqueous solutions. In this context, a number of studies have revealed that the nucleation of calcium sulfate systems occurs nonclassically, involving the aggregation and reorganization of nanosized prenucleation species. In recent work, we have shown that this particle-mediated nucleation pathway is actually imprinted in the resultant micrometer-sized CaSO4 crystals. This property of CaSO4 minerals provides us with the unique opportunity to search for evidence of nonclassical nucleation pathways in geological environments. In particular, we focused on large anhydrite crystals extracted from the Naica Mine in Mexico. We were able to shed light on this mineral's growth history by mapping defects at different length scales. Based on this, we argue that the nanoscale misalignment of the structural subunits, observed in the initial calcium sulfate crystal seeds, propagates through different length scales both in morphological, as well as in strictly crystallographic aspects, eventually causing the formation of large mesostructured single crystals of anhydrite. Hence, the nonclassical nucleation mechanism introduces a “seed of imperfection,” which leads to a macroscopic “single” crystal whose fragments do not fit together at different length scales in a self-similar manner. Consequently, anisotropic voids of various sizes are formed with very well-defined walls/edges. However, at the same time, the material retains in part its single crystal nature.

Electron transparent nanotubes reveal crystallization pathways in confinement

“…1,20,39 Transformation of bassanite to gypsum in solution subsequently typically occurs via dissolution/reprecipitation. 22 The observation of nanoscale co-aligned domains in natural calcium sulfate crystals 1,40 support that the multiple (bassanite) nucleation events followed by maturation into a larger aligned crystal observed in our conned pores are unlikely to be an artefact of connement, but instead are something that occurs across scales in the calcium sulfate system.…”

“…This may be due to their crystallographic orientation (c-axis aligned with the long axis of the control rods rather than perpendicular alignment of the conned rods) or the ethanolic synthesis leading to a more stable bassanite surface structuring. 39,40 Either route could inhibit water from entering into the bassanite structure of the control particles, thus stabilizing them against transformation to gypsum.…”

Formation and properties of spindle-shaped aragonite mesocrystals from Mg-bearing solutions

“…, in a non-classical crystallization process, as was also suggested by previous authors. 6,12,15,26,35 The formation of mesocrystals via the attachment of crystalline nanoparticles in a crystallographically co-aligned fashion was widely described, e.g. , in titanium oxides, 32,48 iron oxides, 34,49,50 and calcium sulphates.…”

“…[31][32][33] However, Zhu et al 34 suggested that mesocrystals could alternatively be produced via conversion of amorphous nanoparticles into nanocrystals near mineral surfaces, in a process that involves both dissolution-re-precipitation and oriented attachment. Regardless of their assumed formation mechanism, the properties of mesocrystals differ from those of bulk single crystals, 35 e.g., have larger surface area and porosity, making them effective substrates for adsorption. 36 With respect to the possible role of an amorphous precursor in the formation of aragonite mesocrystals, Walker et al 10 performed cryo-TEM analysis on freshly precipitated aragonite, synthesized in ethanol-rich aqueous media.…”

“…31–33 However, Zhu et al 34 suggested that mesocrystals could alternatively be produced via conversion of amorphous nanoparticles into nanocrystals near mineral surfaces, in a process that involves both dissolution–re-precipitation and oriented attachment. Regardless of their assumed formation mechanism, the properties of mesocrystals differ from those of bulk single crystals, 35 e.g. , have larger surface area and porosity, making them effective substrates for adsorption.…”