Surface stress dissipation during growth front nucleation as mechanism for spherulitic crystal growth

Spherulitic crystal growth structures are omnipresent in the sedimentary realm. They occur as allochems, such as ooids, as crystal fans in stromatolites or as botryoids in tufa and speleothems. The spherulitic structure is due to radially arranged crystallographic axes which manifests as round outer shapes and the characteristic extinction cross under cross-polarized light. Often, spherulitic structures are ascribed to some mystic biological effect, however, without providing any further explanation of the underlying mechanism.

While the overall driving force for spherulitic growth arises from the crystals attempting to reduce their surface energy, several pathways have been suggested in the literature by which the crystal structure and shape relaxes to the stress field imposed by the surface energy. Prominent is the concept of auto-deformation, where low-angle branching is introduced by crystallographic rearrangement in the interior of the crystal, due to cascades of discrete fracturing. In contrast, a growth front nucleation model has been suggested, in which case low-angle branching already nucleates as atoms or ions are being attached. In this mechanism, the stress field is dissipated before the atoms are incorporated permanently in the crystal lattice, which has the advantage that no bond-breaking event would be necessary (Meister, in press¹). From a non-classical point of view, the growth front model can be modified in the sense that already existing nano-particles are attaching in an oriented way, so that low-angle boundaries are established.

Ultimately, the prevailing crystal growth mode depends on crystal growth kinetics, as a result of both macroscopic factors – such as the supersaturation ratio of the bulk solution and interfacial energy – and molecular-scale factors that shape the nano-scale energy landscape. The mineralogical and crystallographic structure that can be reached by overcoming the lowest possible kinetic barrier should result, just as predicted by Ostwald’s step rule (Meister, in press²). Inorganic and organic co-solutes may act as modifiers, impacting the interface energy landscape and thereby shifting the boundary between step-flow growth and adhesive growth, facilitating non-crystallographic branching, and, thus, provoking configurations different from idiomorphic crystals.

¹⁾ Meister, P. (in press) Spherulitic mineral growth: auto-deformation, growth front nucleation, or semi-oriented attachment? In P. Meister, C. Fischer and N. Preto (Eds.): “Nucleation and growth of sedimentary minerals”, IAS Special Publications, accepted.

²⁾ Meister, P. (in press) Ostwald’s step rule: a consequence of growth kinetics and nano-scale energy landscape. In P. Meister, C. Fischer and N. Preto (Eds.): “Nucleation and growth of sedimentary minerals”, IAS Special Publications, accepted.