Strontium pulse-chase labelling of bivalve shells produces timed ‘snapshots’ of different stages of nacre growth, which yield the first direct evidence for a two-step growth process of nacre that has been inferred as early as in the 1960’s 26: Extensional nacre growth occurs along the local growth direction of the shell normal to the organic interlamellar sheets (Fig. 2c, white arrows) and is complemented by space-filling growth of nacre tablets parallel to the interlamellar sheets (Fig. 2c, grey arrows). NanoSIMS maps of the labelled nacre (Fig. 2 Supplementary Fig. 2) indicate that extensional growth of nacre lamellae occurs simultaneously at multiple growth centres along the organic interlamellar sheet, forming the tablet centres of new nacre tablets. This observation supports literature studies indicating that extensional growth proceeds via mineral bridges via pores in the organic interlamellar sheets 34,35 often associated with screw dislocation28. The interplay of both modes of growth result in a stepped pattern of adjacent Sr-rich (i.e. labelled) and Sr-poor (i.e., unlabelled portions in the same nacre tablet) when shells are studied in cross-section (Fig. 2b and Supplementary Fig. 2, supplementary video).
Individual aragonitic nanogranules, the basic structural units of nacre tablets, are systematically zoned in Sr concentration at nanometre resolution: They display distinct Sr-enriched cortices towards the contours of the nanogranules and Sr-poor central parts. Sr-enrichments are clearly identified through significantly higher counts in the APT analyses (Figs. 3, 4, Supplementary Figs. 5 to 8) and through the presence and intensity of a shoulder indicative for strontianite in the PiFM spectra (Figs. 5, 6). At tablet-scale, nanogranules along the rim of a nacre tablet (Fig. 6a) adjacent to the organic sheet are more enriched in Sr compared to other areas in the nacre tablets.
Cortices of similar dimensions in nacre, including in nacre nanogranules have been previously reported in the literature, however, these structures were only identified by phase composition (i.e., presence or absence of ACC) and not, as presented here, based on their chemical composition: Nassif et al.12 reported similarly sized, 3–5 nm thick ACC layers coating the nacre tablets in Haliotis laevigata. In the shell of Phorcus turbinatus and at spatial resolution of individual nanogranules, Macias-Sanchez et al.33 observed 5–10 nm thick ACC cortices coating crystalline aragonite nanogranules (referred to as ‘nanoglobules’). Both studies12,33 argue that these ACC cortices represent vestiges of the non-classical crystallisation pathway of nacre and are stabilized by trace metal ions and/or organic molecules expelled from ACC upon crystallization of aragonite.
Similarly, we interpret the Sr cortices observed here as a result of phase transformation of ACC to aragonite, created by the exclusion of excess Sr upon crystallization of aragonite. Hence, in these initial stages of crystallization each organic-coated nanogranule30,44,45 serves as a compartment for the ACC to aragonite transformation33. Laboratory studies have shown that the dehydration of ACC serves as an effective trigger for crystallisation46 and it has been suggested that the presence of these small amounts of surface water mediate the formation of the crystalline phase by dissolution and reprecipitation2,46. For the first time here, we provide evidence in naturally formed nacre through the formation of Sr-rich cortices in nanogranules, that the ACC-to-aragonite transformation proceeds via an intragranular and spatially-confined dissolution-reprecipitation mechanism that preserves the fine structure of the material47.
Exposed to Sr-enriched seawater, excess amounts of strontium are non-selectively incorporated in the growing shell due to the lack of long-range order of the ACC structure16,48−50. Upon crystallization, the formation of aragonite is energetically favoured over strontianite51, although both are isostructural52,53. Thus, excess strontium is excluded from the aragonite lattice and forms Sr-rich CaCO3 cortices along the outer contours of the nanogranules. Since Sr ions can stabilize ACC54 these cortices are most likely amorphous when they form and may remain so for extended periods of time.
The systematic presence of strontium-enriched cortices and Sr-poor cores in individual nanogranules throughout the nacre tablet indicates that the major transformation mechanism is the dissolution of the ACC structure to crystallize the CaCO3 as aragonite. Aragonite has a different strontium content governed by its selective crystal chemistry55 and therefore leads to Sr-enriched cortices, while transformation via solid-state mechanisms would not affect the chemical composition. Such very localized dissolution and reprecipitation processes successfully preserve the nanogranular texture of nacre.
Spatially coupled dissolution-reprecipitation reactions that preserve the fine structures and overall shape of the replaced mineral, kinetically outcompete solid-state transformation reactions and are, in fact, very common in mineralized systems 56. They are catalysed by a fluid and are usually driven by small free energy differences between the reaction partner phases. Specifically, in interface-coupled systems with an interfacial fluid47, the dissolution rate of one and the activation energy barrier for nucleation of the other reaction partner create local equilibrium conditions which result in the preservation of very fine structural details after phase transformation57,58.
A common issue encountered in laboratory dissolution-reprecipitation experiments is the significant morphological change occurring during transformation15,16 which fail to explain the preservation of the hierarchical ultrastructure observed in natural biominerals and, hence, seem to support solid state transformation of ACC to the crystalline calcium carbonate phase. Our findings here solve this long-standing question and show that in natural nacre, the fine structural details of the material are preserved due to a spatially coupled dissolution-reprecipitation mechanism that takes place in nanogranular compartments delineated by organic coatings.