Seknazi E et al. (2019), From spinodal decomposition to alternating laye...

ECB-ART-47474

Nat Commun 2019 Oct 08;101:4559. doi: 10.1038/s41467-019-12168-8.

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From spinodal decomposition to alternating layered structure within single crystals of biogenic magnesium calcite.

Seknazi E , Kozachkevich S , Polishchuk I , Bianco Stein N , Villanova J , Suuronen JP , Dejoie C , Zaslansky P , Katsman A , Pokroy B .

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As organisms can form crystals only under ambient conditions, they demonstrate fascinating strategies to overcome this limitation. Recently, we reported a previously unknown biostrategy for toughening brittle calcite crystals, using coherently incorporated Mg-rich nanoprecipitates arranged in a layered manner in the lenses of a brittle star, Ophiocoma wendtii. Here we propose the mechanisms of formation of this functional hierarchical structure under conditions of ambient temperature and limited solid diffusion. We propose that formation proceeds via a spinodal decomposition of a liquid or gel-like magnesium amorphous calcium carbonate (Mg-ACC) precursor into Mg-rich nanoparticles and a Mg-depleted amorphous matrix. In a second step, crystallization of the decomposed amorphous precursor leads to the formation of high-Mg particle-rich layers. The model is supported by our experimental results in synthetic systems. These insights have significant implications for fundamental understanding of the role of Mg-ACC material transformation during crystallization and its subsequent stability.

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Genes referenced: LOC115925415 LOC590371

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	Fig. 1. The layered structure revealed by tomography. a XRF tomography reconstruction slice across a lens (pixel size is 150â€‰nm, red areas correspond to higher Ca content). Corresponding intensity line plot taken from the lens surface towards the center of the bulk (marked by a yellow arrow), shows the measured oscillations of the calcium signal (black) compared with theoretical schematic oscillations of the magnesium content (dashed red), perpendicular to the sample surface. b Typical nano-HT reconstructed dorsal arm plate (DAP) part, showing oscillations in mass-density (pixel size is 35â€‰nm). Corresponding intensity line plot taken from the lens surface towards the center of the bulk (marked by a yellow arrow)
	Fig. 2. The layered structure is caused by a layered distribution of high-Mg nanoparticles. a HRTEM (HAADF-STEM mode) of a FIB-cut lens lamella (cut perpendicularly to a lens surfaceâ€“the lens surface can be seen at the right edge of the image) showing the existence of light (dark) nanoparticles distributed in layers parallel to the surface. b Zoom-in corresponding to the yellow square in (a)
	Fig. 3. The layered structure revealed by electron microscopy a HRSEM (BSE detector) image of a polished lens section. b BSE image of the same specimen after ion-milling in the FIB. Yellow arrows point at visible layers in the milled box. c HRSEM (secondary electron (SE) detector) images from polished DAP. The insets in a and c are a low magnification image of the polished DAP, the yellow rectangles represent the corresponding areas of interest
	Fig. 4. Other calcitic partsâ€™ morphology and structure. aâ€“c HRSEM images of the O. wendtii brittle star, showing a arm vertebrae, b spicules, and c teeth of the organism. d Diffractogram of these calcitic parts at room temperature and after heating treatment at 400â€‰Â°C. e Zoom-in on the (104) diffraction peaks plotted on logarithmic scale
	Fig. 5. Coherent nanoparticles in the brittle star spicules. a HRTEM image of a spicule, revealing brighter nanodomains, which are coherent with the matrix. b HRTEM image after in-situ heating at 400â€‰Â°C, demonstrating growth of the bright nanodomains and loss of coherency
	Fig. 6. 3D morphology and layered structure of spicules. Nano-HT reconstructed parallel section (a) and horizontal section (b) of the brittle star spicule (pixel size is 120â€‰nm)
	Fig. 7. Instability of Mg-CaCO3 for a range of Mg composition. (104) XRD peak from Mg-calcite crystallized from hydrothermally treated Mg-ACC in solution. Data were acquired by synchrotron radiation (Î»â€‰=â€‰0.496 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{A}$$\end{document}AË™). Numbers near the peaks are the Î· (Mg/Caâ€‰+â€‰Mg) in the calcite crystals, obtained from the refined lattice parameters and the equations linking the lattice parameters and Î· obtained by Zolotoyabko et al.24. The diffractograms are normalized to the highest diffraction peaks (calcite (104) peak for the powders precipitated from solutions containing 0 or 50% Mg in solutions, and aragonite peak for the other powders)
	Fig. 8. Proposed mechanism of formation of layered structure. a Gel-like or liquid-like Mg-ACC precursor containing 15% MgCO3. b Decomposition starts from growing fluctuations in concentration (spinodal decomposition of Mg-ACC solution). c Large Mg-concentration fluctuations result in formation of Mg-rich nanoparticles. d At some point, a nucleus of crystalline Mg-depleted calcite appears. e, f First possible crystallization route: e the crystalline nucleus grows through the gel, rejecting small particles (below a critical size) and incorporates large particles (above the critical size). f Local gel viscosity increases owing to the increase in volume fraction of the particles reaching the critical value at which particles are trapped and coherently crystallize. g The process is repeated. hâ€“j Second possible crystallization route: h Mg exclusion from crystallizing calcite and Mg enrichment of the vicinal Mg-ACC leads to secondary spinodal decomposition near the crystallization front. i Resulting formation of additional Mg-rich nanoparticles. j Advancement of crystallization front leading to formation of particle-enriched crystalline layer; thereafter the process of Mg exclusion and Mg-ACC secondary spinodal decomposition is repeated

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Aizenberg, Calcitic microlenses as part of the photoreceptor system in brittlestars. 2001, Pubmed, Echinobase