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

Abstract

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.

Stable Prenucleation Calcium Carbonate Clusters Define Liquid–Liquid Phase Separation

“…While so‐called “polymer‐induced liquid precursors” (PILPs) of calcium carbonate were first described in the late 1990s, it has become evident that these species constitute polymer‐stabilized rather than polymer‐induced states, since they also occur in purely inorganic systems . As a result of their major role in controlling crystal morphologies in biomineralization, research activities on such liquid and solid amorphous intermediates of CaCO 3 are intensive . Indeed, numerous studies have impressively illustrated their great use for controlling calcium carbonate crystallization also in vitro …”

Magnesium-rich nanoprecipitates in calcite: atomistic mechanisms responsible for toughening in Ophiocoma wendtii

“…The nanoprecipitates are believed to form during crystallisation from amorphous calcium carbonate (ACC) into calcite. 10 The solubility of Mg in ACC is much higher than in calcite, therefore during crystallisation Mg rich regions would be expected to form. Coherent nanoparticles, often referred to as Guinier-Preston (GP) zones, 11,12 are already well known in metallurgy, and have been the subject of previous molecular dynamics studies.…”

Stabile Calciumcarbonat‐Pränukleationscluster bestimmen die Flüssig‐flüssig‐Phasenseparation

“…Während bereits in den 1990er Jahren die sogenannten “polymerinduzierten flüssigen Vorstufen” (polymer‐induced liquid precursors, PILPs) von Calciumcarbonat beschrieben wurden, ist mittlerweile erwiesen, dass es sich bei diesen Spezies vielmehr um polymerstabilisierte als um polymerinduzierte Zustände handelt, da sie auch in rein anorganischen Systemen auftreten . Aufgrund ihrer wichtigen Rolle in der Kontrolle von Kristallmorphologien während der Biomineralisation befassen sich viele Forschungsarbeiten intensiv mit diesen flüssigen und festen amorphen Zwischenstufen von CaCO 3 . In der Tat haben viele Studien eindrucksvoll ihre enorme Nützlichkeit zur Kontrolle der Kristallisation von Calciumcarbonat auch in vitro veranschaulicht …”