Effects of Mg and Hydrogel Solid Content on the Crystallization of Calcium Carbonate in Biomimetic Counter-diffusion Systems

Abstract

Carbonate biominerals are nanocomposites with an intimate association of organic and mineral components. Here we investigate the crystallization of CaCO 3 in gelatin hydrogels (2.5 and 10 wt % solid content) in the presence of Mg (0.01 M) in the growth medium. The precipitate consisted mainly of calcite in all experiments. A wide variety of morphologies and incorporated Mg contents (up to 26 mol % in sphere-like aggregates grown in 10 wt % gelatin) was observed. Etching experiments uncovered an intimate relationship between the inorganic component and a polymeric network in the calcite crystal aggregates. The characteristics of this network varied for hydrogels with different solid contents. When Mg was not present in the growth medium, we obtained 200 nm to 1 μm thick incorporations that were bordered on both sides by a delicate gelatin network. As Mg was added, the incorporations became thinner (∼50−60 nm), and the gelatin network became compact. Electron backscatter diffraction evidenced that the calcite usually consists of aggregates of mutually misoriented crystals with an internal mosaic spread. Crystals with high lattice co-orientation, which occur rather rarely, are terminated by regular rhombohedral (104)-type faces. The irregular-shaped and mosaic-structured aggregates occasionally have a rim of such rhombohedral crystallites. In the experiment with 10 wt % solid gelatin content and Mg in the growth medium, the calcite consisted of crystallites with fan-like small-angle misorientations (split growth), leading to spherulitic microstructures. We attribute these frequent and characteristic small-angle boundaries to dislocations that relax misfit strain, which is associated with selective Mg incorporation at acute growth steps. We ascribe our observations to the acidic functional groups of the gelatin promoting the desolvation of the hydrated Mg 2+ ions, leading to an increased incorporation of Mg into calcite and a reduced inhibition of calcite nucleation and growth. ■ INTRODUCTIONCalcite, the stable CaCO 3 polymorph at Earth's surface conditions, is one of the most abundant mineral constituents of mineralized tissues in both fossil and modern organisms. 1−3 Magnesium is a very common additive to calcium carbonate biological hard tissues, occurring in a wide range of concentrations. The most common concentrations vary from ∼0 to 23 mol % MgCO 3 , 4 although magnesium contents as high 45 mol % MgCO 3 are found in the hard tissues of some organisms, such as in sea urchin teeth. 5 Since different phyla incorporate Mg to a variable extent into their hard tissues, 6−10 they are ranked into two major groups: low and high MgCO 3 -producing organisms. 4 Echinoderms, corals, and neritic benthic foraminifera belong to the high-Mg group, while planktonic foraminifera and brachiopods represent low-Mg biocarbonates, respectively. Echinoderm skeletons show the highest variation in Mg concentrations (4.8−15.9 mol % MgCO 3 ), while oktocorals and neritic benthic foraminifera have more or less comparable amounts of Mg in t...

Nucleation and stabilization of Eocene dolomite in evaporative lacustrine deposits from central Tibetan plateau

“…Other interpretations include the mineralization of proteinaceous spheroids produced by tissue decay (Schieber & Arnott, 2003) or a by-product of the preparation procedure (Folk & Lynch, 1997). The viscosity of organic polymers and presence of Mg 2+ in carbonate has been suggested to be responsible for the spherical morphology (Meldrum & Hyde, 2001;Braissant et al, 2003;Meister et al, 2011b;Nindiyasari et al, 2014). Template-directed calcium carbonate formation in solution has been also found to start with the nucleation of amorphous nanoparticles (Pouget et al, 2009).…”

Influence of gel-strength and magnesium doping on the organization of calcite/hydrogel mesocrystal composites

“…As explained in the previous section for calcite/silica gel composites, the incorporation of Mg into calcite induces lattice strain which is released through the formation of dislocations (Nindiyasari et al, 2014b(Nindiyasari et al, , 2015. The amount of Mg incorporated into calcite is up to~5 times higher in gelatin than in silica gel.…”

“…The MUD value is defined as the multiple of uniform (random) orientation (Kocks et al, 2000). A calcite single crystal precipitated from solution, devoid of any gel, has a MUD value of 725 (Kim et al, 2014;Nindiyasari et al, 2014b, Greiner et al, 2018. The mesocrystalline calcite grown in silica and gelatin gels shows a mainly single-crystal-like co-orientation despite the gel occlusion.…”

“…The second effect of Mg on calcite growth, the generation of lattice strain associated with Mg substituting Ca in the calcite structure, leads to the formation of dislocations as a means to release this strain. The distribution of dislocations at regular intervals within the aggregate generates low angle boundaries between crystal subunits (Nindiyasari et al, 2014b(Nindiyasari et al, , 2015, explaining the increased number of crystal subunits that constitute those composites that formed in the presence of higher Mg concentration. The combination of these two effects explains that the occluded silica network appears more dispersed and is distributed between smaller and more numerous crystal subunits in those calcite/silica gel aggregates grown in silica gels with high Mg concentrations compared to aggregates that formed in silica gel with a low Mg concentration or, even, without Mg.…”

“…This correlation strongly relates to the change in mechanical properties of the gel (gel strength) as it becomes denser. Furthermore, by adding a 0.1 M MgCl 2 containing solution to gelatin, agarose and silica hydrogel, we can state that the presence of Mg in the growth medium influences the structuring of calcite-hydrogel composites by contributing to the development of low and high angle boundaries between crystal subunits and promoting split growth phenomena (Nindiyasari et al, 2014b(Nindiyasari et al, , 2015.…”