We investigated the influence of the porosity of the growth medium on the crystallization of calcium carbonate in hydrogels with different gelatin solid contents (2.5, 5, and 10 wt %). In all experiments, the precipitate consisted of calcite with occasional occurrences of some vaterite and aragonite. The calcite grew as compact radial intergrowths of crystals that show rhombohedral external faces. The crystal surfaces consist of identical 1−10 μm sized rhombohedral sub-blocks. Electron backscatter diffraction (EBSD) uncovered the radial intergrowth structure of the aggregates. EBSD also documented the internal microscale mosaicity and mesocrystal-like constitution of the gel-grown calcite. Raman spectroscopy and thermogravimetric analysis confirmed the presence of gelatin within the crystals. It reached up to ∼2 mass % in the calcite-gelatin composites that formed in hydrogels with 10 wt % gelatin content. Calcite morphology and mosaicity varied with the gelatin content of the hydrogel, such that an increase in gelatin content initiated the growth of smaller crystal aggregates having progressively rougher surfaces, increasing amounts of incorporated gel, and increasing degrees of misorientation in the internal mosaic structure. Apart from biospecific morphology, the gel growth experiment successfully mimics many characteristics of calcite biomineralization such as formation of a hierarchical hybrid composite, crystal mosaicity, and mesocrystal-like constitution.
In this paper, we present an experimental study of the interaction between gypsum (010) surfaces and aqueous solutions of Na 2 CO 3 with different concentrations. This interaction leads to the carbonatation (i.e., the transformation into carbonate minerals) of gypsum crystals, which under ambient conditions shows the characteristics of a mineral replacement and leads to the formation of pseudomorphs consisting of an aggregate of calcite crystals. Carbonatation progress was monitored by scanning electron microscopy (SEM) and glancing incidence X-ray diffraction (GIXRD). The carbonatation advances from outside to inside the gypsum crystal and occurs through a sequence of reactions, which involves the dissolution of gypsum and the simultaneous crystallization of different polymorphs of CaCO 3 [amorphous calcium carbonate (ACC), vaterite, aragonite, and calcite], as well as several solvent-mediated transformations between these polymorphs. The sequence in which CaCO 3 phases form is interpreted taking into consideration nucleation kinetics and the qualitative evolution of several chemical parameters in the aqueous solution. The textural characteristics of the transformed regions are described. The degree of faithfulness of the pseudomorphs obtained is related to the kinetics of the carbonatation process, which in turn depends on the initial concentration of carbonate in the aqueous solutions. Finally, changes in the rate at which the transformation front advances are discussed on the basis of both textural and physicochemical considerations.
The extended use of hexavalent chromium Cr(VI) compounds in industrial processes caused a significant increase of the concentration of this highly toxic heavy metal in natural environments. In order to investigate the influence of Cr(VI) in the formation of CaCO 3 , crystallization experiments were carried out in a double diffusion system, using silica hydrogel with different Cr(VI) contents as the growth medium. Crystalline products were examined by scanning electron microscopy, Raman spectroscopy, electron microprobe analysis, and single crystal X-ray diffraction. Increasing Cr(VI) concentration caused inhibition of the nucleation and growth of calcite and promoted the formation of the metastable polymorphs aragonite and vaterite. This effect correlated with a decrease of crystal size. Furthermore, the habit of calcite crystals changed with increasing Cr(VI) concentrations from {104} to forms increasingly elongated parallel to the c-axis. Raman spectroscopy, single crystal X-ray diffraction (XRD), and electron microprobe analysis (EMPA) gave strong indications of an isomorphic anion substitution of trigonal planar carbonate by the tetrahedral chromate within the calcite lattice. The apparent partitioning coefficients of Cr(VI) into calcite determined in this work suggest that the fate of this pollutant in natural environments can be significantly influenced by CaCO 3 precipitation processes.
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...
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