Nanoscopic Characteristics of Anhydrite (100) Growth

The growth of anhydrite (100) surface in contact with supersaturated aqueous solutions (β_anh = 1–3.6) under low hydrothermal conditions (<i>T</i> = 60–120 °C) has been studied by use of a hydrothermal atomic force microscope (HAFM). Our observations show that growth on this surface occurs by lateral spreading of monomolecular layers (3.5 Å in height) and is highly anisotropic, with [001] and [001̅] alternating as fast and slow directions in successive monolayers. This anisotropic growth is evidence of strong structural control, which becomes less intense as temperature and/or supersaturation increases. The growth anisotropy affects the development of spirals, determining the combination of fast-moving and slow-moving steps to form bilayer steps around the emergence point of screw dislocations and leading to nonconstant spread rates. As a result, the overall efficiency of spiral growth mechanism is highly dependent on the interaction between slow-moving bilayers and fast-moving monolayers originating from different dislocations. Formation of two-dimensional nuclei occurs only at <i>T</i> ≥ 80 °C and β_anh ≥ 2, two-dimensional nucleation density always being very low (≤1 nucleus/μm²) under the conditions explored. These facts, together with the slow kinetics of anhydrite growth in comparison to the much faster kinetics of gypsum growth, might explain the frequent metastable formation of gypsum crystals under temperatures corresponding to the stability field of anhydrite

Formation of Strontianite and Witherite Cohesive Layers on Calcite Surfaces for Building Stone Conservation

The formation of micrometric-thick mineral cohesive layers is a novel method to prevent the deterioration of historical buildings. Here, we study the formation of thin, cohesive, pseudomorphic shells of strontianite (SrCO3) and witherite (BaCO3) on the surface of calcite (CaCO3) single crystals reacted with aqueous solutions bearing Sr2+ and Ba2+, respectively. The reaction front moves inward from the calcite–solution interface through a dissolution–crystallization reaction, which stops before the strontianite and witherite shells are barely 40 thick. These shells consist of elongated crystallites that grow oriented on the calcite substrate, with which they share very small contact areas. The calcite–strontianite and −witherite epitaxies are mono-dimensional and involve a parallelism between (101̅4)Cal||(021)Str/Wth. Strontianite and witherite cohesive layers remain strongly attached to the calcite substrates, which appear crack-free even after 2 years of reaction time. The formation of thin, cohesive, and durable replacement layers of strontianite and witherite may provide a long-lasting protection for calcitic marbles and limestones used as building stones in cultural heritage

Influence of Gelatin Hydrogel Porosity on the Crystallization of CaCO₃

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

Effect of Hydrogel Matrices on Calcite Crystal Growth Morphology, Aggregate Formation, and Co-Orientation in Biomimetic Experiments and Biomineralization Environments

We investigate the effect of gelatin, agarose, and silica hydrogel with and without magnesium in the growth medium on calcite single crystal growth and aggregate formation. We characterize the hydrogel and the mineral by cryo-scanning electron microscopy (SEM), high-resolution SEM, and electron backscatter diffraction (EBSD). We image the pristine hydrogel fabric and the fabric of hydrogel incorporated into the mineral. We visualize the hydrogel–mineral interface and investigate the effect of the hydrogels on calcite micro- and mesostructure in the gel/calcite composits. We compare hydrogel fabrics in biomimetic hybrid composites with biopolymer matrices and networks in biological carbonate tissues of bivalves, gastropods, brachiopods, and corraline red algae. In Mg-free environments, silica gel has very little effect on crystal morphology and arrangement; the gel/calcite composite that forms is a single gradient mesocrystal. Agarose and gelatin hydrogels influence mineral organization in gel/calcite aggregates, and these consist of very few subunits separated by hydrogel membranes. With Mg added to the growth medium, large and small angle boundaries highly increase in number: silica gel/calcite aggregates consist of partial spherulites with mesocrystalline subentities; agarose, gelatin gel/calcite aggregates are regular spherulites, and their subentities are single crystals. Thus, calcite crystal organization is influenced by accumulative split growth provoked by incorporation of magnesium

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

Carbonate biominerals are nanocomposites with an intimate association of organic and mineral components. Here we investigate the crystallization of CaCO₃ 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 Mg²⁺ ions, leading to an increased incorporation of Mg into calcite and a reduced inhibition of calcite nucleation and growth