Skeletal biominerals often show complex morphologies and ultrastructures that are important for their mechanical function. The production of these biomineral structures requires exact control over mineral precipitation, which takes place under physiological conditions in the presence of gel-like organic matrices. Understanding the pathway of reproducibly forming elaborate structures with specific mechanical properties may open a way for new technical applications. We have conducted a detailed investigation of the ultrastructural organisation of calcitic octocoral sclerites, which demonstrated that internal fibrous crystals grow by aligned aggregation and fusion of nanoparticles. These nanoparticles were initially precipitated as nano-granular layers on the sclerite surface, where they enabled the smooth shaping of the sclerites. Experimental calcite precipitation in polyacrylamide hydrogel demonstrated that crystal growth via precursornanoparticles can be induced by gelatinous networks. This finding indicates the organic matrices' role in controlling the mineral precipitation and crystal growth in biomineralisation.
In crystal growth of mineral species or different compositional members of a solid solution on one another, the degree of lattice mismatch at their interface affects the growth pattern of the precipitating mineral phase. Fast layer-by-layer growth of magnesian calcite on pure calcite (1014) substrates has been observed at Mg 2þ /Ca 2þ ratios of 2-7 using in situ atomic force microscopy. Under solution conditions of calcite saturation states starting from Ω ≈ 33, depending on Mg 2þ /Ca 2þ ratios and carbonate content, bulging in the epitaxial magnesian calcite thin film led to the formation of networks of ridges along the [441], [481], and [421] directions. Eventually, spreading of monolayers stopped at the ridges and formed stationary multilayer steps, resulting in separate and individually growing crystal segments. Molecular dynamics computational modeling suggests that relaxation of strain energy, caused by the interfacial lattice mismatch between pure calcite and the isostructural magnesiumcontaining phase with smaller lattice constants, leads to a semicoherent interface and disordered linear zones cutting through the thin film. As a consequence, the surface bulges up in a way similar to our laboratory observations. This strain-induced segmentation produces aggregates of aligned microcrystals and increase knowledge of the behavior of strained thin films in general.
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