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

Abstract

Atomistic simulations provide insight into an example of the superiority of biogenic crystals, where Mg-rich nanoprecipitates in calcite inhibit crack propagation.

Unseeded, spontaneous nucleation of spherulitic magnesium calcite

“…8c,d). This observation is consistent with the strengthening and toughening magnesium calcite nanoprecipitates observed in some biominerals [34][35][36].…”

“…Firstly, the Mg 2+ ions are smaller than Ca 2+ and generate defects (misfits, dislocations) and lattice strain within precipitate that destabilize solid structure and lead to increased solubility and reduced solution supersaturation [1,33]. However, in some cases, Mg-rich nanoprecipitates' alignment results in strengthening and toughening the bio-carbonate matrix [34][35][36]. Secondly, because the Mg 2+ ions bind water more strongly than Ca 2+ , they introduce their hydration water into the sediment and, as a result, delay transformations to more stable and usually dehydrated carbonate polymorphs [1,[6][7][8][9][10]33].…”

Biomineralized Materials as Model Systems for Structural Composites: Intracrystalline Structural Features and Their Strengthening and Toughening Mechanisms

“…[ 24 ] MD simulation of a nanodomain (4 nm in diameter) in calcite matrix showed a similar distribution of residual stress (Figure 16d). [ 219 ] Further MD results revealed that the presence of these Mg‐rich nanodomains significantly modifies the fracture behavior of calcite (Figure 16e). [ 219 ] In contrast to the $$\{ {10\bar{1}4} \}$$ cleavage fracture in pure calcite (Figure 16e‐i), the crack bisected the nanodomain along a disordered path with significant bridging (Figure 16e‐ii).…”

Incredible internal strains within a biogenic single crystal viewed by X-ray diffraction tomography

“…[25][26][27][28] This is made possible by their tailored structure, that confers them toughness and optical capabilities. [26][27][28][29][30] Indeed, the DAPs contain hundreds of rounded features, called lenses, whose rounded shape guides the light through and focuses it onto photoreceptor nerve bundles. In addition, lenses's alignment along the optical c-axis of calcite minimizes birefringence.…”

“…27,29 The existence of coherent high-Mg nanoparticles and associated strains are responsible for the enhanced mechanical properties of the otherwise brittle lenses. 27,30 The crystallographic characterization of atomic structure via synchrotron High-resolution Powder X-ray Diffraction (HRPXRD) of powdered samples: the diffraction pattern displayed peaks correponding to a single low-Mg calcite phase. Upon heating, the coherent high-Mg particles grow within the lenses and lose coherency with the matrix, thereby incoherent high-Mg particles' diffraction peaks become visible by HRPXRD.…”