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Structuring over many length scales is a design strategy widely used in Nature to create materials with unique functional properties. We here present a comprehensive analysis of an adult sea urchin spine, and in revealing a complex, hierarchical structure, show how Nature fabricates a material which diffracts as a single crystal of calcite and yet fractures as a glassy material. Each spine comprises a highly oriented array of Mg-calcite nanocrystals in which amorphous regions and macromolecules are embedded. It is postulated that this mesocrystalline structure forms via the crystallization of a dense array of amorphous calcium carbonate (ACC) precursor particles. A residual surface layer of ACC and/or macromolecules remains around the nanoparticle units which creates the mesocrystal structure and contributes to the conchoidal fracture behavior. Nature’s demonstration of how crystallization of an amorphous precursor phase can create a crystalline material with remarkable properties therefore provides inspiration for a novel approach to the design and synthesis of synthetic composite materials.

Biominerals exhibit morphologies, hierarchical ordering and properties that invariably surpass those of their synthetic counterparts. A key feature of these materials, which sets them apart from synthetic crystals, is their nanocomposite structure, which derives from intimate association of organic molecules with the mineral host. We here demonstrate the production of artificial biominerals where single crystals of calcite occlude a remarkable 13 wt% of 20 nm anionic diblock copolymer micelles, which act as 'pseudo-proteins'. The synthetic crystals exhibit analogous texture and defect structures to biogenic calcite crystals and are harder than pure calcite. Further, the micelles are specifically adsorbed on {104} faces and undergo a change in shape on incorporation within the crystal lattice. This system provides a unique model for understanding biomineral formation, giving insight into both the mechanism of occlusion of biomacromolecules within single crystals, and the relationship between the macroscopic mechanical properties of a crystal and its microscopic structure.

The mechanisms by which amorphous intermediates transform into crystalline materials are poorly understood. Currently, attracting enormous interest is the crystallization of amorphous calcium carbonate, a key intermediary in synthetic, biological and environmental systems. Here we attempt to unify many contrasting and apparently contradictory studies by investigating this process in detail. We show that amorphous calcium carbonate can dehydrate before crystallizing, both in solution and in air, while thermal analyses and solid-state nuclear magnetic resonance measurements reveal that its water is present in distinct environments. Loss of the final water fraction—comprising less than 15% of the total—then triggers crystallization. The high activation energy of this step suggests that it occurs by partial dissolution/recrystallization, mediated by surface water, and the majority of the particle then crystallizes by a solid-state transformation. Such mechanisms are likely to be widespread in solid-state reactions and their characterization will facilitate greater control over these processes.

(SPB) or y.y.kim@leeds.ac.uk (YYK). 2Structural biominerals are inorganic/organic composites that exhibit remarkable mechanical properties. However, the structure-property relationships of even the simplest building unitmineral single crystals containing embedded macromolecules -remain poorly understood. Here, by means of a model biomineral made from calcite single crystals containing glycine (0-7 mol%) or aspartic acid (0-4 mol%), we elucidate the origin of the superior hardness of biogenic calcite.We analyzed lattice distortions in these model crystals by using x-ray diffraction and molecular dynamics simulations, and by means of solid-state nuclear magnetic resonance show that the amino acids are incorporated as individual molecules. We also demonstrate that nanoindentation hardness increased with amino acid content, reaching values equivalent to their biogenic counterparts. A dislocation pinning model reveals that the enhanced hardness is determined by the force required to cut covalent bonds in the molecules.3 Biominerals such as bones, teeth and seashells are characterized by properties optimized for their functions. Despite being formed from brittle minerals and flexible polymers, nature demonstrates that it is possible to generate materials with strengths and toughnesses appropriate for structural applications 1 . At one level, the mechanical properties of these hierarchically structured materials are modelled as classical composites consisting of a mineral phase embedded in an organic matrix 2 . However, the single crystal mineral building blocks of biominerals are also composites 3 , containing both aggregates of biomacromolecules as large as 20 nm 4,5 and inorganic impurities 6,7 . While it should be entirely possible to employ this simple biogenic strategy in materials synthesis 8,9 , the strengthening and toughening mechanisms that result from these inclusions are still poorly understood 10,11 . This work addresses this challenge by analyzing hardening mechanisms in a simple model biomineral system: calcite single crystals containing known amounts of amino acids. We report synthetic calcite crystals with hardnesses equivalent to those of their biogenic counterparts, and offer a detailed explanation for the observed hardening.Since plastic deformation in single crystals occurs by the motion of dislocations, hardness is enhanced by features that inhibit dislocation motion. The mechanisms by which guest species may harden ionic single crystals generally fall into two categories. Second phase particles directly block dislocation motion, requiring a dislocation to either cut through (shear) a particle or bypass it by a diffusive process to keep going 12 . Solutes (point defects) do not directly block dislocation motion, but the stress fields of the dislocations interact with those associated with misfitting solutes, retarding dislocation motion 12 . Biominerals, notably calcite, often deform plastically by twinning 11 , but since twins grow by motion of "twinning dislocations" 13 , these concep...

The term mesocrystal has been widely used to describe crystals that form by oriented assembly, and that exhibit nanoparticle substructures. Using calcite crystals co-precipitated with polymers as a suitable test case, this article looks critically at the concept of mesocrystals. Here we demonstrate that the data commonly used to assign mesocrystal structure may be frequently misinterpreted, and that these calcite/polymer crystals do not have nanoparticle substructures. Although morphologies suggest the presence of nanoparticles, these are only present on the crystal surface. High surface areas are only recorded for crystals freshly removed from solution and are again attributed to a thin shell of nanoparticles on a solid calcite core. Line broadening in powder X-ray diffraction spectra is due to lattice strain only, precluding the existence of a nanoparticle sub-structure. Finally, study of the formation mechanism provides no evidence for crystalline precursor particles. A re-evaluation of existing literature on some mesocrystals may therefore be required.

Calcite single crystals containing polystyrene particles are synthesized using a straightforward, one‐pot method, through control of the particle surface structure and the crystallization conditions. Investigation of the mechanical behavior of these composite crystals using nanoindentation shows enhanced fracture toughness via a crack bridging mechanism.

Atomic level defects such as dislocations play key roles in determining the macroscopic properties of crystalline materials 1,2. Their effects range from increased chemical reactivity 3,4 to enhanced mechanical properties 5,6. Dislocations have been widely studied using traditional techniques such as X-ray diffraction and optical imaging. Recent advances have enabled atomic force microscopy to study single dislocations 7 in two-dimensions (2D), while transmission electron microscopy (TEM) can now visualise strain fields in three-dimensions (3D) with near atomic resolution 8–10. However, these techniques cannot offer 3D imaging of the formation or movement of dislocations during dynamic processes. Here, we describe how Bragg Coherent Diffraction Imaging (BCDI) 11,12 can be used to visualize in 3D, the entire network of dislocations present within an individual calcite crystal during repeated growth and dissolution cycles. These investigations demonstrate the potential of BCDI for studying the mechanisms underlying the response of crystalline materials to external stimuli.