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...

Solid-state NMR has become the method of choice for determining details of molecular-level structure in heterogeneous systems. Though spin-1/2 nuclei still form the core of most such studies, quadrupolar nuclei are increasingly being used. This review assesses what is currently possible, from achieving high-resolution spectra for quadrupolar nuclei (a prerequisite for most structure determination work), to forming correlation spectra which give qualitative details of spatial proximity of nuclei and the determination of internuclear distances, between quadrupolar spins and quadrupolar and spin-1/2 nuclei. Examples are given of each method discussed, and the advantages and disadvantages of the various experiments for different possible applications are assessed.

We provide evidence that citrate anions bridge between mineral platelets in bone and hypothesize that their presence acts to maintain separate platelets with disordered regions between them rather than gradual transformations into larger, more ordered blocks of mineral. To assess this hypothesis, we take as a model for a citrate bridging between layers of calcium phosphate mineral a double salt octacalcium phosphate citrate (OCPcitrate). We use a combination of multinuclear solid-state NMR spectroscopy, powder X-ray diffraction, and first principles electronic structure calculations to propose a quantitative structure for this material, in which citrate anions reside in a hydrated layer, bridging between apatitic layers. To assess the relevance of such a structure in native bone mineral, we present for the first time, to our knowledge, 17 O NMR data on bone and compare them with 17 O NMR data for OCP-citrate and other calcium phosphate minerals relevant to bone. The proposed structural model that we deduce from this work for bone mineral is a layered structure with thin apatitic platelets sandwiched between OCP-citrate-like hydrated layers. Such a structure can explain a number of known structural features of bone mineral: the thin, plate-like morphology of mature bone mineral crystals, the presence of significant quantities of strongly bound water molecules, and the relatively high concentration of hydrogen phosphate as well as the maintenance of a disordered region between mineral platelets.NMR crystallography | biomineralization B one is a complex organic-inorganic composite material (1), in which calcium phosphate nanoparticles are held within a primarily collagen protein matrix. The mineral component is a poorly crystalline phase, closely related to hydroxyapatite. The currently accepted model of bone mineral is ∼50-to 150-nmthick stacks of very closely packed apatitic platelets, each of order 2.5-4 nm in thickness (1-4), arranged so that their large (100) faces are parallel to each other and their c axes are strongly ordered (parallel to collagen fibrils) (5). NMR studies show that, in addition to the largely ordered but nonstoichiometric apatitic phase, there is a substantial, highly hydrated, disordered phase containing up to 55% of the bone mineral phosphatic ions (6, 7) but in the form of hydrogen phosphate or phosphate strongly hydrogen-bonded to water rather than apatitic orthophosphate (8). This phase has been assigned as a surface phase, but whether the surface in question is that of individual mineral platelets or the surface of the overall structure formed by a stack of such platelets is not yet clear. There is, however, significant experimental evidence that is not explained by this model as it stands. First, there has never been any observation of an isolated mineral platelet in mature bone, even in preparations in which there have been attempts to disperse the mineral structures (9). This feature suggests that the mineral platelets are not independent structures-indeed, their ordered aggregati...

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International audienceDespite the numerous studies of bone mineral, there are still many questions regarding the exact structure and composition of the mineral phase, and how the mineral crystals become organised with respect to each other and the collagen matrix. Bone mineral is commonly formulated as hydroxyapatite, albeit with numerous substitutions, and has previously been studied by 31P and 1H NMR, which has given considerable insight into the complexity of the mineral structure. However, to date, there has been no report of an NMR investigation of the other major component of bone mineral, calcium, nor of common minority cations like sodium. Here, direct analysis of the local environment of calcium in two biological apatites, equine bone (HB) and bovine tooth (CT), was carried out using both 43Ca solid state NMR and Ca K-edge X-ray absorption spectroscopy, revealing important structural information about the calcium coordination shell. The 43Ca diso in HB and CT is found to correlate with the average Ca–O bond distance measured by Ca K-edge EXAFS, and the 43Ca NMR linewidths show that there is a greater distribution in chemical bonding around calcium in HB and CT, compared to synthetic apatites. In the case of sodium, 23Na MAS NMR, high resolution 3Q-MAS NMR, as well as 23Na{31P} REDOR and 1H{23Na} R3-HMQC correlation experiments give the first direct evidence that some sodium is located inside the apatite phase in HB and CT, but with a greater distribution of environments compared to a synthetic sodium substituted apatite (Na-HA)