Special Issue "Nucleation of Minerals: Precursors, Intermediates and Their Use in Materials Chemistry"

A special issue of Minerals (ISSN 2075-163X).

Deadline for manuscript submissions: closed (31 May 2017)

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A printed edition of this Special Issue is available here.

Special Issue Editor

Guest Editor
PD Dr. Denis Gebauer

Department of Chemistry, Physical Chemistry, University of Konstanz, 78464 Konstanz, Germany
Website | E-Mail
Interests: physical chemistry; materials chemistry; crystallization; nucleation; non-classical crystallization; polyamorphism; pre-nucleation clusters; biomineralization; additive-controlled crystallization; bio-inspired materials

Special Issue Information

Dear Colleagues,

Nucleation is the key event in mineralization, but a general molecular understanding of phase separation mechanisms is still missing, despite more than 100 years of research in this field. In the recent years, many studies have highlighted the occurrence of precursors and intermediates, which seem to challenge the assumptions underlying classical theories of nucleation and growth. This is especially true for the field of biomineralization, where bio-inspired strategies take advantage of the precursors' and intermediates' special properties for the generation of advanced materials. All of this has led to the development of "non-classical" frameworks, which, however, often lack quantitative expressions for the evaluation and prediction of phase separation, growth and ripening processes, and are under considerable debate. It is, thus, evident that there is a crucial need for research into the early stages of mineral nucleation and growth, designed for the testing, refinement, and expansion of the different existing notions. This Special Issue aims to bring together corresponding studies from all these areas, dealing with precursors and intermediates in mineralization processes. We welcome fundamental physical chemical studies, experimental, as well as theoretical, but also detailed analyses and characterizations of the formation mechanisms of both biogenic and bio-inspired, mineral-based (hybrid) materials. We also solicit methodological studies employing cutting-edge in situ analytics. The hope is that this Special Issue will contribute to the achievement of a better understanding of nucleation precursors and intermediates, and their target-oriented use in materials chemistry.

Dr. Denis Gebauer
Guest Editor

Manuscript Submission Information

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Keywords

  • mineral nucleation and growth
  • non-classical crystallization
  • pre-nucleation clusters
  • amorphous intermediates
  • mineral poly(a)morphism
  • formation mechanisms of biominerals
  • additive-controlled mineralization
  • mechanims of bio-inspired mineralization
  • in situ analyses of the early stages of mineralization

Published Papers (18 papers)

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Editorial

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Open AccessEditorial Editorial for Special Issue “Nucleation of Minerals: Precursors, Intermediates and Their Use in Materials Chemistry”
Minerals 2018, 8(6), 239; https://doi.org/10.3390/min8060239
Received: 31 May 2018 / Revised: 1 June 2018 / Accepted: 2 June 2018 / Published: 4 June 2018
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Research

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Open AccessArticle Indications that Amorphous Calcium Carbonates Occur in Pathological Mineralisation—A Urinary Stone from a Guinea Pig
Minerals 2018, 8(3), 84; https://doi.org/10.3390/min8030084
Received: 29 January 2018 / Revised: 22 February 2018 / Accepted: 24 February 2018 / Published: 27 February 2018
Cited by 1 | PDF Full-text (5738 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Calcium carbonate is an abundant biomineral that is of great importance in industrial or geological contexts. In recent years, many studies of the precipitation of CaCO3 have shown that amorphous precursors and intermediates are widespread in the biomineralization processes and can also
[...] Read more.
Calcium carbonate is an abundant biomineral that is of great importance in industrial or geological contexts. In recent years, many studies of the precipitation of CaCO3 have shown that amorphous precursors and intermediates are widespread in the biomineralization processes and can also be exploited in bio-inspired materials chemistry. In this work, the thorough investigation of a urinary stone of a guinea pig suggests that amorphous calcium carbonate (ACC) can play a role in pathological mineralization. Importantly, certain analytical techniques that are often applied in the corresponding analyses are sensitive only to crystalline CaCO3 and can misleadingly exclude the relevance of calcium carbonate during the formation of urinary stones. Our analyses suggest that ACC is the major constituent of the particular stone studied, which possibly precipitated on struvite nuclei. Minor amounts of urea, other stable inorganics, and minor organic inclusions are observed as well. Full article
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Open AccessArticle Structural Transition of Inorganic Silica–Carbonate Composites Towards Curved Lifelike Morphologies
Minerals 2018, 8(2), 75; https://doi.org/10.3390/min8020075
Received: 2 February 2018 / Revised: 14 February 2018 / Accepted: 16 February 2018 / Published: 18 February 2018
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Abstract
The self-assembly of alkaline earth carbonates in the presence of silica at high pH leads to a unique class of composite materials displaying a broad variety of self-assembled superstructures with complex morphologies. A detailed understanding of the formation process of these purely inorganic
[...] Read more.
The self-assembly of alkaline earth carbonates in the presence of silica at high pH leads to a unique class of composite materials displaying a broad variety of self-assembled superstructures with complex morphologies. A detailed understanding of the formation process of these purely inorganic architectures is crucial for their implications in the context of primitive life detection as well as for their use in the synthesis of advanced biomimetic materials. Recently, great efforts have been made to gain insight into the molecular mechanisms driving self-assembly in these systems, resulting in a consistent model for morphogenesis at ambient conditions. In the present work, we build on this knowledge and investigate the influence of temperature, supersaturation, and an added multivalent cation as parameters by which the shape of the forming superstructures can be controlled. In particular, we focus on trumpet- and coral-like structures which quantitatively replace the well-characterised sheets and worm-like braids at elevated temperature and in the presence of additional ions, respectively. The observed morphological changes are discussed in light of the recently proposed formation mechanism with the aim to ultimately understand and control the major physicochemical factors governing the self-assembly process. Full article
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Open AccessArticle Pyrophosphate-Inhibition of Apatite Formation Studied by In Situ X-Ray Diffraction
Minerals 2018, 8(2), 65; https://doi.org/10.3390/min8020065
Received: 28 August 2017 / Revised: 10 February 2018 / Accepted: 12 February 2018 / Published: 13 February 2018
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Abstract
The pathways to crystals are still under debate, especially for materials relevant to biomineralization, such as calcium phosphate apatite known from bone and teeth. Pyrophosphate is widely used in biology to control apatite formation since it is a potent inhibitor of apatite crystallization.
[...] Read more.
The pathways to crystals are still under debate, especially for materials relevant to biomineralization, such as calcium phosphate apatite known from bone and teeth. Pyrophosphate is widely used in biology to control apatite formation since it is a potent inhibitor of apatite crystallization. The impacts of pyrophosphate on apatite formation and crystallization kinetics are, however, not fully understood. Therefore, we studied apatite crystallization in water by synchrotron in situ X-ray diffraction. Crystallization was conducted from calcium chloride (0.2 M) and sodium phosphate (0.12 M) at pH 12 where hydrogen phosphate is the dominant phosphate species and at 60 °C to allow the synchrotron measurements to be conducted in a timely fashion. Following the formation of an initial amorphous phase, needle shaped crystals formed that had an octacalcium phosphate-like composition, but were too small to display the full 3D periodic structure of octacalcium phosphate. At later growth stages the crystals became apatitic, as revealed by changes in the lattice constant and calcium content. Pyrophosphate strongly inhibited nucleation of apatite and increased the onset of crystallization from minute to hour time scales. Pyrophosphate also reduced the rate of growth. Furthermore, when the pyrophosphate concentration exceeded ~1% of the calcium concentration, the resultant crystals had reduced size anisotropy suggesting that pyrophosphate interacts in a site-specific manner with the formation of apatite crystals. Full article
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Open AccessArticle Liquid Cell Transmission Electron Microscopy and the Impact of Confinement on the Precipitation from Supersaturated Solutions
Minerals 2018, 8(1), 21; https://doi.org/10.3390/min8010021
Received: 4 September 2017 / Revised: 28 December 2017 / Accepted: 12 January 2018 / Published: 15 January 2018
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Abstract
The study of nucleation and growth from supersaturated ion solutions is a key area of interest in biomineralization research and beyond with high-resolution in situ imaging techniques such as liquid cell transmission electron microscopy (LCTEM) attracting substantial attention. However, there is increasing experimental
[...] Read more.
The study of nucleation and growth from supersaturated ion solutions is a key area of interest in biomineralization research and beyond with high-resolution in situ imaging techniques such as liquid cell transmission electron microscopy (LCTEM) attracting substantial attention. However, there is increasing experimental evidence that experiments performed with this technique differ from those performed in bulk solutions due to the spatial restriction, which is a prerequisite for LCTEM to provide electron transparent samples. We have performed 2D Finite Elements (FE) simulations to study the impact of confinement on the steady state concentration profiles around a nanoparticle in a supersaturated solution of the constituent ions. We find that confinement below a critical value significantly reduces the concentration of available ions in solutions and hence the stability of the precipitates. These findings could explain the necessity to substantially increase ion activities of Ca2+ and CO32− to induce precipitation in LCTEM. Full article
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Open AccessCommunication A Micro-Comb Test System for In Situ Investigation of Infiltration and Crystallization Processes
Minerals 2017, 7(10), 187; https://doi.org/10.3390/min7100187
Received: 31 August 2017 / Revised: 29 September 2017 / Accepted: 3 October 2017 / Published: 6 October 2017
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Abstract
The investigation of mineralization and demineralization processes is important for the understanding of many phenomena in daily life. Many crystalline materials are exposed to decay processes, resulting in lesions, cracks, and cavities. Historical artifacts, for example, often composed of calcium carbonate (CaCO3), are
[...] Read more.
The investigation of mineralization and demineralization processes is important for the understanding of many phenomena in daily life. Many crystalline materials are exposed to decay processes, resulting in lesions, cracks, and cavities. Historical artifacts, for example, often composed of calcium carbonate (CaCO3), are damaged by exposure to acid rain or temperature cycles. Another example for lesions in a crystalline material is dental caries, which lead to the loss of dental hard tissue, mainly composed of hydroxyapatite (HAp). The filling of such cavities and lesions, to avoid further mineral loss and enable or support the remineralization, is a major effort in both areas. Nevertheless, the investigation of the filling process of these materials into the cavities is difficult due to the non-transparency and crystallinity of the concerned materials. In order to address this problem, we present a transparent, inexpensive, and reusable test system for the investigation of infiltration and crystallization processes in situ, being able to deliver datasets that could potentially be used for quantitative evaluation of the infiltration process. This was achieved using a UV-lithography-based micro-comb test system (MCTS), combined with self-assembled monolayers (SAMs) to mimic the surface tension/wettability of different materials, like marble, sandstone, or human enamel. Moreover, the potential of this test system is illustrated by infiltration of a CaCO3 crystallization solution and a hydroxyapatite precursor (HApP) into the MCTS. Full article
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Open AccessArticle Physicochemical and Additive Controls on the Multistep Precipitation Pathway of Gypsum
Minerals 2017, 7(8), 140; https://doi.org/10.3390/min7080140
Received: 6 July 2017 / Revised: 2 August 2017 / Accepted: 3 August 2017 / Published: 9 August 2017
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Abstract
Synchrotron-based small- and wide-angle X-ray scattering (SAXS/WAXS) was used to examine in situ the precipitation of gypsum (CaSO4·2H2O) from solution. We determined the role of (I) supersaturation, (II) temperature and (III) additives (Mg2+ and citric acid) on the
[...] Read more.
Synchrotron-based small- and wide-angle X-ray scattering (SAXS/WAXS) was used to examine in situ the precipitation of gypsum (CaSO4·2H2O) from solution. We determined the role of (I) supersaturation, (II) temperature and (III) additives (Mg2+ and citric acid) on the precipitation mechanism and rate of gypsum. Detailed analysis of the SAXS data showed that for all tested supersaturations and temperatures the same nucleation pathway was maintained, i.e., formation of primary particles that aggregate and transform/re-organize into gypsum. In the presence of Mg2+ more primary particle are formed compared to the pure experiment, but the onset of their transformation/reorganization was slowed down. Citrate reduces the formation of primary particles resulting in a longer induction time of gypsum formation. Based on the WAXS data we determined that the precipitation rate of gypsum increased 5-fold from 4 to 40 °C, which results in an effective activation energy of ~30 kJ·mol−1. Mg2+ reduces the precipitation rate of gypsum by more than half, most likely by blocking the attachment sites of the growth units, while citric acid only weakly hampers the growth of gypsum by lowering the effective supersaturation. In short, our results show that the nucleation mechanism is independent of the solution conditions and that Mg2+ and citric acid influence differently the nucleation pathway and growth kinetics of gypsum. These insights are key for further improving our ability to control the crystallization process of calcium sulphate. Full article
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Open AccessArticle Fabrication of Single-Crystalline Calcite Needle-Like Particles Using the Aragonite–Calcite Phase Transition
Minerals 2017, 7(8), 133; https://doi.org/10.3390/min7080133
Received: 24 July 2017 / Revised: 28 July 2017 / Accepted: 30 July 2017 / Published: 1 August 2017
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Abstract
Calcium carbonate (CaCO3) occurs in two major polymorphs: rhombohedral calcite and orthorhombic aragonite, the latter is thermodynamically metastable. In this study, we first prepared aragonite needle-like particles by introducing CO2-containing gas into Ca(OH)2 aqueous slurry. Then, the resulted
[...] Read more.
Calcium carbonate (CaCO3) occurs in two major polymorphs: rhombohedral calcite and orthorhombic aragonite, the latter is thermodynamically metastable. In this study, we first prepared aragonite needle-like particles by introducing CO2-containing gas into Ca(OH)2 aqueous slurry. Then, the resulted aragonite particles were heat treated at 500 °C for 1 h, in order to induce the aragonite–calcite phase transition. Particle structures before and after the heat treatment were characterized mainly by powder X-ray diffractometry (XRD), field emission-scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). We found that single-crystalline calcite needle-like particles with zigzag surface structures can be fabricated using the phase transition. Full article
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Open AccessArticle Carbonate Apatite Precipitation from Synthetic Municipal Wastewater
Minerals 2017, 7(8), 129; https://doi.org/10.3390/min7080129
Received: 31 May 2017 / Revised: 18 July 2017 / Accepted: 18 July 2017 / Published: 25 July 2017
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Abstract
An important component of phosphorite (phosphate rock) is carbonate apatite, as it is required for phosphorous fertilizer production due to its increased phosphate solubility caused by carbonate substitution in the apatite mineral lattice. High phosphate concentrations in municipal wastewater treatment plants are commonly
[...] Read more.
An important component of phosphorite (phosphate rock) is carbonate apatite, as it is required for phosphorous fertilizer production due to its increased phosphate solubility caused by carbonate substitution in the apatite mineral lattice. High phosphate concentrations in municipal wastewater treatment plants are commonly reduced by precipitating iron phosphate by addition of iron chloride. We investigated the possibility of precipitating carbonate apatite from a potential range of phosphate concentrations that could be available from municipal wastewater treatment plants with anaerobic digestion reactors (5 mM–30 mM). Synthetic phosphate solutions at neutral pH were mixed in batch experiments with a calcium carbonate solution produced by dissolving calcite in contact with carbon dioxide gas, with and without carbonate apatite seed. Batch experiments were used to identify the carbonate apatite supersaturation ranges for homogeneous and heterogeneous nucleation, and the precipitates analyzed with Raman spectroscopy, powder X-ray diffraction, inorganic carbon coulometry, and scanning electron microscopy. Some precipitates contained carbonate weight fractions within the range reported for geological phosphate rock (1.4–6.3 wt %). The precipitates were spherical, poorly crystalline carbonate apatite, suggesting an amorphous precursor transformed to a poorly crystalline carbonate apatite without changing morphology. Full article
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Open AccessArticle Hydration Effects on the Stability of Calcium Carbonate Pre-Nucleation Species
Minerals 2017, 7(7), 126; https://doi.org/10.3390/min7070126
Received: 1 June 2017 / Revised: 1 July 2017 / Accepted: 14 July 2017 / Published: 20 July 2017
Cited by 4 | PDF Full-text (3378 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Recent experimental evidence and computer modeling have shown that the crystallization of a range of minerals does not necessarily follow classical models and theories. In several systems, liquid precursors, stable pre-nucleation clusters and amorphous phases precede the nucleation and growth of stable mineral
[...] Read more.
Recent experimental evidence and computer modeling have shown that the crystallization of a range of minerals does not necessarily follow classical models and theories. In several systems, liquid precursors, stable pre-nucleation clusters and amorphous phases precede the nucleation and growth of stable mineral phases. However, little is known on the effect of background ionic species on the formation and stability of pre-nucleation species formed in aqueous solutions. Here, we present a systematic study on the effect of a range of background ions on the crystallization of solid phases in the CaCO3-H2O system, which has been thoroughly studied due to its technical and mineralogical importance, and is known to undergo non-classical crystallization pathways. The induction time for the onset of calcium carbonate nucleation and effective critical supersaturation are systematically higher in the presence of background ions with decreasing ionic radii. We propose that the stabilization of water molecules in the pre-nucleation clusters by background ions can explain these results. The stabilization of solvation water hinders cluster dehydration, which is an essential step for precipitation. This hypothesis is corroborated by the observed correlation between parameters such as the macroscopic equilibrium constant for the formation of calcium/carbonate ion associates, the induction time, and the ionic radius of the background ions in the solution. Overall, these results provide new evidence supporting the hypothesis that pre-nucleation cluster dehydration is the rate-controlling step for calcium carbonate precipitation. Full article
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Open AccessCommunication Desiccator Volume: A Vital Yet Ignored Parameter in CaCO3 Crystallization by the Ammonium Carbonate Diffusion Method
Minerals 2017, 7(7), 122; https://doi.org/10.3390/min7070122
Received: 30 May 2017 / Revised: 13 July 2017 / Accepted: 14 July 2017 / Published: 19 July 2017
Cited by 1 | PDF Full-text (2686 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Employing the widely used ammonium carbonate diffusion method, we demonstrate that altering an extrinsic parameter—desiccator size—which is rarely detailed in publications, can alter the route of crystallization. Hexagonally packed assemblies of spherical magnesium-calcium carbonate particles or spherulitic aragonitic particles can be selectively prepared
[...] Read more.
Employing the widely used ammonium carbonate diffusion method, we demonstrate that altering an extrinsic parameter—desiccator size—which is rarely detailed in publications, can alter the route of crystallization. Hexagonally packed assemblies of spherical magnesium-calcium carbonate particles or spherulitic aragonitic particles can be selectively prepared from the same initial reaction solution by simply changing the internal volume of the desiccator, thereby changing the rate of carbonate addition and consequently precursor formation. This demonstrates that it is not merely the quantity of an additive which can control particle morphogenesis and phase selectivity, but control of other often ignored parameters are vital to ensure adequate reproducibility. Full article
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Open AccessArticle Ammonium-Carbamate-Rich Organogels for the Preparation of Amorphous Calcium Carbonates
Minerals 2017, 7(7), 110; https://doi.org/10.3390/min7070110
Received: 31 May 2017 / Revised: 17 June 2017 / Accepted: 18 June 2017 / Published: 27 June 2017
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Abstract
Amine-CO2 chemistry is important for a range of different chemical processes, including carbon dioxide capture. Here, we studied how aspects of this chemistry could be used to prepare calcium carbonates. Chemically crosslinked organogels were first prepared by reacting hyperbranched polyethylene imine (PEI)
[...] Read more.
Amine-CO2 chemistry is important for a range of different chemical processes, including carbon dioxide capture. Here, we studied how aspects of this chemistry could be used to prepare calcium carbonates. Chemically crosslinked organogels were first prepared by reacting hyperbranched polyethylene imine (PEI) dissolved in DMSO with carbon dioxide. The crosslinks of the organogel consisted of ammonium-carbamate ion pairs as was shown by IR spectroscopy. These carbamate-rich organogels were subsequently subjected to aqueous solutions of calcium acetate, and amorphous calcium carbonate (ACC) precipitated. The ACC did not crystalize during the mixing for up to 20 h, as was shown by a combination of IR spectroscopy, X-ray diffraction, scanning electron microscopy, and thermal analysis. Some PEI had been included or adsorbed on the ACC particles. Traces of calcite were observed in one sample that had been subjected to water in a work-up procedure. Full article
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Open AccessArticle Crystallization of Jarosite with Variable Al3+ Content: The Transition to Alunite
Minerals 2017, 7(6), 90; https://doi.org/10.3390/min7060090
Received: 28 March 2017 / Revised: 29 May 2017 / Accepted: 29 May 2017 / Published: 1 June 2017
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Abstract
This study focused on the formation of the jarosite-alunite solid solution at relatively low temperature, 90 °C. It was found that the transition from jarosite to alunite results in significant changes in the powder X-ray diffraction pattern, the infrared spectrum and thermal behavior
[...] Read more.
This study focused on the formation of the jarosite-alunite solid solution at relatively low temperature, 90 °C. It was found that the transition from jarosite to alunite results in significant changes in the powder X-ray diffraction pattern, the infrared spectrum and thermal behavior when the degree of substitution reached ≥50%. The initial Al/(Al + Fe) in solution, however, required to achieve these substitution levels in the solid is ≥90%. The morphology shows that the faceted jarosite form goes through an intergrown transition to a spherical morphology of pure alunite. This morphology has not been previously observed for alunite and most likely reflects the formation temperature. Rietveld analysis shows that the a lattice parameter obeys Vegard’s Rule while the c lattice parameter behavior is more complex. Empirical modelling of the incorporation of Fe into alunite supports the general trends found in the X-ray diffraction data for the behaviour of the a-axis with Al/Fe content. The dehydration of the Al3+ ion could be a significant contribution to the activation energy barrier to alunite formation as found for other minerals. Finally, dynamic light scattering showed that the nucleation behavior for jarosite and Fe-containing alunite are significantly different. Alunite appears to nucleate continuously rather than in a single nucleation event. Full article
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Graphical abstract

Open AccessArticle Crystal Chemistry and Stability of Hydrated Rare-Earth Phosphates Formed at Room Temperature
Minerals 2017, 7(5), 84; https://doi.org/10.3390/min7050084
Received: 29 April 2017 / Revised: 15 May 2017 / Accepted: 17 May 2017 / Published: 19 May 2017
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Abstract
In order to understand the crystal chemical properties of hydrous rare-earth (RE) phosphates, REPO4,hyd, that form at ambient temperature, we have synthesized REPO4,hyd through the interaction of aqueous RE elements (REEs) with aqueous P at room temperature at pH <
[...] Read more.
In order to understand the crystal chemical properties of hydrous rare-earth (RE) phosphates, REPO4,hyd, that form at ambient temperature, we have synthesized REPO4,hyd through the interaction of aqueous RE elements (REEs) with aqueous P at room temperature at pH < 6, where the precipitation of RE hydroxides does not occur, and performed rigorous solid characterization. The second experiment was designed identically except for using hydroxyapatite (HAP) crystals as the P source at pH constrained by the dissolved P. Hydrated RE phosphate that precipitated at pH 3 after 3 days was classified into three groups: LREPO4,hyd (La → Gd) containing each REE from La-Gd, MREPO4,hyd (Tb → Ho), and HREPO4,hyd (Er → Lu). The latter two groups included increasing fractions of an amorphous component with increasing ionic radius, which was associated with non-coordinated water. REallPO4,hyd that contains all lanthanides except Pm transformed to rhabdophane structure over 30 days of aging. In the experiments using HAP, light REEs were preferentially distributed into nano-crystals, which can potentially constrain initial RE distributions in aqueous phase. Consequently, the mineralogical properties of hydrous RE phosphates forming at ambient temperature depend on the aging, the pH of the solution, and the average ionic radii of REE, similarly to the well-crystalline RE phosphates. Full article
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Open AccessArticle In Situ AFM Study of Crystal Growth on a Barite (001) Surface in BaSO4 Solutions at 30 °C
Minerals 2016, 6(4), 117; https://doi.org/10.3390/min6040117
Received: 21 September 2016 / Revised: 21 October 2016 / Accepted: 24 October 2016 / Published: 2 November 2016
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Abstract
The growth behavior and kinetics of the barite (001) surface in supersaturated BaSO4 solutions (supersaturation index (SI) = 1.1–4.1) at 30 °C were investigated using in situ atomic force microscopy (AFM). At the lowest supersaturation, the growth behavior was mainly
[...] Read more.
The growth behavior and kinetics of the barite (001) surface in supersaturated BaSO4 solutions (supersaturation index (SI) = 1.1–4.1) at 30 °C were investigated using in situ atomic force microscopy (AFM). At the lowest supersaturation, the growth behavior was mainly the advancement of the initial step edges and filling in of the etch pits formed in the water before the BaSO4 solution was injected. For solutions with higher supersaturation, the growth behavior was characterized by the advance of the <uv0> and [010] half-layer steps with two different advance rates and the formation of growth spirals with a rhombic to bow-shaped form and sector-shaped two-dimensional (2D) nuclei. The advance rates of the initial steps and the two steps of 2D nuclei were proportional to the SI. In contrast, the advance rates of the parallel steps with extremely short step spacing on growth spirals were proportional to SI2, indicating that the lateral growth rates of growth spirals were directly proportional to the step separations. This dependence of the advance rate of every step on the growth spirals on the step separations predicts that the growth rates along the [001] direction of the growth spirals were proportional to SI2 for lower supersaturations and to SI for higher supersaturations. The nucleation and growth rates of the 2D nuclei increased sharply for higher supersaturations using exponential functions. Using these kinetic equations, we predicted a critical supersaturation (SI ≈ 4.3) at which the main growth mechanism of the (001) face would change from a spiral growth to a 2D nucleation growth mechanism: therefore, the morphology of bulk crystals would change. Full article
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Review

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Open AccessReview In Situ Atomic Force Microscopy Studies on Nucleation and Self-Assembly of Biogenic and Bio-Inspired Materials
Minerals 2017, 7(9), 158; https://doi.org/10.3390/min7090158
Received: 30 May 2017 / Revised: 14 August 2017 / Accepted: 16 August 2017 / Published: 31 August 2017
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Abstract
Through billions of years of evolution, nature has been able to create highly sophisticated and ordered structures in living systems, including cells, cellular components and viruses. The formation of these structures involves nucleation and self-assembly, which are fundamental physical processes associated with the
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Through billions of years of evolution, nature has been able to create highly sophisticated and ordered structures in living systems, including cells, cellular components and viruses. The formation of these structures involves nucleation and self-assembly, which are fundamental physical processes associated with the formation of any ordered structure. It is important to understand how biogenic materials self-assemble into functional and highly ordered structures in order to determine the mechanisms of biological systems, as well as design and produce new classes of materials which are inspired by nature but equipped with better physiochemical properties for our purposes. An ideal tool for the study of nucleation and self-assembly is in situ atomic force microscopy (AFM), which has been widely used in this field and further developed for different applications in recent years. The main aim of this work is to review the latest contributions that have been reported on studies of nucleation and self-assembly of biogenic and bio-inspired materials using in situ AFM. We will address this topic by introducing the background of AFM, and discussing recent in situ AFM studies on nucleation and self-assembly of soft biogenic, soft bioinspired and hard materials. Full article
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Open AccessReview About the Genetic Mechanisms of Apatites: A Survey on the Methodological Approaches
Minerals 2017, 7(8), 139; https://doi.org/10.3390/min7080139
Received: 15 June 2017 / Revised: 21 July 2017 / Accepted: 1 August 2017 / Published: 5 August 2017
Cited by 1 | PDF Full-text (2615 KB) | HTML Full-text | XML Full-text
Abstract
Apatites are properly considered as a strategic material owing to the broad range of their practical uses, primarily biomedical but chemical, pharmaceutical, environmental and geological as well. The apatite group of minerals has been the subject of a huge number of papers, mainly
[...] Read more.
Apatites are properly considered as a strategic material owing to the broad range of their practical uses, primarily biomedical but chemical, pharmaceutical, environmental and geological as well. The apatite group of minerals has been the subject of a huge number of papers, mainly devoted to the mass crystallization of nanosized hydroxyapatite (or carboapatite) as a scaffold for osteoinduction purposes. Many wet and dry methods of synthesis have been proposed. The products have been characterized using various techniques, from the transmission electron microscopy to many spectroscopic methods like IR and Raman. The experimental approach usually found in literature allows getting tailor made micro- and nano- crystals ready to be used in a wide variety of fields. Despite the wide interest in synthesis and characterization, little attention has been paid to the relationships between bulk structure and corresponding surfaces and to the role plaid by surfaces on the mechanisms involved during the early stages of growth of apatites. In order to improve the understanding of their structure and chemical variability, close attention will be focused on the structural complexity of hydroxyapatite (HAp), on the richness of its surfaces and their role in the interaction with the precursor phases, and in growth kinetics and morphology. Full article
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Other

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Open AccessCommentary Polymorphs, Proteins, and Nucleation Theory: A Critical Analysis
Minerals 2017, 7(4), 62; https://doi.org/10.3390/min7040062
Received: 1 April 2017 / Revised: 15 April 2017 / Accepted: 18 April 2017 / Published: 21 April 2017
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Abstract
Over the last eight years new theories regarding nucleation, crystal growth, and polymorphism have emerged. Many of these theories were developed in response to observations in nature, where classical nucleation theory failed to account for amorphous mineral precursors, phases, and particle assembly processes
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Over the last eight years new theories regarding nucleation, crystal growth, and polymorphism have emerged. Many of these theories were developed in response to observations in nature, where classical nucleation theory failed to account for amorphous mineral precursors, phases, and particle assembly processes that are responsible for the formation of invertebrate mineralized skeletal elements, such as the mollusk shell nacre layer (aragonite polymorph) and the sea urchin spicule (calcite polymorph). Here, we summarize these existing nucleation theories and place them within the context of what we know about biomineralization proteins, which are likely participants in the management of mineral precursor formation, stabilization, and assembly into polymorphs. With few exceptions, much of the protein literature confirms that polymorph-specific proteins, such as those from mollusk shell nacre aragonite, can promote polymorph formation. However, past studies fail to provide important mechanistic insights into this process, owing to variations in techniques, methodologies, and the lack of standardization in mineral assay experimentation. We propose that the way forward past this roadblock is for the protein community to adopt standardized nucleation assays and approaches that are compatible with current and emerging nucleation precursor studies. This will allow cross-comparisons, kinetic observations, and hopefully provide the information that will explain how proteins manage polymorph formation and stabilization. Full article
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