Ostwald Step Rule Research Articles

A comprehensive approach to crystallization sequence in novel MgO/CaO-Al2O3-SiO2 glass-ceramics by Ostwald's step rule and composition fluctuation model

An understanding of the crystallization sequence (the formation order of crystalline phases) of glasses is crucial for the processing and composition design of novel glass-ceramics. A comprehensive approach to the crystallization sequence in novel MgO/CaO-Al2O3-SiO2 (MAS/CAS) glass-ceramics was performed from the Ostwald's step rule and the model of the distribution of nanoscale composition fluctuations (DNCF) for the first time. The stoichiometric cordierite Mg2Al4Si5O18 glass exhibits the crystallization sequence predicted by the Ostwald's step rule and a narrow DNCF model, i.e., metastable β-quartz solid solutions (β-QSSs) → high-temperature type indialite → low-temperature type α-cordierite. A broad DNCF model was proposed to present in osumilite Mg2Al4Si11O30 and pyrope Mg3Al2Si3O12 glasses, resulting in the initial formation of β-QSSs or forsterite 2MgO.SiO2 prior to α-cordierite formation. The crystallization sequence in stoichiometric anorthite CaAl2Si2O8 glass was also understood along the Ostwald's step rule and a narrow DNCF model. The importance of nanoscale composition fluctuations, in particular the formation of MgO/CaO-rich fragile regions, was emphasized in the crystallization of MAS and CAS-based glasses having strong Al-O-Si linkages between AlO4 and SiO4 tetrahedra. The present article will contribute to the comprehensive design of new innovative aluminosilicate-based glass-ceramics.

The thermodynamic analysis of physico-chemical processes of microarc oxidation of aluminium and titanium alloys

The paper presents the results of assessing the probability of the occurrenceof physicochemical reactions in the electrolyte and on the electrodes while forming oxide coatings on aluminium and titanium alloys usinga microarc method. Calculations have been performed with the help of the thermodynamic analysis using the Hess’s law and Ostwald step rule. The calculations are coherent with the research results.

Hydrothermal treatment of arsenic sulfide slag to immobilize arsenic into scorodite and recycle sulfur

Arsenic sulfide slag (ASS) is typically by-produced from arsenic-containing wastewater treatment. In this work, a novel hydrothermal treatment method with the assistance of Fe(NO3)3 (HT-Fe(NO3)3) was developed to detoxify ASS by transforming arsenic into scorodite and extracting sulfur in one step. After hydrothermal treatment, As(III) in ASS was oxidized and immobilized into the stable scorodite with a high As immobilization efficiency (~99%), and the toxicity leachability of arsenic-containing solid waste significantly reduced from 634.2 to 2.5 mg/L, well below the discharge standard of solid waste. Further study reveals that the nucleation and growth process was fit well by Avrami-Erofeev model and followed Ostwald step rule, which involved the As2S3 dissolution, formation of amorphous ferric arsenate and then crystallization within the amorphous precursor. In this process, sulfur originated from As2S3 played an important role by serving as the heterogeneous nuclei to decrease the barrier for the formation of amorphous ferric arsenate, and facilitated the transformation of as-formed scorodite from nano-sheet aggregates to the bulk and dense spherical polymorph, which further increased the stability of the arsenic contained solid product. This study will shed light on the development of new technologies for treatment of industrial solid waste and recycle of useful resources.

Tailored Hydrothermal Synthesis of Specific Facet-Dominated TiO2 Nanocrystals from Lepidocrocite-Type Layered Titanate Nanosheets: Systematical Investigation and Enhanced Photocatalytic Performance

A series of samples including leaf-like and rod-like rutile TiO2 nanoparticles with various facets exposed on the surface, parallelepiped-shaped anatase nanoparticles with [111] vertical facet exposed on the surface, irregular anatase nanoparticles, microsized six-point star-like anatase aggregates, and almond-like brookite aggregates had been hydrothermally synthesized from lepidocrocite-type layered titanate nanosheets. A systematical investigation was established to uncover the phase transition and morphological evolution from nanosheets to TiO2 polymorphs, and a phase diagram was determined by adjusting the synthesis parameters of the pH value and temperature. Two kinds of mechanisms composed of the dissolution-deposition process following Ostwald's ripening mechanism and the in situ topochemical conversion process following Ostwald's step rule had been proposed based on the time-dependent hydrothermal experiments. Briefly, the formation of the single-crystalline rutile phase appeared only at high temperatures with very low pH values, and similarly, the brookite phase strictly formed at high temperatures with a very high pH value. Nevertheless, the anatase phase could moderately appear in a wide range of temperatures and pH values. In addition, the single-crystalline rutile adopted a leaf-like morphology at low temperatures with high pH values and a rod-like morphology at high temperatures with low pH values, while the morphological evolution of anatase particles proceeded from irregular to parallelepiped-shaped and finally to six-point star-like morphology, and the crystal size was reduced from 1000 to 5 nm with decreasing pH values. Meanwhile, with the prolongation of the hydrothermal time, the layered titanate nanosheets first dissolved into the amorphous state and further converted into small anatase nanoparticles and finally to rutile or anatase nanoparticles based on the dissolution-deposition process, or the {010}-faceted layered titanate structure first converted into the [111]-vertical faceted anatase nanosheets by the topochemical transformation reaction and then split into the [111]-vertical faceted anatase nanoparticles. More importantly, the mesoporous [111]-vertical faceted anatase nanoparticles exhibited enhanced photocatalytic performance compared to that of Degussa P25, which was ascribed to its superior electronic band structure and effective charge separation. The systematical investigation in this work would be significant for consummating the preparation of the TiO2 polymorphs from layered titanate nanosheets and provided some reference values and guide schemes for the preparation of TiO2 nanoparticles with outstanding photocatalytic performance.

Processes of metastable-mineral formation in oxidation zones and mine waste

AbstractOxidation zones and mine wastes are metal-rich, near-surface environments, natural and man-made critical zones of ore deposits, respectively. They contain a number of minerals which, despite their metastability, occur consistently and in abundance. Field studies, presented as examples in this work, show that metastable minerals form not only directly from aqueous solutions, but also from more complex precursors, such as nanoparticles, gels, X-ray amorphous solids, or clusters. Initial precipitation of metastable phases and their conversion to stable phases is described by the Ostwald's step rule. Thermodynamic data show that there is a tendency, but no rule, that structurally more complex phases are also thermodynamically more stable. The Ostwald's step rule could then state that the initial metastable phases are structurally simple and easily assembled from aqueous solutions, nanoparticles, gels, disordered solids, or clusters. The structural similarity of the precursor and the forming phase is a kinetic factor favouring the crystallisation of the new phase. Calculation of saturation indices for mine drainage solutions show that they are mostly supersaturated with respect to the stable phases and the aqueous concentrations are sufficient to precipitate metastable minerals. In our fieldwork, we often encounter gelatinous substances with copper, manganese or tungsten that slowly convert to metastable oxysalt minerals. Another possibility is the crystallisation of various metastable minerals from solid, homogeneous ‘resins’ that are X-ray amorphous. Minerals typical for near-surface environments may be stabilised by their surface energy at high specific surface areas. For example, ferrihydrite is often described as a metastable phase but can be shown to be stable with respect to nanosised hematite.

Atomic-level mechanism for isothermal crystallization in supercooled liquid tantalum

The morphology and physical properties of crystal as well as the glass-forming ability (GFA) of metals are closely related to the evolution pathway of atomic structures in the early stage of nucleation in supercooled liquids. Therefore, the study of the evolution of atomic structures in the isothermal crystallization process of supercooled liquids, is of great significance not only for predicting and accurately controlling the crystal nucleation and growth, but also for understanding the local atomic structural origin of the GFA. In the present work, the atomic-level mechanism for isothermal crystallization in the supercooled liquid tantalum is studied by molecular dynamics (MD) simulation. The microstructural evolution of metal Ta system is characterized and analyzed by using the potential energy per atom (<i>PE</i>), the pair distribution function (PDF) g(r), and the largest standard cluster (LSC). Two crystallization paths of Ta supercooled liquid can be observed during isothermal relaxations. For each pathway the incubation time of the formation critical nucleus increases with annealing temperature (<i>T</i>) rising. At 1800 K ≤ <i>T</i> ≤ 1850 K, the crystallization of supercooled liquid Ta conforms to the Ostwald's step rule: first, Z12 (i.e. icosahedron) and Z14 (Kasper cluster with 14 coordination number) clusters in supercooled liquids are hinged into medium-range order (i.e., Z-MRO); then the Z-MRO are merged and ordered into A15 crystal phase; finally, BCC crystal nucleus inside of the A15 phase grows rapidly into BCC single crystal at the cost of the atoms in A15 phase. While at 1900 K ≤ <i>T</i> ≤ 1950 K, Ta supercooled liquid is directly transformed into A15 phase. The A15 crystal phase is mainly formed by the continuous merging of the largest Z-MRO with the small Z-MRO, which is similar to the picture of the classical nucleation theory (CNT). However, whether the phase transition from A15 to BCC will occur above 1900 K remains to be further confirmed by a longer-time MD simulation. Relative to the supercooled liquids of monoatomic metals with lower melting point, the good GFA of Ta may originate from the slowly growing A15 crystal nucleus in its supercooled liquid.

Nucleation and Growth of Amino Acid and Peptide Supramolecular Polymers through Liquid-Liquid Phase Separation.

The transition of peptides and proteins from the solution phase into fibrillar structures is a general phenomenon encountered in functional and aberrant biology and is increasingly exploited in soft materials science. However, the fundamental molecular events underpinning the early stages of their assembly and subsequent growth have remained challenging to elucidate. Here, we show that liquid-liquid phase separation into solute-rich and solute-poor phases is a fundamental step leading to the nucleation of supramolecular nanofibrils from molecular building blocks, including peptides and even amphiphilic amino acids. The solute-rich liquid droplets act as nucleation sites, allowing the formation of thermodynamically favorable nanofibrils following Ostwald's step rule. The transition from solution to liquid droplets is entropy driven while the transition from liquid droplets to nanofibrils is mediated by enthalpic interactions and characterized by structural reorganization. These findings shed light on how the nucleation barrier toward the formation of solid phases can be lowered through a kinetic mechanism which proceeds through a metastable liquid phase.

Nucleation and Growth of Amino Acid and Peptide Supramolecular Polymers through Liquid–Liquid Phase Separation

AbstractThe transition of peptides and proteins from the solution phase into fibrillar structures is a general phenomenon encountered in functional and aberrant biology and is increasingly exploited in soft materials science. However, the fundamental molecular events underpinning the early stages of their assembly and subsequent growth have remained challenging to elucidate. Here, we show that liquid–liquid phase separation into solute‐rich and solute‐poor phases is a fundamental step leading to the nucleation of supramolecular nanofibrils from molecular building blocks, including peptides and even amphiphilic amino acids. The solute‐rich liquid droplets act as nucleation sites, allowing the formation of thermodynamically favorable nanofibrils following Ostwald's step rule. The transition from solution to liquid droplets is entropy driven while the transition from liquid droplets to nanofibrils is mediated by enthalpic interactions and characterized by structural reorganization. These findings shed light on how the nucleation barrier toward the formation of solid phases can be lowered through a kinetic mechanism which proceeds through a metastable liquid phase.

One-Pot Formation of Sb-Carbon Microspheres with Graphene Sheets: Potassium-Ion Storage Properties and Discharge Mechanisms.

Low-cost rechargeable batteries are ardently required for large-scale energy storage applications. In this regard, nonaqueous potassium-ion batteries (KIBs) are ascendant candidates due to the abundance of potassium resources, yet their energy density and cycle stability are insufficient for practical use. In this study, we report the Sb-based multicomposite comprising Sb nanoparticles, amorphous carbon (C), and reduced graphene oxide (rGO) as an anode material for KIBs. By adopting the tartaric acid as a carbon source and a chelating agent simultaneously, a multicomposite electrode with uniform and fine-sized Sb particles is realized. The Sb-C-rGO multicomposite exhibits a reversible capacity of 310 mAh g-1 at 0.5 A g-1 and 79% of it is retained after 100 cycles. Electrochemical tests show that the capacity fading in the Sb-C-rGO cell is attributed to the side reactions in the K metal and electrolyte, rather than the degradation of Sb nanoparticles. Furthermore, the formation of the metastable product is elucidated by Ostwald's step rule and density functional theory calculations. The present synthesis approach and the understanding of the failure and working mechanisms provide general insight into developing the alloying-type electrode materials for rechargeable batteries.

Mn(II) oxidation catalyzed by nanohematite surfaces and manganite/hausmannite core-shell nanowire formation by self-catalytic reaction

Abstract The present study investigated the solid products of heterogeneous catalytic oxidation of aqueous Mn(II) by O2 in the presence of hematite nanoparticles (NPs) using transmission electron microscopy (TEM), scanning TEM (STEM), and electron energy-loss spectroscopy (EELS). The oxidation experiments were conducted at room temperature (22 ± 2 °C) in solutions containing 10−3 M Mn(II) at pH 7.5 in the dark. During 48 h of reaction, a single-phase manganite (a Mn oxyhydroxide) nanowire was formed by way of metastable groutite and feitknechtite nanowires, both of which are polymorphs of manganite. Between 48 and 168 h, the manganite further altered to core-shell structured nanowires with hausmannite (a mixed valent Mn oxide) forming a very thin (2 nm) outer-shell on the manganite wire core. The formation of Mn-oxyhydroxide nanowires was catalyzed by the hematite NPs surface through electrochemical pathways. This is described via the electron transfer across a redox couple from adsorbed Mn(II) to another site with adsorbed O2 via the band structure of the semiconducting hematite. On the other hand, the Ostwald step rule was operative in the sequential crystallization from metastable groutite and feitknechtite to stable manganite, which could have resulted from the differences in the surface free energies, sizes, and morphologies of product polymorphs at the nanoscale. The final product (at least in the time range of our experiments) exhibited manganite/hausmannite core-shell structures. This product is most likely to be formed by the reactions catalyzed by the manganite nanowires themselves. Microscopic investigation of interfacial changes in structure and chemistry at the nanoscale is key, and it can be essential to understand the kinetic and thermodynamic phenomena related to the redox chemistry and phase stability of nanoparticles in complex, heterogeneous systems such as natural environments.

Phase transitions in fluctuations and their role in two-step nucleation.

We consider the thermodynamic behavior of local fluctuations occurring in a stable or metastable bulk phase. For a system with three or more phases, we present a simple analysis based on classical nucleation theory that predicts thermodynamic conditions at which small fluctuations resemble the phase having the lowest surface tension with the surrounding bulk phase, even if this phase does not have a lower chemical potential. We also identify the conditions at which a fluctuation may convert to a different phase as its size increases, referred to here as a "fluctuation phase transition" (FPT). We demonstrate these phenomena in simulations of a two dimensional lattice model by evaluating the free energy surface that describes the thermodynamic properties of a fluctuation as a function of its size and phase composition. We show that a FPT can occur in the fluctuations of either a stable or metastable bulk phase and that the transition is first-order. We also find that the FPT is bracketed by well-defined spinodals, which place limits on the size of fluctuations of distinct phases. Furthermore, when the FPT occurs in a metastable bulk phase, we show that the superposition of the FPT on the nucleation process results in two-step nucleation (TSN). We identify distinct regimes of TSN based on the nucleation pathway in the free energy surface and correlate these regimes to the phase diagram of the bulk system. Our results clarify the origin of TSN and elucidate a wide variety of phenomena associated with TSN, including the Ostwald step rule.

Molecular dynamics investigation of the local structure in iron melts and its role in crystal nucleation during rapid solidification.

A comprehensive investigation on local structures in iron melts and their role in nucleation under various cooling rates was performed by means of large-scale molecular dynamics simulations. The embedded atoms method (EAM) was adopted to describe the interactions between iron atoms. Connections between short-range order (SRO), medium-range order (MRO), and crystalline nucleation from iron melts were constructed using several structural analysis techniques, including the radial distribution function, common neighbor analysis method, the Voronoi tessellation, and bond order analysis. The simulation results showed that abundant types of atomic clusters with SRO, mainly including the icosahedral-like (ICO-like) and fcc-like clusters, were predominant in undercooled iron melts. The obtained microstructures were determined by the competition between the ICO-like and crystal-like configurations. There existed a critical cooling rate, below which the fcc-like configurations gain the advantage upon cooling and where crystallization could take place; otherwise, the ICO-like configurations are favored and the glass phases could be obtained. Furthermore, it was proved that the crystal nucleation could be divided into three stages: first, a fluctuation and competition between crystal-like and ICO-like clusters in undercooled melts; second, the formation and growth of MRO clusters via the transformation of atomic configurations from ICO-like to crystal-like; finally, the nucleation of bcc nuclei from the core of steady MRO clusters. This process agrees with the Ostwald's step rule and the findings from other investigations. Based on the analysis of the compositional origin of MRO clusters, we further found that the MRO clusters were mainly composed of fcc-like instead of ICO-like configurations, indicating a negative role of ICO-like configurations in crystal nucleation.

Thermodynamic and Kinetic Origins of Ferroelectricity in Fluorite Structure Oxides

AbstractFerroelectricity in fluorite structure oxides such as HfO2 and ZrO2 has been intensively studied since the first report on it in 2011. The ferroelectricity in this material system is induced by the formation of a non‐centrosymmetric orthorhombic phase, which is not thermodynamically stable under the normal thin‐film processing conditions. Therefore, the thermodynamic and kinetic origins of the formation of the ferroelectric phase have yet to be clearly elucidated. Here, the previously proposed thermodynamic model based on the surface/interface/grain boundary energy argument is critically reviewed, and it is concluded that the thermodynamic model could not account for the emergence of the metastable phase accurately. Subsequently, the probable kinetic paths of phase evolution during the crystallization annealing and cooling processes are reassessed. A phase transition in Hf1−xZrxO2 thin films can be considered a feasible example of the well‐known Ostwald's step rule involving a two‐step phase transition with different kinetic energy barriers. It is concluded that the emergence of the ferroelectric orthorhombic phase can be mainly attributed to a kinetic origin: the transition to a stable nonferroelectric monoclinic phase either from the tetragonal or orthorhombic phase, which must be initially formed during the crystallization anneal, is kinetically suppressed.

Crystal nucleation and metastable bcc phase in charged colloids: A molecular dynamics study.

The dynamic process of homogenous nucleation in charged colloids is investigated by brute-force molecular dynamics simulation. To check if the liquid-solid transition will pass through metastable bcc, simulations are performed at the state points that definitely lie in the phase region of thermodynamically stable fcc. The simulation results confirm that, in all of these cases, the preordered precursors, acting as the seeds of nucleation, always have predominant bcc symmetry consistent with Ostwald's step rule and the Alexander-McTague mechanism. However, the polymorph selection is not straightforward because the crystal structures formed are not often determined by the symmetry of intermediate precursors but have different characters under different state points. The region of the state point where bcc crystal structures of large enough size are formed during crystallization is narrow, which gives a reasonable explanation as to why the metastable bcc phase in charged colloidal suspensions is rarely detected in macroscopic experiments.

Polymorph formation for a zeolitic imidazolate framework composition - Zn(Im)2

We study the effect of synthesis time and temperature on the crystal formation, morphology and size of zeolitic imidazolate frameworks (ZIFs) with the Zn(Im)2 composition by using the solution mixing method. The crystal structure, morphology and thermodynamic properties of the ZIFs were characterized by powder X-ray diffraction, scanning electron microscopy and nitrogen sorption isotherms. Our results indicate that the synthesis time significantly influences the structure and topology of crystal products. The crystal structure transforms from ZIF-coi to the progressively denser polymorphs (ZIF-zec, ZIF-nog and ZIF-zni) upon increasing synthesis time from 18 to 120 hours at 10 °C, in accordance with Ostwald's step rule. Increasing synthesis temperature does not change the formation of the ZIF-zec crystals but affects their morphologies and porosities. Both ZIF-nog and ZIF-zec exhibit relatively large surface areas (>500 m2/g). Furthermore, heating ZIF-nog and ZIF-zec causes recrystallization to ZIF-zni and subsequent melting. Quenching the MOF-liquid results in Zn(Im)2 glass. This work helps to understand and control the crystal formation of ZIFs, and reveals two new MOF glass formers.

Forward flux sampling calculation of homogeneous nucleation rates from aqueous NaCl solutions.

We used molecular dynamics simulations and the path sampling technique known as forward flux sampling to study homogeneous nucleation of NaCl crystals from supersaturated aqueous solutions at 298 K and 1 bar. Nucleation rates were obtained for a range of salt concentrations for the Joung-Cheatham NaCl force field combined with the Extended Simple Point Charge (SPC/E) water model. The calculated nucleation rates are significantly lower than the available experimental measurements. The estimates for the nucleation rates in this work do not rely on classical nucleation theory, but the pathways observed in the simulations suggest that the nucleation process is better described by classical nucleation theory than an alternative interpretation based on Ostwald's step rule, in contrast to some prior simulations of related models. In addition to the size of NaCl nucleus, we find that the crystallinity of a nascent cluster plays an important role in the nucleation process. Nuclei with high crystallinity were found to have higher growth probability and longer lifetimes, possibly because they are less exposed to hydration water.

Nucleation and growth of a core-shell composite nucleus by diffusion.

The critical radius of a core-shell-type nucleus grown by diffusion in a phase-separated solution is studied. A kinetic critical radius rather than the thermodynamic critical radius of standard classical nucleation theory can be defined from the diffusional growth equations. It is shown that there exist two kinetic critical radii for the core-shell-type nucleus, for which both the inner-core radius and the outer-shell radius will be stationary. Therefore, these two critical radii correspond to a single critical point of the nucleation path with a single energy barrier even though the nucleation looks like a two-step process. The two radii are given by formulas similar to that of classical nucleation theory if the Ostwald-Freundlich boundary condition is imposed at the surface of the inner nucleus and that of the outer shell. The subsequent growth of a core-shell-type postcritical nucleus follows the classical picture of Ostwald's step rule. Our result is consistent with some of the experimental and numerical results which suggest the core-shell-type critical nucleus.

Peculiarities in thermal evolution of precipitated amorphous calcium phosphates with an initial Ca/P ratio of 1:1.

Thermal evolution of amorphous calcium phosphate (ACP) powder from a fast nitrate synthesis with a Ca/P ratio of 1:1 were studied in the range of 20-980 °C. The powder consisted of amorphous dicalcium phosphate anhydrate (CaHPO4) after heating to 200 °C. CaHPO4 gradually condensed to amorphous calcium pyrophosphate Ca2P2O7 (CPP) between 200 to 620 °C. Amorphous CPP crystallized at 620-740 °C to a metastable polymorph α'-CPP of the high-temperature phase α-CPP and β-CPP. The α'-CPP/ β-CPP phase ratio reached a maximum at 800 °C (60 wt% α'-CPP/40 wt% β-CPP), and α'-CPP gradually transformed to β-CPP at a higher temperature. Some β-TCP occurred at 900 °C, so that a three-phasic mixture was obtained in the powder heated to 980 °C. The occurrence of metastable α'-CPP is attributed to Ostwald's step rule, and a mechanism for β-TCP formation is proposed. The advantages of prospective biomaterials from these powders are discussed.

Phase evolution during heat treatment of amorphous calcium phosphate derived from fast nitrate synthesis

The phase evolution in amorphous calcium phosphate (ACP, with a Ca/P ratio of 1 : 1), derived from the fast nitrate synthesis using different conditions, was studied in temperature range 20-980?C. ACP crystallized within 600-700?C and the phase composition depended on the synthesis duration. It was firstly revealed that for an extremely short synthesis (1min) two metastable phases ??-CPP and ??-TCP of the high-temperature calcium pyrophosphate ?-CPP and tricalcium phosphate ?-TCP were crystallized. For a longer synthesis (5min), ??- CPP and minor ?-CPP crystallized. The metastable phases gradually transformed to stable polymorphs ?-CPP and ?-TCP above 800?C, and a biphasic mixture ?-CPP/?-TCP or ?-CPP formed at 980?C. The crystallization of the metastable phases was attributed to the Ostwald step rule. A mechanism for the formation of TCP (Ca/P = 1.5) from ACP (Ca/P = 1) was proposed. The prepared powders of ?-CPP/?-TCP, ?-CPP or initial ACP were fine-grained and would have enhanced sinterability. Contribution to the densification was demonstrated due to the thermal transformation of the metastable polymorphs into stable phases having higher densities.

Nonclassical crystallization in vivo et in vitro (II): Nanogranular features in biomimetic minerals disclose a general colloid-mediated crystal growth mechanism.

Recent research has shown that biominerals and their biomimetics (i) typically form via an amorphous precursor phase, and (ii) commonly display a nanogranular texture. Apparently, these two key features are closely related, underlining the fact that the formation of biominerals and their biomimetics does not necessarily follow classical crystallization routes, and leaves a characteristic nanotextural imprint which may help to disclose their origins and formation mechanisms. Here we present a general overview of the current theories and models of nonclassical crystallization and their applicability for the advance of our current understanding of biomineralization and biomimetic mineralization. We pay particular attention to the link between nonclassical crystallization routes and the resulting nanogranular textures of biomimetic CaCO3 mineral structures. After a general introductory section, we present an overview of classical nucleation and crystal growth theories and their limitations. Then, we introduce the Ostwald's step rule as a general framework to explain nonclassical crystallization. Subsequently, we describe nonclassical crystallization routes involving stable prenucleation clusters, dense liquid and solid amorphous precursor phases, as well as current nonclassical crystal growth models. The latter include oriented attachment, mesocrystallization and the new model based on the colloidal growth of crystals via attachment of amorphous nanoparticles. Biomimetic examples of nanostructured CaCO3 minerals formed via these nonclassical routes are presented which help us to show that colloid-mediated crystal growth can be regarded as a wide-spread growth mechanism. Implications of these observations for the advance in the current understanding on the formation of biomimetic materials and biominerals are finally outlined.