Phase transformation of Mg-calcite to aragonite in active-forming hot spring travertines

HFG would like to thank the University of St Andrews for the studentship and Mr Ross Blackley for his help on using the SEM and TEM microscopes. WZZ thanks EPSRC for financial support on FEG-SEM equipment (EP/F019580/1) and a Platform (EP/K015540/1).A travertine specimen collected from the western part of Yunnan Province of China was subjected to microstructural analysis by powder X-ray diffraction, scanning electron microscopy, high resolution transmission electron microscopy and energy dispersive X-ray spectroscopy. A new formation mechanism was proposed whereby polycrystalline rhombohedral particles of magnesium-containing calcite underwent a phase transformation into sheaf-like clusters of aragonite microrods. It is proposed that a high concentration of magnesium ions and embedded biological matter poisoned the growth of calcite and therefore instigated the phase transformation of the core of the rhombohedral calcite particles to an aragonite phase with a higher crystallinity. The single crystalline aragonite microrods with a higher density than the Mg-calcite nanocrystallites grew at the expense of the latter to generate sheaf-like clusters. This newly discovered formation mechanism is expected to enhance previous knowledge on this geologically important phase transformation from a morphology point of view.PostprintPeer reviewe

Reversed crystal growth of calcite in naturally occurring travertine crust

WZZ thanks EPSRC for 510 financial support on FEG-SEM equipment (EP/F019580/1) and a Platform (EP/K015540/1).A microstructural investigation by electron microscopy on a travertine specimen collected from Munigou National Park, Sichuan Province, China revealed evidence of a non-classical reversed crystal growth route previously only discovered in synthetic materials. Examination of the travertine specimen suggests that the presence of organic matter initiates the oriented aggregation of calcite nanocrystallites. Surface re-crystallisation of the aggregates leads to a single crystalline rhombohedral shell with a polycrystalline core. This core-shell structure carries a strong resemblance to synthetic calcite prepared in the presence of chitosan, where the growth of calcite was found to follow the so-called reversed crystal growth process. It is proposed that the similar roles of biomolecules in naturally occurring travertine and chitosan in the synthetic system are based on their isoelectric points and the polymerizable property of long chain chemical structures. This study is important so that the structural similarities between naturally occurring biominerals and biomimetic materials can be further understood.Publisher PDFPeer reviewe

Dipole field driven morphology evolution in biomimetic vaterite

Morphology evolution is an important process in naturally occurring biominerals. To investigate the interaction between biomolecules and inorganic components in the construction of biominerals, biomimetic hexagonal prism vaterite crystals were hydrothermally prepared through a reaction of urea with calcium nitrate tetrahydrate, whilst gelatin was added as a structure directing agent. An extraordinary morphology evolution was observed. The time dependent growth was investigated by using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and thermogravimetric analysis. In the early stages, vaterite nanocrystallites, ~5 nm in diameter, underwent aggregation with gelatin molecules and precursor molecules into 50 nm sized clusters. Some nanoneedles, consisting of self-orientated nanocrystallites embedded within a soft gelatin matrix, were developed on the surface of disordered cores to form spherulite particles, with a similar morphology to natural spherulite biominerals. Further growth was affected by the high viscosity of gelatin, resulting in ellipsoid particles composed of spherulitically ordered needles. It is proposed that surface adsorbed gelatin induces the formation of dipoles in the nanocrystallites and interaction between the dipoles is the driving force of the alignment of the nanocrystallites. Further growth might create a relatively strong and mirror-symmetric dipolar field, followed by a morphology change from ellipsoidal with a cell-division like splitting, to twin-cauliflower, dumbbell, cylindrical and finally to hexagonal prism particles. In this morphology evolution, the alignment of the crystallites changes from 1D linear manner (single crystal like) to 3D radial pattern, and finally to mirror symmetric 1D linear manner. This newly proposed mechanism sheds light on the microstructural evolution in many biomimetic materials and biominerals.PostprintPeer reviewe

Formation, crystal growth and colour appearance of Mimetic Tianmu glaze

WZZ thanks EPSRC for financial support on FEG-SEM equipment (EP/F019580/1).Mimetic Tianmu glaze has been synthesized and analysed by using X-ray diffraction, energy-dispersive X-ray spectroscopy, scanning electron microscopy and transmission electron microscopy. It was found that the main body of the glaze was amorphous aluminium silicate with many embedded polycrystalline spherical particles of metal oxides containing manganese, cobalt, vanadium, bismuth and tungsten. Two dimensional spinel dendrites crystals of manganese, cobalt and aluminium oxide formed on the surface of the glaze. The formation mechanism of the microstructures in the Tianmu glaze is proposed. The colour appearance of the glaze has also been discussed. It has been found that the crystal thickness dependant light interference could be an important factor for the appearance of rainbow-like colour in the glaze layer.PostprintPeer reviewe

Growth mechanism of dendritic hematite via hydrolysis of ferricyanide

W.Z. thanks EPSRC for financial support to purchase the FEG SEM (EP/F019580/1).The detailed process of the hydrolysis of ferricyanide into dendritic α-Fe2O3 (hematite) crystals with snowflake-like, feather-like and leaf-like morphologies has been investigated. [Fe(CN)6]3– anions were found to polymerize into large, disordered soft matter aggregates at an early stage. The nucleation of hematite crystals took place near the surface of these aggregates via further hydrolysis. After the crystals grew to a certain size, branches started to appear. When the concentration of ferricyanide was low (i.e. 2 mM to 3.8 mM), growth was preferentially along the six equivalent directions, resulting in a flat snowflake-like shape, while high concentrations (i.e. 9 mM to 500 mM) of ferricyanide led to the growth of selective directions along the zone axes, forming a feather-like or leaf-like morphology. Highly selective adsorption and surface hydrolysis of [Fe(CN)6]3– anions on α-Fe2O3 crystals was found to be a crucial process in the formation of these novel morphologies. It was found that the polymeri- sation of ferricyanide led to a reduction of pH value and that the formation of Fe2O3 increased pH value. The pH value of the solution at the point when the branches start to grow can significantly affect the distribution of Lewis acidic sites on different surfaces and, therefore, change the growth direction. The newly established mechanism is complementary to the classical theories of crystal growth.PostprintPeer reviewe

International genome-wide meta-analysis identifies new primary biliary cirrhosis risk loci and targetable pathogenic pathways.

Primary biliary cirrhosis (PBC) is a classical autoimmune liver disease for which effective immunomodulatory therapy is lacking. Here we perform meta-analyses of discovery data sets from genome-wide association studies of European subjects (n=2,764 cases and 10,475 controls) followed by validation genotyping in an independent cohort (n=3,716 cases and 4,261 controls). We discover and validate six previously unknown risk loci for PBC (Pcombined<5 × 10(-8)) and used pathway analysis to identify JAK-STAT/IL12/IL27 signalling and cytokine-cytokine pathways, for which relevant therapies exist

What can electron microscopy tell us beyond crystal structures?

WZ thanks Professor Jun Yuan for a useful discussion during the revision of this article. The authors wish to thank EPSRC for financial support to the electron microscopy facility (No. EP/F019580/1) and a Platform grant (No. EP/K015540/1). Date of Acceptance: 08/01/2016Transmission electron microscopy is a powerful tool to directly image crystal structures. Not only that, it is often used to reveal crystal size and morphology, crystal orientation, crystal defects, surface structures, superstructures, etc. However, due to the 2D nature of TEM images, it is easy to make mistakes when we try to recover a 3D structure from them. Scanning electron microscopy is able to provide information on the particle size, morphology and surface topography. However, obtaining information on crystallinity of particles using SEM is difficult. In this microreview article, some practical cases of transmission and scanning electron microscopy investigations of inorganic crystals are reviewed. Commonly occurring uncertainties, imperfection and misunderstandings are discussed.Publisher PDFPeer reviewe

Non-classical crystal growth of inorganic and organic materials

Investigation of the early stage morphologies of some organic and inorganic systems has established that their crystal growth does not always follow the classical route established 120 years ago. The classical theory assumes that a highly symmetric crystal develops from the nucleation and repeated attachment of atoms, molecules or ions to a single nucleus. In instances where surfactants or polymers were introduced the crystal growth was often found to follow a reversed crystal growth route. This non-classical growth route was initiated by organic molecules adsorbed onto the surface of the nanocrystallites in early growth stages. In the reversed route these nanocrystallites undergo aggregation into large disordered aggregates followed by surface crystallisation to create a thin highly crystalline and dense shell and finally an extension of the crystallisation from the surface to the core. Although systems following a non-classical crystal growth route have been known for many years their formation mechanisms have a much shorter history. Discovery of the reversed crystal growth route in several hollow, core shell and twin-crystal type morphologies not only assists in the designing of materials for industrial applications but also helps to understand the interactions between organic and inorganic components. This article highlights some recent research by featuring some organic and inorganic systems whose growth follows the reversed crystal growth route

Non-classical crystal growth of inorganic and organic materials

Investigation of the early stage morphologies of some organic and inorganic systems has established that their crystal growth does not always follow the classical route established 120 years ago. The classical theory assumes that a highly symmetric crystal develops from the nucleation and repeated attachment of atoms, molecules or ions to a single nucleus. In instances where surfactants or polymers were introduced the crystal growth was often found to follow a reversed crystal growth route. This non-classical growth route was initiated by organic molecules adsorbed onto the surface of the nanocrystallites in early growth stages. In the reversed route these nanocrystallites undergo aggregation into large disordered aggregates followed by surface crystallisation to create a thin highly crystalline and dense shell and finally an extension of the crystallisation from the surface to the core. Although systems following a non-classical crystal growth route have been known for many years their formation mechanisms have a much shorter history. Discovery of the reversed crystal growth route in several hollow, core shell and twin-crystal type morphologies not only assists in the designing of materials for industrial applications but also helps to understand the interactions between organic and inorganic components. This article highlights some recent research by featuring some organic and inorganic systems whose growth follows the reversed crystal growth route