Seeds of imperfection rule the mesocrystalline disorder in natural anhydrite single crystals.

In recent years, we have come to appreciate the astounding intricacies associated with the formation of minerals from ions in aqueous solutions. In this context, a number of studies have revealed that the nucleation of calcium sulfate systems occurs nonclassically, involving the aggregation and reorganization of nanosized prenucleation species. In recent work, we have shown that this particle-mediated nucleation pathway is actually imprinted in the resultant micrometer-sized CaSO4 crystals. This property of CaSO4 minerals provides us with the unique opportunity to search for evidence of nonclassical nucleation pathways in geological environments. In particular, we focused on large anhydrite crystals extracted from the Naica Mine in Mexico. We were able to shed light on this mineral's growth history by mapping defects at different length scales. Based on this, we argue that the nanoscale misalignment of the structural subunits, observed in the initial calcium sulfate crystal seeds, propagates through different length scales both in morphological, as well as in strictly crystallographic aspects, eventually causing the formation of large mesostructured single crystals of anhydrite. Hence, the nonclassical nucleation mechanism introduces a "seed of imperfection," which leads to a macroscopic "single" crystal whose fragments do not fit together at different length scales in a self-similar manner. Consequently, anisotropic voids of various sizes are formed with very well-defined walls/edges. However, at the same time, the material retains in part its single crystal nature.

Metallic lead nanospheres discovered in ancient zircons.

Zircon (ZrSiO4) is the most commonly used geochronometer, preserving age and geochemical information through a wide range of geological processes. However, zircon U-Pb geochronology can be affected by redistribution of radiogenic Pb, which is incompatible in the crystal structure. This phenomenon is particularly common in zircon that has experienced ultra-high temperature metamorphism, where ion imaging has revealed submicrometer domains that are sufficiently heterogeneously distributed to severely perturb ages, in some cases yielding apparent Hadean (>4 Ga) ages from younger zircons. Documenting the composition and mineralogy of these Pb-enriched domains is essential for understanding the processes of Pb redistribution in zircon and its effects on geochronology. Using high-resolution scanning transmission electron microscopy, we show that Pb-rich domains previously identified in zircons from East Antarctic granulites are 5-30 nm nanospheres of metallic Pb. They are randomly distributed with respect to zircon crystallinity, and their association with a Ti- and Al-rich silica melt suggests that they represent melt inclusions generated during ultra-high temperature metamorphism. Metallic Pb is exceedingly rare in nature and previously has not been reported in association with high-grade metamorphism. Formation of these metallic nanospheres within annealed zircon effectively halts the loss of radiogenic Pb from zircon. Both the redistribution and phase separation of radiogenic Pb in this manner can compromise the precision and accuracy of U-Pb ages obtained by high spatial resolution methods.

Enzymatic phosphatization of fish scales-a pathway for fish fossilization.

Phosphatized fish fossils occur in various locations worldwide. Although these fossils have been intensively studied over the past decades they remain a matter of ongoing research. The mechanism of the permineralization reaction itself remains still debated in the community. The mineralization in apatite of a whole fish requires a substantial amount of phosphate which is scarce in seawater, so the origin of the excess is unknown. Previous research has shown that alkaline phosphatase, a ubiquitous enzyme, can increase the phosphate content in vitro in a medium to the degree of saturation concerning apatite. We applied this principle to an experimental setup where fish scales were exposed to commercial bovine alkaline phosphatase. We analyzed the samples with SEM and TEM and found that apatite crystals had formed on the remaining soft tissue. A comparison of these newly formed apatite crystals with fish fossils from the Solnhofen and Santana fossil deposits showed striking similarities. Both are made up of almost identically sized and shaped nano-apatites. This suggests a common formation process: the spontaneous precipitation from an oversaturated solution. The excess activity of alkaline phosphatase could explain that effect. Therefore, our findings could provide insight into the formation of well-preserved fossils.

Metallic lead (Pb) nanospheres discovered in Hadean and Eoarchean zircon crystals at Jack Hills.

Here, we report small randomly-distributed crystalline lead (Pb) nanospheres occurring in detrital zircon grains obtained from a weakly metamorphosed Archean conglomerate at Jack Hills, Western Australia, making this the third known global example of this phenomenon. They form in zircon crystals ranging from Hadean (> 4 billion years-Ga) to Eoarchean (> 3.6 Ga) in age, but are absent from Paleoarchean (~ 3.4 Ga) crystals. Unlike previous discoveries of nanospheres in zircon from Precambrian gneisses in Antarctica and India, detrital zircon from Jack Hills shows no evidence of ever undergoing ultra-high temperature (UHT) metamorphism, either before or after deposition, therefore implying that nanospheres can form at temperatures lower than ca. 900 °C. The nanospheres are composed of radiogenic Pb released by the breakdown of uranium (U) and thorium (Th) and are present in zircon irrespective of its U, Th and water contents, its oxygen isotopic composition, and the degree of discordance due to Pb loss or gain. The nanospheres pre-date annealed cracks in the crystals, showing that, once formed, they effectively 'freeze' radiogenic Pb in the zircon structure, precluding any further interaction during subsequent geological processes. Both Pb nanoclusters and nanospheres are now reported from Jack Hills, and it appears likely the former is a precursor stage in the formation of the latter. Although the precise mechanism for this transition remains unresolved, a later thermal event is required, but this likely did not reach UHT conditions at Jack Hills.

Magmatic immiscibility and the origin of magnetite-(apatite) iron deposits.

The origin of magnetite-(apatite) iron deposits (MtAp) is one of the most contentious issues in ore geology with competing models that range from hydrothermal to magmatic processes. Here we report melt inclusions trapped in plagioclase phenocrysts in andesite hosting the MtAp mineralization at El Laco, Chile. The results of our study reveal that individual melt inclusions preserve evidence of complex processes involved in melt immiscibility, including separation of Si- and Fe-rich melts, the latter hosting Cu sulfide-rich, phosphate-rich, and residual C-O-HFSE-rich melts, with their melting temperature at 1145 °C. This association is consistent with the assemblages observed in the ore, and provides a link between silicate and Fe-P-rich melts that subsequently produced the magnetite-rich magmas that extruded on the flanks of the volcano. These results strongly suggest that the El Laco mineralization was derived from crystallization of Fe-P-rich melts, thus providing insight into the formation of similar deposits elsewhere.

High-pressure highly reduced nitrides and oxides from chromitite of a Tibetan ophiolite.

The deepest rocks known from within Earth are fragments of normal mantle ( approximately 400 km) and metamorphosed sediments ( approximately 350 km), both found exhumed in continental collision terranes. Here, we report fragments of a highly reduced deep mantle environment from at least 300 km, perhaps very much more, extracted from chromite of a Tibetan ophiolite. The sample consists, in part, of diamond, coesite-after-stishovite, the high-pressure form of TiO(2), native iron, high-pressure nitrides with a deep mantle isotopic signature, and associated SiC. This appears to be a natural example of the recently discovered disproportionation of Fe(2+) at very high pressure and consequent low oxygen fugacity (fO(2)) in deep Earth. Encapsulation within chromitite enclosed within upwelling solid mantle rock appears to be the only vehicle capable of transporting these phases and preserving their low-fO(2) environment at the very high temperatures of oceanic spreading centers.

The parisite-(Ce) enigma: challenges in the identification of fluorcarbonate minerals.

A multi-methodological study was conducted in order to provide further insight into the structural and compositional complexity of rare earth element (REE) fluorcarbonates, with particular attention to their correct assignment to a mineral species. Polycrystals from La Pita Mine, Municipality de Maripí, Boyacá Department, Colombia, show syntaxic intergrowth of parisite-(Ce) with röntgenite-(Ce) and a phase which is assigned to B 3 S 4 (i.e., bastnäsite-3-synchisite-4; still unnamed) fluorcarbonate. Transmission electron microscope (TEM) images reveal well-ordered stacking patterns of two monoclinic polytypes of parisite-(Ce) as well as heavily disordered layer sequences with varying lattice fringe spacings. The crystal structure refinement from single crystal X-ray diffraction data - impeded by twinning, complex stacking patterns, sequential and compositional faults - indicates that the dominant parisite-(Ce) polytype M 1 has space group Cc. Parisite-(Ce), the B 3 S 4 phase and röntgenite-(Ce) show different BSE intensities from high to low. Raman spectroscopic analyses of parisite-(Ce), the B 3 S 4 phase and röntgenite-(Ce) reveal different intensity ratios of the three symmetric CO3 stretching bands at around 1100 cm-1. We propose to non-destructively differentiate parisite-(Ce) and röntgenite-(Ce) by their 1092 cm-1 / 1081 cm-1 ν1(CO3) band height ratio.

Phase decomposition upon alteration of radiation-damaged monazite-(ce) from moss, Østfold, norway.

The internal textures of crystals of moderately radiation-damaged monazite-(Ce) from Moss, Norway, indicate heavy, secondary chemical alteration. In fact, the cm-sized specimens are no longer mono-mineral monazite but rather a composite consisting of monazite-(Ce) and apatite pervaded by several generations of fractures filled with sulphides and a phase rich in Th, Y, and Si. This composite is virtually a 'pseudomorph' after primary euhedral monazite crystals whose faces are still well preserved. The chemical alteration has resulted in major reworking and decomposition of the primary crystals, with potentially uncontrolled elemental changes, including extensive release of Th from the primary monazite and local redeposition of radionuclides in fracture fillings. This seems to question the general alteration-resistance of orthophosphate phases in a low-temperature, 'wet' environment, and hence their suitability as potential host ceramics for the long-term immobilisation of radioactive waste.

Formation and Preservation of Microbial Palisade Fabric in Silica Deposits from El Tatio, Chile.

Palisade fabric is a ubiquitous texture of silica sinter found in low temperature (<40°C) regimes of hot spring environments, and it is formed when populations of filamentous microorganisms act as templates for silica polymerization. Although it is known that postdepositional processes such as biological degradation and dewatering can strongly affect preservation of these fabrics, the impact of extreme aridity has so far not been studied in detail. Here, we report a detailed analysis of recently silicified palisade fabrics from a geyser in El Tatio, Chile, tracing the progressive degradation of microorganisms within the silica matrix. This is complemented by heating experiments of natural sinter samples to assess the role of diagenesis. Sheathed cyanobacteria, identified as Leptolyngbya sp., were found to be incorporated into silica sinter by irregular cycles of wetting, evaporation, and mineral precipitation. Transmission electron microscopy analyses revealed that nanometer-sized silica particles are filling the pore space within individual cyanobacterial sheaths, giving rise to their structural rigidity to sustain a palisade fabric framework. Diagenesis experiments further show that the sheaths of the filaments are preferentially preserved relative to the trichomes, and that the amount of water present within the sinter is an important factor for overall preservation during burial. This study confirms that palisade fabrics are efficiently generated in a highly evaporative geothermal field, and that these biosignatures can be most effectively preserved under dry diagenetic conditions.

Pb nanospheres in ancient zircon yield model ages for zircon formation and Pb mobilization.

Nanospheres of lead (Pb) have recently been identified in zircon (ZrSiO4) with the potential to compromise the veracity of U-Pb age determinations. The key assumption that the determined age is robust against the effects of Pb mobility, as long as Pb is not lost from the zircon during subsequent geological events, is now in question. To determine the effect of nanosphere formation on age determination, and whether analysis of nanospheres can yield additional information about the timing of both zircon growth and nanosphere formation, zircons from the Napier Complex in Enderby Land, East Antarctica, were investigated by high-spatial resolution NanoSIMS (Secondary Ion Mass Spectrometry) mapping. Conventional SIMS analyses with >µm resolution potentially mixes Pb from multiple nanospheres with the zircon host, yielding variable average values and therefore unreliable ages. NanoSIMS analyses were obtained of 207Pb/206Pb in nanospheres a few nanometres in diameter that were resolved from 207Pb/206Pb measurements in the zircon host. We demonstrate that analysis for 207Pb/206Pb in multiple individual Pb nanospheres, along with separate analysis of 207Pb/206Pb in the zircon host, can not only accurately yield the age of zircon crystallization, but also the time of nanosphere formation resulting from Pb mobilization during metamorphism. Model ages for both events can be derived that are correlated due to the limited range of possible solutions that can be satisfied by the measured 207Pb/206Pb ratios of nanospheres and zircon host. For the Napier Complex zircons, this yields a model age of ca 3110 Ma for zircon formation and a late Archean model age of 2610 Ma for the metamorphism that produced the nanospheres. The Nanosphere Model Age (NMA) method constrains both the crystallization age and age of the metamorphism to ~±135 Ma, a significant improvement on errors derived from counting statistics.

Magnetic properties of core-shell nanoparticles possessing a novel Fe(ii)-chromia phase: an experimental and theoretical approach.

Room-temperature ferrimagnetic and superparamagnetic properties, and the magnetic interactions between the core and shell, of our iron-incorporated chromia-based core shell nanoparticles (CSNs) have been investigated using a combination of experimental measurement and density functional theory (DFT) based calculations. We have synthesized CSNs having an epitaxial shell and well-ordered interface properties by utilizing our hydrothermal nanophase epitaxy (HNE) technique. The ferrimagnetic and superparamagnetic properties of the CSNs are manifested beyond room temperature and magnetic measurements reveal that the exchange bias interaction between the antiferromagnetic (AFM) core and ferrimagnetic (FiM) shell persists close to ambient temperature. The DFT calculations confirm the FiM ordering of the Fe-chromia shell. Our calculations show that the FiM ordering is associated with a band gap reduction, Fe-O d-p orbital hybridization, and AFM type Fe-Cr σ type superexchange interaction in the α-Fe0.40Cr1.60O2.92 shell of the CSNs. The novel magnetic core-shell nanoparticles possess a shell comprised of a metastable Fe(ii)-chromia phase, resulting in unique magnetic properties that make them ideal for magnetic device and medicinal applications.

Mineral dissolution and reprecipitation mediated by an amorphous phase.

Fluid-mediated mineral dissolution and reprecipitation processes are the most common mineral reaction mechanism in the solid Earth and are fundamental for the Earth's internal dynamics. Element exchange during such mineral reactions is commonly thought to occur via aqueous solutions with the mineral solubility in the coexisting fluid being a rate limiting factor. Here we show in high-pressure/low temperature rocks that element transfer during mineral dissolution and reprecipitation can occur in an alkali-Al-Si-rich amorphous material that forms directly by depolymerization of the crystal lattice and is thermodynamically decoupled from aqueous solutions. Depolymerization starts along grain boundaries and crystal lattice defects that serve as element exchange pathways and are sites of porosity formation. The resulting amorphous material occupies large volumes in an interconnected porosity network. Precipitation of product minerals occurs directly by repolymerization of the amorphous material at the product surface. This mechanism allows for significantly higher element transport and mineral reaction rates than aqueous solutions with major implications for the role of mineral reactions in the dynamic Earth.

Authigenesis of biomorphic apatite particles from Benguela upwelling zone sediments off Namibia: The role of organic matter in sedimentary apatite nucleation and growth.

Sedimentary phosphorites comprise a major phosphorus (P) ore, yet their formation remains poorly understood. Extant polyphosphate-metabolizing bacterial communities are known to act as bacterial phosphate-pumps, leading to episodically high dissolved phosphate concentrations in pore waters of organic-rich sediment. These conditions can promote the precipitation of amorphous precursor phases that are quickly converted to apatite-usually in carbonate fluorapatite form [Ca10 (PO4 ,CO3 )6 F2-3 ]. To assess the mechanisms underpinning the nucleation and growth of sedimentary apatite, we sampled P-rich sediments from the Namibian shelf, a modern environment where phosphogenesis presently occurs. The P-rich fraction of the topmost centimetres of sediment mainly consists of pellets about 50-400 µm in size, which in turn are comprised of micron-sized apatite particles that are often arranged into radial structures with diameters ranging from 2 to 4 µm, and morphologies that range from rod-shapes to dumbbells to spheres that resemble laboratory-grown fluorapatite-gelatin nanocomposites known from double-diffusion experiments in organic matrices. The nucleation and growth of authigenic apatite on the Namibian shelf is likely analogous to these laboratory-produced precipitates, where organic macromolecules play a central role in apatite nucleation and growth. The high density of apatite nucleation sites within the pellets (>109 particles per cm3 ) suggests precipitation at high pore water phosphate concentrations that have been reported from the Namibian shelf and may be attributed to microbial phosphate pumping. The intimate association of organic material with the apatite could suggest a possible role of biological substrata, such as exopolymeric substances (EPS), in the nucleation of apatite precursors. Importantly, we do not observe any evidence that the apatite particles are actual phosphatized microbes, contradicting some earlier studies. Nevertheless, these results further evidence the potential importance of microbially derived (extracellular) organic matter as a template for phosphatic mineral nucleation in both recent and ancient phosphorites.

Templated and self-limiting calcite formation directed by coccolith organic macromolecules.

The formation of intricately shaped crystalline minerals by organisms is orchestrated by specialized biomacromolecules. The macromolecules associated with coccoliths, nanometer-sized calcite crystal arrays produced by marine microalgae, can form a distinct calcium-rich phase via macromolecular recognition. Here, we show that this calcium-rich phase can be mineralized into a thin film of single-crystalline calcite by the balanced addition of carbonate ions. Such a crystallization process provides a strategy to direct crystalline products via local interactions between soluble macromolecules and compatible templates.

Trace-Element Incorporation into Intracellular Pools Uncovers Calcium-Pathways in a Coccolithophore.

Many organisms form minerals from precursor phases that crystallize under strict biological control. The dynamic intracellular processes of formation, transport, and deposition of these precursor phases are challenging to identify. An unusual situation is recently revealed for the calcifying alga Emiliania huxleyi, as the cells contain a compartment filled with a concentrated Ca and P phase but the final calcite crystals, which are nucleated in a different compartment, are P-free. Thus, the connection of the Ca-P-rich pool to the mineralization process remains unclear. Here, pulse-chase experiments are used with Sr to label the Ca-P-rich phase in E. huxleyi cells, and cryo X-ray absorption spectroscopy and analytical transmission electron microscopy to follow the Sr within cells. It is found that Sr is first found in the Ca-P-rich phase and then becomes incorporated into the calcite. This demonstrates that the calcium used by the cells to build calcite originates from the Ca-P-rich pool.

Formation and transformation of amorphous calcium phosphates on titanium alloy surfaces during atmospheric plasma spraying and their subsequent in vitro performance.

Hydroxyapatite and 'duplex' hydroxyapatite + titania bond coat layers were deposited onto Ti6Al4V substrates by atmospheric plasma spraying (APS) at moderate plasma enthalpies. From as-sprayed coatings and coatings incubated in simulated body fluid (r-SBF) electron-transparent samples were generated by focused ion beam (FIB) excavation and investigated by STEM/TEM in conjunction with energy-dispersive X-ray analysis (EDX), electron diffraction (ED), and electron energy loss spectroscopy (EELS). Adjacent to the metal surface a thin layer of amorphous calcium phosphate (ACP) was deposited whose Ca/P ratio is determined by the presence or absence of the bond coat. No clear indication of a Ca-Ti oxide reaction layer was found at the interface titania bond coat/calcium phosphate. After in vitro incubation of duplex coatings for 24 weeks Ca-deficient defect apatite needles precipitated from ACP. During incubation of hydroxyapatite without a bond coat for 1 week diffusion bands were formed within the ACP of 1-2 microm width parallel to the interface metal/coating, presumably by a dissolution-precipitation sequence.

Macromolecular recognition directs calcium ions to coccolith mineralization sites.

Many organisms form elaborate mineralized structures, constituted of highly organized arrangements of crystals and organic macromolecules. The localization of crystals within these structures is presumably determined by the interaction of nucleating macromolecules with the mineral phase. Here we show that, preceding nucleation, a specific interaction between soluble organic molecules and an organic backbone structure directs mineral components to specific sites. This strategy underlies the formation of coccoliths, which are highly ordered arrangements of calcite crystals produced by marine microalgae. On combining the insoluble organic coccolith scaffold with coccolith-associated soluble macromolecules in vitro, we found a massive accretion of calcium ions at the sites where the crystals form in vivo. The in vitro process exhibits profound similarities to the initial stages of coccolith biogenesis in vivo.

Straightforward and robust synthesis of monodisperse surface-functionalized gold nanoclusters.

Gold nanoclusters are small (1-3 nm) nanoparticles with a high surface area that are useful for biomedical studies and drug delivery. The synthesis of small, surface-functionalized gold nanoclusters is greatly dependent on the reaction conditions. Here, we describe a straightforward, efficient and robust room temperature one-pot synthesis of 2 nm gold nanoclusters using thioglucose as a reducing and stabilizing agent, which was discovered by serendipity. The resultant monodisperse gold nanoclusters are more stable than those generated using some other common methods. The carboxylic acid contained in the stabilizing agent on the cluster surface serves as anchor for nanocluster functionalization. Alternatively, the addition of thiols serves to functionalize the nanoclusters. The resulting non-cytotoxic nanoclusters are taken up by cells and constitute a tuneable platform for biomedical applications including drug delivery.

A vacuole-like compartment concentrates a disordered calcium phase in a key coccolithophorid alga.

Coccoliths are calcitic particles produced inside the cells of unicellular marine algae known as coccolithophores. They are abundant components of sea-floor carbonates, and the stoichiometry of calcium to other elements in fossil coccoliths is widely used to infer past environmental conditions. Here we study cryo-preserved cells of the dominant coccolithophore Emiliania huxleyi using state-of-the-art nanoscale imaging and spectroscopy. We identify a compartment, distinct from the coccolith-producing compartment, filled with high concentrations of a disordered form of calcium. Co-localized with calcium are high concentrations of phosphorus and minor concentrations of other cations. The amounts of calcium stored in this reservoir seem to be dynamic and at a certain stage the compartment is in direct contact with the coccolith-producing vesicle, suggesting an active role in coccolith formation. Our findings provide insights into calcium accumulation in this important calcifying organism.

Noble metal nanoclusters and nanoparticles precede mineral formation in magmatic sulphide melts.

In low temperature aqueous solutions, it has long been recognized by in situ experiments that many minerals are preceded by crystalline nanometre-sized particles and non-crystalline nanophases. For magmatic systems, nanometre-sized precursors have not yet been demonstrated to exist, although the suggestion has been around for some time. Here we demonstrate by high temperature quench experiments that platinum and arsenic self-organize to nanoparticles, well before the melt has reached a Pt-As concentration at which discrete Pt arsenide minerals become stable phases. If all highly siderophile elements associate to nanophases in undersaturated melts, the distribution of the noble metals between silicate, sulphide and metal melts will be controlled by the surface properties of nano-associations, more so than by the chemical properties of the elements.