1,721,001 research outputs found
Cohenite (iron carbide) and native iron formation within garnet included in polycrystalline diamonds by redox freezing in the cratonic lithosphere
No full textCohenite, native iron and troilite inclusions in garnets from polycrystalline diamond aggregates
No full textSyngenetic garnet of eclogitic/pyroxenitic composition included in a polycrystalline diamond aggregate from the Venetia kimberlite, Limpopo Belt, South Africa shows multiple inclusions of spherules consisting of 61+/-5 vol% Fe3C (cohenite), 30+/-2 vol% Fe-Ni and 9+/-3 vol% FeS (troilite). Troilite forms shells around the native iron-cohenite assemblage, implying that both compositions were immiscible melts and were trapped rapidly by the silicate. It is proposed that this polycrystalline diamond-silicate-metallic spherule assemblage formed in very local pressure and fO(2) conditions in cracks at the base of the subcratonic lithosphere from a C-H-O fluid that reacted with surrounding silicate at about 1,300-1,400 degreesC. In a mantle fluid consisting of CH4>H2O>H-2 near fO(2)=IW, the H-2 activity increases rapidly when carbon from the fluid is consumed by diamond precipitation, driving the oxygen fugacity of the system to lower values along the diamond saturation curve. Water from the fluid induces melting of surrounding silicate material, and hydrogen reduces metals in the silicate melt, reflected by an unusually low Ni content of the garnet. The carbon isotopic composition of delta(13)C=-13.69parts per thousand (PDB) and the lack of nitrogen as an impurity is consistent with formation of the diamond from non-biogenic methane, whereas delta(18)O=7.4parts per thousand (SMOW) of the garnet implies derivation of the silicate from subduction-related material. Hence, very localized and transient reducing conditions within the subcratonic lithosphere can be created by this process and do not necessarily call for involvement of fluids derived from subducted material of biogenic origin
A comparison of angiographic CT and multisection CT in lumbar myelographic imaging
No full textBACKGROUND AND PURPOSE: The purpose of this work was to provide an intraindividual comparison of angiographic CT (ACT) and multisection CT (MSCT) in lumbar myelographic imaging and to evaluate possible benefits of ACT, which is a further development of rotational angiography providing image data of high spatial and CT-like contrast resolution. MATERIALS AND METHODS: In 26 patients with degenerative lumbar spine disease a lumbar ACT was performed in combination with conventional lumbar myelography and followed by postmyelographic. MSCT. Conventional lumbar myelography and lumbar ACT were performed with a flat panel detector-equipped angiographic device. Postmyelographic MSCT was performed with a 16-section CT scanner. Three experienced neuroradiologists rated anonymized sets of multiplanar reformatted CT and ACT images regarding diagnostic and technical parameters. The ratings were repeated after 2 months. Weighted K Statistics were calculated to describe the levels of intraobserver and interobserver agreement. RESULTS: The analysis shows that MSCT achieves higher ratings than ACT in all of the parameters asked. An adequate diagnostic quality was only assigned to 80% of the ACT acquisitions compared with 97% of the MSCT acquisitions. All of the mean K values were above 0.60, demonstrating a substantial intraobserver and interobserver agreement for MSCT, as well as for ACT. CONCLUSION: Using ACT, radiographic myelography and myelographic CT can be performed at the same imaging system. However, the results of our study show that the current myelographic ACT image quality fails to apply diagnostic standards. We, therefore, cannot recommend ACT as a general alternative to postmyelographic MSCT
Long-term simulation of Indonesian rainfall with the MPI regional model
No full textSimulations of the Indonesian rainfall variability using the Max Planck Institute regional climate model REMO have been performed using three different lateral boundary forcings: Reanalyses from the European Centre for Medium-Range Weather Forecasts (ERA15), the National Centers for Environmental Prediction and National Center for Atmospheric Research (NRA) as well as from ECHAM4 climate model simulation. The result of those simulations are compared to station data. REMO simulations were performed at 0.5degrees horizontal resolution for the whole archipelago and at 1/6degrees for Sulawesi Island. In general the REMO model, reproduces the spatial pattern of monthly and seasonal rainfall well over land, but overestimates the rainfall over sea. Superiority of REMO performance over land is due to a high-resolution orography, while over sea, REMO suffers from erroneously low surface fluxes. REMO reproduces variability during El Nino-Southern Oscillations years well but fails to show a good (wet and dry) monsoon contrast. Despite strong influences of the lateral boundary fields, REMO shows a realistic improvement of a local phenomenon over Molucca. Significant improvement for the step from the relatively high global 1.125degrees to 0.5degrees resolution is noticeable, but not from 0.5degrees into 1/6degrees. The REMO simulation driven by ERA15 has the best quality, followed by NRA and ECHAM4 driven simulations. The quality of ERA15 is the main factor determining the quality of REMO simulations. A predictability study shows small internal variability among ensemble members. However, there are systematic intrinsic climatological errors as shown in the predictability analysis. These intrinsic errors have monthly, seasonal and regional dependencies and the one over Java is significantly large. The intrinsic error study suggests the presence of the spring predictability barrier and a high level of predictability in summe
Nano-inclusion suite and high resolution micro-computed-tomography of polycrystalline diamond (framesite) from Orapa, Botswana
No full textA single polycrystalline diamond aggregate from the Orapa kimberlite (Botswana) contains a syngenetic micro- and nano-inclusion suite of magnetite, pyrrhotite, omphacite, garnet, rutile and C-O-H fluid in order of abundance. This suite of inclusions is distinctly different from those in fibrous diamonds, although the presence of sub-micrometer fluid inclusions provides evidence for a similarly important role of fluids in the genesis of polycrystalline diamond. It is the first study of polycrystalline diamond by High resolution mu-CT (Computed Tomography) reaching a resolution of 1.3 mu m using polychromatic X-rays. Combined with Focused Ion Beam assisted Transmission Electron Microscopy the method reveals epigenetic replacement coatings of hematite and late stage sheet silicates around the magnetites showing that magnetites are often (but not always) interstitial to the diamond and, thus, were open to late stage overprinting. It is proposed that the polycrystalline diamond formed by a redox reaction between a small-scale carbonatitic melt and a sulfide-bearing eclogite. The water released from the melt during diamond precipitation fluxed local melting of the surrounding eclogite, and oxidation of sulfide phases to magnetite, which mingled with the carbonatitic melt and (re-)precipitated locally. (C) 2011 Elsevier B.V. All rights reserved
Multi-stage Ag–Bi–Co–Ni–U and Cu–Bi vein mineralization at Wittichen, Schwarzwald, SW Germany: geological setting, ore mineralogy, and fluid evolution
No full textThe Wittichen Co-Ag-Bi-U mining area (Schwarzwald ore district, SW Germany) hosts several unconformity-related vein-type mineralizations within Variscan leucogranite and Permian to Triassic redbeds. The multistage mineralization formed at the intersection of two fault systems in the last 250 Ma. A Permo-Triassic ore stage I with minor U-Bi-quartz-fluorite mineralization is followed by a Jurassic to Cretaceous ore stage II with the main Ag and Co mineralization consisting of several generations of gangue minerals that host the sub-stages of U-Bi, Bi-Ag, Ni-As-Bi and Co-As-Bi. Important ore minerals are native elements, Co and Ni arsenides, and pitchblende; sulphides are absent. The Miocene ore stage III comprises barite with the Cu-Bi sulfosalts emplectite, wittichenite and aikinite, and the sulphides anilite and djurleite besides native Bi, chalcopyrite, sphalerite, galena and tennantite. The mineral-forming fluid system changed from low salinity (< 5 wt.% NaCl) at high temperature (around 300A degrees C) in Permian to highly saline (around 25 wt.% NaCl + CaCl2) at lower temperatures (50-150A degrees C) in Triassic to Cretaceous times. Thermodynamic calculations and comparison with similar mineralizations worldwide show that the Mesozoic ore-forming fluid was alkaline with redox conditions above the hematite-magnetite buffer. We suggest that the precipitation mechanism for native elements, pitchblende and arsenides is a decrease in pH during fluid mixing processes. REE patterns in fluorite and the occurrence of Bi in all stages suggest a granitic source of some ore-forming elements, whereas, e.g. Ag, Co and Ni probably have been leached from the redbeds. The greater importance of Cu and isotope data indicates that the Miocene ore stage III is more influenced by fluids from the overlying redbeds and limestones than the earlier mineralization stages.Alfried-Krupp foundatio
Modellierung des Effektes von Spurenelementen in Biomineralien durch biomimetische Synthese
Full text linkThis work aims to determine whether the different crystallisation pathways of amorphous calcium carbonate (ACC) have an impact on the element and oxygen isotope partitioning of the final crystalline phase. In this study, special emphasis is placed on the sparsely investigated pathway of the so-called pseudomorphic transformation, which is a crystallisation process that preserves the disequilibrium morphology of the amorphous precursor. Since it is shown that the composition of ACC can be preserved under pseudomorphic conditions, the influence of a range of synthesis parameters and conditions on the material properties and chemical composition of ACC was scrutinised.
While solid-state transformation retains the chemical composition of doped ACC during crystallisation, the chemical composition is altered during dissolution-reprecipitation pathways. An alternative crystallisation pathway, namely the shape-preserving pseudomorphic transformation, is induced by additives such as poly(acrylic acid), polyaspartic acid and trace amounts of phosphate. Since ACC crystallisation in biominerals occurs in the presence of these or similar additives – especially aspartate-rich domains were found in unusually acidic biomineralisation proteins – a thorough mechanistic understanding of the pseudomorphic transformation is of importance for paleoclimate reconstructions. This work revealed that the pseudomorphic transformation not only preserves the morphology of the amorphous precursor, but also retains the partition coefficients, e.g., in case of Sr-doped ACC. Furthermore, the influence of the pseudomorphic transformation on the oxygen isotope composition demonstrated that this transformation is a quasi-solid to solid phase transformation process, although it takes place in aqueous solutions. Mechanistically, it seems that the presence of certain surface-active additives limits diffusional exchange with the surrounding liquid environment so that pseudomorphic transformation preserves the partition coefficients of ACC even in the final crystalline product.
As the pseudomorphic transformation preserves the composition of the amorphous precursor, it is necessary to understand how the formation of ACC and its partitioning coefficients, as well as its material properties, is influenced by the synthesis conditions. For instance, it was recently shown that it is possible to precipitate ACC with distinct short-range order by simply altering the pH slightly. Therefore, it is of high significance to analyse the influence of the synthesis conditions on ACC. In this thesis, three different approaches were conducted to achieve a better understanding of the structural and compositional synthesis dependence of ACC:
#1 The influence of the synthesis procedures revealed that material properties such as particle size, level of hydration, crystallisation temperature, and density are sensitive to simple changes in the synthesis conditions, which have much less effect on the chemical composition of ACC. Notably, density measurements indicated that synthesis-dependent microstructures of ACC structures exist. Furthermore, a microfluidic setup allowed for ACC synthesis at an exceptionally low pH (pH 7.5) by using ethanol as anti-solvent. Under these conditions, synthesis of ACC at varying pH revealed a significant increase of barium incorporation by decreasing pH.
#2 The influence of the mixing kinetics on the element partition was analysed by precipitating magnesium-, strontium-, and barium-doped ACC under varying flow rates. While less magnesium was incorporated at increasing flow rates, increasing partition coefficients were determined for Sr-doped ACC by increasing flow rates. Notably, no flow rate influence was determined for Ba-doped ACC. These results demonstrate that element partitioning is highly sensitive to changing mixing kinetics, which indicates that prenucleation clusters play a role during ACC formation and control element partitioning to a certain extent.
#3 To mimic ACC formation in a natural environment, ACC was synthesised in artificial seawater under varying synthesis conditions as flow rate, temperature, and pH, which resulted in multiple-doped ACC. Besides material properties such as particle size, the chemical composition was also affected by the synthesis conditions. A significant influence of the flow rate and temperature on magnesium and sulphur partitioning was determined. While less magnesium was incorporated by increasing flow rates, increasing sulphur incorporation was detected. Furthermore, both additives were better incorporated at enhanced temperatures. Notably, increasing concentrations of all dopants were determined with increasing pH.
In the final chapter, a feasibility study was conducted to assess whether a flow-through synthesis enables the synthesis of the fundamental building blocks of calcareous biominerals, namely calcium carbonate nanograins coated with organic matrices. This study demonstrated that the precipitation of Mg-doped ACC particles coated with negatively charged polyelectrolytes, such as polyacrylates or polystyrene sulfonate, is possible by using a flow-through synthesis.Diese Arbeit befasst sich mit dem Einfluss der verschiedenen Kristallisationspfade auf die Einbauraten von Ionen und Sauerstoffisotopen in amorphem Kalziumkarbonat (ACC). Besonderes Augenmerk liegt dabei auf dem Einfluss der wenig untersuchten und sogenannten pseudomorphen Umwandlung (pseudomorphic transformation), welche die Morphologie während der Kristallisation nicht verändert. Da in dieser Arbeit gezeigt wird, dass unter pseudomorphen Bedingungen auch die chemische Zusammensetzung erhalten werden kann, untersucht diese Arbeit zudem den Einfluss der Synthesebedingungen auf die Materialeigenschaften und die chemische Zusammensetzung von ACC.
Während eine sogenannte Festphasenumwandlung (solid-state transformation) die chemische Zusammensetzung von dotiertem ACC während der Kristallisation nicht verändert, wird diese bei einem Kristallisationspfad verändert, bei welchem ACC sich auflöst und neugebildetes kristallines Kalziumkarbonat ausfällt (dissolution-reprecipitation). Jedoch gibt es einen weiteren Kristallisationspfad, die sogenannte pseudomorphe Umwandlung, welche sich dadurch auszeichnet, dass die Nichtgleichgewichtsmorphologie des amorphen Vorläufers während der Kristallisation erhalten wird. Dieser Kristallisationspfad wird durch die Präsenz von Additiven wie Polyacrylsäure, Polyasparaginsäure und sogar von Spurenelementen von Phosphat induziert, wie in dieser Arbeit gezeigt wird. Aufgrund der Tatsache, dass die Kristallisation von ACC in Biomineralien unter der Anwesenheit dieser Additive abläuft – besonders Asparaginsäure ist ein Bestandteil von ungewöhnlich sauren Biomineralisationsproteinen – ist ein tiefergehendes Verständnis der pseudomorphen Umwandlung von Bedeutung für die Paläoklimatik. Diese Arbeit weist erstmals nach, dass die pseudomorphe Umwandlung sogar die Verteilungskoeffizienten während der Kristallisation bewahrt, nachweisbar insbesondere für die Einbauraten von Strontium in ACC. Weiterhin zeigt sich im Einfluss der pseudomorphen Umwandlung auf die Sauerstoffisotopie, dass es sich quasi um eine Festphasenumwandlung handelt, obwohl die Reaktion in Lösung stattfindet. Oberflächenaktive Substanzen scheinen den diffusiven Austausch mit der umgebenden Lösung zu hemmen, sodass die Verteilungskoeffizienten der amorphen Phase dem des kristallinen Endproduktes unerwarteterweise entsprechen.
Aufgrund der Tatsache, dass die pseudomorphe Umwandlung die chemische Zusammensetzung der amorphen Vorphase erhält, ist ein fundierter Kenntnisstand über den Einbau von Dotierelementen in ACC und der Einfluss der Synthesebedingungen auf die Materialeigenschaften und die chemische Zusammensetzung notwendig. Da vor kurzem demonstriert wurde, dass durch eine Veränderung des pH-Wertes unterschiedliche ACC-Nahstrukturen hergestellt werden können, ist eine Untersuchung des Syntheseeinflusses auf ACC von zusätzlicher Bedeutung. Um ein tiefergehendes Verständnis zu erreichen, wurden drei verschiedene Ansätze verfolgt:
#1 Untersuchungen zum Einfluss der Synthesen verdeutlichten, dass Materialeigenschaften wie die Partikelgröße, die Hydratisierung der Proben, die Kristallisationstemperatur und die Dichte durch eine einfache Veränderung der angewendeten Synthese beeinflusst werden können. Im Gegensatz hierzu wird die chemische Zusammensetzung durch die Anwendung einer anderen Synthese nur geringfügig beeinflusst. Dichtemessungen der Pulver deuteten auf die beachtenswerten Möglichkeiten hin, verschiedene ACC-Microstrukturen über die Auswahl der Synthesemethode herstellen zu können. Weiterhin war es durch einen mikrofluidischen Ansatz möglich, Ethanol als Ausfällungsmittel zu verwenden, was eine Synthese von ACC bei pH 7,5 erlaubte. Eine Synthese von ACC bei verschiedenen pH-Werten zeigte einen verstärkten Einbau von Barium in ACC bei verringertem pH.
#2 Um den Einfluss der Kinetik auf den Einbau von Ionen zu untersuchen, wurde mit Magnesium, Strontium und Barium dotiertes ACC bei verschiedenen Fließraten hergestellt. Während bei höheren Fließraten weniger Magnesium eingebaut wurde, wurde das Gegenteil für den Einbau von Strontium festgestellt. Beachtenswert ist, dass der Einbau von Barium nicht von der Fließrate beeinflusst wurde. Diese Ergebnisse verdeutlichen, dass der Einbau von Ionen sehr sensitiv gegenüber veränderten Herstellungsbedingungen ist, was darauf hinweist, dass sogenannte Pränukleationscluster (PNCs) eine gewichtige Rolle während der Bildung von ACC spielen. Weiterhin deutet dies darauf hin, dass PNCs in einem gewissen Rahmen den Einbau von Ionen in ACC kontrollieren.
#3 Um die Herstellung von ACC unter natürlichen Umweltbedingungen zu simulieren, wurde ACC in künstlichem Meerwasser ausgefällt, was in der Bildung von mehrfachdotiertem ACC resultierte. Zusätzlich wurden während der Synthese Bedingungen wie die Fließrate, die Temperatur und der pH-Wert systematisch verändert. Diese Veränderungen beeinflussen nicht nur Materialeigenschaften wie die Partikelgröße, sondern ebenfalls die chemische Zusammensetzung von ACC. So wurde ein deutlicher Einfluss der Fließrate und der Temperatur auf die Einbauraten von Magnesium und Schwefel festgestellt. Während steigende Fließraten zu einer Verringerung des Einbaus von Magnesium führten, erreichten sie einen verstärkten Einbau von Schwefel. Eine Erhöhung der Temperatur führte zu einem erhöhten Einbau beider Ionen. Beachtenswert war der verstärkte Einbau aller Ionen bei steigendem pH-Wert.
Im abschließenden Kapitel des experimentellen Teils dieser Arbeit wurde eine Machtbarkeitsstudie durchgeführt, die aufzeigen soll, ob es mit Hilfe einer sogenannten Flow-through Synthese möglich ist, die fundamentalen Bauteile von kalkhaltigen Biomineralien herzustellen. Diese Bauteile bestehen aus nanogranulärem Kalziumkarbonat, welches mit einer organischen Matrix beschichtet ist. Diese Machtbarkeitsstudie verdeutlichte, dass eine Herstellung von mit Magnesium dotiertem ACC, welches mit negativ geladenen Polyelektrolyten wie Polyacrylsäure oder Polystyrolsulfonat beschichtet wurde, möglich ist
Element substitution by living organisms: The case of manganese in mollusc shell aragonite
Full text linkDetermining the manganese concentration in shells of freshwater bivalves provides a unique way to obtain information about climate and environmental changes during time-intervals that pre-date instrumental data records. This approach, however, relies on a thorough understanding of how manganese is incorporated into the shell material-a point that remained controversial so far. Here we clarify this issue, using state-of-the-art X-ray absorption and X-ray emission spectroscopy in combination with band structure calculations. We verify that in the shells of all studied species manganese is incorporated as high-spin Mn 2+, i.e. manganese always has the same valence as calcium. More importantly, the unique chemical sensitivity of valence-to-core X-ray emission enables us to show that manganese is always coordinated by a CO 3-octahedron. This, firstly, provides firm experimental evidence for manganese being primarily located in the inorganic carbonate. Secondly, it indicates that the structure of the aragonitic host is locally altered such that manganese attains an octahedral, calcitic coordination. This modification at the atomic level enables the bivalve to accommodate many orders of magnitude more manganese in its aragonitic shell than found in any non-biogenic aragonite. This outstanding feature is most likely facilitated through the non-classical crystallization pathway of bivalve shells.Fil: Soldati, Analía Leticia. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Johannes Gutenberg-Universität; Alemania. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; ArgentinaFil: Jacob, Dorrit E.. Macquarie University; AustraliaFil: Glatzel, Pieter. European Synchrotron Radiation Facility; FranciaFil: Swarbrick, Janine C.. European Synchrotron Radiation Facility; FranciaFil: Geck, Jochen. Paris Lodron University Salzburg; Austri
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