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Eric H Oelkers - One of the best experts on this subject based on the ideXlab platform.
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does Variscite control phosphate availability in acidic natural waters an experimental study of Variscite dissolution rates
Geochimica et Cosmochimica Acta, 2011Co-Authors: Teresa Roncalherrero, Eric H OelkersAbstract:Abstract The dissolution rates of natural, well crystallized Variscite (AlPO4·2H2O) were determined from the evolution of aqueous Al and P concentrations in closed and open-system mixed-flow reactors at 25 °C and pH from 1.5 to 9.0. Measured dissolution rates decrease with increasing pH, from 6 × 10−16 mol/cm2/s at pH 1.5 to 5 × 10−17 mol/cm2/s at pH 5.89, and then increase with increasing pH to 4 × 10−16 mol/cm2/s at pH 9.0. Geochemical modeling calculations, performed using measured dissolution rates, indicate that it would take no more than a few weeks or months to equilibrate a mildly acidic, Al and P-free solution with Variscite. Hence, Variscite can buffer aqueous phosphate concentrations in mildly acidic near surface environments. This conclusion is confirmed by consideration of the compositions of natural waters.
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Variscite dissolution rates in aqueous solution does Variscite control the availability of phosphate in acidic natural waters
Mineralogical Magazine, 2008Co-Authors: Teresa Roncalherrero, Eric H OelkersAbstract:The dissolution rates of natural well-crystallized Variscite (AlPO4.2H2O) were measured from the evolution of aqueous Al and P concentrations in closed and mixed-flow through reactors at 25°C and from 1.5 to 9 pH. Measured dissolution rates decrease with increasing pH from 5.05 × 10−16 mol/cm2/s at pH = 1.51 to 4.92 × 10−17 mol/cm2/s at pH = 5.89 and then increase with increasing pH to 1.64 × 10−17 mol/cm2/s at pH = 8.99. Estimates of the time required to equilibrate a mildly acidic, initially Al- and P-free solution with Variscite based on measured dissolution rates and solubility products suggests it takes no more than several weeks to equilibrate this mineral with soil pore fluids. This result suggests that Variscite can buffer aqueous phosphate concentrations in a significant number of near surface environments.
Teresa Roncalherrero - One of the best experts on this subject based on the ideXlab platform.
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does Variscite control phosphate availability in acidic natural waters an experimental study of Variscite dissolution rates
Geochimica et Cosmochimica Acta, 2011Co-Authors: Teresa Roncalherrero, Eric H OelkersAbstract:Abstract The dissolution rates of natural, well crystallized Variscite (AlPO4·2H2O) were determined from the evolution of aqueous Al and P concentrations in closed and open-system mixed-flow reactors at 25 °C and pH from 1.5 to 9.0. Measured dissolution rates decrease with increasing pH, from 6 × 10−16 mol/cm2/s at pH 1.5 to 5 × 10−17 mol/cm2/s at pH 5.89, and then increase with increasing pH to 4 × 10−16 mol/cm2/s at pH 9.0. Geochemical modeling calculations, performed using measured dissolution rates, indicate that it would take no more than a few weeks or months to equilibrate a mildly acidic, Al and P-free solution with Variscite. Hence, Variscite can buffer aqueous phosphate concentrations in mildly acidic near surface environments. This conclusion is confirmed by consideration of the compositions of natural waters.
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Variscite dissolution rates in aqueous solution does Variscite control the availability of phosphate in acidic natural waters
Mineralogical Magazine, 2008Co-Authors: Teresa Roncalherrero, Eric H OelkersAbstract:The dissolution rates of natural well-crystallized Variscite (AlPO4.2H2O) were measured from the evolution of aqueous Al and P concentrations in closed and mixed-flow through reactors at 25°C and from 1.5 to 9 pH. Measured dissolution rates decrease with increasing pH from 5.05 × 10−16 mol/cm2/s at pH = 1.51 to 4.92 × 10−17 mol/cm2/s at pH = 5.89 and then increase with increasing pH to 1.64 × 10−17 mol/cm2/s at pH = 8.99. Estimates of the time required to equilibrate a mildly acidic, initially Al- and P-free solution with Variscite based on measured dissolution rates and solubility products suggests it takes no more than several weeks to equilibrate this mineral with soil pore fluids. This result suggests that Variscite can buffer aqueous phosphate concentrations in a significant number of near surface environments.
George P. Demopoulos - One of the best experts on this subject based on the ideXlab platform.
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raman spectroscopic study of the hydrogen and arsenate bonding environment in isostructural synthetic arsenates of the Variscite group m3 aso4 2h2o m3 fe al in and ga implications for arsenic release in water
Journal of Raman Spectroscopy, 2011Co-Authors: Mario A Gomez, Jeanfrancois Le Berre, Hassane Assaaoudi, George P. DemopoulosAbstract:The Raman spectra of synthetic compounds equivalent to the Variscite group: FeAsO4·2H2O AlAsO4·2H2O, GaAsO4·2H2O, and InAsO4·2H2O are reported. In particular, upon comparison of FeAsO4·2H2O to AlAsO4·2H2O, it is observed that the Type II (weak) H-bond lengths in the latter are slightly longer, which is postulated to affect the stability (As release) in water at pH 5 and 7. Arsenate stretching and bending vibrations were found to be distinct in terms of spectral structure and therefore well suited for fingerprinting. The calculated AsO bond strengths from existing crystallographic data showed no significant variations. The strongest ν1 (AsO43−) stretch was used to monitor the AsO bonding interactions in the four AsOM units, where a shift of 114 cm−1 was observed in the order FeAsO4·2H2O (lowest) < InAsO4·2H2O < GaAsO4·2H2O < AlAsO4·2H2O (highest); this order also followed exactly the measured arsenic release of these phases. This shift in ν1 (AsO43−) position was rationalized to stem from the differences in the electronegativities of the M3+ cations. The trends mentioned above were verified and found to also hold for the isostructural phosphate analogues strengite (FePO4·2H2O) and Variscite (AlPO4·2H2O) using published data. Therefore, it is postulated that, as observed with the stability of solution complexes, there may be a correlation between the electronegativity of the M3+ cation in these isostructural phases and their stability (As or P release) in water. Copyright © 2010 John Wiley & Sons, Ltd.
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The Stability of Hydrated Aluminium Phosphate, ALPO4·1.5H2O
Environmental Technology, 2006Co-Authors: F. Lagno, George P. DemopoulosAbstract:The dissolution mechanism and stability of a synthetic hydrated aluminium phosphate, AlPO4·1.5H2O (AlPO4-H3) was investigated at 22 °C over the pH range 2.4 to 8.8. AlPO4·1.5H2O was found to be more soluble than the better known dihydrate (Variscite, AlPO4·2H2O) hence proving to be metastable. This material was determined to undergo incongruent dissolution at pH around 3.0 and higher producing initially amorphous Al(OH)3 which gradually (within 30-day dissolution period) converted to the stable gibbsite phase. Upon evaluation of the experimental data with PHREEQC it was possible to calculate the solubility product (log Ksp ) and the standard Gibbs free energy of formation (ΔGf,0 ) for AlPO4·1.5H2O to be respectively −20.46±0.40 and −1980.5±2.0 kJ mol−1 at 22 °C.
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the stability of hydrated aluminium phosphate alpo4 1 5h2o
Environmental Technology, 2006Co-Authors: F. Lagno, George P. DemopoulosAbstract:The dissolution mechanism and stability of a synthetic hydrated aluminium phosphate, AlPO4·1.5H2O (AlPO4-H3) was investigated at 22 °C over the pH range 2.4 to 8.8. AlPO4·1.5H2O was found to be more soluble than the better known dihydrate (Variscite, AlPO4·2H2O) hence proving to be metastable. This material was determined to undergo incongruent dissolution at pH around 3.0 and higher producing initially amorphous Al(OH)3 which gradually (within 30-day dissolution period) converted to the stable gibbsite phase. Upon evaluation of the experimental data with PHREEQC it was possible to calculate the solubility product (log Ksp ) and the standard Gibbs free energy of formation (ΔGf,0 ) for AlPO4·1.5H2O to be respectively −20.46±0.40 and −1980.5±2.0 kJ mol−1 at 22 °C.
Gerhard Franz - One of the best experts on this subject based on the ideXlab platform.
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the system al2o3 p2o5 h2o at temperatures below 200 c experimental data on the stability of Variscite and metaVariscite alpo4 2h2o
American Mineralogist, 2007Co-Authors: Kirsten Druppel, Andreas Hosch, Gerhard FranzAbstract:The system Al2O3-P2O5-H2O contains many phosphate minerals that occur in various geologic environments. The natural occurrence of Variscite (AlPO4·2H2O, orthorhombic), including its monoclinic polymorph metaVariscite, is largely restricted to soils and aluminous rocks like Al-rich igneous rocks and shales, which interacted with P-rich hydrothermal solutions or groundwater. Variscite dehydrates to berlinite (AlPO4) and a hydrous P-Al-rich fluid. This dehydration reaction AlPO4·2H2O = AlPO4 + 2H2O is, however, metastable, at low concentrations of P in the fluid, because berlinite breaks down at lower temperature to augelite [Al2PO4(OH)3] + H3PO4 + H2O and trolleite [Al4(PO4)3(OH)3] + H3PO4 + H2O. The Variscite/metaVariscite = berlinite equilibrium has been investigated by synthesis experiments from mixtures of γ-Al2O3 and excess phosphoric acid at pressures between 1 and 5 kbar and temperatures of 100–200 °C using standard cold-seal vessels. The hydration-dehydration equilibrium is mainly controlled by temperature and only weakly by pressure. At 4 and 5 kbar, Variscite/metaVariscite were found at temperatures of ~150 °C, at lower pressure at 115–125 °C in accordance with their natural mode of occurrence. In this T range, however, Variscite/metaVariscite is not the sole phase but is always accompanied by variable but generally minor amounts of wavellite and trolleite Al4(PO4)3(OH)3. Secondary wavellite Al3(PO4)2(OH)3·5H2O, hydrated Al-phosphate AlPO4· x H2O (1.1 ≤ x ≤ 1.3) and Al-metaphosphate hexahydrate Al(H2PO4)3 formed during quenching and/or drying of the runs. Berlinite is the reaction product at temperatures 200 °C/4–5 kbar and 150 °C/1–3 kbar and may be associated with augelite and trolleite.
Guirec Querré - One of the best experts on this subject based on the ideXlab platform.
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Iberian Variscite: ICP-MS-LA and PIXE analysis of recent prehistory beads and pendants from Spain and Portugal
2019Co-Authors: Salvador Domínguez-bella, Guirec Querré, Thomas Calligaro, Javier Martínez LópezAbstract:At the European recent Prehistory Variscite was a precious gemmological material that appears in scarce megalithic burials of the Atlantic facade, especially at the Iberian peninsula and West France. Here we describe the main results of the mineralogical and geochemical analysis of a great and diverse group of archaeological and geological Variscite and turquoise samples from the Iberian peninsula, especially from the Atlantic facade and the SW of Spain. Samples were analysed last years by the use of different analytical techniques as XRD, WDXRF, ICP-MS-LA and PIXE and stereomicroscopy. The geochemical results, obtained especially by PIXE and ICP-MS-LA techniques, on these archaeological samples were discussed in relation to their possible provenance areas, related with the three main geological areas with Variscite and turquoise outcrops in the Iberian peninsula: Palazuelo de las Cuevas-San Vicente area, that included other sites as El Bostal and Bragança district in Portugal; Pico Centeno mines, Encinasola area, in Huelva province, SW Spain; Gavá mines in Catalonia, NE Spain, and other possible sources in Europe as Pannecé, France. Geochemical data were subjected to statistical analysis selecting different chemical variables in Variscites and turquoises. A relatively good discrimination has been obtained through an interactive geochemical model using specially the Fe-Cr-V relationship, represented by a triangular diagram and the elementary relationships between other chemical elements. These results show the existence, in Neolithic and Chalcolithic times, of great exchange networks of exotic or prestige lithic products that extended in Western Europe, hundreds and thousands of kilometres away from their source areas.
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iberian Variscite icp ms la and pixe analysis of recent prehistory beads and pendants from spain and portugal
La parure en callaïs du Néolithique européen, 2019Co-Authors: Salvador Dominguezbella, Guirec Querré, Thomas Calligaro, Javier Martinez Lopez, Joao Luis CardosoAbstract:At the European recent Prehistory Variscite was a precious gemmological material that appears in scarce megalithic burials of the Atlantic facade, especially at the Iberian peninsula and West France. Here we describe the main results of the mineralogical and geochemical analysis of a great and diverse group of archaeological and geological Variscite and turquoise samples from the Iberian peninsula, especially from the Atlantic facade and the SW of Spain. Samples were analysed last years by the use of different analytical techniques as XRD, WDXRF, ICP-MS-LA and PIXE and stereomicroscopy. The geochemical results, obtained especially by PIXE and ICP-MS-LA techniques, on these archaeological samples were discussed in relation to their possible provenance areas, related with the three main geological areas with Variscite and turquoise outcrops in the Iberian peninsula: Palazuelo de las Cuevas-San Vicente area, that included other sites as El Bostal and Braganca district in Portugal; Pico Centeno mines, Encinasola area, in Huelva province, SW Spain; Gava mines in Catalonia, NE Spain, and other possible sources in Europe as Pannece, France. Geochemical data were subjected to statistical analysis selecting different chemical variables in Variscites and turquoises. A relatively good discrimination has been obtained through an interactive geochemical model using specially the Fe-Cr-V relationship, represented by a triangular diagram and the elementary relationships between other chemical elements. These results show the existence, in Neolithic and Chalcolithic times, of great exchange networks of exotic or prestige lithic products that extended in Western Europe, hundreds and thousands of kilometres away from their source areas.
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Provenance des parures en Variscite du néolithique européen : élaboration d’un modèle chimiométrique
2019Co-Authors: Guirec Querré, Thomas Calligaro, Serge Cassen, Marie-pierre Dabard, Salvador Domínguez-bellaAbstract:La découverte d'éléments de parure en callaïs, minéraux de couleur verte, dans les tombes néolithiques en Europe dès le XIXème siècle, a posé la question de leur provenance. Un programme d'analyses PIXE étalé sur plusieurs années a été mis en place afin de répondre à cette question sur la base de données objectives. L'inventaire exhaustif des gisements ouest européens décrit leurs principales caractéristiques géologiques et archéologiques. Leur implantation géographique par rapport aux sites archéologiques permet de faire un premier tri quant à leurs potentialités d'exploitation au Néolithique. Un large échantillonnage représentatif des principaux gisements a été constitué et les échantillons ont été analysés par la méthode PIXE à l'aide d'un faisceau extrait pour obtenir leur composition chimique élémentaire de manière relativement rapide et non destructive. Les concentrations en éléments majeurs permettent de connaître la nature minéralogique des filons et des nodules (Variscite et/ou turquoise). Le spectre des éléments mineurs et en traces montre des variations intrinsèques, propres à chaque gisement. Les teneurs en fer, vanadium, chrome, arsenic, silicium, calcium, zirconium, uranium, leurs variations absolues et celles de leur rapport de concentration sont particulièrement pertinentes pour discriminer les 5 principales sources géologiques potentielles. Sur la base de ces variations de composition, il est possible d'élaborer un modèle chimiométrique de provenance. Ce modèle permet de déterminer la provenance d'un ensemble archéologique de parure en Variscite parmi les 5 plus grands gisements européens de Variscite susceptibles d'avoir été exploités par les groupes humains vivant durant le Néolithique dans l'ouest européen.
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provenance des parures en Variscite du neolithique europeen elaboration d un modele chimiometrique
2019Co-Authors: Guirec Querré, Thomas Calligaro, Serge Cassen, Marie-pierre Dabard, Salvador DominguezbellaAbstract:La decouverte d'elements de parure en callais, mineraux de couleur verte, dans les tombes neolithiques en Europe des le XIXeme siecle, a pose la question de leur provenance. Un programme d'analyses PIXE etale sur plusieurs annees a ete mis en place afin de repondre a cette question sur la base de donnees objectives. L'inventaire exhaustif des gisements ouest europeens decrit leurs principales caracteristiques geologiques et archeologiques. Leur implantation geographique par rapport aux sites archeologiques permet de faire un premier tri quant a leurs potentialites d'exploitation au Neolithique. Un large echantillonnage representatif des principaux gisements a ete constitue et les echantillons ont ete analyses par la methode PIXE a l'aide d'un faisceau extrait pour obtenir leur composition chimique elementaire de maniere relativement rapide et non destructive. Les concentrations en elements majeurs permettent de connaitre la nature mineralogique des filons et des nodules (Variscite et/ou turquoise). Le spectre des elements mineurs et en traces montre des variations intrinseques, propres a chaque gisement. Les teneurs en fer, vanadium, chrome, arsenic, silicium, calcium, zirconium, uranium, leurs variations absolues et celles de leur rapport de concentration sont particulierement pertinentes pour discriminer les 5 principales sources geologiques potentielles. Sur la base de ces variations de composition, il est possible d'elaborer un modele chimiometrique de provenance. Ce modele permet de determiner la provenance d'un ensemble archeologique de parure en Variscite parmi les 5 plus grands gisements europeens de Variscite susceptibles d'avoir ete exploites par les groupes humains vivant durant le Neolithique dans l'ouest europeen.
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Origine des bijoux néolithiques en callaïs de l'ouest de la France
2019Co-Authors: Guirec Querré, Thomas Calligaro, Serge CassenAbstract:747 perles et pendeloques en callaïs sur les 841 connues actuellement provenant de 27 occupations néolithiques de la moitié nord de la France ont été analysées par PIXE, afin de déterminer leur nature minéralogique et leur provenance. La grande majorité de ces éléments de parure est concentrée autour du golfe du Morbihan, dans la région de Carnac (56). Les concentrations en aluminium et phosphore constituant les deux éléments majeurs de la Variscite ont été détectés dans la très grande majorité des cas. Seules cinq perles, notamment deux du Tumulus Saint-Michel et une du Mané Lud présentent également des teneurs élevées en cuivre, typiques de la turquoise. L'utilisation du modèle chimiométrique élaboré sur la base des analyses PIXE des principaux gisements ouest-européens selon un ensemble de critères géochimiques discriminants permet, pour 22 sites, de déterminer la provenance des ensembles de parure en Variscite. Les résultats confirment qu'aucun objet ne provient de Pannecé, l'unique gisement français, ni des mines néolithiques de Can Tintorer dans la région de Barcelone. La totalité des perles et pendeloques provient uniquement des deux gisements ibériques orientés vers la façade atlantique : Encinasola (Huelva) au sud-ouest et Palazuelo (Zamora) au nord-ouest. L'étude de la chronologie des approvisionnements montre que les plus anciennes perles et pendeloques proviennent d'Encinasola et qu'à partir de 4000-4300 ans avant notre ère s'effectue un basculement et que les éléments de parure en Variscite proviennent de Palazuelo. Durant le Néolithique, des transferts sur de longues distances, pouvant atteindre près de 2000 kilomètres, se sont produits sur la façade atlantique, réseaux de transfert qui ont évolué au cours des trois millénaires concernés par ce type de mobilier.
