Study of Al-substituted hematites, prepared from thermal treatment of lepidocrocite     

E. Van Van San  E. De Grave  R. E. Vandenberghe  H. O. Desseyn  L. Datas  V. Barrón  A. Rousset 《Physics and Chemistry of Minerals》2001,28(7):488-497

A study of the characteristics of the Morin transition in aluminous hematites, α-(Fe_1?xAl_x)₂O₃, produced from thermally transformed lepidocrocites, is reported. Six compositions with Al contents between 0.2 and 10 at% have been considered. It is argued that these samples present the advantage that they contain smaller amounts of hydroxyl and water as compared to hematites obtained by other preparation methods. The samples were characterised by a variety of conventional techniques, including thermal analyses, X-ray diffraction, FTIR, TEM/EDX, BET surface-area measurements and diffuse reflectance spectroscopy. All results indicate that the Al is structurally incorporated in the hematite lattices. Transmission Mössbauer spectra were recorded at various temperatures between 80?K and room temperature in order to precisely determine the Morin-transition region and the spin structure in both the low-temperature antiferromagnetic and weakly ferromagnetic states. It was found that the Morin-transition temperatures are markedly higher as compared to similar hematites made from aluminous goethites and that a transition phenomenon persists to an Al substitution of up to at least 10 at%. This different behaviour is ascribed to lower concentrations of structural hydroxyl groups in these lepidocrocite-based hematites.  相似文献   

How to model shallow water‐table depth variations: the case of the Kervidy‐Naizin catchment,France     

Jrme Molnat  Chantal Gascuel‐Odoux  Philippe Davy  Patrick Durand 《水文研究》2005,19(4):901-920

The aim of this work is threefold: (1) to identify the main characteristics of water‐table variations from observations in the Kervidy‐Naizin catchment, a small catchment located in western France; (2) to confront these characteristics with the assumptions of the Topmodel concepts; and (3) to analyse how relaxation of the assumptions could improve the simulation of distributed water‐table depth. A network of piezometers was installed in the Kervidy‐Naizin catchment and the water‐table depth was recorded every 15 min in each piezometer from 1997 to 2000. From these observations, the Kervidy‐Naizin groundwater appears to be characteristic of shallow groundwaters of catchments underlain by crystalline bedrock, in view of the strong relation between water distribution and topography in the bottom land of the hillslopes. However, from midslope to summit, the water table can attain a depth of many metres, it does not parallel the topographic surface and it remains very responsive to rainfall. In particular, hydraulic gradients vary with time and are not equivalent to the soil surface slope. These characteristics call into question some assumptions that are used to model shallow lateral subsurface flow in saturated conditions. We investigate the performance of three models (Topmodel, a kinematic model and a diffusive model) in simulating the hourly distributed water‐table depths along one of the hillslope transects, as well as the hourly stream discharge. For each model, two sets of parameters are identified following a Monte Carlo procedure applied to a simulation period of 2649 h. The performance of each model with each of the two parameter sets is evaluated over a test period of 2158 h. All three models, and hence their underlying assumptions, appear to reproduce adequately the stream discharge variations and water‐table depths in bottom lands at the foot of the hillslope. To simulate the groundwater depth distribution over the whole hillslope, the steady‐state assumption (Topmodel) is quite constraining and leads to unacceptable water‐table depths in midslope and summit areas. Once this assumption is relaxed (kinematic model), the water‐table simulation is improved. A subsequent relaxation of the hydraulic gradient (diffusive model) further improves water‐table simulations in the summit area, while still yielding realistic water‐table depths in the bottom land. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

Non-Fickian mixing: Temporal evolution of the scalar dissipation rate in heterogeneous porous media     

Tanguy Le Borgne  Marco Dentz  Diogo Bolster  Jesus Carrera  Jean-Raynald de Dreuzy  Philippe Davy 《Advances in water resources》2010

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Discrete clay diagenesis in a very low-permeable sequence constrained by an isotopic (K–Ar and Rb–Sr) study     

Davy?Rousset  Norbert?ClauerEmail author 《Contributions to Mineralogy and Petrology》2003,145(2):182-198

Multimethod analyses of several size fractions of clays were used to reconstruct the diagenetic history of the shallow buried claystones within the Paris Basin. A systematic decrease occurred in K-Ar dates relative to the decrease in size of the clay fractions, signifying higher amounts of newly formed clay material in the finer fractions. We suggest that the authigenic clay minerals occurring in the fine fractions had an Al-montmorillonite composition. By assuming that the isotopic K-Ar and Rb-Sr dates obtained on a bentonite layer in the sedimentary sequence stand for pure, authigenic clay minerals, one may interpret all K-Ar dates as mixtures of one authigenic and two detrital end-members. The results imply that a period of low sea level favoured diagenetic smectite-type clay formation about 10-15 million years after deposition of the sediments. Signatures of limited-scale chemical and isotopic homogenisation mean that the rock volumes affected by the diagenetic modifications had to be quite limited. The study of clay minerals extracted from some stylolites further suggests that any overpressure related to the origin of stylolites had no effect on clay authigenesis.  相似文献   

Thickening history of the Western Alps     

Ph. Gillet  P. Choukroune  M. Ballvre  Ph. Davy 《Earth and Planetary Science Letters》1986,78(1)

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A precipiton‐based approach to model hydro‐sedimentary hazards induced by large sediment supplies in alluvial fans          下载免费PDF全文

Thomas Croissant  Dimitri Lague  Philippe Davy  Tim Davies  Philippe Steer 《地球表面变化过程与地形》2017,42(13):2054-2067

Mountain ranges are frequently subjected to mass wasting events triggered by storms or earthquakes and supply large volumes of sediment into river networks. Besides altering river dynamics, large sediment deliveries to alluvial fans are known to cause hydro‐sedimentary hazards such as flooding and river avulsion. Here we explore how the sediment supply history affects hydro‐sedimentary river and fan hazards, and how well can it be predicted given the uncertainties on boundary conditions. We use the 2D morphodynamic model Eros with a new 2D hydrodynamic model driven by a sequence of flood, a sediment entrainment/transport/deposition model and a bank erosion law. We first evaluate the model against a natural case: the 1999 Mount Adams rock avalanche and subsequent avulsion on the Poerua river fan (West Coast, New Zealand). By adjusting for the unknown sediment supply history, Eros predicts the evolution of the alluvial riverbed during the first post‐landslide stages within 30 cm. The model is subsequently used to infer how the sediment supply volume and rate control the fan aggradation patterns and associated hazards. Our results show that the total injected volume controls the overall levels of aggradation, but supply rates have a major control on the location of preferential deposition, avulsion and increased flooding risk. Fan re‐incision following exhaustion of the landslide‐derived sediment supply leads to sediment transfer and deposition downstream and poses similar, but delayed, hydro‐sedimentary hazards. Our results demonstrate that 2D morphodynamics models are able to capture the full range of hazards occurring in alluvial fans including river avulsion aggradation and floods. However, only ensemble simulations accounting for uncertainties in boundary conditions (e.g., discharge history, initial topography, grain size) as well as model realization (e.g., non‐linearities in hydro‐sedimentary processes) can be used to produce probabilistic hazards maps relevant for decision making. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献