Phase de Laves dans la première opale CT bidisperse

A Mexican opal from the state of Jalisco has a bidisperse, perfectly ordered structure with framboid silica lepispheres of two different diameters. Its structure has been determined by the observation of a vicinal section, close to a dense plane. Fresh breaks along a vicinal plane emphasized regular steps, which were studied by means of SEM. We have been able to propose unit cell models, theoretical positions of the two sets of spheres, and probable space group, P6₃/mmc. This structure arranges similarly to that of a diatomic compound having an AB₂ stoichiometry and corresponds to a cubic (MgCu₂-type) or hexagonal (MgZn₂-type) Laves phase. This last variety seems to be the most probable structure. To cite this article: J.-P. Gauthier et al., C. R. Geoscience 336 (2004).     

A Compatible Single-Phase/Two-Phase Numerical Model: 1. Modeling the Transient Salt-Water/Fresh-Water Interface Motion

Abstract A numerical model (NEWVAR) to simulate the transient movement of a discrete interface between salt water and fresh water has been developed. NEWVAR is designed to allow the analysis of a regional two-dimensional ground-water flow in coastal aquifers. The numerical solution permits the prediction of both regional fresh-water levels and two-dimensional fresh-water/salt-water interface by using nested square meshes.
The numerical solution is based on the finite-difference method; the Gauss-Jordan direct method is used for solving steady- and unsteady-state linear equations. Different procedures are used to avoid numerical difficulties in the transient position of the interface toe for two-dimensional areal flow.
The numerical solution was tested against the analytical ones for the cases of an advancing interface and of a floating fresh-water lens over sea water. These tests showed good agreement, thus verifying the finite-difference approximation. The results of an application of this model to a real aquifer are discussed in a companion paper entitled: "A Compatible Single-Phase/Two-Phase Numerical Model 2. Application to a Coastal Aquifer in Mexico."     

Responding to unmet food needs of displaced persons or refugees:

This paper identifies two types of displaced persons in two different ecological zones in Cameroon as a result of two major causes; a poisonous lake gas explosion in the North West and the effects of a civil war in Chad on Kousseri town in North Cameroon. The displaced persons, through no fault of theirs had to look up to both international and local donor organizations for assistance. This paper addresses the usual unmet food needs, analyses some of the performances that often go wrong but which can be corrected. The paper points out that donor agencies and the host government played major roles in raising food, cash, etc. but regrets to note that unsurmountable logistical problems, improper choice and use of personnel with wider expertise to handle specialized problems, inexistent buffer stocks and non-involvement of the displaced persons in managing some of the problems hindered hard-generated resources from reaching most of the beneficiaries. Better performances lie in reducing dependency on continuous external supplies and also in the local hosts allowing room for small scale agro-pastoral productions which should satisfy and even improve on the unmet nutritional difficiencies and rations of the displaced persons.     

Structures and tectonic evolution of the external zone of Alpine Corsica

This paper presents a structural analysis of the external zone of Alpine Corsica, including the autochthonous domain and overlying external nappes (Santa Lucia and Balagne nappes). Two stages of nappe emplacement are identified occurring prior to and after the deposition of the Eocene sediments which were laid down upon first generation thrust contacts but are imbricated with their composite (continental and ophiolitic) basement by second generation thrusts. Five generations of structures with regional extent have been distinguished. However, the first generation has not been recognized within the visible part of the autochthon domain.Eoalpine first generation structures, restricted to allochthonous units, and Late Eocene to Early Oligocene second generation structures were nearly contemporaneous with the two stages of thrusting. The precise significance of E-W third generation structures is poorly understood. Broadly N-S fourth generation structures resulted from Oligocene compressive tectonics (folding and local backthrusting). Finally, fifth generation structures were generated during a Miocene extensional stage.These results are partly consistent with structural features previously reported in the southern and the northern outcrops of the Schistes lustrés, i.e. the main part of the allochthonous domain. A summary of a regional tectonic evolution is thus proposed for Alpine Corsica from Eoalpine obduction to Miocene extension.     

Experience of produced water treatment in the North Sea

This paper describes the experience of the Elf company regarding produced water treatment and disposal in the North Sea. It focuses on three fields where difficulties were faced: it evaluates the various treatment techniques used (plate separators, hydrocyclones, vapour stripping units, injection into a disposal well), the reasons for the problems encountered during the life of these fields and the efforts made to obtain a satisfactory quality of the produced water discharged. The contribution of recent or new technologies put on the market is discussed. It shows that, because of the nature of the effluents to be treated, and the fact that produced water characteristics depend largely on upstream treatment and processes, it is still very difficult to anticipate the correct design of an installation or to extrapolate from laboratory tests. For these reasons and others, the improvement of performance requires extensive effort by operators, process and design engineers in order to improve the quality of effluent discharged. Finally, it expounds the limits of the efforts made by industry to reduce the quantity of hydrocarbons released into the sea or to comply with present regulations. It emphasizes that not only costs are to be taken into account but also the adverse consequences of a further reduction of the limit for discharges, if this should happen. Such a measure would to the use of techniques which, in many cases, would increase the total impact of the produced water treatment and the human risk.     

On the entropy of glaucophane Na2Mg3Al2Si8O22(OH)2

The heat capacity of glaucophane from the Sesia-Lanza region of Italy having the approximate composition (Na_1.93Ca_0.05Fe_0.02) (Mg_2.60Fe_0.41) (Al_1.83Fe_0.15Cr_0.01) (Si_7.92Al_0.08)O₂₂(OH)₂ was measured by adiabatic calorimetry between 4.6 and 359.4 K. After correcting the C _p ⁰ data to values for ideal glaucophane, Na₂Mg₃Al₂Si₈O₂₂(OH)₂ the third-law entropy S ₂₉₈ ⁰ -S ₀ ⁰ was calculated to be 541.2±3.0 J·mol^-1·K^-1. Our value for S ₂₉₈ ⁰ -S ₀ ⁰ is 12.0 J·mol^-1·K^-1 (2.2%) smaller than the value of Likhoydov et al. (1982), 553.2±3.0, is within 6.2 J·mol^-1·K^-1 of the value estimated by Holland (1988), and agrees remarkably well with the value calculated by Gillet et al. (1989) from spectroscopic data, 539 J·mol^-1·K^-1.     

Design charts for a deep circular tunnel under non-uniform loading

Summary This paper describes design charts which can be used to quickly predict the size and shape of the failed rock regions caused by the excavation of a deep tunnel, and the induced closure for cases of non-uniformin-situ stress fields. Through a simple graphical construction, the design charts can also be used to evaluate the performance of a support system.Previously of J. F. T. Agapito and Associates, U.S.A.