Bernard Sanjuan

Aluminum hydroxide solubility in aqueous solutions containing fluoride ions at 50°C

Aluminum hydroxide solubility experiments were performed at 50°C using the method of May et al. (1979b). The shift observed by May at 25°C between acidic and alkaline solution was also observed in our experiments. Infrared spectra of solids separated from solutions show that the low solubility values observed in alkaline medium can be related to the formation of a new solid phase: bayerite (or nordstrandite) (pK∗s4 = 13.4 at 50°C) in the presence of initial gibbsite. A second set of experiments was performed in solutions containing fluoride ions in the 10−4−10−3 M concentration range. Fluorocomplexes are important in acidic solution; Al(OH)mFp mixed complexes can be important in the minimum of Al solubility when total fluoride exceeds 10−3 M. We propose values of thermodynamic data for Al(OH)2F2−, Al(OH)F3−, Al(OH)2F0dg and for aluminum trifluoride solid. The difference of aluminum hydroxide solubility between acidic and alkaline medium is still observed.

Significance of the depth-related transition montmorillonite-beidellite in the Bouillante geothermal field (Guadeloupe, Lesser Antilles)

Abstract The crystal structure and crystal chemistry of dioctahedral smectites in high-enthalpy geothermal systems were investigated through samples collected in two wells drilled in the Bouillante geothermal area to understand the factors that control their vertical variation. Smectites were examined by optical and scanning electron microscopy, electron microprobe analysis, X-ray diffraction (XRD), Fourier transform infrared spectrometry (FTIR), and oxygen-isotope analysis. Smectites predominate within the upper part of the drill holes (up to 260 m depth; temperature range: 67-163 °C). The XRD, FTIR, and chemical microanalyses clearly demonstrate a transition from montmorillonite to beidellite with increasing depth and temperature that proceeded through interstratification of beidellite-like and montmorillonite-like layers. Montmorillonite predominates at temperatures below 100 °C, whereas beidellite predominates between 110 and 163 °C. However, this transition is not explained by a thermally controlled beidellitization process but appears to be related to hydrothermal fluids from which these smectites precipitated. The δ18O values of the fluids that equilibrated with smectites (-3.3 to 0.4‰) indicate that beidellitic smectite precipitated from the hot geothermal fluid associated with minor amounts of residual solutions resulting from local boiling. In the same way, montmorillonitic smectite precipitated from reacting solutions whose origin lies in the phreatic water table (±seawater contribution) associated with minor amounts of liquids resulting from the condensation of vapors escaped from the boiling zones. The mixing rate of geothermal fluid with meteoric waters exerted a major influence on the montmorillonite vs. beidellite ratio of the smectite material as underlined by the irregular depth-related smectite transition.

Origine des substances dissoutes dans les eaux des sources thermales et des forages de la région Asal-Ghoubbet (République de Djibouti)

Abstract From a chemical and Sr isotopic study of hot fluids from the Asal area, Republic of Djibouti, we conclude that the fluids result from basalt-seawater interaction, associated with variable evaporation. Some spring waters result from a mixing between geothermal water and seawater or Asal lake water. In contrast with previous studies, dissolution from evaporitic rocks appears unimportant. The negative boron anomaly in the fluids from the boreholes suggests the presence of a vapour-dominated subsystem in the Asal geothermal field.

Influence of the temperature of CO2-rich springs on their aluminium and rare-earth element contents

Abstract In a set of thermomineral waters of about constant major-element composition, Al, Fe and rare-earth element (REE) concentrations are strongly dependent on the spring temperature. Al and REE concentrations are not affected by CO2 degassing at the outflow of the waters. Al concentrations vary by one order of magnitude between the coldest and the hottest waters. This change in total concentration is related to a change in the speciation: fluoride complexes (mainly AlF03) are the dominant species in cold waters; hydroxide or fluoro-hydroxide complexes are more important at higher temperature. REE patterns vary smoothly as a function of the atomic number from a regular decrease for the hottest water ( Ce Yb = 30 ) to a large increase from Ce to Dy, followed by a slight decrease from Dy to Yb for the coldest waters ( Ce Yb ∼ 0.3 ; Ce Dy ∼ 0.15 ).