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URA 724 est intégrée dans l'UMR6113 ISTO CNRS Université d'OrléansInternational audienceA semiquantitative analysis of the infrared spectra from 650 to 4000 cm-' of six vitrains and the coals associated with three of them is presented. This analysis supplies information on the concentration of several functionalities in the samples. Relationships between infrared data and several parameters (carbon content, vitrinite reflectance, pyrolysis data) are discussed. The results thus obtained contribute to a better understanding of vitrinite structure. They also help to explain the discrepancies between rank parameters obtained from chemical analyses and those determined by petrographic measurements. The comparison between the infrared data of the Cretacic-Albian vitrain from Teruel (Spain) and its associated coal reveals the unusual chemical composition of this vitrain. The infrared analysis of this vitrain and the vitrian from Asturias (Jurassic-Malm) shows unusual features in comparison with “normal” coals. The intensity of the aliphatic peaks relative to the aromatic (A factor) in these vitrains is characteristic of immature samples, closer to those of type I1 kerogens than type 111. This result agrees with the perhydrous character of these samples and helps to explain volatile matter and pyrolysis data. However, for the vitrain from Asturias, the intensity of the bands due to aromatic hydrogen and the ratio between aromatic and aliphatic hydrogen are consistent with a stage of evolution that is higher than the diagenesislcatagenesis transition in agreement with other chemical and petrographic data. In the vitrains from Asturias and Teruel an absorption band at 1500 cm-l is assigned to aromatic C=C bonds. In these vitrains, the band centered at 815 cm-l is the most prominent in the 900-700 cm-l region. These spectral features are unusual in coal spectra, but they seem to be characteristic of perhydrous coals

At neutral pH and 25 °C, the interaction of arsenate-bearing aqueous solutions with gypsum results in surface precipitation of pharmacolite (CaHAsO 4 • 2H 2 O) crystals. The crystals grow oriented onto the gypsum surface, forming an epitaxy. Using an A-centered unit-cell setting for both pharmacolite (Aa) and gypsum (A2/a), the epitaxial relationship is found to be ( 010) Gy | (010) Ph and [101] Gy | [101] Ph . Pharmacolite forms thick three-dimensional crystals elongated on [101] with {010}, { j 111}, and {1 j 1 j 1} as major forms. Both the crystal morphology and the epitaxial orientation are interpreted on the basis of the bond arrangement in the structure of both phases. The reaction can be envisaged as a sort of solvent-mediated replacement of gypsum by pharmacolite. Under these experimental conditions, the process stops at a "pseudo-equilibrium" end point in which the reactive solids become completely isolated from the aqueous solution by the epitaxial coating of pharmacolite crystals. The thermodynamic solubility product of pharmacolite was determined at this stage and found to be pK ) 4.68 ( 0.04. The reaction paths actually followed by the system and the "true equilibrium" end point are modeled using the geochemical code PHREEQC.

UMR 6531/FR 09 CNRS/Université d'Orléans est remplacé par CNRS Univ. d'Orléans, UMR6113 - ISTOThe effect of the progressive increase in temperature (off-line pyrolysis in an open-medium system) on a monomaceralic coal (low rank and pure vitrinite) has been studied on the basis of changes in its chemical and physical structure. The properties of the solid residues and liquid effluents obtained were studied using a wide range of petrographic and geochemical techniques. Results support the validity of this pyrolysis method for following the physico–chemical processes that occur in vitrinite of bituminous coal rank. Of special interest is the information concerning the control of the structure on the vitrinite reflectance parameter in the bituminization stage. The evolution of properties and changes occurring at this stage show that the increase in vitrinite reflectance is strongly related and depends on changes in the chemical structure of this maceral during heating rather than on changes in elemental composition. However, any relationship between reflectance and chemical properties needs to be established empirically. The mechanism during heating consists of a complex mixture of depolymerization and condensation reactions with the progressive homogenisation of the botanical structure of the vitrinite. Moreover, pyrolysis conditions lead to a rapid and massive expulsion of the thermally generated products, causing a significant decrease in the H/C atomic ratio and higher thermostability of the solid residues. The enhancement of cross-linking with the formation of C–O–C bridges, which have high activation energy also contributes to an increase in thermal stability. Results obtained from the production and expulsion of oil suggest that this method is suitable for providing an estimation of the amount of hydrocarbons that might be produced and migrate from coals of similar rank. This is of special interest for coals with significant petroligenous potential or for coals that are gas-prone, depending on their composition and rank

Under slightly acidic conditions and 25 °C, the interaction between phosphate-rich aqueous solutions and gypsum cleavage fragments results in the surface precipitation of brushite (CaHPO 4 ·2H 2 O) crystals, which grow epitaxially on the (010) surface of gypsum. Using an A-centered unit-cell setting for both brushite (Aa) and gypsum (A2/a), the epitaxial relationship implies matching of the planes (010) of both structures and correspondence between equivalent crystallographic directions within these planes. The crystal habit of the overgrowing brushite is thin tabular to laminar on {010} with {111} and {111} as side faces and a clear elongation on [101]. There are two orientations of the brushite plates on the gypsum surface related to each other by a twofold axis on [010]. Thus, the overgrowth is an aggregate of parallel brushite crystals that may be twin-related, with the twofold axis as the twin law. During the interaction, gypsum dissolution is coupled with brushite growth until saturation with respect to both minerals is reached. A model of this thermodynamically driven dissolution-crystallization process is presented using the geochemical code PHREEQC. The epitaxial relationships are explained by comparing the bond system and the crystallographic properties of both minerals.