The French Massif Central and Massif Armoricain belong to three tectonic and paleogeographic domains of the Medio-Europa Variscan Orogen. The entire Massif Central and southern part of the Massif Armoricain belong to the North Gondwana margin, the Central Armorican Domain is a part of Armorica microcontinent and the Leon Domain is a piece of another microcontinent. The N. Gondwana margin and Leon Domain are made of a stack of metamorphic nappes, conversely, the Central Armorican Domain consists of a Proterozoic basement built up by the Neoproterozoic Cadomian orogeny and a Paleozoic sedimentary cover weakly deformed by upright folds related to wrenching. The architecture of the North Gondwana margin results of three main tectonic-metamorphic events that follow an early Late Silurian (ca 415 Ma) high-pressure metamorphism whose associated structures are poorly documented. The Early Devonian D1 event is responsible for top-to-the-SW nappes coeval with migmatization and exhumation of high-pressure rocks around 385-380 Ma. The Late Devonian-Early Carboniferous D2 event is a top-to-the-NW shearing coeval with an intermediate pressure-temperature metamorphism dated around 360-350 Ma. The Visean D3 event is a top-to-the-south shearing widespread in the south Massif Central whereas in north Massif Central, D3 corresponds to the onset of synorogenic extension. The Variscan Belt is also characterized by a widespread magmatism. The Early-Middle Devonian calc-alkaline magmatism is related to the southward subduction of the Rheic Ocean. The Carboniferous magmatic events are the crustal melting response of D2 and D3 tectonic events. Late Visean, Namurian and Westphalian magmatic stages are coeval with extensional tectonics controlled by NW-SE stretching. These structural, metamorphic and magmatic events are replaced in a geodynamic evolution model involving two cycles of microcontinent drifting, rewelding and continental collision.

In the southern part of the French Armorican massif, the Ligerian domain is located along the boundary between Gondwana and Armorica. Lithological, geochemical and structural data on the Saint-Georges-sur-Loire Unit, which is the northern part of the Ligerian domain, allow us to distinguish two sub-units. A southern sub-unit, formed by various blocks (chert, limestone, sandstone, rhyolite, mafic rocks) of Silurian to Middle Devonian age included as olistoliths in a Middle-Late Devonian terrigeneous matrix, overthrusts a sandstone-pelite northern sub-unit. Both units experienced two deformation events. The first one is a top-to-the-NW thrusting and the second one is a left-lateral wrenching. The Saint-Georges-sur-Loire Unit is an accretionary prism formed during the Late Devonian closure of the Layon rift, coeval with the main phase of the Variscan orogeny. The Layon rift, which according to the mafic olistoliths was partly floored by oceanic crust, appears as a buffer structural zone that accounts for the lack in Central Brittany of any tectonic or sedimentary echo of the closure of the Medio-European Ocean. The tectonic evolution of the Saint-Georges-sur-Loire Unit supports a polyorogenic model for this part of the Variscan Belt.

L'unite de Saint-Georges-sur-Loire situee dans le domaine ligerien entre la ride de Lanvaux-Les-Ponts-de-Ce au Nord et la faille de Nort-sur-Erdre au Sud constitue une zone clef pour la comprehension de la geodynamique hercynienne. Cette unite est constituee de deux sous-unites distinctes, l'unite sans blocs schisto-greseuse au Nord (Ordovicien inferieur a Silurien inferieur) et l'unite a blocs, schisto-greseuse et volcanique au Sud. L'unite a blocs meridionale est consideree comme un olistostrome devonien a blocs mono- ou poly-lithologiques de nature et d'âges varies : radiolarites (Silurien), calcaires (Silurien sup. a Devonien moy.), laves et tufs basiques (Silurien), rhyolites et gres. Ces blocs sont remanies au sein d'une matrice schisto-greseuse d'âge probable Devonien moyen a superieur. Cette unite a blocs est deformee par des plis synchisteux deverses vers le Nord et presente une lineation d'allongement N-S a N60E ; elle chevauche l'unite sans blocs. Des plis droits ou legerement deverses vers le Nord sont rencontres dans l'unite sans blocs, ainsi qu'une lineation minerale et d'allongement NW-SE de plus en plus marquee a mesure que l'on se rapproche de la bande ductile de Lanvaux, ou elle s'exprime pleinement. L'unite a blocs constitue egalement une zone intermediaire entre des terrains paleozoiques peu metamorphiques a vergences nord dans le sous-domaine ligerien septentrional et des terrains impliques dans la structure de nappes de Champtoceaux et le metamorphisme de haute pression, a structures vers le Sud dans le sous-domaine meridional. Une nouvelle interpretation geodynamique integrant toutes les donnees disponibles est donc proposee afin de replacer l'unite de Saint-Georges-sur-Loire dans un schema interpretatif a plus grande echelle. Cette interpretation precise l'evolution du domaine nord gondwanien et de la marge sud de la microplaque Armorica au Paleozoique. Elle souligne l'existence de zones de subductions diachrones et de vergences opposees au sein de la microplaque Armorica, met en valeur le role de la faille de Nort-sur-Erdre, consideree comme une zone de suture polyphasee majeure en Armorique et precise la nature geotectonique de l'unite de Saint-Georges-sur-Loire, formation chaotique deposee dans un prisme d'accretion lie a la fermeture du « rift du Layon ».

A new approach to explain the Western Hercynian Belt is given by a lithological and structural study of the St‐Georges‐sur‐Loire Unit (southern part of the French Armorican Massif). This unit is interpreted as a Devonian olistostrome sheared to the North. It is formed in an accretionary prism linked with a south‐dipping subduction zone, called the Layon subduction zone. It results from a chaotic sedimentation including exotic blocks of various lithologies and ages in a Middle Devonian pebbly mudstone matrix. An inovative geodynamic model involving microblock rifting and collision is discussed to explain the presence of the St‐Georges‐sur‐Loire Unit in the Armorican Massif. This model involves two Palaeozoic subductions and emphasizes the role of the Nort‐sur‐Erdre fault, considered as a major polyphase suture zone separating South and Central Armorica.

This paper was presented as a session lecture at the XXXIII ICCC in Florence (29 August–4 September 1998). It intends to point out some recent achievements in the chemistry and physics of transition metal polycyanides in the field of molecular magnetism. Prussian blue is sometimes considered as the first coordination compound and the paper shows how it is possible to obtain brand new results with Prussian blue analogues when looking at these antique systems with fresh eyes. Hexacyanometalates revealed in the last few years as very flexible molecular precursors to build three-dimensional molecule-based magnets with tunable and high Curie temperatures or to grow high nuclearity clusters with tunable high spins and anisotropy. The use of a localized electron orbital model allowed the authors’ team to push the Curie temperatures from 5.6 K in the Prussian blue itself to above room temperature in a vanadium–chromium Prussian blue analogue. Several groups confirmed the result and are improving it. In the same way, high spin molecules with ground spin states ranging from S=3/2 to 27/2 were obtained. The paper reviews some of the steps which lead to these spectacular findings and some of the prospects opened in molecular materials by this revival of polycyanide chemistry.

In the Armorican Massif the St-Clement-de-la-Place antiform results from two distinct but homoaxial deformations. The younger one corresponds to the emplacement of a granodiorite dated here at 312 Ma (Ar-40/Ar-39 on biotite). Although coeval to the dextral motion of the South Armorican shear zone, this pluton does not present the specific features of a synkinematic granite, the early deformation is responsible for the mylonitisation of an orthogneiss with a subhorizontal foliation and NW-SE trending lineation. Top-to-the-NW shearing, incompatible with the Carboniferous wrench tectonics, is the kinematic peculiarity of this orthogneiss. (C) Academie des sciences/Elsevier, Paris.

The influence of electronic and structural effects in FeCl4 and FeCl6 entities has been investigated through X-ray absorption spectroscopy and theoretical calculations of electronic transition energies using the MS-LSD method. The relative importance of the formal oxidation degree of the metal, the metal-ligand distance, and the symmetry of the site on the energy of near-edge structures are studied. The principal effect is the stabilization of the iron Is orbital in the ground state when the oxidation degree increases. The accuracy of the theoretical determination regarding the experimental spectra is discussed. The incidence of the same electronic parameters on the intensities of near-edge structures is also investigated through a transition cross section calculation.

A series of titanium silicalites of MFI structure, active in n-hexane oxyfunctionalization was investigated by XANES and EXAFS. A multiple scattering EXAFS analysis has been performed using the FEFF6 code. Titanium atoms are sited in substitutional framework sites of Td symmetry, with a large TiOSi angle (≈160°). At high Ti content, extra framework anatase TiO2 is formed.

An extended X-ray absorption fine structure study at the copper K-edge of Nasicon-type phosphates with formulas Cu1M2(PO4)3 (M = Ti, Zr), A1-xCuIxZr2(PO4)3 (0 < x ≤ 1, A = Na; x = 0.5, A = H) and CuII0.5M2(PO4)3 (M = Ti, Zr) is reported. For Cu1Zr2(PO4)3, H0.5CuI0.5Zr2(PO4)3, and Na1-xCuIxZr2(PO4)3 (0 <: x ≤ 1) copper pairs have been evidenced (Cu1-Cu1 = 2.40 Å) in the M1 site of the Nasicon structure. In contrast, for the copper(II) phosphates, the Cu2+ ions are surrounded by four oxygen atoms at about 1.95 Å. Copper distribution in these various materials is discussed in relation to structural parameters.

A series of titanium silicalites of MFI structure, active in n-hexane oxyfunctionalization, were investigated by FT-IR, XPS, XANES and EXAFS spectroscopies to characterize the titanium sites. Most of the titanium ions are sited in a non-substitutional framework sites of C4v symmetry rather than T-d. The framework IR bands reveal that the [SiO4] units, linked to titanium via double Ti-O-Si bridges, have a symmetry lowered from T-d to at least C-2v. The decrease of the 960 cm(-1) IR band upon the effect of adsorption of water or H2O2 is attributed to the partial hydrolysis of the double bridges leading to a linkage by single bridges. A molecular simulation investigation shows that such sites can be accommodated in the structure by disruption of the four (Si) membered rings.

X-ray absorption measurements (XANES-EXAFS) have been performed on the series of glasses with composition xTiO2·yNa2O·zP2O5 (y/z = 43and x = 10.7 mol%). Two TiO distances of about 1.65 Å and 1.95 Å have been found, and TiTi correlations (about 3.4 Å) have been observed for x = 26.4 and 34.6%. A model is proposed to explain the progressive modifications of XANES and EXAFS spectra with increasing x. Two different types of site for titanium have been identified in these glasses, in which Ti is respectively sixfold- and fivefold-coordinated. The rate of fivefold-coordinated Ti increases with increasing x.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

With a dedicated cell which allows us to investigate on line the structural modifications occuring during different electrochemical reactions by X‐ray absorption spectroscopy, we have been able to follow directly the successive steps of the inter/deintercalation process of lithium within the V2O5 matrix. The in situ measurements performed on LixV2O5 compounds in the range (0<×<0.8) allows one to point out small and significant modifications of both electronic state and atomic surroundings of vandium. Combining the X‐ray absorption results, we suggest a model for the evolution of the cathode material.