Cristallochimie des biotites ferro-alumineuses dans le système Na2O-K2O-FeO-Fe2O3-Al2O3-SiO3-SiO2-H2O-HF. Analyse par spectrométries vibrationnelles et Mössbauer.

CE TRAVAIL CONCERNE L'ETUDE CRISTALLOCHIMIQUE DES BIOTITES FERRO-ALUMINEUSES DE SYNTHESE DANS LE SYSTEME NA20-K20-FE0-FE203-AL203-SI02-H20-HF, EN FONCTION DE LA TEMPERATURE ET DE LA FUGACITE D'OXYGENE. LES METHODES DE CARACTERISATION EMPLOYEES SONT LA DIFFRACTION X ET LES SPECTROMETRIES INFRAROUGE, RAMAN ET MOSSBAUER. EN L'ABSENCE DE FLUOR, LE FER FERRIQUE SE REPARTIT ENTRE SITES TETRA- ET OCTAEDRIQUES, SA TENEUR GLOBALE AUGMENTE AVEC LA FUGACITE D'OXYGENE. EN CONDITIONS HYDROTHERMALES, L'OXYDATION DU FER FAIT INTERVENIR AU MOINS TROIS MECANISMES, ET N'EST PAS ASSOCIEE A LA SEULE DEPROTONATION. LE POLE ANNITE THEORIQUE N'EST PAS STABLE ; L'ANNITE NE PEUT ETRE DECRITE QUE COMME UNE SOLUTION SOLIDE A CINQ CONSTITUANTS. LA SOLUBILITE DE F EST BEAUCOUP PLUS ELEVEE DANS L'ANNITE (XF = 0,5), OU ELLE PROVOQUE UNE DIMINUTION DE LA TENEUR EN FER FERRIQUE, QUE DANS LES BIOTITES ALUMINEUSES (XF = 0,2) DANS LESQUELLES LA PROPORTION DE FER FERRIQUE DIMINUE. CET ENSEMBLE DE DONNEES MONTRE UNE ADAPTATION DE LA STRUCTURE DE CES MICAS A LA FOIS AUX CONDITIONS DE GENESE ET A LA COMPOSITION GLOBALE DES SYSTEMES. EN PARTICULIER, LES MODIFICATIONS DES DISTRIBUTIONS CATIONIQUES INTERSITES ET INTERCOUCHES PERMETTENT LE MAINTIEN DE L'ELECTRONEUTRALITE LOCALE, ET L'ADAPTATION DIMENSIONNELLE DES COUCHES. LE DOMAINE DE STABILITE DES BIOTITES SODIQUES EST LIMITE AUX COMPOSITIONS LES PLUS ALUMINEUSES. D'UN POINT DE VUE METHODOLOGIQUE, L'AVANCEE LA PLUS SIGNIFICATIVE CONCERNE LA SPECTROMETRIE MOSSBAUER AVEC LA MISE EN EVIDENCE DE TROIS DOUBLETS FERREUX DANS CERTAINS CAS, ET D'ORDRES: FER FERREUX EN M1, FER FERRIQUE ET ALUMINIUM EN M2.Pas résumé en anglai

Structural effects of OH Þ F substitution in trioctahedral micas of the system K2O-FeO-Fe2O3-Al2O3-SiO2-H2O-HF.

The OH => F Substitution in trioctahedral ferrous micas has been investigated at 720 °C. 1 kbar PH2o- under /02conditions set by the MW (FeiOj-Fe^O) buffer.The starting compositions belong to the annite-siderophyllite join:K(Fe-, xAlx)(Si3.xAlUx)01(,(OH): with x 0 (annite). 0.5 (Fe-eastonile). and 0.75 (Es). In F-bearing system. thecompositions investigated belong to (OH.F)-annite. (OH.F)-Fe-eastonile and (OH.F)-Es joins. A single mica phase wasobserved lor (OH.F)-annite in 0 F Substitution induces local cationic changes and consequently a dimensional adaptation of sheets (limitedin such micas to a 5.5°). which in turn conlrols the fluorine solubility in these studied micas. The results alsoshow lhat the Fe-'*/Fe,„Ia| ratio in F-bearing micas is not only controlled by /02 but also by structural constraints.Thefluorine content of natura] biotiles has to be taken into aecount to cslimate oxygen fugacities prevailing in the rocks

Fe-F and Al-F avoidance rule in ferrous-aluminous (OF, F) biotites.

International audienceThe results of infrared and Raman spectroscopic investigations in the OH-slrelching and lattiee-mode regions in synthetic ferrous-aluminous (OH.F)-biotites are presented. In the OH-stretching region (3800-3200 cm '). all micas studied present a high intensity peak at high frequencies [3669 cm ' for (OH)-annite and 3641 cm-' for (OH)-Es] which can be decomposed into two bands and a low intensity peak at low frequencies [3535 er1 for (OH)-annite and 3589 cm ' lor (OH)-Es] which suggests rather a vacant octahedral site. Along the (OH.F)-annite join. the intense peak at 3669 cm ' shifts to lower frequencies as XF increases from 0 to 0.4. In contrast. this peak shifts to higher frequencies along the (OH.F)-Es join (Es K(Fe:25Al())(Si225Al,75)Ol()(OH.F)2). The low-intensity V-band re¬ mains roughly unchanged. The two bands that compose the 3669 cm ' peak are assigned to a N-band resulting from OH-Fe2+Fe2+Fe2+ (Tri-6p vibrations and a Ib-band due to OH-Fe2*Fe2+AP* (Tri-7-) vibrations. As the amount of fluorine increases in micas of the (OH.F)-annite join. the N-band frequency varies weakly but its intensity decreases significantly, while the Ibband becomes more intense. In contrast, these two bands show an opposite behaviour in (OH.F)-Es micas. TTie Nband intensity increases whereas that of the Ib-band decreases. The opposite evolution of the two main bands in the OH-stretching region shows that F is preferentially linked to Fe rather than to AI in (OH.F)-anmte, whereas F is preferentially linked to AI rather than to Fe in (OH.F)-Es. Conse¬ quently. the bond strengths Al-F or Fe-F are not controlled by the Fe-F or Al-F avoidance rule (which would predict lhat fluorine is preferentially associated to Fe in all micas). but by structural constraints. The Al-F or Fe-F avoidance rule may not play a determining role on the fluorine content of the micas as it is generally agrecd.The maximum fluorine contents in micas mainly depend on the ability of the dimensional adaptation of tetrahedral and octahedral layers

Infrared spectra of annite in the interlayer and lattice vibrational range

Fe-rich trioctahedral micas hydrothermally synthesized are characterized by infrared spectroscopy, the spectra are collected from powder suspensions. In the lattice vibrational range, the vibrations Si–Onb, Si–Ob, Al–Onb, Al–O–Si, Si–O–Si and delta OH are all characterized by the presence of doublets in the annite KBr-absorption spectra. In agreement with observations in the vibrational range of the OH-groups, this feature is interpreted to reflect the chemical heterogeneity of the octahedral and tetrahedral layers, imposed by crystallochemical constraints. With increasing Al content of the micas along the annite-siderophyllite join, the evolution of the bands shows that Al and Si become more ordered in the tetrahedral layer. In the interlayer vibrational range, the OH-annite end-member shows clearly five of the six predicted vibrations. The bands occuring at 66 cm-1 and at 120–130 cm-1 are related to vibrations involving interlayer cation, whereas the band observed at 152 cm-1 is assigned to basal oxygen vibrations around the interlayer cations. The Tschermak substitution (starting from the annite end-member) increases the misfit between the octahedral and tetrahedral layers. However, variations of fO2 do not affect significantly the band frequencies resulting from motions related to the interlayer cations, suggesting that the geometry of the interlayer site is not significantly disturbed by variation of the Fe3+/Fe2+ ratio in annite

Infrared spectra of annite in the OH-stretching vibrational range.

In the OH-stretching range, most of the micas studied exhibit partial dioctahedral character, which increases with increasing fO2. At both the Ni-NiO and wüstite-magnetite oxygen buffers, our spectra of annite indicate the presence of aluminum and Fe3+ in octahedral sites, which is required to articulate the octahedral layers. The decomposition of the infrared spectra allows one to redefine the assignment of annite absorption bands. Three V-bands of "dioctahedral-type" (in the range 3580–3520 cm-1) are characterized by the bulk number of charges of the two adjacent cations: two bands due to OH groups adjacent to an octahedral vacancy and bonded to Fe2+Al3+square or Fe2+Fe3+square (range 3580–3540 cm-1); a band due to OH groups bonded to Fe3+Fe3+square at 3530 cm-1. For Al-rich annite, the band assigned to OH–Fe2+Fe3+square is replaced by a band due to OH groups adjacent to Fe3+Al3+square. An 8-fold charged environment OH–Fe2+Al3+Al3+ does not seem likely in the micas studied. At higher wavenumbers, two I-bands of "trioctahedral-type" (OH–Fe2+Fe2+Fe3+ and Fe2+Fe2+Al3+) and one N-band corresponding to OH–Fe2+Fe2+Fe2+ have been resolved. For all trioctahedral-type bands (I- and N-bands), a systematic shift is observed toward lower wavenumbers with increasing aluminum content of the mica. When compared to N- or I-bands, V-bands show an opposite behaviour

Multi-decadal evolution of the Moroccan Atlantic shoreline: A case study from the Essaouira coastal sector

The degradation of coastlines is a result of global warming, which is closely accompanied by rising sea and ocean levels, as well as anthropogenic activities. The systematic fixation of mobile dunes has proven to be a positive environmental practice thus far. To accurately assess and predict future trends, it is essential to analyse the position of the coastline and understand its dynamics over time. In line with this goal, a study was conducted to investigate the decadal evolution of the sand shoreline along a 25 km stretch of the high-energy low-coast of Essaouira (Morocco, NW Africa). Landsat satellite images were used spanning a period of 30 years (1990, 2000, 2010, and 2020) and employed the digital shoreline analysis system model (DSAS) in conjunction with morphosedimentary dynamics analysis. Additionally, Google Earth satellite images from 2007 to 2022 were consulted to gain insights into the cyclical morpho-sedimentary processes that affect the beach/dune system. The findings revealed that the Essaouira coast has experienced landward migration of the coastline, leading to the retreat of beach/dune systems. Transgressive dunes, which had been fixed previously, were reactivated as part of this process, contributing to the morpho-sedimentary impacts observed along the coast