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Cristallochimie du fer dans les chlorites métamorphiques : approche analytique multiéchelle, expérimentale, et implications pétrologiques

Abstract : Chorite is a phyllosilicate crystallizing in a wide range of pressure and temperature conditions, among a diversity of rocks : from diagenesis to metamorphic conditions, typically in greenschist facies but also in blueschist and amphibolite facies. Chlorite is useful for metamorphic petrology as a geothermometer, as its composition is sensitive to the temperature of crystallization and chlorite is commonly found within mineral assemblages. Chlorite shows many solid solutions, among which Fe may be incorporated in large proportions in divalent (Fe2+) and/or trivalent (Fe3+) state. Unfortunately current models for geothermometry on chlorite either neglect Fe3+ or require evaluating the thermodynamic properties of Fe3+-rich chlorite for increased accuracy. This study aims at providing new crystal-chemistry data on ferric chlorite from a wide range of composition and origin, and to answer the following questions : (1) how much Fe3+ may be incorporated in chlorite ? (2) what are the cationic substitutions and end-members for Fe3+ incorporation ? (3) how is Fe3+ distributed within the chlorite structure ? and (4) what are the dependencies between Fe3+ in chlorite and the pressure-temperature conditions, and oxygen fugacity, at crystal and mineral scales ? This work investigates the speciation of iron in chlorite with different techniques; X-ray absorption near-edge spectroscopy at the Fe-K edge (K XANES), and electron energy-loss spectroscopy (EELS) combined with electron microprobe major element analyzes. X-ray diffraction investigation brings structural information on the structure of ferric chlorite. This analytical strategy is focused on chlorite from natural rock samples and experimental syntheses made at fixed pressure and temperature, and under buffered oxygen fugacity. A new database on ferric chlorite crystal-chemistry is the major output of this work, which highlights substitutions involving Fe3+. A Fe3+ -rich, vacant endmember is required to account for the di-trioctahedral substitution, and a magnesian end-member with 1 Fe3+ replacing Al for the homovalent Al - Fe3+ substitution. These results are observed in natural specimens and confirmed by experimental synthsesis of ferric chlorite. In addition, chlorite with Fe3+ > 1.5 p.f.u. shows systematic deviation from the ideal O10(OH)8 anionic basis, where proton deficit has been inferred from indirect measurements of H+ content. These results are consistent with the existence of an "oxychlorite" group within the classification of phyllosilicates. At mineral scale, the variations of the oxidation state take place at nanoscale and are unrelated to variations in the amount of Fe. Chlorite crystallized via experimental synthesis shows similar features. These results explain the poor success of geothermometry on some iron-rich chlorite, and allow us to propose improved cation distribution algorithms for geo thermometry. Our study paves the way for future experimental synthesis focused on oxychlorite.
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Lorella Masci. Cristallochimie du fer dans les chlorites métamorphiques : approche analytique multiéchelle, expérimentale, et implications pétrologiques. Sciences de la Terre. Sorbonne Université, 2019. Français. ⟨NNT : 2019SORUS256⟩. ⟨tel-02968387⟩

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