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Synthetic and natural Fe-Mg chloritoid

¹ GeoForschungsZentrum Potsdam, Telegrafenberg, Projektbereich 4.1, 14473 Potsdam, Germany.
² Institut für Angewandte Geowissenschaften I, Technische Universität Berlin, Ernst-Reuter-Platz 1, 10587 Berlin, Germany e-mail: koch1637{at}mailszrz.zrz.tu-berlin.de
³ Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Germany

Fe-Mg bearing chloritoids were synthesized at 18–35 kbar and 590–720°C in a piston cylinder apparatus under the f_O2 defined by the Fe/FeO buffer (IW). We investigated the synthetic crystals by electron microprobe, X-ray powder diffraction, high-resolution transmission electron microscopy, Mössbauer spectroscopy and diffuse reflectance spectroscopy in the UV-NIR (30000-6000 cm^-1). Structural studies reveal that the high-pressure chloritoids crystallize in the triclinic space group C. The molar volume of the chloritoid solid solutions depends linearly on composition. The crystal lattices are nearly perfect with almost no defects, while some of the triclinic chloritoids may contain minor amounts of monoclinic intergrowths. Mössbauer spectroscopy shows that in the synthetic chloritoids a small amount of Fe_tot is incorporated as Fe³⁺. This result is consistent with estimates from electron microprobe analyses and the results of diffuse reflectance spectroscopy. The diffuse reflectance spectra exhibit a broad band at about 15000 cm^-1 caused by Fe²⁺Fe³⁺ charge transfer and two small crystal-field bands at about 11000 and 8200 cm^-1 due to Fe²⁺ in the strongly distorted M1B octahedron (Hålenius et al., 1981). The crystal-field effect on H^excess (CFSE^excess) of Fe²⁺ in chloritoid was determined by an analysis of the crystal-field spectra. CFSE^excess is negative within the whole Fe-Mg chloritoid solid-solution series. It is larger in the Fe-rich than in the Mg-rich part of the system, which indicates a non-ideal, asymmetrical mixing behaviour of Fe-Mg chloritoid.

Key-words: Fe-Mg chloritoid, syntheses, Mössbauer spectroscopy, electron microprobe, diffuse reflectance spectroscopy, crystal-field spectra, CFSE^excess.

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