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Synthetic and natural Fe-Mg chloritoid; structural, spectroscopic and thermodynamic studies

Monika Koch-Mueller, Irmgard Abs-Wurmbach, Klaus Langer, Cliff Shaw, Richard Wirth and Matthias Gottschalk
Synthetic and natural Fe-Mg chloritoid; structural, spectroscopic and thermodynamic studies
European Journal of Mineralogy (April 2000) 12 (2): 293-314

Abstract

Fe-Mg bearing chloritoids were synthesized at 18-35 kbar and 590-720 degrees C in a piston cylinder apparatus under the f (sub 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, Mossbauer spectroscopy and diffuse reflectance spectroscopy in the UV-NIR (30000-6000 cm (super -1) ). Structural studies reveal that the high-pressure chloritoids crystallize in the triclinic space group c1. 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. Mossbauer spectroscopy shows that in the synthetic chloritoids a small amount of Fe (sub tot) is incorporated as Fe (super 3+) . 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 (super -1) caused by Fe (super 2+) Fe (super 3+) charge transfer and two small crystal-field bands at about 1 1000 and 8200 cm (super -1) due to Fe (super 2+) in the strongly distorted M l B octahedron (Halenius et al., 1981). The crystal-field effect on Delta H (super excess) (CFSE (super excess) ) of Fe (super 2+) in chloritoid was determined by an analysis of the crystal-field spectra. CFSE (super 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.


ISSN: 0935-1221
Serial Title: European Journal of Mineralogy
Serial Volume: 12
Serial Issue: 2
Title: Synthetic and natural Fe-Mg chloritoid; structural, spectroscopic and thermodynamic studies
Affiliation: GeoForschungsZentrum Potsdam, Potsdam, Federal Republic of Germany
Pages: 293-314
Published: 200004
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 57
Accession Number: 2000-041807
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 7 tables
Secondary Affiliation: Technische Universitaet Berlin, DEU, Federal Republic of GermanyUniversitaet Bayreuth, DEU, Federal Republic of Germany
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2020, American Geosciences Institute.
Update Code: 200014
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