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High-pressure IR-spectra and the thermodynamic properties of chloritoid

¹ Geophysical Laboratory, Center of High Pressure Research, Carnegie Institution of Washington, 5251 Broad Branch Road N.W., Washington, D.C. 20015, U.S.A.
² GeoForschungsZentrum Potsdam, Projektbereich 4.1, Telegrafenberg, 14473 Potsdam, Germany
³ Department of Earth and Planetary Science, Washington University, Box 1169, 1 Brookings Drive, St. Louis, Missouri 63130, U.S.A.

Correspondence: ^* E-mail: mkoch{at}gfz-potsdam.de

Using IR radiation from a synchrotron source, high-quality absorbance spectra were obtained from polycrystalline powder of chloritoid (cld) from ambient conditions up to pressures of 10 GPa over 50 to 4000 cm^–1. The idealized chemical composition of the chloritoid group is M₂Al₄O₂(SiO₄)₂(OH)₄ where M = Fe or Mg in our experiments. All of the 42 expected fundamental IR modes were observed. These data, combined with the response of the IR bands to substitutions of Fe for Mg, and of D for H, constrained the band assignments. Heat capacity (C_P) and entropy (S_o) for the triclinic and monoclinic polymorphs of Fe- and Mg-cld were calculated from the Kieffer-type model, using our detailed band assignments. The calculated heat capacity and entropy for the monoclinic and triclinic polymorphs differ negligibly. The results at temperatures above 298 K are described by the following polynomial expressions in J/(mol·K): C_P = 7.835 · 10² – 5.170 · 10³T^–0.5 –1.648 · 10⁷T^–2 + 1.934 · 10⁹T^–3 for Mg-cld and C_P = 7.848 · 10² – 5.185 · 10³T^–0.5 – 1.548 · 10⁷T^–2 + 1.783 · 10⁹T^–3 for Fe-cld. At room temperature, S_s = 293 J/mol·K for Mg-cld and 335 J/mol·K for Fe-cld. These values differ somewhat from entropy estimated from various internally consistent databases (–3 to –9% for Mg-cld and –9 to +5% for Fe-cld). However, using our new S_o and C_P values in conjunction with the enthalpy of formation, H_f = –7101 kJ/mol for Mg-cld or H_f = –6422 kJ/mol for Fe-cld (estimated in this study), we can closely reproduce the experimental data for the reactions Mg-chloritoid + talc = clinochlore + kyanite (Chopin 1985) and Fe-chloritoid = almandine + diaspore + water (Vidal et al. 1994).

This article has been cited by other articles:

				K.-D. GREVEL, W.-A. KAHL, J. MAJZLAN, A. NAVROTSKY, C. LATHE, and T. FOCKENBERG Thermodynamic properties of magnesiochloritoid European Journal of Mineralogy, August 1, 2005; 17(4): 587 - 598. [Abstract] [Full Text] [PDF]