• journal_title：American Mineralogist
• Contributor：Monika Koch-Müller ; Anne M. Hofmeister ; Yingwei Fei ; Zhenxian Liu
• Publisher：Mineralogical Society of America
• Date：2002-
• Format：text/html
• Language：en
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：87
• issue：5-6
• firstpage：609
• section：Articles

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).