CHARACTERIZATION AND THERMODYNAMIC PROPERTIES OF ANDRADITE, CA3FE2SI3O12

Garnet with composition of almost pure andradite (Ca3Fe2Si3O12) occurs in fissures cutting the foliation of pyroxenites at the Herbeira ultramafic massif (Cabo Ortegal, northwest Spain). The infrared and Raman spectra of the natural andradite have been recorded at room pressure and room temperature in order to model the thermodynamic properties. The IR spectrum exhibits three modes (112, 704, and 1070 cm-1) not previously described in the literature. Using these new data in the lattice vibrational model of Kieffer (1979a, 1979b, 1979c), the thermodynamic properties of andradite have been calculated. Agreement between measured and calculated specific heat and relative enthalpy is excellent (ca. 1% difference between 300 and 1100 K). The specific heat (C(P,298) almost-equal-to 346.5 J/mol.K) and entropy (S298 almost-equal-to 313.6 J/mol.K) at room temperature agree with reported experimental values. The model of density of state proposed for andradite is used to calculate C(P)(T), H(T), and S(T) of Ca3Fe2Si3O12 between 0 and 1500 K. Equations expressing the temperature dependence of heat capacity and relative heat content of andradite are thus proposed that allow its thermodynamic properties C(P)(T), H(T), and S(T) to be calculated at high temperatures (T > 298 K).