Thermodynamic properties of MgSiO₃ ilmenite from vibrational spectra

Unpolarized infrared (IR) reflectance spectra for MgSiO₃ ilmenite taken from a single-crystal and from a densly packed polycrystalline sample possessed all eight peaks mandated by symmetry between 337 and 850 cm^-1. Polarizations were inferred from intensity differences between the two samples. IR peak positions differ by up to 250 cm^-1 from recent calculations, but on average are within 11%. Heat capacity C_pcalculated from these data by using a Kieffer-type model are within the experimental uncertainty of calorimetric measurements from 170 to 700 K. Outside this range, calculated C_pis probably accurate within a few percent, based on recent results for garnets. Calculated entropy is only slightly less accurate, giving S⁰(298.15 K) as 54.1 ±0.5 J/ mol-K, which is 10% lower than recent estimates based on phase equilibria. The slope of the phase boundary between ilmenite and perovskite is used to predict S⁰(298.15 K) of perovskite as 58.7 ±1.4 J/mol-K, which is 10% lower than previous values.