MgSiO3 ilmenite: Heat capacity, thermal expansivity, and enthalpy of transformation

Abstract

A new determination, using high temperature drop-solution calorimetry, of the enthalpy of transformation of MgSiO3 pyroxene to ilmenite gives ΔH298 = 59.03 ±4.26 kJ/mol. The heat capacity of the ilmenite and orthopyroxene phases has been measured by differential scanning calorimetry at 170–700 K; Cp of MgSiO3 ilmenite is 4–10 percent less than that of MgSiO3 pyroxene throughout the range studied. The heat capacity differences are consistent with lattice vibrational models proposed by McMillan and Ross (1987) and suggest an entropy change of -18 ± 3 J-K-1 ·mol-1, approximately independent of temperature, for the pyroxene-ilmenite transition. The unit cell parameters of MgSiO3 ilmenite were measured at 298–876 K and yield an average volume thermal expansion coefficient of 2.44 × 10-5 K-1. The thermochemical data are used to calculate phase relations involving pyroxene, β-Mg2SiO4 plus stishovite, Mg2SiO4 spinel plus stishovite, and ilmenite in good agreement with the results of high pressure studies.