The reaction MgCr2O4+SiO2=Cr2O3+MgSiO3 and the free energy of formation of magnesiochromite (MgCr2O4)

Low-temperature heat capacity measurements for MgCr2O4 have only been performed down to 52 K, and the commonly quoted third-law entropy at 298 K (106 J K-1 mol(-1)) was obtained by empirical extrapolation of these measurements to 0 K without considering the magnetic or electronic ordering contributions to the entropy. Subsequent magnetic measurements at low temperature reveal that the Neel temperature, at which magnetic ordering of the Cr3+ ions in MgCr2O4 occurs, is at similar to 15 K. Hence a substantial contribution to the entropy of MgCr2O4 has been missed. We have determined the position of the near-univariant reaction MgCr2O4 + SiO2 = MgSiO3 + Cr2O3. The reaction, which has a small positive slope in P-T space, has been bracketed at 100 K intervals between 1273 and 1773 K by reversal experiments. The reaction is extremely sluggish, and lengthy run times with a flux (H2O, BaO-B2O3 or K2O-B2O3) are needed to produce tight reversal brackets. The results, combined with assessed thermodynamic data for Cr2O3, MgSiO3 and SiO2, give the entropy and enthalpy of formation of MgCr2O4 spinel. As expected, our experimental results are not in good agreement with the presently available thermodynamic data. We obtain Delta(f)H(298)degrees = -1759.2 +/- 1.5 kJ mol(-1) and S(298)degrees = 122.1 +/- 1.0 J K-1 mol(-1) for MgCr2O4. This entropy is some 16 J K-1 mol(-1) more than the calorimetrically determined value, and implies a value for the magnetic entropy of MgCr2O4 consistent with an effective spin quantum number (S') for Cr3+ of 1/2 rather than the theoretical 3/2, indicating, as in other spinels, spin quenching.