O'Neill, HughScott, Dean2015-12-130935-1221http://hdl.handle.net/1885/84674Synthetic qandilite (Mg2TiO4) is an inverse spinel in which random mixing of Mg and Ti on the octahedral site should give rise to a configurational entropy of 11.5 J K-1 mol -1. The free energy of formation of Mg2TiO 4 has been determined from the location of the univariant reaction MgTiO3 + MgO = Mg2TiO 4 in pressure-temperature space between 0 and 20 kbar; together with literature data for MgTiO3 and MgO, the results give S(98K)0 = 111.2 ± 0.6 J K-1 mol-1 for Mg2TiO4. This entropy is only 7.6 ± 0.9 J K-1 mol-1 more than the calorimetrically determined value of 103.6 ± 0.7 J K-1 mol-1. The discrepancy implies either considerable short-range order of Mg and Ti in Mg2TiO4, or perhaps an error in the high-temperature heat-capacity data for MgTiO 3 caused by disordering in this substance.Keywords: energy; phase equilibrium; spinel; thermodynamics Order-disorder; Phase equilibria; Spinel; Thermodynamic dataThe free energy of formation of Mg2TiO4 (synthetic qandilite), an inverse spinel with configurational entropy200510.1127/0935-1221/2005/0017-03152015-12-12