Synthetic 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 Mg2TiO4 has been determined from the location of the univariant reaction MgTiO3 + MgO = Mg2TiO4 in pressure-temperature space between 0 and 20 kbar; together with literature data for MgTiO3 and MgO, the results give So(298K)= 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 MgTiO3 caused by disordering in this substance.

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