The free energy of formation of Mg (sub 2) TiO (sub 4) (synthetic qandilite), an inverse spinel with configurational entropy
The free energy of formation of Mg (sub 2) TiO (sub 4) (synthetic qandilite), an inverse spinel with configurational entropy (in EMPG X; Experimental mineralogy, petrology and geochemistry, A. B. Woodland (editor) and S. F. Foley (editor))
European Journal of Mineralogy (April 2005) 17 (2): 315-323
Syntheic qandilite 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 (super -1) mol (super -1) . The free energy of formation of Mg (sub 2) TiO (sub 4 ) has been determined from the location of the univariant reaction MgTiO (sub 3) + MgO = Mg (sub 2) TiO (sub 4) in P-T space between 0 and 20 kbar; together with published data for MgTiO (sub 3) and MgO, the results give S (super 0) (sub (298 K)) = 111.2 + or - 0.6 J K (super -1) mol (super -1) for Mg (sub 2) TiO (sub 4) . This entropy is only 7.6 + or - 0.9 J K (super -1) mol (super -1) more than the calorimetrically determined value of 103.6 + or - 0.7 J K (super -1) mol (super -1) . The discrepancy implies either considerable short-range order of Mg and Ti (super ) in Mg (sub 2) TiO (sub 4) , or perhaps an error in the high-temperature heat-capacity data for MgTiO (sub 3) caused by disordering.
