1Lyubov P. Ogorodova, 1,2Yuliya D. Gritsenko, 1Marina F. Vigasina, 1Andrey Y. Bychkov, 1Dmitry A. Ksenofontov, 1Lyubov V. Melchakova
American Mineralogist 103, 1945-1952 Link to Article [https://doi.org/10.2138/am-2018-6475]
1Geological Faculty, M.V. Lomonosov Moscow State University, Leninskie Gory, Moscow, 119234, Russia
2Fersman Mineralogical Museum RAS, Leninskiy prospect 18-2, Moscow, 117071, Russia
Copyright: The American Mineralogical Society
In the present study, four samples of natural melilites were characterized using electron microprobe analysis, powder X-ray diffraction, FTIR, and Raman spectroscopy, and their thermodynamic properties were measured with a high-temperature heat-flux Tian-Calvet microcalorimeter. The enthalpies of formation from the elements were determined to be: –3796.3 ± 4.1 kJ/mol for Ca1.8Na0.2(Mg0.7Al0.2Fe2+0.1Fe0.12+)Si2O7, –3753.6 ± 5.2 kJ/mol for Ca1.6Na0.4(Mg0.5Al0.4Fe2+0.1Fe0.12+)Si2O7, –3736.4 ± 3.7 kJ/mol for Ca1.6Na0.4(Mg0.4Al0.4Fe2+0.2Fe0.22+)Si2O7, and –3929.2 ± 3.8 kJ/mol for Ca2(Mg0.4Al0.6)[Si1.4Al0.6O7]. Using the obtained formation enthalpies and estimated entropies, the standard Gibbs free energies of formation of these melilites were calculated. Finally, the enthalpies of the formation of the end-members of the isomorphic åkermanite-gehlenite and åkermanite-alumoåkermanite series were derived. The obtained thermodynamic properties of melilites of different compositions can be used for quantitative modeling of formation conditions of these minerals in related geological and industrial processes.