Enthalpy of Formation of Brazilianite: Calorimetric Data

• Published in: Geochemistry international 2022-11, Vol.60 (11), p.1114-1121
• Main Authors: Ogorodova, L. P., Gritsenko, Yu. D., Vigasina, M. F., Vyatkin, S. V., Melchakova, L. V., Ksenofontov, D. A.
• Format: Article
• Language: English
• Subjects: Analysis; Analytical methods; Boron oxides; Calorimetry; Diffraction; Earth and Environmental Science; Earth Sciences; Electron microprobe; Electron probe microanalysis; Electron probes; Enthalpy; Entropy; Entropy of formation; Free energy; Geochemistry; Heat measurement; Heat of formation; Phosphates; Vanadium; X ray powder diffraction; X-ray diffraction; X-rays
• ISSN: 0016-7029; 1556-1968
• DOI: 10.1134/S0016702922110106

Brazilianite Na 0.91 Al 3.03 (PO 4 ) 1.98 (OH) 4.06 (Galilea, Minas Gerais, Brazil) has been studied by thermal and electron microprobe analysis, powder X-ray diffraction, IR, Raman, and EPR spectroscopy. The EPR method has established that the peculiar feature of the studied sample is the presence of a structural impurity of vanadium. The enthalpy of formation of brazilianite NaAl 3 (PO 4 ) 2 (OH) 4 from the elements ∆ f H 0 (298.15 K) = −5322.3 ± 5.1 kJ/mol was determined by the method of calorimetry of dissolution in a melt of lead borate 2PbO∙B 2 O 3 on a Setaram (France) Calvet microcalorimeter. The value of the absolute entropy S 0 (298.15 K) = 303.8 ± 0.9 J/(mol K) was estimated, and the entropy of formation ∆ f S 0 (298.15 K) = − 1407.1 ± 1.1 J/(mol K) and the Gibbs energy of formation from elements ∆ f G 0 (298.15 K) = − 4902.8 ± 5.1 kJ/mol were calculated.