Thermal stability and dehydration of anapaite

• Published in: Thermochimica acta 1998-11, Vol.320 (1), p.223-230
• Main Authors: Eeckhout, S.G, Vochten, R, Blaton, N.M, De Grave, E, Janssens, J, Desseyn, H
• Format: Article
• Language: English
• Subjects: Activation energy; Anapaite; Chemistry; Dehydration; Exact sciences and technology; Inorganic chemistry and origins of life; IR spectroscopy; Preparations and properties; Salts; Thermal stability
• ISSN: 0040-6031; 1872-762X
• DOI: 10.1016/S0040-6031(98)00446-8

The thermal behavior of anapaite, Ca 2Fe 2+(PO 4) 2·4H 2O, has been studied by TG/DTG and DSC techniques, complemented by Fourier-transform IR spectroscopy. The anapaite sample, originating from Bellaver de Cerdena (Spain) was identified as such using X-ray diffraction and qualitative energy-dispersive analysis of X-rays. 57Fe Mössbauer spectroscopy at various temperatures could not detect any Fe 3+. It was found from thermal analyses and IR spectroscopy that two types of hydrogen–bonded water molecules exist in the structure. This feature is related to the distance between the hydrogen atom of a water molecule and the oxygen atom of a phosphate group, the distance between both oxygen atoms and the angle O(H 2O)–H⋯O(PO 4). The dehydration process proceeds in two partially overlapping steps. The removal of the last two, strongly bonded water molecules is accompanied by the decomposition of the crystal structure. From TG curves, the activation energy was calculated for different intervals of dehydration reaction. For this purpose, five slow heating rates between 0.4 and 2°C/min were applied. The activation energy for the entire process was also obtained from DSC (223 kJ/mol) and found to be in reasonable agreement with the average of the various values from the TGA (233 kJ/mol). The heat of reaction for the complete dehydration was found to be 177 kJ/mol.