THERMAL-STABILITY AND DEHYDRATION OF ANAPAITE

The thermal behavior of anapaite, Ca2Fe2+(PO4)(2).4H(2)O, has been stu died by TG/DTG and DSC techniques, complemented by Fourier-transform I R spectroscopy. The anapaite sample, originating from Bellaver de Cerd ena (Spain) was identified as such using X-ray diffraction and qualita tive energy-dispersive analysis of X-rays. Fe-57 Mossbauer spectroscop y at various temperatures could not detect any Fe3+. It was found from thermal analyses and IR spectroscopy that two types of hydrogen-bonde d water molecules exist in the structure. This feature is related to t he distance between the hydrogen atom of a water molecule and the oxyg en atom of a phosphate group, the distance between both oxygen atoms a nd the angle O(H2O)-H ... O(PO4). 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 cry stal structure. From TG curves, the activation energy was calculated f or different intervals of dehydration reaction. For this purpose, five slow heating rates between 0.4 and 2 degrees C/min were applied. The activation energy for the entire process was also obtained from DSC (2 23 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 fo r the complete dehydration was found to be 177 kJ/mol. (C) 1998 Elsevi er Science B.V.