MOSSBAUER-EFFECT STUDY OF THE OXIDATION OF VIVIANITE

Mössbauer spectroscopy has been used to study the oxidation of vivianite crystals treated thermally in air and in vacuo. The rate of changes of the fractional ferrous content shows very complicated dehydration-oxidation processes depending on the duration of heating, on the surrounding atmosphere, and on the sample preparation procedure. The highest rate of the conversion Fe2+→Fe3+ was found in air around 390 K for pure vivianite, while samples ground together with boron nitride show a very different behaviour under the same heating conditions. Apparently, a loss of water occurs either the oxidation takes place in vacuo or in air. The oxidation process seems to be due to two competing processes: Oxidation by oxygen diffusing into the vivianite from outside and auto-oxidation by decomposition of crystal water. © 1990.