Identification of a green rust mineral in a reductomorphic soil by Mossbauer and Raman spectroscopies

Mossbauer and Raman spectroscopies are used to identify for the first time a green rust as a mineral in a reductomorphic soil from samples extracted in the forest of Fougeres (Brittany-France). The Mossbauer spectrum displays two characteristic ferrous and ferric quadrupole doublets, the abundance ratio Fe(II)/Fe(III) of which is close to 1. Comparison with synthetic mixed valence Fe(II)Fe(III) hydroxides supports the conclusion that the most probable formula is Fe-2(OH)(5), i.e., according to the pyroaurite-like crystal structure [Fe-(III)Fe-(III)(OH)(4)](+) [OH](-). The microprobe Raman spectrum exhibits two bands at 518 and 427 cm(-1) as for synthetic green rusts. When exposed to the air, the new mineral goes rapidly from bluish-green to ochrous. The formula is compatible with the values of ionic activity products Q for equilibria between aqueous iron species and minerals obtained from soil waters, which suggests that this new mineral is likely to control the mobility of Fe in the environment. Copyright (C) 1997 Elsevier Science Ltd.