Fougerite: From field experiment to the homologation of the mineral

Fougerite has been approved as a new mineral by the International Mineralogical Association in February 2004. Its characteristic blue-green colour is used as a universal criterion to identify the gley soils. By oxidation, it transforms into lepidocrocite. The discovery of fougerite in 1996 was the result of 10 years of a study of in situ seasonal dynamics of the system soil–solution, which led to the confirmation of the ancient assumption of the existence of a mixed hydroxide of the group of green rusts (GRs) in gleys. Synthetic GRs consist of a brucitic layer of Fe(II)–Fe(III), whose excess positive charge, due to Fe³⁺, is compensated in the interlayer by different anions, such as sulphate, chloride, oxalate, carbonate, etc. Physical, chemical, and mineralogical characterizations have been performed as a complement, and have confirmed the first results. In fougerite from Fougères, Mg is present in the layer in addition to Fe; the nature of the interlayer anion, which can vary depending on the environment, remains an open question. To cite this article: F. Trolard, C. R. Geoscience 338 (2006).