Résumé : Infrared absorption spectra of the amorphous and phosphorus-rich ferric mineral associated with the biofilm of Montacuta ferruginosa, a marine bivalve, were obtained by applying the KBr disc method. Phosphate absorption bands due to P-O stretching vibrations were observed at 1100 and 1020 cm-1. This result, as well as the similarity of spectra with other well known (bio)minerals, suggests that the mineral is an amorphous iron oxyhydroxide gel with phosphate sorbed on its surface rather than a pure ferric phosphate. It is suggested that phosphate ions are of microbial origin because phosphate-producing enzymes were detected in vivo. Apart from iron, the mineral phase is devoid of other heavy metals. Despite its similarity with other abiotically precipitated phases, it is argued that the mineral is the result of in situ microbial biomineralization processes in the biofilm and not the result of a simple bioaccumulation process. This is supported by microscopic observations. A geological implication is that the simple presence of such iron minerals in ancient sedimentary environments, where microbes have not been fossilized, might be considered as an indicator of microorganisms performing biomineralization. By comparison with the present-day environment of M. ferruginosa, precise paleoenvironmental conditions may be inferred. © 2001 Elsevier Science B.V. All rights reserved.