The specimens from a microbial biofilm formed on the wall of an excavated granite vault in a deep underground laboratory were examined by 57 Fe Mossbauer spectroscopy. In the aerobic face of the biofilm, iron was found in a form of protoferrihydrite, whereas in the anaerobic face at the rock-biofilm boundary, it was found as very fine particles of siderite, typically 2-3 nm in size. The same iron compounds were formed in aerobically and anaerobically cultured samples of the biofilm. To the best of our knowledge, this is the first clear experimental evidence of biogeochemically formed siderite and ferrihydrite in the natural environment. Finding microbial consortia capable of precipitation of Fe (super 2+) and Fe (super 3+) in close proximity may have a bearing on the development of early forms of life and, in particular, on the deposition of banded iron-formations.