Accumulations of oxidized iron especially iron corrosion products are typically referred to by shape e.g. tubercles (mounds) whiskers chimneys rusticles and combinations of shapes e.g. tubercles with chimneys. Mineralogical data suggest that both tubercle and rusticle morphologies provide the possibility for reducing environments. Abiotic mechanisms have been proposed for tubercle formation and for adsorption of metal cations on tubercle surfaces. In contrast biotic mechanisms have been proposed for rusticle formation and cation adsorption e.g. biological extraction and bioaccumulation. To date identification of microorganisms in tubercles has been limited to surfaces and core regions. The precise locations of microorganisms associated with rusticles have not been established.