Elongated, segmented forms found on fracture surfaces within the martian meteorite ALH84001 have been proposed to be martian ‘nanofossils’1, even though they appear too small to be fossilized bacteria2,3,4. We have examined similar forms and find that the majority are (non-biological) lamellar growth steps on pyroxene and carbonate crystals. Their segmented surface microstructures are laboratory artefacts resulting from the deposition of conductive heavy-metal coatings.