Magnetite is a black, cubic iron oxide of chemical formula Fe3O4 belonging to the spinel group. It is the most magnetic mineral on Earth. The formula of magnetite may also be written as (Fe2+Fe23+O4), which indicates the occurrence of two iron ions having different valences that occupy specific sites in the crystal structure. This arrangement causes a transfer of electrons between the different irons generating the magnetic field. Magnetite occurs in almost all igneous and metamorphic rocks and in several types of sedimentary rocks, including banded iron formations. Crystals of magnetite are found in some bacteria called magnetobacteria (e.g., Magnetospirillum magnetotacticum). Magnetic minerals were found in the Martian meteorite ALH 84001 and interpreted as the best evidence of indigenous bacterial activity.