A typical BIF consists of repeated , thin layers ( a few millimeters to a few centimeters in thickness ) of silver to black iron oxides , either magnetite ( Fe3O4 ) or hematite ( Fe2O3 ) , alternating with bands of iron-poor shales and cherts , often red in color , of similar thickness , and containing microbands ( sub-millimeter ) of iron oxides .
They consist of repeated thin layers of iron oxides , either magnetite ( Fe3O4 ) or hematite ( Fe2O3 ) , alternating with bands of iron-poor shale and chert .