A typical electron microprobe printout of an oxide analysis shows weight percent concentrations of different elements in terms of their oxides, Fe is usually expressed as FeO. For minerals, it also shows calculated number of cations on the basis of an assumed (theoretical) number of oxygen atoms. For examplejn olivines, (Mg,Fe)2SiO4, the theoretical number of oxygen atoms is four and the sum of cations should add up to three if the analysis is good. However, in case of spinels such as magnetite, Fe3O4, which contain significant amounts of Fe2O3, the analysis total falls below 100 and the cation sum (based on four oxygens) exceeds the theoretical cation sum. in other words, the formula calculation shows an oxygen deficiency. Geologists use this information to calculate the amount of Fe2O3 in the spinel assuming the other cations have only one oxidation state.