Chemical Composition of Meteorites
Magnesium & Iron
MgO, Fe2O3(T), & MgO / Fe2O3 (T) vs. Fe2O3 (T) + MgO
Among meteorites, MgO necessarily increases with Fe2O3(T) + MgO. Some terrestrial rocks lie off the trend because they have MgO/Fe2O3(T) ratios outside the range for meteorites (below). High-MgO terrestrial rocks are mainly ultramafic rocks like dunites and serpentinites.
The same is largely true for Fe2O3(T). High-Fe2O3 terrestrial rocks are iron ores.
MgO / Fe2O3(T)
The ratio of magnesium to iron does not vary greatly among most kinds of stony meteorites. Rocks with MgO/Fe2O3(T) <0.2 or >6 are not probably meteorites. Only rare lunar granites have low MgO/Fe2O3(T), and none of these has been found as a lunar meteorite.