The molecular formula of the mineral haematite is $\mathrm{Fe}_2 \mathrm{O}_3$ and that of magnetite is $\mathrm{Fe}_3 \mathrm{O}_4$ respective. Both contain iron ( Fe ) but in different oxidation states.

Oxidation state of iron ( Fe ) in :

(a) Haematite $\left(\mathrm{Fe}_2 \mathrm{O}_3\right)$

Let the oxidation state of iron ( Fe ) be $x$

$$ \begin{aligned} 2 x+3 \times(-2) & =0 \\\\ 2 x & =6 \\\\ x & =+3 \end{aligned} $$

Iron exist as Fe (III).

(b) Magnetite $\left(\mathrm{Fe}_3 \mathrm{O}_4\right)$

The mineral magnetite is made by two iron oxides $\mathrm{FeO} . \mathrm{Fe}_2 \mathrm{O}_3$.

The oxidation state of iron in $\mathrm{Fe}_2 \mathrm{O}_3$ is +3 and oxidation state of iron in FeO can be calculated as :

$$ \begin{aligned} x+2 \times(-2) & =0 \\\\ x & =+2 \end{aligned} $$

Iron exist as Fe (II).

Hence, the oxidation state of iron in haematite is III and that in magnetite is II, III respectively.