Reaction of iron with steam
What type of a reaction is it?


I would describe it first and foremost as a redox reaction. Iron is oxidised and hydrogen is reduced; the half equations would be:
$$\ce{2Fe -&gt; Fe^2+ + Fe^3+ + 5e-}$$ $$\ce{H2O + 2e- -&gt; H2 + O^2-}$$


  Why have we combined $\ce{FeO}$ and $\ce{Fe2O3}$ resulting in the formation of $\ce{Fe3O4}$?


This is a good observation. $\ce{Fe3O4}$ contains both iron(II) and iron(III) ions and is sometimes written as $\ce{FeO \cdot Fe2O3}$. The dot in the middle signifies that there is some form of bonding between the two compounds (another common example of this is water of crystallisation). $\ce{Fe3O4}$ is a compound and not a mixture because it does not consist of two separate $\ce{FeO}$ and $\ce{Fe2O3}$ phases but rather it is a single crystal structure containing $\ce{Fe^2+}$, $\ce{Fe^3+}$ and $\ce{O^2-}$ ions.


  $\ce{Fe3O4}$ has a cubic inverse spinel structure which consists of a cubic close packed array of oxide ions where all of the $\ce{Fe^2+}$ ions occupy half of the octahedral sites and the $\ce{Fe^3+}$ are split evenly across the remaining octahedral sites and the tetrahedral sites.