Why is the melting point of magnesium oxide higher than aluminium oxide?

There's a graph of the melting points of period three oxides. The melting point of magnesium oxide is several hundred Kelvin higher than aluminiumoxide. I can't find any explanations for this on the Internet or in any of my A-level textbooks — perhaps this is something more advanced I haven't come across.

The reason I am questioning the value of its melting point is I'd expect aluminium oxide to be higher. Aluminium is a 3+ ion whereas magnesium is only 2+. Aluminium is also a smaller ion so the effective electrostatic attraction on the valence electrons should be greater. I'm also pretty sure aluminum oxide has greater covalent charterer than magnesium oxide. All of these would lead me to believe it has a higher melting point, so what is the explanation for it being less?

• You could apply Kapustinskii's Equation to obtain the lattice energy and see if that provides an explanation. – Tyberius Apr 15 '17 at 19:38
• For what it's worth, based on your plot of melting points for these 3rd row oxides, MgO appears to fall right in line and $\ce{Na2O}$ seems to be the odd-ball. – airhuff Apr 15 '17 at 19:46

2) Alumina, being more covalently bound than magnesia, could have polyatomic species like $\ce {AlO+}$ surviving in the liquid. This would imply that alumina has to break down fewer bonds than magnesia.