AL inorganic chem

JUST GUESS.

think about the nature of melting and boiling.
melting means particles can freely flow over each other, while boiling means particles have to overcome all attraction against each other, such that particles come out individually as gas.

in your case, melting requires only weakening of attraction between "ions" and sea of electron, such that "ions" can escape from the lattice.
ionic radii of Mg(2+) and Al(3+) are similar (diagonal relationship), so their size of lattice and separation are similar.

in boiling, however, the net charge difference plays the major role in attraction. aluminium is more positively-charged than magnesium, so attraction will be much stronger.