When the mineral dolomite (caco3·mgco3) is heated, it gives off carbon dioxide and forms a mixture of a metal oxide and a metal carbonate. which carbonate is formed?

Now, question is why MgO but not CaO.

At a dissociation pressure of 1 atm, the temperature at which CO2 and metal oxide is formed is :

For MgCO3 =  540 °C
For CaCO3  =  900 °C” 
Also,
The driving force is the larger lattice energy of MgO(3795) vs that of CaO(3414 kJ mol^-1) . More the lattice energy, the more chances of that compound to form.
So MgO is formed first.

Carbonate is Calcium carbonate