12,18
1,14
12,12
14,18
A ccp structure has 4 atoms/ions per unit cell, 4 octahedral voids and 8 tetrahedral voids. Thus, in the unit cell of the mineral, there are 4 oxide ions, 4m Al³⁺ ions and 8n Mg²⁺ ions respectively. For charge balance, 4(-2) + 4m(3+) + 8n(2+) = 0. The above equation will be balanced when m = 12 and n = 18. Hence, m and n, respectively, are 12, 18.