Answer
The formula unit of garnet is $Ca_{3}Al_{2}(SiO_{4})_{3}$.
Work Step by Step
3 $SiO_{4}^{4-}$ ions contribute a total of -12 charge. This negative charge is neutralized by 3 $Ca^{2+}$ ions and 2 $Al^{3+}$ ions. Hence, formula unit of garnet is $Ca_{3}Al_{2}(SiO_{4})_{3}$.
