Question

How many oxygen atoms are shared among the tetrahedron of a silicate anion in the mineral beryl?

Answer 1

The chemical formula of beryl is $B{e}_{3}A{l}_{2}S{i}_6{O}_{18}$. The cyclic structure of $S{i}_6{O}_{18}^{12-}$ is:

So the number of $O$-atoms shared among the tetrahedron is 6.