Beryl contains the cyclic silicate anion $Si X_{6} O X_{18} X^{12 -}$ in which every $SiO X_{4}$ tetrahedron shares exactly two corners with neighbours. The number of non-bridging (terminal) oxygen atoms in the ring is

Answer: A

Each of the 6 tetrahedra has 2 shared corners, each oxygen is shared by two tetrahedra, so bridging O

= 6 \times 2/2 = 6

. Terminal O

= 18 - 6 = 12

, which matches the charge

- 12

(one negative charge per terminal O).