# Why can't two Silicon tetraoxide tetrahedrons share an edge or a face?

Our teacher told us about structural types of silicate minerals (neosilicate, sorosilicate, cyclosilicates ...) saying that those tetrahedrons can only share one corner (oxygen), but he didn't give us any explanation, could anyone help me with this?

• That would be too much of angular strain. – Ivan Neretin Mar 1 '16 at 15:18
• What if the angles facing each other, symmetrical in respect to the potential shared edge, were equal? – Vlad Uskar Mar 1 '16 at 17:38
• They are equal; so what? – Ivan Neretin Mar 1 '16 at 18:07
• In that case, both angles would be 109°, which does not qualify as "abnormal" for a tetrahedron, i don't see source of the strain here. – Vlad Uskar Mar 1 '16 at 18:20
• I suppose you are talking about the $\ce{O-Si-O}$ angles. OK, but what about $\ce{Si-O-Si}$? – Ivan Neretin Mar 1 '16 at 18:23