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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?

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    $\begingroup$ That would be too much of angular strain. $\endgroup$ Mar 1, 2016 at 15:18
  • $\begingroup$ What if the angles facing each other, symmetrical in respect to the potential shared edge, were equal? $\endgroup$
    – Vlad Uskar
    Mar 1, 2016 at 17:38
  • $\begingroup$ They are equal; so what? $\endgroup$ Mar 1, 2016 at 18:07
  • $\begingroup$ 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. $\endgroup$
    – Vlad Uskar
    Mar 1, 2016 at 18:20
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    $\begingroup$ I suppose you are talking about the $\ce{O-Si-O}$ angles. OK, but what about $\ce{Si-O-Si}$? $\endgroup$ Mar 1, 2016 at 18:23

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In solid-state chemistry, we know a certain set of rules called the Pauling rules. One of them states that sharing faces or edges is unfavorable. Imagine your silicate-tetrahedra to share faces. That would equal the closest distance between two highly charged silicon cations. They both have a charge of +4, so there is really some repulsion. Same goes for the edges which is also quite close.

If you increase the overall size of the tetrahedron however by exchanging oxide by sulfide you will end up in the silicon(IV)sulfide structure which is essentially long chains of tetrahedra that share edges. Here by introducing a much larger anion the distance between the two silicon centers is increased and edges can be shared.

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