Dear Bob,
You are of course quite right, there is no SiO2 molecule, because quartz and
all its polymorphs are „periodic nets“. You have to select a „cluster“ and to
make it neutral you would have to add terminal OH groups.
This is a problem we face also in zeollite chemistry (many of them are
polymorphs of SiO2) and a number of attempts had been made to define suitable
clusters that describe a structure. So far the best approach for periodic nets
seems to be that of "natural tilings“, defined and described by V. A. Blatov,
O. Delgado-Friedrichs, M. O'Keeffe & D. M. Proserpio, Acta Crystallogr. A 63,
418-425 (2007). These natural tilings are further discussed by N. Anurova, V.
A. Blatov, G. D. Ilyushin and D. M. Proserpio, J. Phys. Chem. C 114,
10160-10170 (2010).
You can see examples of such tilings on our zeolite website. For example, for
the natural zeolite faujasite in
http://izasc.ethz.ch/fmi/xsl/IZA-SC/Tilings/FAU.pdf
Its framework of faujasite can be viewed with JSmol on
http://izasc.ethz.ch/fmi/xsl/IZA-SC/ftc_3d_JSmol.php
These natural tilings have been done for all zeolites.
I am not sure, if this is what Hans Horn was looking for, but it would
certainly be nice if Jmol could display such drawings ;-)
Thanks and best wishes
Christian
On 20 Sep 2014, at 04:24, Robert Hanson <hans...@stolaf.edu> wrote:
> Well, I think it is an interesting request. Help me define what the idea is a
> bit more, and I can see what I can do. Given a few choice examples, how would
> you define your request exactly?
>
> For example, we have quartz (SiO2):
> http://chemapps.stolaf.edu/jmol/simple2.htm?load%20jsmol/data/quartz.cif%20{1%201%201}
>
> In this unit cell we have
>
> Si 1 + 4(1/2) = 3 atoms
> O 6 atoms
>
> But which three atoms would you choose to represent the model for "SiO2"? Is
> it always possible to identify such a subset? Some cases probably involve
> multiple options, and they are not equivalent. For example, in this case,
> there is not a perfect tetrahedron around that Si atom. So picking different
> O atoms would give different structures for the formula unit. Which one would
> be the correct one to load? Sounds very challenging to me.
>
> Bob
>
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