McCaw Chemistry

Resources written by Chas McCaw for sixth form chemistry teaching and beyond.

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α-Quartz 4: a polyhedral description of the helical chains

A silicon atom is in the centre of each tetrahedron, bonded to each of the four oxygen atoms occupying the corner positions of the tetrahedra. The blue tetrahedra mark out one of the helical chains. Three full rotations of these corner-sharing tetrahdera are seen as three unit cells along the direction of the helical chains are shown. A black wireframe for one of the unit cells is shown.

Looking down the trigonal axis you can see that the pale brown tetrahedra, if taken with the column of blue tetrahedra adjacent to them, form another right-handed helical chain. Indeed the three columns of blue tetrahedra can combine with a neighbouring column (in neighbouring unit cells in the case of the other two blue columns) to form right-handed helical chains. Therefore all the silicon atoms in the structure can be considered to be shared between two such helical chains. As such one can appreciate by symmetry (considering the trigonal screw axis running between the columns) that all the silicon atoms in the structure are equivalent. A similar argument can be applied to the oxygen atoms in the structure.

Since a right-handed helix is different to its mirror image, the left-handed helix, the structure to the left illustrates optical activity. Quartz can crystallise in right- or left-handed forms, and samples can even arise containing both forms.

Go to page 5 to look at the helix in terms of silicon-oxygen covalent bonds.

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