Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay
|Rutile is one of the most important titanium minerals, with the formula TiO2. It has tetragonal symmetry. Viewed down the fourfold symmetry axis, the rutile structure looks like this. Octahedra with Ti at their centers enclose square tunnels.|
Strictly speaking, the structure only seems to have twofold symmetry because the horizontally oriented octahedra are at different elevations than the vertically oriented ones. However, if we go around a square tunnel from the point of one octahedron to the next, we find ourselves spiralling up or down. This is a good example of a screw axis. The symmetry really is fourfold, but is screw axis symmetry rather than simple rotational symmetry.
|Seen from the side, the rutile structure looks like this. Alternating chains of octahedra run parallel to the fourfold symmetry axis.|
|At first glance rutile looks very different from the close-packed atomic structures, but it really isn't. Close-packed oxygen sheets are slightly kinked (by about 20 degrees) to permit the fourfold symmetry. The kinked sheets are shown in dark blue.|
|Viewed from the side we see this. A close-packed oxygen layer is shown in blue. On the right, two layers of octahedra are shown. The light purple faces and the hidden face of the intervening octahedron chain make up one of the close-packed sheets.|
A number of other fairly important minerals have this structure, notably cassiterite(SnO2), the principal ore of tin, and pyrolusite (MnO2), a major source of manganese. In addition, a high pressure silica polymorph, stishovite (SiO2),has this structure. Note that the Si atoms no longer have 4-fold coordination. It takes about 100 kbar pressure to break down silica tetrahedra, and stishovite is found principally at meteor impact sites. In contrast to the rather open structure of quartz, with density 2.67, stishovite has density 4.35.
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Created 15 October 2009, Last Update