Andalusite Structure

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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Andalusite is a nesosilicate and the low temperature and low pressure polymorph of Al2SiO5.

Like sillimanite, andalusite consists of chains of octahedra joined at the edges. The unit cell is nearly square as viewed down the c-axis.

Each group of four octahedra encloses a flattened octahedron. Silica tetrahedra point inward along the long axis of the octahedron.

Each octahedron contains an aluminum aton and six oxygens, four of which are shared. The net formula is thus AlO4.

The two silica tetrahedra and extra oxygens suggest that they must be counted as shared with the next layer of octahedra up or down. The net formula thus becomes AlSiO5.

The remaining aluminum atom is caged by the in-pointing tetrahedra. Like the silica tetrahedra, there are actually two of them but they must be counted as shared by two layers.

The coordination number of the non-octahedral aluminum is 5. Like most unusual coordinations, it results not from the symmetry demands of bonding but from mechanical trapping of an atom between other, more symmetrical units.

These coordination polyhedra are usually described as dipyramids. That's a technically accurate but poor description since the polyhedra really don't have rotational symmetry around the axis of the dipyramid.

The lower vertices of the octahedra shown here are linked by tetrahedra. The upper vertices are linked by pairs of irregular dipyramids. The gaps between the free octahedron vertices are spanned alternately by tetrahedra and dipyramids.

The structure consists of chains of octahedra linked alternately by layers of tetrahedra and layers of dipyramids.

The octahedra and pairs of dipyramids share edges. All of the other linkages are at vertices.

Above is a sketch showing the structure in more detail. At left we see only the tetrahedron linkages with the dipyramids shown in skeletal form only. As we move down and right we see additional layers of the structure.


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Created 08 April, 2005, Last Update 14 Dec 2009

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