Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay
Zircon (ZrSiO4) is the principal repository for zirconium. It hasa fairly simple structure. It has a tetragonal unit cell with a = 6.6 Angstromunits and c = 6. Zircon is perhaps the most important tetragonal mineral.Zirconium atoms are at the corners of the unit cell, the center, and ataltitudes of 1/4c and 3/4 c along the midlines of each vertical  face.Silica tetrahedra are centered at top and bottom center, the midpoints of eachedge parallel to c, and at 3/4c and 1/4 c along the midlines of each verticalface.
|The zircon structure seen from the top, looking along the c axis. Silica tetrahedra are in blue, zirconium atoms in orange, with darker hues for increasing distance. The coordination polyhedron for the central zirconium atom is shown in purple.|
|The zircon structure seen from the side, looking perpendicular to the c axis. Silica tetrahedra are in blue, zirconium atoms in orange, with darker hues for increasing distance. The silica tetrahedra are slightly skewed for perspective, but are really symmetrical about the faces. Silicon atoms are in green |
.The coordination polyhedron for the central zirconium atom is shown in purple. The central zirconium atom, which is inside the purple polyhedron, is outlined in gray.
The one somewhat odd feature of this structure is the coordination polyhedronof the zirconium atoms. The zirconium atoms are surrounded by eight oxygens, twosets of four at slightly different distances, but is not a slightly distortedcube. It is very nearly a shape called a snub disphenoid or Siamesedodecahedron. The ideal shape has 12 equilateral triangle faces and tetragonaldisphenoid symmetry. One way to picture it as a very flat disphenoid cut intotwo pairs of faces and pulled apart along the symmetry axis, with a zigzag bandof 8 triangles added in between. Another way is to imagine gluing two pentagonalpyramids together base to base along three of the five edges, then squeezingfrom the side and adding two more triangles to fill the opening.
Created 26 February, 2001, Last Update 31 May 2020