6 In the mineral zircon (Zrsio.), zirconium (z ) is in cubic coordination (light

Question

6 In the mineral zircon (Zrsio.), zirconium (z ) is in cubic coordination (light gray polyhedra) with oxygen and silicon (Sify is in tetrahedral coordination (dark gray polyhedra) with oxygen. The cubic sites share edges with other cubic sites as well as tetrahedral sites. The tetrahedral sites, however, do not share any elements (faces, edges, or corners) with other tetrahedral sites. Use the Multi-Cation Repulsion Rule to explain why Si and Zr behave differently despite their similar ors(close)-ctes -Corners(prhoet) valence. (6)

Explanation / Answer

The zircon crystal structure suggests that the cubic sites are characterized by the sharing of edges with SiO4 tetrahedra and within the cube itself. Although the presence the two or more cations creates repulsive forces, the shared edges of the tetrahedra causes the negatively charged oxygen to close the gap between Zr and Si. The shared oxygen promotes Zr-Si stability, but at the same time it causes the Si-centered tetrahedra to may away from each other. Hence, the rule of multi-cation repulsion is used to explain the distinctive behaviour of Si4+ and Zr4+ cations.

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