Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay
Or maybe we should ask, why are any coordination polyhedra regular?
The most regular coordination polyhedra are found among the native elements and simple ionic compounds like oxides, halides and sulfides. The elements are easy to understand: all the atoms are identical and their structures are dominated by close packing arrangements. Simple ionic compounds typically have a single anion and single cation, leading to a single coordination geometry. In some cases, like spinel or chalcopyrite, there are two cations. In chalcopyrite, they are similar enough in size and ionic charge to have the same coordination with the anion.
The minerals with complex anions, like sulfates, phosphates, tungstates, vanadates, molybdates, and so on, have geometrically regular anions, usually tetrahedra, and the remaining ions are confined in the irregular voids between. For many of these minerals, it is easier to visualize their structure by looking solely at the complex anions and the cations, and ignoring the cation polyhedra altogether. For many of these minerals, the cation coordination polyhedra are so irregular that merely visualizing their shape is a challenge, much less visualizing them connected to one another.
Yet silicates, which have complex anions with tetrahedral coordination, tend to have reasonably regular coordination polyhedra, although slight distortions are common. Why do minerals with complex anions have oddball coordination polyhedra, yet silicates do not? The answer seems to lie with the charges on the central cation in complex anion groups, and the resulting charges on the anion. Look at the following list:
| Cation | Charge | Complex Anion | Charge |
| S | +6 | SO4 | -2 |
| P | +5 | PO4 | -3 |
| W | +6 | WO4 | -2 |
| Mo | +6 | MoO4 | -2 |
| V | +5 | PO4 | -3 |
| Cr | +6 | CrO4 | -2 |
| Si | +4 | SiO4 | -4 |
In contrast to all the other complex ion forming elements, silicon has a smaller charge and a larger charge on the complex anion. Thus, all the other complex anions have a much stronger attraction between the central cation and the surrounding oxygens. More importantly, the complex anions as a whole have smaller charges. That means a smaller number of cations to balance the complex anions.
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Created 9 May 2002, Last Update