Bonding With                     

The Silica Tetrahedron           

How does the negatively charged silica tetrahedron form stable, electrically neutral minerals?

The most obvious way for a silicon tetrahedron to reduce its negative charge would be to bond with positive ions (cations) (i.e., form an ionic bond).

If you need to review your definitions of chemical bonding...

For example, a tetrahedron could bond with two Mg2+ ions.

This balanced arrangement (Mg2SiO4) is actually the mineral forsterite.

 

However, because silicon and oxygen are so much more abundant than the cations such as magnesium, we would expect to run out of positive ions long before we used up the silicon and oxygen!

So if there is a surplus of silica tetrahedra...

...What if tetrahedra bonded together by sharing an oxygen ion between them (i.e., covalently bonding with another tetrahedron)?...

...Or by sharing more than one oxygen? 

 Let's analyze the pattern...

  Case 1:  SiO44-

1 tetrahedron
No shared oxygens

1 Si

4-

-4 / Si

  Case 2:  Si2O76-

2 tetrahedra
1 shared 
oxygen/tetrahedron

2 Si

6-

-3 / Si

  Case 3:  Si5O1312-

5 tetrahedra
1 or 2  shared oxygens/tetrahedron

5 Si

12-

-2.4 / Si

As more oxygens are shared between tetrahedra, the overall charge per tetrahedron decreases.

 

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In fact, if a tetrahedron shared each of it's 4 oxygens with neighboring tetrahedra (i.e., kept half for itself and gave half to each neighbor), then it would have no charge (neutral). 

The balanced arrangement SiO2 is actually the mineral quartz, one of the most common minerals in the continental crust.

 


© 2001 Wayne G. Powell