Atomic Bonds 

X-ray crystallography reveals the basic atomic building block of the 
silicates: the silica tetrahedron, a cluster of atoms composed of a 
central silicon ion with a charge of +4 surrounded by four oxygen 
ions, each with a charge of -2. The entire group, thus has a net 
charge of -4. The silicon ion has a radius of 0.39 angstrom, the 
oxygen ion a radius of 1.40 angstroms. (One angstrom equals one 
ten-billionth of a meter.) The Si-O bond length, the sum of these 
radii, averages about 1.79 angstroms. To give an idea of how small 
this unit is, the distance across two tetrahedra in a silicate chain 
(one form of tetrahedral grouping) is 5.4 angstroms, and about 45 
million tetrahedra compose a chain 2.5 cm (1 in) long. 

A single oxygen ion may bond to two silicon ions, forming clusters or 
polymers of smlica tetrahedra. Two silicon ions with an oxygen in 
common will form a silica heptahedra group, with a charge of -6. 
Further clustering gives rise to more and more complex polymers. 

Because different minerals contain different kinds of clusters, 
silicates can be classified according to the type of cluster they 
contain. The common silicates may consist of isolated tetrahedra or of 
polymers in the form of rings, chains, sheets, or frameworks. 

Silica tetrahedra share only corners, not edges or faces; that is, two 
silicon ions may have one oxygen in common, but not two or three. The 
latter cases, although geometrically possible, would bring the highly 
charged silicon ions too close to each other, setting up strong 
repulsive forces. Aluminum (Al) ions, because they are similar in size 
to silicon ions, readily substitute for silicon in minerals called 
aluminosilicates. The aluminum ion has a charge of +3; wherever it 
substitutes for silicon (+4), a charge deficiency must be made up by 
some other positively charged ion packed between the tetrahedra. 

The interstitial ions in silicates occupy positions in which they are 
surrounded by a cluster or coordination polyhedron of the oxygen ions 
of the silica tetrahedra. The larger the interstitial ion, the more 
oxygen ions that cluster around it. Of the common elements in the 
Earth's crust, aluminum may be surrounded by four or by six oxygen 
ions at the corners of a double, four-sided pyramid, forming what is 
called octahedral coordination. Magnesium (Mg) and iron (Fe) occur 
almost exclusively in octahedral coordination; calcium (Ca) may have 
6, 8, or 12 neighbors, and sodium (Na) and potassium (K) commonly have 
8 or more oxygen neighbors.