Silicate Structures 

As we discussed in a previous lecture, the relative abundance of elements in the Earth's crust determines what minerals will form and what minerals will be common.  Because Oxygen and Silicon are the most abundant elements, the silicate minerals are the most common.  Thus, we will spend some time here discussing the structure, chemistry, and occurrence of silicate minerals.  Our systematic discussion of the common rock forming minerals will follow in the lectures throughout the remainder of the course.

Element Wt% Atomic% Volume% O 46.60 62.55 ~94 Si 27.72 21.22   ~6 Al   8.13   6.47 Fe   5.00   1.92 Ca   3.63   1.94 Na   2.83   2.34 K   2.59   1.42 Mg   2.09   1.84 Total 98.59 100.00 100

In order to discuss the silicates and their structures it is first necessary to remember that the way atoms are packed together or coordinated by larger anions, like oxygen depends on the radius ratio of the cation to the anion, Rx/Rz.

Rx/Rz C.N. Type 1.0 12 Hexagonal or Cubic Closest Packing 1.0 - 0.732 8 Cubic 0.732 - 0.414 6 Octahedral 0.414 - 0.225 4 Tetrahedral 0.225 - 0.155 3 Triangular <0.155 2 Linear

Since oxygen is the most abundant element in the crust, oxygen will be the major anion that coordinates the other other cations. Thus, for the major ions that occur in the crust, we can make the following table showing the coordination and coordination polyhedra that are expected for each of the common cations.

Ion	C.N.  (with Oxygen)	Coord. Polyhedron	Ionic Radius,  Å
K+	8 - 12	cubic to closest	1.51 (8) - 1.64 (12)
Na+	8 - 6	cubic to octahedral	1.18 (8) - 1.02 (6)
Ca+2	8 - 6		1.12 (8) - 1.00 (6)
Mn+2	6	Octahedral	0.83
Fe+2	6		0.78
Mg+2	6		0.72
Fe+3	6		0.65
Ti+4	6		0.61
Al+3	6		0.54
Al+3	4	Tetrahedral	0.39
Si+4	4		0.26
C+4	3	Triangular	0.08

The radius ratio of Si+4 to O-2 requires that Si+4 be coordinated by 4 O-2 ions in tetrahedral coordination.  

In order to neutralize the +4 charge on the Si cation, one negative charge from each of the Oxygen ions will reach the Si cation. Thus, each Oxygen will be left with a net charge of -1, resulting in a SiO4-4 tetrahedral group that can be bonded to other cations.  It is this SiO4-4 tetrahedron that forms the basis of the silicate minerals.

Since Si+4 is a highly charged cation, Pauling's rules state that it should be separated a far as possible from other Si+4 ions.  Thus, when these SiO4-4 tetrahedrons are linked together, only corner oxygens will be shared with other SiO4-4 groups.  Several possibilities exist and give rise to the different silicate groups