EENS 211

Earth Materials

Tulane University

Prof. Stephen A. Nelson

Silicate Structures

 


 

Silicate Structures and Structural Formula

 

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, 
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.

 

Nesosilicates  (Island Silicates)

If the corner oxygens are not shared with other SiO4-4 tetrahedrons, each tetrahedron will be isolated. Thus, this group is often referred to as the island silicate group.  The basic structural unit is then SiO4-4.  In this group the oxygens are shared with octahedral groups that contain other cations like Mg+2, Fe+2, or Ca+2.  Olivine is a good example: (Mg,Fe)2SiO4.

 

 

Sorosilicates  (Double Island Silicates)

If one of the corner oxygens is shared with another tetrahedron, this gives rise to the sorosilicate group.  It is often referred to as the double island group because there are two linked tetrahedrons isolated from all other tetrahedrons.  In this case, the basic structural unit is Si2O7-6.  A good example of a sorosilicate is the mineral hemimorphite - Zn4Si2O7(OH).H2O.  Some sorosilicates are a combination of single and double islands, like in epidote - Ca2(Fe+3,Al)Al2(SiO4)(Si2O7)(OH).

 

Cyclosilicates (Ring Silicates)

If two of the oxygens are shared and the structure is arranged in a ring, such as that shown here, we get the basic structural unit of the cyclosilcates or ring silicates.  Shown here is a six membered ring forming the structural group Si6O18-12.  Three membered rings, Si3O9-6, four membered rings, Si4O12-8, and five membered rings Si5O15-10 are also possible.  A good example of a cyclosilicate is the mineral Beryl - Be3Al2Si6O18.

Inosilicates (Single Chain Silicates)

If two of the oxygens are shared in a way to make long single chains of linked SiO4 tetrahedra, we get the single chain silicates or inosilicates. In this case the basic structural unit is Si2O6-4 or SiO3-2.  This group is the basis for the pyroxene group of minerals, like the orthopyroxenes (Mg,Fe)SiO3 or the clinopyroxenes Ca(Mg,Fe)Si2O6.

Inosilicates (Double Chain Silicates)

If two chains are linked together so that each tetrahedral group shares 3 of its oxygens, we can from double chains, with the basic structural group being Si4O11-6.  The amphibole group of minerals are double chain silicates, for example the tremolite - ferroactinolite series - Ca2(Mg,Fe)5Si8O22(OH)2.

 

Phyllosilicates (Sheet Silicates)

If 3 of the oxygens from each tetrahedral group are shared such that an infinite sheet of SiO4 tetrahedra are shared we get the basis for the phyllosilicates or sheet silicates.  In this case the basic structural group is Si2O5-2. The micas, clay minerals, chlorite, talc, and serpentine minerals are all based on this structure.  A good example is biotite - K(Mg,Fe)3(AlSi3)O10(OH)2.  Note that in this structure, Al is substituting for Si in one of the tetrahedral groups.

 

Tectosilicates (Framework Silicates)

If all of the corner oxygens are shared with another SiO4 tetrahedron, then a framework structure develops.  The basic structural group then becomes SiO2.  The minerals quartz, cristobalite, and tridymite all are based on this structure.  If some of the Si+4 ions are replaced by Al+3 then this produces a charge imbalance and allows for other ions to be found coordinated in different arrangements within the framework structure.  Thus, the feldspar and feldspathoid minerals are also based on the tectosilicate framework.  

 

General Formula for Silicates

Based on these basic structural units, we can construct a general structural chemical formula for the silicates.  But one substitution in particular tends to mess things up a bit.  This is Al+3, the third most abundant element in the Earth's crust.  Al+3 has an ionic radius that varies between 0.54 and 0.39 depending on the coordination number.  Thus, it could either fit in 6-fold coordination with oxygen or 4-fold coordination with oxygen. Because Al+3 will go into 4-fold coordination with oxygen, it sometimes substitutes for Si+4.  If such a substitution takes place, it creates a charge imbalance that must be made up elsewhere in the silicate structure.  

The other common elements in the Earth's crust that enter the silicates do so in other types of coordination.  Ions like Al+3, Mg+2, Fe+2, Fe+3, Mn+2, and Ti+4 enter into 6-fold or octahedral sites. Larger ions like Ca+2, and Na+1, are found in octahedral coordination or 8-fold, cubic coordination sites.  Very large cations like K+1, Ba+2, and sometimes Na+1 are coordinated by 12 oxygens in 12-fold coordination sites.

We can thus write a general structural formula for the silicates as follows:

XmYn(ZpOq)Wr

where X represents an  8 to 12 fold coordination site for large cations like K+, Rb+, Ba+2, Na+, and Ca+2.

Y represents a 6-fold (octahedral) site for intermediate sized cations like Al+3, Mg+2, Fe+2, Fe+3, Mn+2, and Ti+4.

Z represents the tetrahedral site containing Si+4, and Al+3.

the ratio p:q depends on the degree of polymerization of the silica (or alumina) tetrahedrons, or the silicate structural type as discussed above.

O is oxygen, 

and W is a hyrdoxyl (OH-1) site into which can substitute large anions like F-1 or Cl-1.

The subscripts m, n, and r depend on the ratio of p to q and are chosen to maintain charge balance.

This is summarized in the table shown here.  In this table note that there is very little substitution that takes place between ions that enter the X, Y, and Z sites.  The exceptions are mainly substitution of Al+3 for Si+4, which is noted in the Table, and whether the X site is large enough to accept the largest cations like K+1, Ba+2, or Rb+1.

Site C.N. Ion
Z 4 Si+4
Al+3
Y 6 Al+3
Fe+3
Fe+2
Mg+2
Mn+2
Ti+4
X 8 Na+1
Ca+2
8 - 12 K+1
Ba+2
Rb+1

 

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