SILICATE STRUCTURES

 

A.   Introduction

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. 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, RC/RA.

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

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 SiO-44 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.

B.   Different Types of Silicates Groups 

a)     Nesosilicates (Free Silicates)

(O shared O2 atoms and ratio of Si:O = 1:4)

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.

Other details:

·       Isolated (SiO4)4-tetrahedra and bounded to one another via ionic bonds with interstitial cations.

·       Dense packing –high density.

·       Independent tetrahedral –crystal habits are equidimensional, and lack pronounced cleavage.

·       Al3+ substitution in T-site generally low.

·       Many nesosilicates (but not all) have orthogonal crystallographic systems.

       i.          Olivines

The olivines consist of a complete solid solution between Mg2SiO4 (forsterite, Fo) and Fe2SiO4 (fayalite, Fa). There is limited substitution of the following end members:

Ca2SiO4 – Larnite, Mn2SiO4 – Tephritic, CaMgSiO4 - monticellite (which is commonly found in metamorphosed dolomites)

     ii.          Garnets

Garnets are isometric minerals and thus isotropic in thin section, although sometimes they are seen to be weakly birefringent (slightly anisotropic). They are also nesosilicates, and therefore based on the SiO4 structural unit. The general formula for garnets is:

A3B2(Si3O12)

where the A sites are cubic sites containing large divalent cations, usually Ca, Fe, Mg, or Mn, and the B sites are octahedral sites occupied by smaller trivalent cations, like Al and Fe+3. Garnets with no Ca in the A site and Al in the B site are called the Pyralspite series. These consist of the end members:

·       Pyrope - Mg3Al2Si3O12

·       Almandine - Fe3Al2Si3O12, Spessartine - Mn3Al2Si3O12

·       Garnets with Ca in the A site are called the Ugrandite series and consist of the end members: Uvarovite - Ca3Cr2Si3O12

·       Grossularite - Ca3Al2Si3O12

·       Andradite - Ca32Fe3+3SiO12

   iii.          Al2SiO5 Minerals

The Al2SiO5 minerals are common in aluminous metamorphic rocks (meta-shales and meta- mudstones) and sometimes found in aluminous igneous rocks. In metamorphic rocks the Al2SiO5 polymorphs provide rather general estimates of the pressure and temperature of metamorphism, with Kyanite indicating relatively high pressure, andalusite indicating low temperature and pressure, and sillimanite indicating high temperature. Better estimates of pressure and temperature are provided if two of the minerals are present in the same rock.

b)    Sorosilicate

(1 shared O2 atoms and ratio of Si:O = 2:7)

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 SiO-6. A good example of a sorosilicate is the mineral hemimorphite - Zn4Si2O7(OH). H2O. Some sorosilicate’s are a combination of single and double islands, like in epidote - Ca2(Fe+3,Al)Al2(SiO4)(Si2O7)(OH).

Other details:

·       SiO4 tetrahedrane combination with Si2O7 unit.

·       These commonly form edge-sharing linked octahedral chains.

·       These chains in-turn form sheets by binding with isolated SiO4 tetrahedra.

·       While bonds between chains and within sheets are quite strong (large coordination spaces that accept large cations), bonds between sheets are weak leading to the common phenomenon of only 1 direction of perfect cleavage.

Sorosilicate’s are the double island silicates. Hemimorphite and epidote group, has this structure.

Epidote, Clinozoisite, Zoisite:

The important minerals in the epidote group are epidote, clinozoisite, and zoisite. Since the sorosilicates are based on the Si2O7 -6 group, the structural formula can be written as:

Ca2(Al,Fe+3)Al2O(SiO4)(Si2O7)(OH)

Thus, the epidote group contains both the double tetrahedra and the single tetrahedron, separated by groups of AlO6 octahedra and Ca in nine to 10-fold coordination with Oxygen or OH. The formula can be rewritten as:   

Ca2 (Al,Fe+3)Al2Si3O12(OH)

Epidote is the Fe-rich variety and has the above general formula. Clinozoisite is the Fe-free variety with the chemical formula:

Ca2Al3Si3O12(OH)

Both clinozoisite and epidote are monoclinic (2/m). Zoisite has the same chemical formula as clinozoisite but is orthorhombic.

c)     Cyclosilicates (Ring Silicates)

(2 shared O2 atoms and ratio of Si:O = 1:3)  

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 cyclosilicates or ring silicates. Shown here is a six membered ring forming the structural group SiO-12. Three membered rings, Si3O9-6, four membered rings, SiO-8, and five membered rings Si5O15-10 are also possible. A good example of a cyclosilicate is the mineral Beryl - Be3Al2Si6O18. 3, 4, 6, 8, 9, or 12 membered rings of Si4+ tetrahedra (though 6-membered are the most common). The high concentration of strongly linked bonds yield relatively dense minerals that are quite hard. The cyclosilicates are based on rings of SiO4 tetrahedra, with a Si:O ratio of 1:3. The most common minerals based on this structure are

Beryl - Be3Al2Si6O18,

Cordierite - (Mg,Fe)2Al4Si5O18.nH2O

Tourmaline - Na(Mg,Fe,Mn,Li,Al)3Al6Si6O18(BO3) 3 (OH)4

d)    Inosilicates            

Single Chain                               Double chain

       i.          Single Chain Silicates        

(2 shared O2 atoms and ratio of Si:O = 1:3)  

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 Si O-4 or SiO-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.

     ii.          Double Chain Silicates          

(2, then 3 shared O2- ions & Si:O = 4:11)

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 - ferro actinolite series - Ca2(Mg,Fe)5Si8O22(OH)2.

e)     Phyllosilicates (Sheet Silicates)

(3 shared O2- ions & Si:O = 2:5)

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 SiO-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.

Many members have a platy or flaky habit with one very prominent cleavage. Minerals are generally soft, low specific gravity, may even be flexible. Most are hydroxyl bearing. Each tetrahedra is bound to three neighboring tetrahedra via three basal bridging oxygens. The apical oxygen of each tetrahedral in a sheet all point in the same direction. The sheets are stacked either apice-to-apice or base-to-base. All unshared oxygens point away from the tetrahedra: “apical oxygens”. In an undistorted sheet the hydroxyl (OH) group sits in the center and each outlined triangle is equivalent.

f)      Tectosilicates (Framework Silicates)

(4 shared O2- ions & Si:O = 1:2)

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. Infinite 3-dimensional network of (SiO4)4- or (Si3Al)O81--(Si2Al2)O82- building blocks. All oxygen atoms are shared between two SiO44-tetrahedron. Quartz is useful as a source of Si and for silica. It is used in electronics as an oscillator and is piezoelectric.

                                                                                                                     Quartz

The structure of feldspar is similar to that of the SiO2 polymorphs, consisting of an infinite network of tetrahedra inter-connected via bridging oxygen atoms. In contrast to the SiO2group, the tetrahedra may be AlO4 as well as SiO4. Minerals are rendered electrically neutral as a result of being “stuffed” with alkali or alkali-earth element cations in available voids.                

                             Feldspar-tetrahedra                                     Felds-structure