SILICATE CRYSTAL STRUCTURES

This information has been copied (with minor modifications shown in blue) from the website of the Geology Department at the Univ. of Texas at San Antonio)

A.    The silicon tetrahedron:  Silicon (Si⁺⁴) is typically surrounded by four oxygen atoms (O^-2) to form a stable silicon tetrahedron (SiO₄).  This geometric structure is very strong, because the tiny silica atom nestles perfectly between four large oxygens, covalently bonded to each.  On the other hand, this structure results in an excess charge of -4.

One or more of the oxygens are often “shared” by adjacent silica tetrahedra, reducing the charge difference.

2 tetrahedra not sharing oxygen:             SiO₄        -8

2 tetrahedra sharing one oxygen:             Si₂O₇        -6

2 tetrahedra sharing two oxygens:           SiO₃        -4

2 tetrahedra sharing 3 oxygens:              Si₂O₅        -2

tetrahedra sharing all 4 oxygens:            SiO₂         charge balanced

To balance the remaining charge, metals cations, commonly Mg⁺², Fe⁺², Ca⁺², Na⁺¹, K⁺¹, and Al⁺³, are added by ionic bonding.

In the following list of structures I have "greyed-out" the ones that we are not talking about in GEOL-111. I have highlighted the structure terms that we are using.

B. Silicate structures:                          basic unit:         mineral examples:

1.       Nesosilicates:  independent tetrahedra      SiO₄                      olivine,

garnet

2.     Sorosilicates:  linked by one oxygen          Si₂O₇                     epidote

3.     Cyclosilicates:  rings of tetrahedra            Si_xO_3x                      beryl

4.     Inosilicates:      single chains                      SiO₃                      pyroxenes

                 double chains                    Si₄O₁₁                    amphiboles

5.        Phyllosilicates:  sheet silicates                  Si₄O₁₀                   micas,

 clays

6.     Tectosilicates:  framework silicates             SiO₂                    quartz

         3-D (not draw-able)                       AlSi₃O₈              feldspars

C.    Common silicate minerals (Typical mineral formulae are shown with ferromagnesian silicates highlighted

in green, and non-ferromagnesian silicates highlighted in pink.)

1.      Olivine: (Mg,Fe)₂SiO₄

      nesosilicate

      T=1890ºC (Mg) to 1205ºC (Fe)

2.      Pyroxene:  XYSi₂O₆ where X=Na, Ca, Mn, Fe, Mg, Li

 Y=Mn, Fe, Mg, Mg, Al, Cr, Ti

       and Al can substitute for Si

Single chain inosilicate

Therefore 2 different directions of cleavage at 87 to 93º

Augite, a common pyroxene, is Ca(Mg,Fe)Si₂O₆

3.      Amphibole:    X_0-1Y₂Z₅Si₈O₂₂ where  X=Na, K

Y=Ca, Na, Mn, Fe, Mg, Li

Z=Mn, Fe, Mg, Al, Ti

and Al can substitute for Si

                        double chain inosilicate

                        Therefore 2 directions of cleavage  near 60 and 120º

                                      Hornblende is Ca₂(Fe,Mg)₅Si₈O₂₂(OH)₂

4.      Micas:  phyllosiliciates

Therefore cleaves into sheets

a.       Biotite:  K(Mg,Fe)₃AlSi₃O₁₀(OH)₂—dark mica

b.      Muscovite:  KAl₂AlSi₃O₁₀(OH)₂—white mica

5.      Feldspars:     (Al,Si)₄O₈ tectosilicates

KAlSi₃O₈  --  NaAlSi₃O₈  --  CaAl₂Si₂O₈

            Alkali feldspars] - [plagioclase feldspars

a.       plagioclase feldspars:

--continual series Ca >> Na

--key to ID is twinning:    A plane along which a crystal mirrors itself. 

Seen as fine thin straight parallel lines.

b.      alkali feldspars:

--orthoclase (often salmon pink):  plutonic

--microcline (often green):  pegmatites

--sanidine (often tiny shiny phenocrysts):  volcanic

--perthitic texture: thick wavy intergrowth of 2 feldspars at high temp

6.      Quartz: SiO₂ tectosilicates

Polymorphic

D.   Bowen’s Reaction Series:

Olivine

                                             Ca-plagioclase    

  Pyroxenes                            feldspar

          Amphiboles

                          Biotite        Na-plagioclase

                                         feldspar

                           Potassium Feldspar

                           Muscovite

Quartz