Mineralogy
Nesosilicates

"Nesosilicates" or "Orthosilicates": Island Silicates - SiO4

    Isolated, silica tetrahedra bonded together by cations that sit in octahedral sites between them:

    Si:O ratio is 1 : 4

    Examples of Nesosilicates: 

      Easily-weathered Nesosilicates:
        Olivine (Fe,Mg)2SiO4 
         Garnet (Ca,Fe,Mg)3(Al,Cr,Fe)2(SiO4)3

        Nesosilicates do not have linked silica tetrahedrons. Because bonds between Si-tetrahedrons tend to be strong, this lack of bonds makes these minerals generally weaker. In addition, garnets & olivines commonly contain iron, which is easily oxidized. Both minerals also form at high temperatures relative to surface conditions, which puts them out of equilibrium with those conditions.

      In olivine and garnet, fracture is irregular because the isolated tetrahedral structure produces no preferred plane of weakness

      Olivine alters to serpentine (a phyllosilicate)

    Resistant Nesosilicates:
      Topaz Al2SiO4(OH,F)2 
       Kyanite, Andalusite, Sillimanite (polymorphs): Al2SiO5 = Al2·SiO4
      The three Al2SiO5 polymorphs - Kyanite, Andalusite, and Sillimanite - form under different temp. and press. conditions, thus good indicators of formational environment

       

      Back to Main Silicate Page

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