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What is the structural difference between …

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What is the structural difference between orthosilicates and pyrosilicates?
  • 1 answers

Sia ? 2 years, 9 months ago

  • No tetrahedral Oxygens shared with other tetrahedra
  • Based on SiO4 4-
  • Also called nesosilicates
  • A diverse group of minerals, examples covered or touched on here or in lab:
    • olivine
    • garnet
    • aluminosilicates
      • kyanite
      • andalusite
      • sillimanite
    • topaz
    • staurolite
    • zircon
    • titanite or sphene
    • chloritoid
    • humite group
  • structures determined by size(s) and charge(s) of cations in the structure
  • often rather dense minerals, high S.G., hard (lots of references to the Gems course), high R.I. - mantle phases
  • equant grains with generally poor cleavage
  • low amounts of Al substitution for Si

    Pyrosilicates are silicates which contain Si2O76-. They are formed by joining two tetrahedral Si2O44-. When two Si2O44- are joined, there is removal of one oxygen atom and the two units join at the corner oxygen atom. Example is thortveitite, Sc2Si2O7. The structure of pyrosilicate is:

    The reds are oxygen atoms and greens are silicon.

    Cyclic silicates contains (SiO3)n2n- which are formed by linking three or more tetrahedral SiO44- units cyclically. Each SiO44- shares two oxygen atoms with other unit. Example is Benitoite, BaTi(SiO3)3. Here three SiO44- units are arranged cyclically.

    <i>Orthosilicates: </i>In simple Orthosilicates the SiO4 4- tetrahedron units do not share any of its oxygen atoms with any of the other tetrahedra units. E.g. Willmenite ZnSiO4, Zircon ZrSiO4.

    <i> </i><i>Pyrosilicates or Island silicates</i>

    In Pyrosilicates one oxygen atom is shared by two silicate units General formula is (Si2O7)6-

    E.g. Thortevitite Sc2[Si2O7] and Hemimorphite Zn(OH)2(Si2O7)H2O

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