Mineralogy
Inosilicates
"Inosilicates" = Chain silicates 
SINGLE CHAIN SILICATES = "Pyroxenes" and Pyroxenoids - SiO3

    Each tetrahedron shares a total of 2 oxygens with 2 different neighboring tetrahedra:

    Si:O ratio is 1 : 3

      PYROXENES are subdivided into two groups: 
        Orthopyroxenes - do not contain Ca - contain smaller cations so both the M1 and M2 cation sites are in 6CN. This results in a Orthorhombic crystal structure.

        Clinopyroxenes - contain Ca (which is a large cation) along with smaller cations - the M1 site is a 6CN site that takes the smaller cations, whereas the M2 site becomes a 7-8CN site to accommodate the large Ca ions. This results in a lower symmetry, and a Monoclinic crystal structure.

        Examples of Pyroxenes:

          Enstatite MgSiO3
          Hypersthene (Mg,Fe)Si03
          Ferrosilite FeSi03

          Augite (Ca,Na)(Mg,Fe,Al)(Si,Al)2O6 
           Diopside CaMgSi206
          Hedenbergite CaFeSi206

          Also click here for a triangular compositional diagram of the pyroxenes

    • In PYROXENES the chains extend untwisted along the c-axis

    • In PYROXENOIDS the chain structure is twisted, resulting in:
      1. Lower symmetry - Triclinic system 
      2. Longer repeat distances*
        Examples of Pyroxenoids: 
          Wollastonite CaSiO3 
           Rhodonite MnSiO3

    • Single chain inosilicates display "Square Cleavage" (2 good cleavages at 87° and 93°)




DOUBLE CHAIN SILICATES = "Amphiboles" - Si4O11

    Tetrahedra alternate sharing 2, then 3, then 2, then 3... oxygens with neighboring tetrahedra:

    Si:O ratio is 4 : 11


    Examples of Amphiboles: 

      Hornblende (Ca,Na)2-3(Mg,Al,Fe)5Si6(Si,Al)2O22(OH)2 
       Tremolite Ca2Mg5Si8O22(OH)2 
       Actinolite Ca2(Mg,Fe)5Si6(Si,Al)2O22(OH)2 

      Also click here for a triangular compositional diagram of the amphiboles

  • In the structure of double chain inosilicates, the two linked chains have a large "hole" between them

      - this hole is filled by large anions like OH-, Cl-, F-

      - water (OH-) is common in the holes at lower temperatures but can be driven out of structure with heating

      - when the water is driven out, the structure may convert to a pyroxene

  • Double chain inosilicates display "Diamond Cleavage" (2 good cleavages at 56° and 124°)

 

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