GeoClassroom Physical Geology Historical Geology Structure Lab Mineralogy Petrology

Oxides & Hydroxides

  • Oxides are abundant in nature and include many important ores

  • Oxides are grouped on the basis of their metal(A) to oxygen(O) ratios into the following subgroups:

    1. A2O Group

      • does not include many minerals & the minerals in this group are not very abundant in nature

      • examples:   Cu2O - cuprite
          - minor ore of copper
          - found in the oxidized zone of supergene enrichment settings

    2. AO Group

      • this group is a little more abundant in nature

      • examples:  ZnO - zincite - minor ore of zinc
              MgO - periclase - refractory mineral - very high melting point

          - both are formed in Si-poor metamorphic environments
          (because if Si present, it would combine to form silicate mineral)

          e.g.,

          -if H2O is also present in metamorphic environment periclase may alter further to form brucite

    3. A2O3 Group - Hematite Group

      • large group, abundant in nature

      • examples:  Fe2O3 - hematite - major ore of Fe
            high temp. forms - igneous or metamorphic
            low temp. forms - sedimentary

          Al2O3 - corrundum - gem (sapphire & ruby) & abrasives
            forms in high temp. environments

    4. AO2 Group - Rutile Group

      • examples:   TiO2 - rutile - ore of Ti, high temp. igneous
              SnO2 - cassiterite - ore of tin, high temp. igneous
              MnO2 - pyrolusite - ore of Mn, low temp & sedimentary

    5. AB2O4 Group - Spinel Group

      COMPOSITION:
          A = 2+ cations
          B = 3+ cations

        3 Major Compositional Groups:

        1. Spinel group - B = Al3+
        2. Chromite group - B = Cr3+
        3. Magnetite group - B = Fe3+

        *within each group - good solid solution
        *between groups - limited solid solution

      STRUCTURE:

      • Need 8 AB2O4 molecules for unit cell

      • Oxygens are close-packed with one another
      • Interstices between oxygens include:   64 tetrahedral sites &
                     32 octahedral sites

      • * However only 8 tetrahedral sites, 16 octahedral sites are actually filled with cations

        Spinel structures:

        1. Normal spinels:
          -the 8 tetrahedral sites are filled by A2+
          -the 16 octahedral sites are filled by B3+
          -this happens because cations have an octahedral site preference*

        2. Inverse spinels:
          -the 8 tetrahedral sites are filled by 8B3+
          -the 16 octahedral sites are filled by 8B3+ and 8A2+

        *Octahedral Site Preference (OSP) - function of stablity of crystal

        Highest to lowest:

        Cr3+, Mn3+, Ni2+, Cu2+, V2+, Co2+, Ti2+, Fe2+, Fe3+, Mn2+, (Al3+ and Mg2+ also very low)

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