• 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

• 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
  

• 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

• 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

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

• 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)