EXTINCTION IN UNIAXIAL MINERALS

Uniaxial minerals will exhibit all four types of extinction discussed earlier.

The type is dependent on:

  1. the orientation that the mineral is cut
  2. the presence of cleavage(s) in the grain

Tetragonal minerals

  1. Zircon ZrSi04- poor prismatic
  2. Rutile Ti02 - good prismatic

Depending on how the crystal is cut, and how its indicatrix is cut, dictates what will be seen in thin section.

Hexagonal Minerals

  1. Quartz - SiO2 - no cleavage
  2. Apatite - Ca5(PO4)3(F,C1,OH) - rare pinacoidal, prism
  3. Calcite - CaC03 - 1 of two cleavages rhombohedral
  4. Nepheline - NaAlSiO4 - no cleavage

Hexagonal minerals will exhibit the following forms prisms, pinacoids, pyramids and rhombohedrons which will exhibit prismatic, pinaciodal and rhombohedral cleavages.

The birefringence, interference colours and any cleavage displayed by hexagonal minerals is a function of how the grain has been cut.

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