EXTINCTION

Now we want to examine other properties of minerals which are useful in the identification of unknown minerals.

Anisotropic minerals go extinct between crossed polars every 90° of rotation. Extinction occurs when one vibration direction of a mineral is parallel with the lower polarizer. As a result no component of the incident light can be resolved into the vibration direction of the upper polarizer, so all the light which passes through the mineral is absorbed at the upper polarizer, and the mineral is black.

Upon rotating the stage to the 45° position, a maximum component of both the slow and fast ray is available to be resolved into the vibration direction of the upper polarizer. Allowing a maximum amount of light to pass and the mineral appears brightest.

The only change in the interference colours is that they get brighter or dimmer with rotation, the actual colours do not change.

Many minerals generally form elongate grains and have an easily recognizable cleavage direction, e.g. biotite, hornblende, plagioclase.

The extinction angle is the angle between the length or cleavage of a mineral and the minerals vibration directions.

The extinction angles when measured on several grains of the same mineral, in the same thin section, will be variable. The angle varies because of the orientation of the grains. The maximum extinction angle recorded is diagnostic for the mineral.


Types of Extinction

  1. Parallel Extinction
    The mineral grain is extinct when the cleavage or length is aligned with one of the crosshairs.
    The extinction angle (EA) = 0°

e.g.

    • orthopyroxene
    • biotite

 

  1. Inclined Extinction
    The mineral is extinct when the cleavage is at an angle to the crosshairs.
    EA > 0°


e.g.

    • clinopyroxene
    • hornblende

 

  1. Symmetrical Extinction
    The mineral grain displays two cleavages or two distinct crystal faces. It is possible to measure two extinction angles between each cleavage or face and the vibration directions. If the two angles are equal then Symmetrical extinction exists.
    EA1 = EA2

e.g.

    • amphibole
    • calcite

 

  1. No Cleavage
    Minerals which are not elongated or do not exhibit a prominent cleavage will still go extinct every 90° of rotation, but there is no cleavage or elongation direction from which to measure the extinction angle.

e.g.

    • quartz
    • olivine

Exceptions to Normal Extinction Patterns

Different portions of the same grain may go extinct at different times, i.e. they have different extinction angles. This may be caused by chemical zonation or strain.

Chemical zonation

The optical properties of a mineral vary with the chemical composition resulting in varying extinction directions for a mineral. Such minerals are said to be zoned.
e.g. plagioclase, olivine

Strain


During deformation some grains become bent, resulting in different portions of the same grain having different orientations, therefore they go extinct at different times.
e.g. quartz, plagioclase