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.
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.
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
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