Section: Topic 9 | Rock Minerals in Thin Sections

MSWPC-E

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Extinction angle

The extinction angle of a given grain is the angle between any specified crystallographic direction and either of the two vibration directions. It can be an important distinguishing character for different minerals. It is crucial to record whether the nature of the extinction shown by most grains of a mineral is straight or inclined

Minerals belonging to the tetragonal, hexagonal, trigonal or orthorhombic crystal systems in general show straight extinction.
Minerals belonging to the triclinic and monoclinic system in general show inclined extinction.

Straight extinction (PX)

Inclined extinction (PX)

Undulatory extinction. Quartz (PX)

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