Extinction (Optical Mineralogy)

Extinction is one of the most important observations made under crossed polarizers. It is the position in which an anisotropic mineral grain appears completely dark, and it occurs predictably at specific orientations of the microscope stage. Understanding why extinction happens and what it reveals about the mineral is fundamental to all optical mineralogy work.

Why Extinction Occurs

Unless the optic axis of the mineral is oriented vertically - the one direction in which double refraction does not occur - every anisotropic mineral goes dark between crossed polarizers exactly once in every 90° of stage rotation. Extinction occurs when the vibration directions of the two rays inside the mineral happen to be parallel to the vibration directions of the lower and upper polarizers. In this orientation, the plane-polarized light from the lower polarizer enters the mineral already aligned with one of the mineral’s vibration directions, so no component of that light can be resolved into the direction of the upper polarizer. All the light passing through the mineral retains its original vibration direction and is entirely absorbed at the upper polarizer - the grain appears black. ^[1]

What Happens Away from Extinction

When the stage is rotated so that the mineral’s vibration directions are at 45° to the polarizers (the NE-SW and NW-SE positions, also called the “diagonal position”), the plane-polarized light from the lower polarizer can be resolved into both slow and fast ray directions within the mineral in equal proportions. Both rays form with equal amplitude, travel through the mineral with different velocities, exit with a retardation between them, and then interfere at the upper polarizer to produce an interference color. As the stage is rotated, the interference color brightens and dims - brightest at 45° from extinction - but does not change hue, because the retardation between the two rays remains constant for a given grain thickness and orientation. ^[1]

Practical Use: Locating Vibration Directions

The most important practical application of extinction is that it allows the observer to pin down the vibration directions of the slow and fast rays in a mineral grain. When the grain is at extinction, its two vibration directions are aligned exactly N-S and E-W in the microscope field. The observer marks which crystallographic or morphological direction in the grain aligns with which compass direction at extinction. An accessory plate can then be inserted to determine which of those two directions carries the slow ray and which the fast ray. ^[1]

This matters because the relationship between a mineral’s crystallographic directions and its vibration directions - called the optic orientation - is a diagnostic property for mineral identification, particularly for distinguishing monoclinic and triclinic minerals.

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