Determination of the optic sign of uniaxial minerals

 

Positive: epsilon < omega (velocities) and n epsilon > n omega (refractive indices)

Negative: epsilon > omega (velocities) and n epsilon < n omega (refractive indices)

 

Correct mineral cuts

The correc mineral cuts for the determination of the optic sign of uniaxial mineral are:

cut perpendicular to the optic axis is used ,

cut slightly inclined one, but not one that is so inclined that the centre of the cross disappears from the field of view.

 

Determination of the optic sign of uniaxial minerals

Additive effects (retardation) between the waves of the mineral and those of the compensating plate will occur in some quadrants of the interference figure (the fast-fast and slow-slow components coincide) and the colour blue will appear. In other quadrants, the interference colour will get lower to produce yellow, as subtractive effects (compensation) take place (fast-slow and slow-fast).

The "first order red" compensating plate is introduced. The isogyre will turn red (as here the waves of the mineral have no retardation).

If quadrants 1 and 3 turn blue (and 2 and 4 turn yellow) the mineral is positive.

If the opposite occurs (blue in 2 and 4, and yellow in 1 and 3) the crystal is negative.

When the figure contains isochromatic curves, the mica and quartz wedge compensator can be used to determine the optic sign, and if the 1st order red compensator is used, the adjacent quadrants or areas must be compared in order to find out the chromatic effects (previous page).

 

Compensation and retardation

Let us consider the reason for the chromatic changes shows in previous screens. As the figure shows, the component omega always vibrates in a tangential direction, whilst epsilon vibrates in a radial direction.

Let us look at the first quadrant of the figure, which is blue. As the compensating plate has introduced a red colour, if blue now appears, it is because the colour has risen in value, there has been an intensification, which means that the fast component of the mineral has coincided with the fast one of the compensator (similarly slow with slow).

As the directions of the fast and slow components of the compensator are known, those of the mineral can be established. In the first quadrant, the fast component of the compensator vibrates in a tangential position and the slow one in a radial direction, thus in the mineral, omega is the fast and epsilon is the slow one. Therefore nw is the small refractive index and ne is the large one (the velocity and the refractive index are inverse), and this is the definition of a positive uniaxial crystal!

The situation is the same in the third quadrant and is inverted in quadrants 2 and 4. In the latter quadrants, the fast component of the compensator now vibrates in a radial direction whilst the slow one does so in a tangential direction. As a result, quadrants 2 and 4 are in a subtractive situation (fast-slow) and the interference colour goes down to yellow.

In negative uniaxial crystals the colours are inverted: 1 and 3 are yellow, whilst 2 and 4 are blue.

 

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