Geo 406 Lecture 06 - Extinction, Uniaxial minerals, optics, optic sign

Reading: Ch. 53-55, 65

next: pp. 55-65

Goals

Learn angle of extinction

Understand differences in directions in uniaxial minerals

Learn optic sign for uniaxial minerals

Angle of extinction

Angle between prominent feature or features (cleavages, growth faces) and extinction.

inclined

parallel

symmetric

none

undulatory

Uniaxial minerals

fast vs. slow

ordinary (omega) vs. extraordinary (epsilon)

double refraction

in lab, we saw that as you tilt the rhomb, the two rays get closer together

there is a direction in which the difference between the ordinary and the extraordinary rays is at a maximum

in other paths, the difference is less than the maximum.

vibration directions

one ray (the ordinary ray) always vibrates in the plane perpendicular to the c-axis

other vibrates at 90 to it, and at about 90° to propagation direction

Example: Calcite structure

two rays at 90° in (0001) plane see the same environment - same n

if ray is traveling near the c-axis, then e-ray vibrates almost in the (0001) plane - has n near n_epsilon

as angle between ray path and c-axis grows towards maximum of 90°, electronic environment of e-ray gets more and more different from o-ray, and thus n gets more different from n_omega

indices of refraction

if we measured the indices of refraction for these rays, we'd find that n_omega < n_epsilon' < n_epsilon OR n_omega > n_epsilon' > n_epsilon

optic axis - light traveling along optic axis does not split into fast & slow.

optic sign

positive: n_epsilon > n_omega

negative: n_epsilon < n_omega

Uniaxial Indicatrix

Recall definition and construction of indicatrix

measure n's vibrating in every direction, and plot those n's as distances from a center - gives an ellipsoid

Optical axes:

X - smallest n

Z - largest n

Y - intermediate n

might be equal to X or Z

For uniaxial minerals, section of indicatrix normal to c-axis must be a circle - all n's are equal

this is n_omega

For positive minerals, n_epsilon is larger, so we have a peanut m&m

For negative, n is smaller, so we have a plain m&m