lab 5 cross polarized light

How to measure retardation?

Extinction

When the vibration directions of light passing through the
mineral are parallel to the vibration directions of the lower
and upper polarizer, the mineral turns black

Unless an optic axis is vertical (recall that an OA is a direction in an
anisotropic mineral along which light is NOT doubly refracted), all
anisotropic minerals go dark (extinct) between cross-polarizers once in
every 90° of stage rotation.

Extinction

parallel extinction= 0

inclined extinction >0

symmetrical extinction

undulatory extinction

birds-eye ectinction

How to measure the extinction
angles?

1. rotate stage unil crystallographic feature is parallel to cross hairs

2. note degree on stage

3. rotate stage until mineral is extinct

4. note degree and calcualte difference

Twinning

Twinning represents a symmetrical intergrowth of two or
more crystal segments of the same mineral
• Contact twins
• Penetration twins
• Simple & Multiple twins
• Cyclic twins

twins

Exsolution

the process where a solid solution phase unmixes into two separate phases in the solid state

Steps for Determining Elongation:

1. rotate crystal 45 degrees between north-south

2. add the gypsum plate

3. If slow on slow ray = Increase Interference Color (Add;
   (+) Positive Elongation)

4. if fast on slow = Decrease Interference Color (Subtract;
   (-) Negative Elongation)