Roll over for circularly cross polarized (CP)
Dacite - Linearly cross polarized
(XP)
Roll over for circularly cross
polarized (CP)
| Gabbro - Linearly cross
polarized (XP) Roll over for circularly cross polarized (CP) |
Dacite - Linearly cross polarized
(XP)
Roll over for circularly cross
polarized (CP)
|
|
|
Linear cross-polarized optical images reveal the birefringence of mineral grains and are very useful in petrography. However, if the vibration directions of the crystal are not at 45° to the polarizers, then the intensity of the interference colors is reduced, and in some orientations, to zero (extinction). This reduction in intensity, and extinction, can be eliminated if circular polarizers are used in place of linear polarizers. The interference colors of colorless minerals then correspond exactly with those on the Michel-Lévy chart. Similarly, isogyres are eliminated from conoscopic images. This simple, inexpensive technique can be used with ordinary petrographic microscopes and scanners. It can be very useful for quantifcation of textures (microstructures) from single images.
On my petrographic microscope I use a 1/4 wave plate in the accessory
plate slot and a 25 mm diameter mica plate balanced on the top of the
condenser lens.The latter must be rotated to get extinction with no thin
section in the light path.
JM Derochette web page on microscopy.
