Plane Polarized Light (PPL)
➢ Transparent – surrounding minerals that allow light to pass through
➢ Opaque – does not let light pass through
- cannot characterize more on this type of microscope
- Common opaque minerals are magnetite, ilmenite, and pyrite.
• Color – Observed color or lack of color under PPL only
- Common colored minerals are biotite, amphibole, and pyroxene.
➢ Pleochroism – changes color when the stage is being rotated
• Relief
✓ Positive – when the becke line is moving into our mineral as the stage is being
rotated
✓ Negative – becke line moves outside of the mineral
o High – very dark line, significantly thick and high pronounced
o Medium –
o Low – very faint line
➢ Becke Line – can only be seen in PPL
- band of light along grain boundaries
➢ Isotropy – mineral light behaves differently depending on orientation
Cross Polarized Light (XPL)
• Interference Color – observed under XPL only
➢ Anisotropic minerals – rotating the stage at full 360 degrees, the interference colors
fade to black for four times (90 degrees)
➢ Isotropic – extinct at all orientations
- Never displays interference color in XPL
- Can be transparent in PPL
• Birefringence – the refraction of light in an anisotropic mineral
