Optical Mineralogy Review

Clinopyroxene

End-section

• High relief

• Almost perpendicular cleavage

• Yellow-orange when you see cleavages

• Low interference colors

C-axis parallel

• High relief

• High order interference colors

• Inclined extinction

Can see some very slight, more-so ivory pleochroism

K-Feldspar

Very low relief, almost clear non pleochroic

With polar on you see cross-hatch twinning

Exsolution

Orthopyroxene

Very low interference colors

Brownish-orange

Pink-greenish light pleochroism

High relief

Parallel extinction

Plagioclase

Low relief, clear, no pleochroism

When cross polars are on, there can be some twinning sometimes not

Albite will have very good twinning

Sometimes can look like quartz

Olivine

Parallel extinction

Range of interference colors, some low and some pretty high

High interference colors + parallel extinction + higher relief = Olivine

Curved fractures(?)

Sometimes has cleavage..

Biotite

Pleochroism

• Darkest when cleavage planes run east-west(?)

• Normal pleochroism

With cross polars on

• High order interference colors (pink, green)

Overall flaky appearance

Bird’s eye extinction, pebble appearance coarse

Amphibole

60/120 cleavage

Lower relief

Can have inclined extinction (clinoamphiboles, like hornblende)

Can also have parallel extinction (orthoamphiboles)

Quartz

Very low relief, clear

No cleavage

Very flat appearance

Muscovite

Low to moderate relief, almost colorless

Again, flaky

Med-high order interference colors, VIBRANT colors

Bird’s eye extinction

Garnet

High relief

No pleochroism

COMPLETELY black with cross polars on

Calcite

It’s super pastel and bright and very hard to miss