Minerals in thin section: common in mafic igneous rocks

What relief does olivine have?

Olivine has high relief

Does olivine have high or low birefringence?

Olivine has high birefringence

Does olivine show good cleavage in thin section? Or other cracks?

Olivine rarely shows good cleavage but commonly have randomly orientated cracks

What does the different order of interference colours represent in olivine?

Olivine has different order of interference colours depending on the orientation of cutting of the crystals

What order interference colours does olivine have?

First, second and third

Why is there zoning in olivine crystals and where is this seen?

Olivine shows zoning around the edges as the edges are slightly more rich in iron

What colour is olivine in PPL?

Olivine is colourless or slightly green in PPL

What colour is orthopyroxene in PPL?

Orthopyroxene is pleochroic pink and green in PPL

Is cleavage visible in orthopyroxene?

Cleavage is visible in orthopyroxene with other irregular cracks

What order interference colours does orthopyroxene have?

Orthopyroxene has first order interference colours only

Does orthopyroxene have high or low birefringence?

Orthopyroxene has low birefringence

What extinction does orthopyroxene show and in which sections?

Orthopyroxene shows straight extinction in all sections showing one cleavage

What colour is clinopyroxene in PPL and XPL?

Light brown/green

What colour is hornblende (amphibole) in PPL?

Different shades of brown

What are the thick black rims around hornblendes caused by?

Due to the formation of iron oxide due to oxidation

What are the maximum order interference colours for hornblende?

Second order interference colours

Are hornblendes/amphiboles pleochroic in PPL?

Yes

What cleavage does biotite have?

1 perfect basal

Describe the colour change pleochroism of biotite

Changes from light to medium/dark brown

When is the strongest absorption colour of biotite seen in PPL?

When the cleavage is parallel to the polarizer

How can biotite be distinguished from hornblende?

Biotite has a speckled surface when its close to extinction

What interference colours does plagioclase have?

Low first order grey and white

What colour is chlorite in PPL?

Greenish yellow

What colour change does chlorite undergo in PPL when rotated 90 (pleochroism)

Green to yellow

What word describes the interference colours of chlorite in XPL?

Anomalous

What does ‘anomalous interference colours’ mean?

These colours do not appear in the interference chart

What colours does chlorite appear in XPL?

Brown and blue

What 2 words describe quartz in thin section?

Invariably clear and unaltered

Does quartz cleavage visible in thin section?

No

What colour is quartz in XPL?

Grey or white

What does it mean if quartz is yellow in XPL?

The thin section is slightly too thick

What is undulose extinction?

When the rpc