Minerals in thin section: Properties

What does anisotropy look like in XPL?

Show extinction and interference colours in XPL

Which crystal system is not anisotropic?

Cubic crystal system

Are cleavage planes regular or irregular patterns?

Regular planes

What is cleavage?

The regular planes along which minerals tend to break along

What do conchoidal fractures look like?

Curved fractures

What type of minerals are conchoidal fractures common in? Give an example

Strong minerals, eg. quartz

Do minerals with conchoidal fractures have cleavage?

Generally have no or very weak cleavage

What type of minerals are always in extinction?

Isotropic

Why are isotropic minerals always in extinction?

The polarised light is not doubly refracted when passing through

How many extinction positions do anisotropic minerals show during a 360 rotation?

4 symmetrical

What are the 3 types of extinction, relative to crystal faces or cleavage?

Parallel, inclined and symmetrical

Are fractures regular or irregular?

Irregular

What are fractures?

The irregular lines along which minerals tend to break along

What are interference colours?

The colour of anisotropic minerals in XPL

When do isotropic minerals show extinction?

In all positions in XPL

Give examples of isotropic substances

Cubic crystal systems, eg. garnet, and glass

What is lustre?

The reflective property of a mineral

What is pleochroism?

Minerals exhibit different colours when observed at different angles in PPL

What is a reentrant angle?

Crystal faces that intersect at an angle between 180 and 360

What do reentrant angles mark?

Twinning in a crystal

What crystal properties do twins have different values for?

Interference colour and extinction angle