Optical Mineralogy: Reflected Light Microscopy

What type of mineral is optical light microscopy useful for?

Opaque minerals

What groups of minerals tend to be opaque?

Oxides and sulfides

What types of slides do you look at for reflected light microscopy?

Polished thin sections or plugs

What is a plug?

Chunks of rock embedded in epoxy

When you see brighter illumination while looking at a mineral, what does this mean?

That more light is being relfected

What’s generally observable in reflected light microscopy?

Crystal and aggregate habit, crystal shape, how euhedral a crystal is, and the orientation of a grain

How does zoning or layering appear in reflected light microscopy, and is it always visible?

Usually you only see black and white, and acid etching may be necessary to reveal it

How does cleavage appear in reflected light microscopy?

Parallel cracks, or triangular pits if three directions of cleavage are present

How do indentations appear in reflected light microscopy?

They will be darker than the surface

Can you see twinning in reflected light microscopy?

You may be able to infer them from the crystal’s overall shape

How does exolution appear in reflected light?

Blebs, patches, or lamellea will be present, often crystallogrphically controlled

How do alterations appear in reflected light?

A grain will look moth-eaten, patchy, or heteorgeniuns

Polishing hardness becke line effect

Softer minerals abrade more during polishing, causing a concavity which creates a becke line effect

Polishing hardness

Whether or not a grain was scratched by mineral used to polish it, revealing it’s hardness to be less than or greater to that mineral

What are additional ways to test hardness

You can scratch your sample or use microindentation

PPRL

Plane Polarized Reflected Light

What does PPRL measure?

Reflectivity, denoted R, which is essentially brightness

What is the formula for reflectivity

intensity of reflected light over light applied

Are colors visible in PPRL?

Yes, reflectance varies with wavelength, creating a tint

Why is oil added to the slide sometimes?

Minerals look brighter in air, but more colorful in oil

Why do silicates often appear dark in PPRL?

Because they have very low reflectance

Bireflection

A change in reflectance with orientation

Why does bireflection occur?

Light penetrates the mineral a small amount, causing it to break into two rays and be reflected differently