Thin Section

Minerals and Thin Sections

Green Thin Section

Black Thin Section

A thin section is a slice of a rock, usually about 30 microns thick, or 30 μm. A thin section is looked at under a special microscope to study the minerals in the rock. As discussed previously, minerals have certain traits that are unique to that specific mineral. Some traits, however, can only be viewed with thin sections. This is a photo of a rock with black and green sections. Click the black ovals to view the thin sections of this rock.

What is a thin section?

Polarizing

Light travels from the bottom of the microscope through a polarizing lens to have all the light travel in one singular direction. The "straightened" light then travels through the thin section and microscope, ending in our eyes. This is the "regular" mode, called Plane Polarized Light or PPL. The other mode that is used is called XPL, or Cross Poliazed Light. This works by adding another polarizing lens to the microscope above the thin section. This means that only the light that is warped from the crystals can make it through the second polarizer and to our eyes.

How do these thin sections work?

• Colour
• Extinction

• Twinning
• Cleavage

XPL Traits

PPL Traits

• Colour
• Relief
• Pleochroism

• Cleavage

Common Traits to use for identifications in Thin Sections

-Certain Minerals have certain colours in XPL and are very diagnostic-Michel-Levy Chart

M-L Chart

Examples

Examples

-Certain Minerals have certain colours in PPL -The most comon colours are colourless, green, brown, and blue.

XPL

PPL

Colour

Pleochoism is a trait that some minerals have where when they are rotated, the colour will slightly change.

Video Example

Pleochroism

Hornblend has perfect 120 degree clevage, making diamond shapes.

Biotite has basal clevage.

Just like with regular hand samples, minerals in thin sections can have clevage too.

Clevage

Relief is how much of an edge/boundary the mineral has. The higher the topology, the higher the relief.

Bioitie has moderate/medium relief.

Olivene has high relief.

Plagioclase has low relief.

Relief

Example Video

Minerals always go extinct every 90 degrees rotated when on XPL. This means that they turn black and no light is permitted to travel though them. Some minerals, however, are always extict because they are isotropic.

Extinction

Tartan

Simple

Carslbad

Twinning happenes from intergrowth inside of crystals. It results in lines inside of a mineral that go opposite in colour to the rest of the mineral and is very diagnostic.

Twinning

The thin section from the black section of this rock.

This is the thin section from the green part of this rock.

A polarizing lens has a grid that blocks all light except for the light waves that travel in the same direction as the grid. When two polarized lenses are crossed, like in a microscope, it blocks all light because the second grid blocks the light allowed through the first grid.

The Michel-Levy Chart is used to catagorize minerals in XPL. The left axis shows the thinkness of the thin section, which earlier we said is usualy 30 microns. We then look at the colours on the mineral and mach it up with the colours on the chart. We then follow the diagonal line it hits to find the birefringence. Each mineral has a specific birefringence.

Epidote

Epidote is a light green in PPL but has really high M-L colours. Its colour can be used to differentiate between Chlorite (the light teal/green mineral).

Biotite

Biotite has a really strong brown, occasionally green colour in PPL. because of how strong the colour is, it usually looks the same in XPL, masking the colour of the ML chart, which is really high.

Quartz

Quartz is compley colourless in PPL and in XPL. It has a really low M-L chart colour of just white and grey.

Epidote

Epidote has really high birefringence.

Biotite

Biotite has really high birefringence but is blocked by the strong brown.

Quartz

Quartz has really low birefringence so it is only white and grey.

Spinel is one of those isotropic minerals.

A video to view pleochoism in Biotite.

Plagioclase Feldspar has simple twinning.

Orthoclase Feldspar can have Carlsbad twinning. This happens when a whole new crystal starts growing out of the existing one, resulting in half of the crystal to have solid twinning

Alkali Feldspar has very distictive Tartan twinning. Tartan comes from the Scottish Language. It refers to their kilts, which have a plaid pattern.