Minerals in thin section
Tutor - Mike Horne FGS
University of Hull
Thin sections are slices of rock cut to 0.03 mm thick and stuck on a class slide with Canada Balsam. They are studied with a petrological microscope, using polarised transmitted light.
The optical properties observed help us to identify the minerals present in the rock. Always remember that we are looking at a two dimensional slice of a three dimensional grain or crystal, which has been cut at random - so you should examine several grains/crystals of the same mineral to get the full picture. Here is a list of features to observe.
Size - Are the grains/crystals larger than the grains/crystals of other minerals. Large crystals are called phenocrysts in an Igneous Rock and porphyroblasts in a metamorphic rock.
Shape - do the crystals have well defined edges and angles (euhedral), or are they rounded (anhedral), or something in between (subhedral)?
Angles between crystal faces - measure the angle between the crystal faces if they are well defined
Colour - the absorption colour in plane polarised light - this is not the same as the colour of the mineral in hand specimen. Most minerals are colourless in thin section. Some are opaque, light cannot pass through them and they appear black, so we cannot identify them using this type of microscopy. Magnetite, haematite and pyrite are opaque. Chlorite is green, Biotite is brown.
Pleochroism - changes in the absorption colour as the stage of the microscope is rotated. Biotite shows shades of brown, Chlorite shows shades of green
Cleavage and cleavage angles - cleavage appears as parallel lines within the crystal, if there is more than one cleavage you can calculate the angle between them.
Relief - do some minerals appear to 'stand out' more than others? This is due to their refractive index - the mineral with the higher RI has higher relief. You can check this by using the "Becke Line".
Alteration - some minerals start to weather and break down to form new minerals. Feldspars often have a mucky appearance due to alteration to clays.
Birefringence - the interference colour observed in crossed polars (with the analyser in). Look at several grains/crystals of the same mineral to find the maximum birefringence. The birefringence depends on the thickness of the thin section (normally they are 30 microns = 0.030 mm), the crystal system, the refractive indices of the mineral and the orientation of the crystal. Glass, cubic system minerals, and some crystals cut parallel to a crystal axis will show no birefringence.
Extinction angles - minerals that exhibit interference colours go dark when the microscope stage is rotated. Does this 'extinction' coincide with a cleavage direction or elongation of the crystal, or is it an angle?
Twinning - twinned crystals are often optically reversed, with the twinned parts going into extinction at different times. Twinning can be simple (two), multiple or crosshatched giving a tartan pattern
Zoning - different parts of the crystal go into extinction at different times as the stage is rotated - there is a change from the centre to the outside of the crystal.
Strain - different parts of the crystal go into extinction at different times as the stage is rotated - there is a change from one side of the crystal to the other due to stress (pressure) on the crystal.
Links to other sites about optical properties of minerals:
Mickey's Optical Mineralogy page