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Mike Horne FGS

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.

When looking at a thin section I recommend the following procedure - first have a good look at the slide and try to work out how many different minerals are present and observe the textures; then methodically describe each mineral (bearing in mind that you will have to look at several examples to check the maximum birefringence and pleochroism) - look at them in plane polarised light (ppl) and then crossed polars (xpl) to measure the birefringence (bif) and extinction angles; finally calculate roughly the percentage of each mineral present and describe the rock texture in detail.

The minerals -

Opaque minerals - appear black in ppl and xpl and cannot be identified unless you use a reflecting microscope to study a polished section. May be haemetite, magnetite, pyrite or many other ore minerals. If it has a square shape it may be pyrite.

Quartz - clear in ppl; no cleavage; low first order interference colours in xpl - greys and whites, sometimes cream; often exhibits strained extinction. Occurs in igneous, metamorphic and sedimentary rocks.Trigonal.

Feldspars - tend to be oblong in shape; clear or 'muddy' in ppl; low first order interference colours in xpl - greys and whites; exhibits different forms of twinning that helps us to divide the feldspars into different types:- simple twinning, multiple (bar-code) twinning and crosshatch (tartan) twinning; decompose to clay - giving a muddy appearance. Monoclinic.
Orthoclase - Potassium Feldspar (K Feld) simple twinning.
Plagioclase - multiple twinning. There is a solid solution series from Sodium to Calcium Feldspars and they can be further subdivided into Albite, Oligoclase, Andesine, Labradorite, Bytownite and Anorthite using their Refractive indices and extinction angles.
Microcline - tartan twinning.

Nephaline - generally looks very much like quartz, but the two minerals never occur together; birefringence 1st order grey. Hexagonal symetry.

Pyroxenes - shape prismatic or octagonal; pink-brown-green in ppl; 2 good cleavages at 90 deg.; high relief; 1st to 2nd order birefringence; extintionct angles 45 deg or 0; sometimes twinned; occur in basic & ultrabasic igneous rocks and metamorphics.
Orthopyroxenes - e.g. Hypersthene - 1st order bif; straight extinction; faint pink to green pleo. Orthorhombic.
Clinopyroxenes - e.g. Augite - moderate 2nd order bif.; 45 degrees extinction. Monoclinic.

Amphiboles - e.g. Hornblende - prismatic shape; brown, yellow, green or bluish in ppl; sometimes strongly pleochroic; 2 good cleavages at 124 deg.; high positive relief; up to mid 2nd order bif.; extinction angles up to 25 deg.; twinning not common; occurrence acid igneous and hugh grade metamorphics. Monoclinic.

Biotite Mica - tend to be long platy xtals; good cleavage; brown or occasionally green in ppl.; pleochrosim shades of yellow and brown; in xpl interference colours masked by the brown absorption colour; straight extinction; occurrence igneous and metamorphic rocks. Monclinic.

Muscovite Mica - platy or radiating xtals; good cleavage; clear in ppl; low to moderate positive relief; bright interference colours in xpl. high 3rd order; occurrence granites, schists, mudrocks; can form from alteration of feldspars. Monoclinic.

Chlorite - clear to green in ppls; pleochrosim is shades of green; good cleavage; interference colours masked by green colour; straight extinction; occurs in low grade metamorphic rocks and ironstones (but do not confuse with glauconite).Monoclinic..

Olivine - Prismatic, clear to yellow-brown in ppl; pleochroic; poor cleavage and often has random crack in xtals; high 3rd order bif.; straight extinction; twinning rare; occurs in basic igneous rocks; partial alteration to Magnetite common. Orthorhombic.

Garnet - polygonal or almost circular in shape; clear to browny-pink in ppl; may have inclusions of other minerals; no cleavage; high positive relief; isotropic - so dark in xpls; occurs in metamorphic rocks. Cubic symmetry.

Calcite - interlocking xtals; clear to yellow in ppl; no pleochroism; very good cleavage; relief varies from low to high so it 'twinkles' when rotated in ppl; high birefringence (off the chart) pastel pinks and greens; sometimes twinned; occurs in marbles and limestones; Trigonal. note - other carbonates such as dolomite are very similar; note - two forms of calcite/carbonate cement seen in limestones are 'sparry' (nice xtals) or 'micritic' (muddy).

Serpentine - green in ppl; low 1st order bif.; appears to have 'mesh like' structure; occurs in altered basic and ultrabasic igneous rocks. Monoclinic.

Zircon - colourless in ppl; cleavage not conspicuous; very high positive relief; 3rd or 4th order bif; straight extinction in elongated xtals; common accessory mineral in acid and intermediate igneous rocks. Tetragonal.

Sphene - clear to slight brown in ppl; small diamond shaped xtals; cleavage not normally visible; very high positive relief; very high bif. = white; common accessory mineral in acid and intermediate igneous rocks and some metamorphic rocks. Monoclinic.

Andalucite - colourless or very pale pink in ppl; sometime pleochroic; 2 cleavages at 90 deg; moderate positive relief; low 1st order bif; straight extinction; occursin metamorphose argillaceous (clayey) rocks; variety Chiastolite has Maltese Cross twinning. Orthorhombic.

Kyanite - colourless to pale blue in ppl; 2 cleavages at 90 deg; high positive relief; bif up to 1st order red; extinction 30 deg to cleavage; occurs in schists and gniesses; associated with garnet and staurolite. Triclinic.

Staurolite - pale yellow in ppl; pleochroic; one poor cleavage; high positive relief; high 1st order bif; straight extinction; occurs in schists and gneisses. Orthorhombic.

Sillimanite - colourless in ppl; cleavage not normally seen; high positive relief; 2nd order bif; elongated xtals have straight extinction; occurs in high grade metamorphic rocks. Orthorhombic.

Tourmaline - shades of brown, green or blue in ppl; strongly pleochroic; zoning common. Trigonal.

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