Optical Microscopy – Exercise 2
PUT THE THIN SECTIONS BACK WHERE YOU FOUND THEM!
Examine the following thin sections in any order.
Thin section #Z1 (E16 & E17 are also good but have no garnet)
Carefully look at this thin section of a glaucophane schist, a high-pressure metamorphic rock.
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On the basis of colour and relief, how many main minerals can you see in this thin section?
What is the grain size range in this thin section?
Find a good example of the highest relief mineral, which is colourless to pale pink and forms the largest
grains in this sample:
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Describe the mineral in terms of grain size, crystal form/shape, colour/pleochroism,
cleavage/fracture and relief (PPL). (Note: You should notice lots of small fragments of other
minerals inside the crystal. These are called inclusions);
Now insert the analyser (i.e. XN). Which polarisation colour do you see? Now find more grains of
the same mineral and check whether the polarisation colour changes. Is this mineral isotropic or
anisotropic? To which crystal system does it belong?
This is the mineral GARNET. It is by far the most common isotropic mineral you will see, common in many
igneous and metamorphic rocks. It forms large grains or porphyroblasts in this sample.
Now look at the strongly coloured blue/purple mineral. This is GLAUCOPHANE, a type of amphibole:
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Describe the mineral in terms of grain size, shape, colour/pleochroism, cleavage/fracture and
relief (PPL);
Focussing on one grain, insert the analyser. You should see a polarisation colour. Which colour and
order is shown? (Tip: look for colour fringes at the grain boundary and, with reference to the
Michel-Levy chart, see if you can match the colour and order). What is the retardation (in nm) of this
grain);
Look through the thin section and find the maximum polarisation colour of this mineral;
What is the retardation and birefringence of this mineral?
Now look at the colourless to pale yellow mineral. This is EPIDOTE, another common metamorphic
mineral:
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Describe the mineral in terms of grain size, shape, colour/pleochroism, cleavage/fracture and
relief (PPL);
Focussing on one grain, insert the analyser. You should see a polarisation colour. Which colour and
order is shown? (Tip: look for colour fringes). What is the retardation (in nm) of this grain);
Look through the thin section and find the maximum polarisation colour of this mineral;
What is the retardation and birefringence of this mineral?
Now look at the lowest relief, colourless mineral. This is QUARTZ:
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Look through the thin section and find the maximum polarisation colour of quartz;
What is the retardation and birefringence of quartz?
Find a part of the thin section that shows all the minerals you have described and do a detailed labelled
drawing. Remember to add a scale….
Thin section #Y13 (Y08 & G107 are also good)
Sample Y13 is a volcanic rock comprising larger grains of early-formed crystals (phenocrysts) in a finergrained groundmass:
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Two minerals form the phenocryst minerals, a colourless mineral and a pale-brown mineral. Using
mineral ID sheets, draw each and describe them in terms of grain size, crystal form/shape,
colour/pleochroism and relief (PPL).
The colourless mineral has no cleavage but is commonly fractured. The pale brown mineral shows
cleavage – how many cleavage planes are there and what is the angle between them?
Which of these two minerals has the higher relief?
Focus on a grain of one of the minerals and insert the analyser (XN). What do you see? Rotate the
grain through 360°. How often does the mineral go into extinction and at what angular
relationship? Explain this effect;
Both minerals should show polarisation colours. Are the minerals isotropic or anisotropic?
Look through the thin section and find the maximum polarisation colour for each of these
minerals. What are these (colour and order)? What is the birefringence of each?
The two minerals are OLIVINE (high relief, high birefringence, fractures) and the common clinopyroxene
AUGITE (moderate to high relief, moderate birefringence, 2 cleavages). Both minerals are common in
mafic igneous rocks such as basalts and gabbros. You should notice that many grains of augite do not go
into extinction at exactly the same position. This is because of chemical zoning.
Thin section #X1 (Z24 & G100 are also good)
There are many minerals in this metamorphic rock, larger porphyroblasts sitting in a finer-grained
matrix. Ignoring the higher-relief porphyroblasts, most of the rock is made up of (a) two moderate relief
minerals, one brown, one colourless, both with one good cleavage; and (b) low relief colourless minerals:
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Examine the moderate relief minerals and describe them in terms of grain size, crystal form,
colour/pleochroism, cleavage/fracture and relief (PPL);
Do the mineral occur with a random or preferred orientation?
Insert the anlayser (XN) and rotate the grain until it is almost (but not quite) black. What do you see
(can you see any unusual textures)? Are these minerals isotropic or anisotropic?
Focus on one grain of one of each of the minerals (XN). Using fringe counting if possible, determine
the polarisation colour and order. What is the retardation of these grains?
Examining the entire slide, find the maximum polarisation colour for each of these two minerals.
What are these (colour and order)? What is the birefringence of each?
These minerals are MICAS (brown = biotite; colourless = muscovite). Both are common in metamorphic
rocks and occur in many igneous rocks. They are easily distinguished by their ‘speckled’ appearance when
they are close to their extinction positions.
Now look at the other common colourless grains that show no or very low relief. Most of these are
QUARTZ, one of the most common rock forming minerals.
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Describe the mineral in terms of grain size, crystal form and cleavage/fracture (PPL);
Using the Becke test, does quartz have positive or negative relief?;
Examining the slide and find the maximum polarisation colour of quartz. What are these (colour
and order)? What is the birefringence of quartz?