|University of Bristol|
Minerals under the MicroscopeCreated by Charlotte Gladstone with a bit of help
from Paul Browning (firstname.lastname@example.org)
In Practicals 5 and 6 we introduce an important skill that will underpin
practical work in the rest of the Level 1 Geology course - use of the polarising
(or petrological microscope). Like many skills worth having, use of the polarising
microscope takes practise to perfect. To help you in this task we provide a
number of supporting learning materials:
Both the OUVM and the Optical Mineralogy module can be accessed from computers in Earth Sciences' Computer Suite (G36) by selecting "Coursware" in the Launcher and opening the folder "UKESCC".
- The Open
University Virtual Microscope (OUVM) provides a computer-based simulation
of a polarising microscope
- The UKESCC Optical
Mineralogy module which provides a computer-based tutorial on the entire
- This hypertext document (which you may be reading for the first time in a paper
form) which will help you use the OUVM and Optical Mineralogy module as
productively as possible
Two bits of advice:
- The OUVM is not a substitute for the "real thing" - make sure you are able to
transpose the techniques you learn on the simulation to real thin sections
- The Optical Mineralogy module contains much material of a content and a level
that will not be covered until the Level 2 Mineralogy course. Do not be intimidated
by it! By all means look at the advanced material but use this document to direct
your study on the parts of the module that are essential background for the Level
As you work through this document you will find two sorts of "maps" that look like
These represent the "top pages" of the OUVM and Optical Mineralogy module. The map
on the left means "if you want to see an example of this look at slide 6 on the
OUVM". The map on the right means "if you want more background on this topic look
at pages 1-11 on the section on Birefringence in the Optical Mineralogy module".
Click on either of these maps to see what the "top pages" really look like.
You will also see a few warning triangles like this:
These represent pages of the Optical Mineralogy module which cover subjects which
you haven't come across yet in the course. These will be taught to you in the second
year course. Naturally, you are welcome to read these pages anyway.
The Polarising Microscope
Thin sections and the polarising microscope
Up to now you've looked at rocks in hand specimen and have found that it is
sometimes rather difficult to be confident about identifying minerals, especially
in rocks that are fine-grained. Thin sections change all this!
A thin section is made by grinding down a slice of rock which has been glued to
a glass slide until it reaches a thickness of about 0.03mm (30 microns). At this
thickness most minerals become more or less transparent and can therefore be
studied by a microscope using transmitted light. Thin sections are time consuming
and costly to prepare - each is worth £10 - so please treat them with care.
The instrument we use to look at thin sections is a polarising microscope.
These are expensive (£1500!) items and also need to be treated with care, especially
Before using a polarising microscope it is important to know a bit about polarised
light and the optical properties of minerals.
You should be familiar with the following (from bottom to top, following the
- Illumination control
- Substage assembly (polariser, iris diaphragm and condenser)
- Rotating stage with clips
- Objectives in rotating nose piece
- Coarse/fine focussing wheel
- Analyser : make sure it is out
- Bertrand Lens : make sure it is out
- Eyepiece with crosshairs
If you're not sure what all the bits are (the analyser, the Bertrand Lens, etc.)
click on the picture. This will give you a full labelled microscope.
The nature of polarised light
Light travels as electro-magnetic
vibrations in which the vibration direction is transverse to the direction of
propagation. Transverse wave-motions of this type are said to be plane polarised
when all the vibrations lie in one plane. Light from the sun is unpolarised but when it
reflects off a surface it becomes partly polarised as shown opposite.
Most crystalline substance are anisotropic - their physical properties
(including refractive index) differ if measured in different directions.
Crystals belonging to the cubic system are the exception and are said
to be isotropic - their physical properties do not vary with direction.
When a ray of ordinary (unpolarised) light enters an anisotropic crystal it is
in general split into two rays - this is called double refraction.
Shape and Cleavage
The form of crystals and the arrangement of cleavage planes within them are useful
Consider the mineral augite, a member of the pyroxene group. This
diagram shows the crystal form. It has two cleavages at 90 degrees to each other
running parallel to the length of the crystal (so-called "prismatic" cleavage).
This thin secton shows a euhedral phenocryst of augite.
These two photos show cleavage well. Diagram A illustrates one cleavage
(so-called "basal" cleavage). Diagram B shows two cleavages
(prismatic). Arrows have been added to help you pick out the cleavages.
You will immediately notice when you look at thin sections that some minerals
are clearly visible (that is, details of surface texture,
cleavage, etc., are obvious) while others appear almost featureless and, if
colourless, barely visible. This is the property known as relief.
Minerals which have refractive indices which differ markedly from that of the
mounting medium (the glue used to stick the rock slice to the glass slide and
the cover slip to the rock) show up clearly in thin section and are said to have
high relief. Minerals with low relief have refractive indices close to that of
the mounting medium of about 1.54.
Relief is a useful distinguishing property for the the igneous rock-froming
minerals; all the mafic minerals show high relief but all the felsic minerals
(with the exception of muscovite) show low relief.
This thi section shows examples of contrasting relief. The high relief
mineral is clinopyroxene and the low relief mineral is plagioclase feldspar.
Colour and Pleochroism
We've already observed that augite can appear slightly pink in plane polarised
light (PPL) - this is the result of selective absorption of certain of the wavelengths
that comprise the white light supplied by the illumination.
The anistropy shown by non-cubic crystals in their physical properties can also be
shown by their absorption - this phenomenon is called pleochroism and is a
useful distinguishing property.
Pleochroism is apparent in thin section when minerals undergo a colour change as
they are rotated in plane polarised light.
Here are two pictures of the same crystal of biotite, under PPL. The first is taken
with the cleavage oriented E-W along the crosshairs.
In this second picture, the stage has been rotated 90 degrees so that the cleavage
is oriented N-S along the crosshairs.
Look at the variation in colour between these two; this is pleochroism.
(The black spots are caused from radiation damage by small inclusions of uranium- or
thorium-bearing minerals like zircon.)
Some minerals, typically the ore minerals (oxides and sulphides), are not
transparent in thin section - they are opaque. We really need another form of
microscope - a reflecting light microscope - before we could be sure. It is
important to realise that whilst an opaque mineral might appear isotropic
this may not be the case. Magnetite, belonging to the cubic system, is isotropic;
haematite, belonging to the trigonal system, is anisotropic. However, both the
iron oxide minerals are opaque and appear isotropic in transmitted light.
What is going to happen if we place an anisotropic mineral on a polarising
microscope? Consider first what will happen in just plane polarised light
(with the analyser out):
The light illuminating the specimen is already polarised in an E-W direction.
In certain positions of the specimen the plane of polarisation will be parallel
with one or other of the vibration directions of the crystal. In this situation
all the light passing through the crystal utilises one of the vibration directions
only since it has no component of vibration in the plane of the other. If, on the
other hand, the plane of polarisation of the illumination is NOT parallel to
either of the vibration directions of the grain, light will pass through the
crystal utilising both of the possible vibration directions.
Now consider the function of the analyser (with its vibration direction N-S):
The mineral grain will become dark in four positions 90 degrees apart as the stage is
rotated the extinction positions are reached whenever either of the vibration
directions of the grain all into parallelism with the vibration direction of
the polariser. In these positions all the light transmitted through the crystal
utilizes one vibration direction only (W-E) and this light is completely cut out
by the analyser (N-S). In between the four extinction positions some light will
always pass through the analyser because the light passing through the crystal
utilises both vibration directions, neither of which is normal to the vibration
direction of the analyser.
Light from an original single, plane polarised (white) beam passes through
anisotropic crystals as two rays with different velocities.
When the analyser is inserted the two rays are recombined into a single
N-S plane and interfere with each other as they will be out of phase.
The phase difference depends on:
Thus for a given grain in a particular section, wavelength is the only variable
and the phase difference will vary for the different wavelengths in the white
light supplied. Hence certain colours will be reinforced because of path
differences which happen to coincide with a whole range of wavelengths, and
others will be cut out because of phase differences involving a half wavelength.
- The thickness of the section
- The wavelengths of the light
- The difference in refractive index between the two rays (constant for
all wavelengths if no dispersion) which is called the birefringence
The result is a coloured ray, i.e. a coloured appearance of the grain (as seen on
the left), due tothe removal of certain wavelengths from the original white. For a
more rigorous discussion of interference colours see the Optical Mineralogy module
- however, do not worry if you find this material challenging.
This maximum colour is often diagnostic of an anistropic mineral and it is observed
in sections that display simultaneously the maximum and minimum refractive indices.
The numerical difference between the two indices is the birefringence. For
example, augite has a maximum and minimum refractive indices of 1.724 and 1.700
giving a birefringence of 0.024. The Michel Levy chart summarizes the relationships
between interference colours, birefringence and thin section thickness.
The extinction angle of a given grain is the angle between any specified
crystallographic direction and either of the two vibration directions. It can
be an important distinguishing character for different minerals.
As with interference colours, a mineral in different orientations will show
different kinds of extinction. It is important to record either the nature of
the extinction shown by MOST grains of a mineral (i.e. straight or inclined)
and if inclined extinction is displayed to record the MAXIMUM extinction angle shown.
The following generalizations apply:
- Minerals belonging to the tetragonal, hexagonal, trigonal or orthorhombic
crystal systems will in general show straight extinction.
- Minerals belonging to the monoclinic system will often show inclined extinction
but may sometimes show straight extinction.
- Minerals belonging to the triclinic system will in general show inclined
(If you need more information on crystal systems see Lecture 1 of the Earth
Materials or the Crystallography Courseware Module)
This first photograph shows a pyroxene crystal with cleavage aligned E-W along
the crosshairs. The second photograph shows the same crystal in an extinction
position. This shows inclined extinction as the cleavage has a different orientation
to either of the vibration directions.
One consequence of the symmetry of the internal structure of crystals is the possible
growth of twinned crystals. A twinned crystal is a single crystal divided into
two (or more) parts in which the crystal lattice of one part is differently oriented
with respect to the next.
Repeat twinning is a prominent feature of many minerals, particularly the plagioclase
feldpars. The stripey plagioclase feldspar crystal on the right shows this. The
crystal is divided up into narrow lamellae with alternate orientations. The black and
white stripes are caused by lamellae of one orientation being in an extinction
position, while lamellae of the second orientation are not.
Successful mineral identification when using thin sections is (like when looking at
rocks and mineral in hand specimen) a matter of asking yourself the right questions.
With practise you will build yourself an automatic "expert system" to lead you to
the right answer. An example of an expert system is shown below - providing you answer
the questions correctly (by making accurate observations) then you will lead youself
to the correct answer. But note that
- this is not the only expert system that would be designed for igneous rock
- it is not watertight; for example hornblende can show brown pleochroism (and
biotite when it alters to chlorite can show green pleochroism)
- in such cases you would need to check that the other thin section properties are
consistent with your interpretation (e.g. brown hornblende will not in general
show straight extinction where as biotite will).
Battey - Mineralogy for students (QE363.2 BAT)
Phillips - Optical Mineralogy: the non-opaque minerals (QE369.06 PHI)
Kerr - Optical Mineralogy (QE369.06 KER)
(The code in brackets after each title refers to the Library catalogue number)