UBUniversity of Bristol
EARTH
SCIENCES

Minerals under the Microscope

Created by Charlotte Gladstone with a bit of help from Paul Browning (paul.browning@bris.ac.uk)

Introduction
The polarising microscope
Plane polarised light (PPL)
Cross polarised light (XPL)
Shape and Cleavage Interference colours
Relief Extinction angle
Colour and Pleochroism Twinning
Opacity
Vibration directions
Mineral Identification
Additional reading

Introduction

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".

Two bits of advice:

Two maps

As you work through this document you will find two sorts of "maps" that look like this:

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.

Warning Sign

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.

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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 the lenses. 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 light path):

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.

Double Refraction

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.

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Shape and Cleavage

The form of crystals and the arrangement of cleavage planes within them are useful for identification.

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).

Augite Phenocryst

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.

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Relief

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.

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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.

Biotite E-W

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.

Biotite N-S

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.)

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Opacity

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.

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Vibration Directions

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.

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Interference colours

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 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.

Birefringence

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.

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Extinction Angle

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:

(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.

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Twinning

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.

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Mineral Identification

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

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Additional Reading

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)

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