University of Bristol EARTH SCIENCES

• 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 subject

• 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 1 course.

Two maps

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

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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Thin sections and the polarising microscope

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):

• 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

Double Refraction

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.

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.

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 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.

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:

• 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.

Birefringence

• 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 extinction.

(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.

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 forming minerals
• 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).

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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