Studying the OU S260 Thin Rock Sections Under The Microscope.

  An Introduction to Mineralogical Terms and Observations with the Leica CME.

By Ian Walker.  U.K.

Please Note: Geology and the study of rocks, minerals and crystals is a very complex subject, as an amateur microscopist I am trying to convey my own enthusiasm and interest relating to these slides, I suggest anyone requiring detailed knowledge of rocks, minerals and thin rock sections look to professional  sources.

The OU S260 thin rock section collection consists of 24, 2" X 1" slides designed to be used with a modest polarizing microscope and are made for the Open University (UK) Geology Course S260. I have owned a box of these for some time and thought a general overview may be of interest to anyone wishing to study a wide range of thin rock samples under a microscope for the first time. The impression I get is that some geology courses are going 'virtual' for their study of rock sections where a CD-ROM will contain pictures of the samples and software allows you to view them under different lighting conditions, slides may become a thing of the past!

The Open University S260 thin rock sections with some examples.

I am certainly not an expert on geology or rocks and minerals but I enjoyed studying these slides and writing up my own notes; this article is based upon those notes. You can expect to pay premium prices, typically £17 per slide, for thin rock sections for the microscope from a source such as Gregory, Bottley & Lloyd of London. Older samples on eBay also command high prices especially if a large collection is on offer (although larger sets on eBay can be good value per slide if funds allow). The OU slides work out at less than £3 per slide, are contained in a smart cardboard box and come with an A4 printed page containing details of the rock type and locality. Rather than provide images and notes on all the slides, I have selected several which I found particularly interesting. For general viewing the slide size can be a problem for many biological microscopes because of the short length. I have got round this problem by cutting a small piece of thick card and placing it to one side of the slide so my slide carrier can grip them correctly - so far I have not had any problems with this method, this does prevent you from rotating the slide however!

Home made adapter to bring the width of the slide to a standard 3".

The sections themselves have been made to a reasonable standard but are spoilt by the sealing agent (without coverslip) which is far from optically perfect and gives the slides a slightly 'milky' appearance and spoils them both visually under the microscope and photographically where a loss of sharpness and definition occurs. However that does not detract from my use of them to learn about rocks and minerals. Each one of the slides has a label marking them from A to X; A to W are rock sections and X is a small wedge of gypsum. Below are tables showing all the slides but divided into the main rock types.

What is Petrography? It's the study and description of rocks and petrogenesis, the study of the origin of rocks.

What are rocks? Combinations or aggregates of minerals except those principally made of glass, [silicon dioxide].

What are minerals? Chemical compounds, inorganic, with defined crystal structures and naturally occurring.

Several books and sources of information have been useful to understanding rocks and minerals, these include:

A Colour Atlas Of Rocks And Minerals In Thin Section by W.S. MacKenzie and A. E. Adams - available at the Amazon (UK) on-line store; a current work excellent for beginners with good photographs of thin sections and an introductory chapter on understanding the main terms of use in the study of minerals, thin rock sections and the polarizing microscope.

Optical Mineralogy by Paul. F. Kerr, published at various times, mine is a late reprint 1977 but it goes back to the 1940's. Only available second hand. I got mine from U.S.A. via the ABE Books network. Prices vary enormously so shop around. This book takes you through a more thorough tour of crystal structure and mineralogy but has a good number of line drawings, tables and B&W photographs throughout.

Microsoft Encarta [CD-ROM or DVD], excellent source for general terms of rocks, minerals and gemstones; if you want a no-nonsense, simple, concise explanation of rocks and minerals it's excellent, with plenty of images.

Encyclopaedia Britannica [CDROM or DVD] another good source of information but rather less in the way of images.

Molecular Expressions, a professional website with a huge resource of high quality material; it can be a bit daunting at first, but if you put in specific terms into the search engine you should be able find what you are looking for, including many references to the theory of light, polarization and the polarizing microscope.

Home made analyzer with calibration markings fitted beneath binocular head of the Leica CME and the polarizer slide fitted beneath the condenser.

To view the thin sections properly you will need a polarizer/analyzer fitted to your biological scope, many modern designs can have them fitted as accessories but I made my own. Of course if you have a dedicated polarizing microscope also with rotating stage, it makes things much easier. Since my Canon Ixus camera has been adapted to my Leica CME for use with the remote control software, I have used this combination for this article, also to show that you can get acceptable results with a simple set-up. The normal 'orientation' for the polarizer is East-West and can be a simple piece of polarizer sheet cut to fit your filter tray beneath the condenser or possibly resting upon the light source, but any imperfections or dust are likely to show up on your images if placed here. The analyzer, placed beneath the binocular head or attached to a monocular eyepiece tube is always placed at right angles to the polarizer so the orientation of the polarizing sheet is now North-South. One of the slides, Biotite [H] shown here can be used to set the orientation of the polarizer and hence the analyzer.

  Biotite .

Left image shows biotite crystals with cleavage plane orientated N-S and the right image, a selected crystal 1/3rd of the way down facing E-W, 40x magnification under plane polarised light.

When ordinary light [consisting of waves vibrating in all directions] is passed through a polarizer it becomes plane polarized light, [vibrations in a single plane - the plane of polarization] here after I will call this PPL. The interaction of plane polarized light with some minerals such as biotite can be very useful, firstly for setting up your polarizer for correct alignment and secondly other minerals can behave quite differently to biotite and you can use PPL as a diagnostic to mineral type. The effect of some minerals to change their colour when rotated in PPL is known as pleochroism and the colour, its absorption colour.

Many minerals can be easily broken in certain planes, this is related to weaknesses in the atomic structure along these planes - biotite is an excellent example of this phenomenom known ascleavage because it is easily visible in the structure as well defined straight lines separating different parts of the mineral and can be seen in the above images. Not all minerals have such definite cleavage planes however and some may have more than one. Two prominent examples with cleavage planes include the pyroxenes and feldspars, the cleavage planes can be at 90 degrees to each other but alternative angles are equally possible and this is also a good method of identifying minerals, especially if they have another strong characteristic. Minerals may have even more cleavage planes - fluorite as used in some microscope objectives has four.

So to set up your microscope polarizer you can orientate some well defined crystals of biotite so that the crystal structure runs exactly horizontal as seen through your microscope [fine cross-hairs fitted to your eyepiece can help you achieve this and remember not to have your analyzer in situ] now rotate your polarizer for the best extinction, ie: the darkest colour of the crystals. To do this it's best to observe just one or two well aligned crystals. When you have aligned your polarizer it's a simple case of putting your analyzer at some convenient point above the slide such as attaching to a monocular tube eyepiece and rotate it for the darkest colour whilst looking at a clear part of your slide, ideally this should go competely black cutting off all light but don't be surprised if it is just dark grey or has some 'banding' this is normal if you don't have 'strain free' objectives and condenser. When you have completed this you have crossed-polars andfrom the above observations we can deduce that biotite appears darkest when its cleavage plane is parallel to the polarization vibrational axis when viewed under PPL.

What is Biotite? Biotite is one form of mica known as brown or dark mica, another form is muscovite [colourless], I have already mentioned cleavage and that biotite has what is termed - perfect cleavage in one direction and is the reason why mica is easily broken into very thin sheets along its cleavage plane. Mica was used for many years in electrical and electronic apparatus for its insulating properties but now has largely been superceded by modern synthetic materials.

Biotite crystals, slide [H], showing interference colours , orientated with their cleavage at approximately 45 degrees to the polarizer-analyzer, 40x magnification.

The Open University slide [H] which has been used for our alignment of the polarizer-analyzer is actually marked biotite gneiss, the term gneiss tell us that the mineral biotite has formed into layers or bands creating a laminated effect in rock which also includes other minerals, this shows up well in the crossed-polar image. You can see some of the other minerals in the crossed-polar image, these are mainly feldspars showing up grey and if you look to the middle of the right hand edge you can see an effect closely associated with feldspars and that is multiple twinning showing up as alternate light and dark bands. Feldspars make up one of the commonest rock forming minerals in the earth's crust and can be divided into alkali and plagioclase , it is the plagioclase which are associated with the multiple twinning effect so the mineral is probably plagioclase feldspar. Alkali and plagioclase are group terms for minerals of similar chemical composition, an example would be orthoclase which is part of the alkali feldspar group, the chemical formulae can be quite complex and is beyond what we need to know here!

The colours shown by minerals under crossed-polars can be useful in determining their type by measuring what is called birefringence, this is the difference between the maximum and minimum refractive index and is associated with the interference colours you see when you rotate the sample on a rotating stage, unfortunately some minerals like biotite are quite darkly coloured physically and can mask their true maximum interference colours.

To show up colours under crossed-polars a mineral must have more than one refractive index, minerals with only one refractive index [isotropic ] have structures made up of atoms in a very regular arrangement, this causes light to pass through a crystal with the same velocity irrespective in which way the light is traveling through it. This should not be confused with opal and glass which are also  isotropic, in this case their internal atomic structure lacks regularity - no matter which way you turn a piece of glass round under crossed-polars it will always appear black under good conditions. The birefringence can be measured accurately from charts and is a good reason to have a rotating stage on your microscope to allow you to check the maximum interference colour , this is difficult using a standard X-Y stage. Minerals creating these colours are said to be anisotropic - the light traveling through the medium is doubly refracted. If polarized light is directed at an anisotropic mineral the light may not only be refracted but split into two beams as it enters the air - mineral junction, these beams will have a different velocity as they travel through the medium and thus become out of phase with each other, when the two light beams reach the other side of the mineral the two interfere with each other and if viewed with crossed-polarization sheets will show interference colours.

The colours generated by anisotropic minerals when cut in thin section for viewing under a microscope will depend on:

a. Thickness of section.

b. Orientation in which the mineral has been cut.

c. The birefringence value of the mineral.

Quality mineral and rock sections designed for microscopical study are cut to a standard 30 micrometre [0.03mm] thickness so this is one variable we don't have to be too concerned about and usually we are only interested with the maximum interference colour. This allows us to be able to directly read off a value of birefringence from a standard chart from the colour we see under crossed-polars, these are split into orders such as 1st, 2nd, 3rd orders and higher. A single crystal of any mineral will show colours from black to its maximum interference colour as the orientation of the crystal is rotated under crossed-polars. Some of the older charts such as those made by Zeiss and Leitz were works of art in themselves and also included overlaid graphs on top of the colours to take into account different section thicknesses, the interference colours shown on these different charts seem to vary depending on printing quality etc. The diagram below shows a very much simplified birefringence chart, the colours are obtained from inserting a quartz wedge between crossed-polars, the thickness of the wedge goes from nearly zero at the left hand side to about 0.15mm on the right, three common minerals are shown. The various numbers beneath the minerals gives an indication of the birefringence as you go up the chart and some of the highest birefringence figures are from the calcite group with figures way off the end of my small chart in the region of 0.17-0.18.

   A Simple Birefringence Chart.

The figure of 551nm is significant in the fact that this value is used in 'sensitive tint' compensation plates, these are usually inserted into the body tube above the nosepiece [mounted at 45 degrees with respect to the polarizer-analyzer] and come as accessories to polarization microscopes and have the property to add or subtract from the interference colour of the sample [depending on the orientation of the crystal]. This can aid in diagnosing minerals that you may be uncertain about with respect to theirinterference colour but this is going into a whole new area beyond the requirements of this article. Some suggestions for homemade compensating plates are described in my earlier article. For further details see e.g. the Molecular Expressions website or type in suitable keywords into a search engine.

  Gabbro .

Gabbro, slide [K] under PPL, 40x magnification, showing the complex deep fractures and imperfect cleavage of the mineral Olivine.

Two images of Gabbro, slide [K], the left showing a blue-yellow interference colour of Olivine under crossed-polars and right, the slide rotated to show the minimum interference colour of the same crystal.

What is Gabbro? Gabbro is an igneous rock composed of feldspar, and of the darker minerals - pyroxenes, hornblende or olivine.

What are Igneous Rocks? these are rocks formed from the solidification of liquids, normally these are silicate based, more commonly known as magma.

What is Olivine? Olivine is a mineral composed of magnesium and iron silicate, its physical colour can range from olive-green to brown.

  Granite .

Granite, slide [E], two examples of cross-hatched twinning under crossed-polars typically associated with alkali feldspar, 40x magnification, some biotite can be seen on the right hand side of the image below.

What is Granite? Granite is an igneous rock composed of feldspar, quartz and small amounts of biotite or muscovite and trace amounts of several other minerals depending on location. Granite crystallizes from magma cooling slowly well below the earth's surface and together with other crystalline rocks provide the foundation of the continental masses and can be found in the very old Precambrian shields formed around 4 billion years ago when the Earth was young, including perhaps, these samples from Scotland!

  Shelly Limestone.

Shelly Limestone, slide [R] under PPL, 40x magnification.

What is Limestone? Limestone is a sedimentary rock that is mostly made up of calcium carbonate, [usually taking the form of the mineral calcite] and with this sample has probably been produced biologically. The ocean floors contain vast amounts of dead marine organisms like foraminifera and over a period of time their calcareous shells can form into limestone. When baked in a very hot furnace limestone is changed into lime - calcium oxide, this is useful as a fertilizer.

Shelly limestone, slide [R] under crossed-polars, 40x magnification showing the high-order [pink] interference colours.

  Porphyritic Rhyolite.

Porphyritic rhyolite, slide [O] on the left in PPL and on the right under crossed-polars, 40x magnification.

What is Rhyolite? Rhyolite is an igneous rock chiefly composed of feldspars and quartz. The porphyritic variety contain larger embedded crystals of feldspar and quartz known as phenocrysts and in this case can clearly be seen against the groundmass of much smaller crystals.

An accompanying web page in this February 2004 Micscape issue is 'Open University S260 Thin Rock Sections Selected Image Gallery'.

  Comments to the author, Ian Walker, are welcomed.

Also of interest may be:

The polarizing microscope, modifications to a biological scope and using the JNOEC XPT-7, [Bunel SP-70] this is another article by me going into more detail on how to modify your existing scope for polarization work and some personal notes on using the JNOEC dedicated polarization microscope.

Notes on simple polarization experiments with micas using a student microscope an excellent introduction to interference patterns generated by mica when viewed under crossed-polars, this is another fascinating area of mineralogy for the microscope and Dave shows how you can get good results with the minimum amount of accessories.

The End.

Editor's footnote: The OU S260 set and other sets of rock sections can be obtained from some UK microscopy suppliers and also UK geological suppliers. It's worth comparing prices, as the S260 set in 2003 varied from ca. £59 to £102 between suppliers.
Details of the Open University's Geology course S260 and course materials can be found here .

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