Optical microscopy

How do geologists examine rocks and decide what mineral are present – or what structure the rock has – both of which are essential to identifying the rock? We are very lucky if the particle size is large enough to see with the naked or even with a hand lens. A microscope would come in handy, but rocks don’t sit neatly on a microscope stage like a biological specimen, nor is it easy to imagine seeing through a piece of rock. But that is what we need to do to be able to use the magnifying power of a microscope to see any details in the rock. Microscope slides for rock identification are perfect.

Thin Sections

So we need to make a microscope slide -but how? Well first, we saw through the rock with a diamond tipped circular saw, then we have to polish the cut face to a very smooth surface, typically using grinding paste as fine as 1 micron [1/1000 millimetre] to remove any scratches or cut marks that would show up under high power magnification. That polished face is then glued to a glass microscope slide. So, we have a lump of rock stuck to a glass slide – what next?

Using the diamond tipped circular saw once again, we cut off as much of the rock as possible leaving a slice of rock as thin as possible stuck to the glass. Next we grind off the rest of the rock using a diamond impregnated grinding wheel or paste until the rock is only 30 microns thick – and very uniformly thick across the section. Again this surface needs to be highly polished, then a cover slip is attached, either glued on or using immersion oil. This is called a thin section. This sequence of videos takes you through the whole process.

Most rock components are now transparent and can be viewed through the microscope.

Petrological Microscope

Oh to have the simple life of a biologist, but no, rocks are more complicated. We do now have a thin section, thin enough for most mineral /crystals to be transparent but identifying those minerals is more than just a visual task. We want to identify how light passes through each ‘crystal’ [note they might not look like crystals but each grain in the rock is a crystalline structure – in sedimentary rocks they may have been eroded and re-shaped but internally they have their original crystal structure.]

So we use a petrological microscope (call it a geological microscope if you wish). It differs from a biological microscope in two main ways, firstly, the stage rotates, and secondly there is a polararising filter in the body tube – in this one, its the in/out rotating knob by the eyepiece – and another below the stage [see photo below]. These two adaptations allow us to observe and measurements of how light passes through the crystal – sometimes called interfering with the light. [Note to be pedantic, these filters should be pieces of calcite and called Nicol prisms but polaroid is infinitely cheaper and almost as good]. Petrology is the study of the structure and composition rocks – its twin is mineralogy, the study of the structure and composition of minerals that make up the rocks.

Mineralogy

So we start by placing our thin section on the microscope stage and look at it under the microscope in normal light as if it was a biological specimen. [Note we have three types of light -normal, polarized and cross polarized]. The photo below shows a collection of thin sections of rocks from North Wales.

This video introduces optical mineralogy

In normal light we can describe shape, natural colour, transparency, any internal features such as cleavage or fractures. Some minerals are not transparent which, together with their shape, is about as far as we can go. (Reflected light microscopy is needed for them)

In polarized light (only the polarising filter below the stage in place) we can describe colour, pleochroism (does colour change as we rotate the stage), ‘relief’ [how much the mineral stands out against its neighbours.

In cross polarized light [with both polarising filters in place] we can describe interference [‘birefringence’] colours [see chart below] , twinning [where two crystals are intergrown], extinction [when the mineral goes black as the stage is rotated] and how that direction aligns with features in the crystal [long axis or twinning or cleavage] or with the cross hairs [NS or EW] in the microscope. We have enough information now to identify most minerals, at least to their family group.

This YouTube channel from GeoHub Liverpool has a a range of videos both explaining optical properties and showing you a shows a number of rock types.

Michel -Levy Chart

Technical details you might want to skip ! (The birefringence of a anisotropic material can be estimated when observed in a polarized light microscope using the Michel-Levy interference color chart, presented above. The Michel-Levy chart is utilized by comparing the highest-order interference colors displayed by the specimen in the microscope to those contained on the chart. Once the appropriate color has been located, the nearest vertical line along the interference color is followed to the nearest horizontal line representing the known thickness. Birefringence is determined by selecting the diagonal line crossing the ordinate at the intersection of the specimen interference color and thickness value).

More advanced users can take this further looking for the optic axes, whether there are one or two, their alignment with other features in the crystal. This video takes you to more advanced issues

What do we expect to find under the microscope?

Depending on the rock type we might be expecting quartz, mica, feldspar, amphibole, pyroxene, olivine, calcite, clay ……. Apart from quartz and calcite these are large families of mineral though each family tends to share similar characteristics, for example most feldspars are twinned, most amphiboles have inclined extinction and so on. This page on shutterstock illustrates a wide variety of thin sections

This thin section in plane polarized shows concentric rings of orientated calcite crystals illustrating the internal structure of ooliths.

The two sections above show the same thin section, firstly in plane polarized light where little detail can be seen except the shape of the large quartz crystal. The second [apologies for fuzzy image] in cross polarized light shows first order interference colours typical of quartz or feldspar, but the absence of any kind of twinning suggests quartz [which is never twinned and never has any cleavage] whereas most feldspar crystals show some form of twining. The groundmass is a mixture of very small quartz and feldspar crystals. this is an igneous rock with a two stage crystallisation, very slow at first allowing the quartz to grow very large, then rapid chilling leading to the very fine groundmass.

The thin section above shows mica schist in crossed polarized light. Three bands can be seen running roughly horizontal across the image, the top and bottom are mica – identified by the very vivid interference colours. The middle band is quartz, identified by the grey-white colours of larger, blocky grains which seem to be intergrown and their colours seem ‘shaded’ rather than clear and uniform. This results from the crystals being strained during the pressure causing metamorphism [contrast with the cleaner look of the quartz grain in the quartz porphyry above].

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