Note the colourful chondrules as viewed under cross-polarized light. Such views are used for mineral identification and demonstrate the interaction of light with real-world samples. You too can make your own petrographic microscope! Here's how...
Here at York, as we welcome our newest class of students, we're taking a few minutes to pause and start thinking more about our recruitment efforts in the future. One question that comes up often is how we can stand out in a sea of power point presentations. The solution - to try to be a little more hands-on by bringing in some props to illustrate some of the principles behind our work. To that end, some of us have come up with little inexpensive demonstrations that capture a lot of what we do here in the Earth and Space Science and Engineering Department. Look for these at university nights at high schools everywhere in the coming months!
For my part, I put together a little, inexpensive rig that gives students a taste of a few different themes. Basically, you could call it the poor man's petrographic microscope. It presents a neat little introduction to some interesting optics with serious engineering applications as well as some earth science. By using meteorite thin sections as my specimen, I can springboard into planetary science and spacecraft and how we can use simple techniques to learn a great deal from the materials we encounter in our exploration.
For those of you not familiar with petrographic microscopy, here is the simplest version of the operating principle. You take a continuous visible light source, such as a halogen lamp, and you add a linear polarizing filter over top of it. This gives you a source of plane-polarized illumination which you use to light your sample from underneath. However, you don't just observe the sample directly in transmission, you add a second linear polarizing filter between the sample and your microscope. The trick here, is to make sure that the polarization angles of the two filters are offset by 90 degrees, thus you are observing the sample in "cross-polarized" light.
Those of you who have played around with polarized sunglasses might be scratching your heads right about now. If the two polarizers are crossed, how can you see anything at all? Wouldn't the second polarizing filter just eliminate all light that made it through the first filter? That would be true if there wasn't a sample in between. The key here is that different minerals interact with light in different ways, rotating the incoming waveform and outputting it at a different polarization angle that can make it through the second filter. Thus, by crossing the polarizing filters we are eliminating any light that does not interact and seeing only the light which has interacted with the minerals.
Since this polarization is a function of wavelength, the effective result is that different minerals show up in different colours where before they all looked a similar slightly-tinted translucent. An example can be seen below for a terrestrial Gabbro taken by wikipedia user Slim Sepp. It's notable that this is also the principle under which LCD displays function, so it has a number of uses beyond mineral identification.
Typical petrographic microscope rigs run about $5,000 or so. But if you don't need the kind of precision that academic work entails, here's how you can buld your own for about $100. I purchased dichroic linear polarizing film from Thor Labs for $8 per 2" x 2" square. You'll need at least two of these, but more can be helpful since the film scratches easily and exposure to the high temperature of a lamp can cause melting in the plastic.
You'll also need a sheet of plastic or glass and a way of sticking together each piece. I opted for double-sided tape and an 11" x 14" sheet of PolyCarbonate (typically under the trade name "Lexan"), available from Home Depot for $10-$15. An important note here - glass is obviously the best choice, but it is fragile and difficult to work with. Making straight cuts (e.g. with a glass cutter) isn't difficult, but creating the sample holder would be tough. Acrylic shares the transparency of glass and is much cheaper than is polycarb, but will fracture easily with home tools. The only drawback of polycarb, aside from its cost, is that polycarb will also interact with polarized light which means that you won't get colours that are as pure as you would otherwise. That said, the result isn't bad and for me, it's worth it to work with a much more forgiving material.
In addition to a lamp, here's what you'll need:
The microscope I used was a Veho VMS-004 with a magnification of 400x, however, I would reccomend something with less magnification. You just don't need that much, and the depth of field such a powerful magnification gives you can make it difficult to achieve good focus or to navigate the sample to look for interesting features. If you want to bring the cost way down, you can use a smartphone camera, and it is this that I have used to take the pictures that will be shown in the rest of this article and the intro image at the very top.
The biggest challenge is the sample holder. you need to have three separate layers. One to serve as the base, the second to raise up the top layer to leave a gap for your sample holder. The final layer secures the sample in place and provides a platform for the second piece of polarizing film. You will also need a slider with a notch to move the sample in and out of the holder. I made mine in the shape of an octagon so that I could observe my sample at offsets of 45 degrees, though circular sliders are more common so that you can see the full range of colour changes. The last thing you will want to do is to tape on the polarizing film to the top and bottom of the holder - keep this on the outside to prevent scratching as you move the slider and make sure that the orientations of the film are crossed.
Next you'll need a sample to investigate. Thin sections of chondrites work best and you can find many quality specimens on eBay for about $100-$250. This one came from a dealer in Germany. The NWA prefix, by the way, is an indication of where the meteorite was recovered. In this case "NorthWest Africa" which means that it fell somewhere in the Sahara desert and was recovered by Bedouins. These nomadic people have discovered that the strange black rocks they sometimes find amid the sands have value to collectors.
Now is the time to bring everything together. Place your thin section in the slider, place the entire apparatus on top of the lamp (shown here is a Grono lamp from Ikea, which gives you a nice surface to work with) and plug in the microscope, or observe the sample visually or with a geologist's loupe.
Rotated 90 degrees, note the changes in colour. For instance, there's a striped triangular clast near the center whose colour changes from blue to yellow from the top to the bottom.
One strong recommendation - if at all possible use an LED bulb in your lamp! This will keep the sample holder from heating up too quickly and can prevent the film from melting.
As a final note, Chondrules are the subject of intense study in the planetary sciences because they are some of the oldest solids in the solar system. They record an event that took place only a few million years after condensation from the solar nebula which flash-heated an estimated 50-80% of solar system solids. This heating was not enough to vaporize the grains, but was enough to melt them. Under zero-gravity conditions, they formed spherical blebs and crystals grew within them as those blebs cooled. If you look at solar system materials of just the right age, you find them utterly shot through with these objects, such as our ordinary chondrite above.