Minerals with Cross-Polarized Light

Using some minerals samples that we had lying around at work and low budget tools (my fingernail to scrape off pieces of the minerals), I looked at crystal structures with a cross-polarized light Foldscope (see this post for instructions).

First, some muscovite mica which forms in very thin sheets and has high birefringence (which is what gives us beautiful colours when put under cross-polarized light).
Muscovite Mica - Polarized Muscovite Mica - Polarized 2

Next, some celestite. I had never heard of this mineral before, but some quick googling taught me that it is the principal source of strontium. It has lower birefringence than muscovite but we still get some lovely colours.
Celestite - Polarized Celestite - Polarized 2

And lastly, my favourite, calcite. I was expecting great things from the calcite sample since I know that calcite has many special optical properties. I wasn’t disappointed. Calcite has very high birefringence compared to the other two minerals. And, of course, I had to bust out the high-magnification lens to capture the beautiful cleavage.
Calcite - Polarized Calcite - Polarized 4 Calcite - Polarized 2

I did have a couple of issues while doing this activity. I believe they both stem from the samples being too thick. The first issue I had was with focus. The foldscope seemed to have a hard time focussing on the samples and I think it’s because the sample was too close to the lens. The other problem was that the sliding mechanism kept getting stuck on chunks of mineral. Perhaps with better tools and more careful slide preparation, these can be easily avoided.

4 Comments Add yours

  1. laksiyer says:

    Muscovite mica my favorite. Calcite cleavage nice.

  2. Manu Prakash says:

    Beautiful work. Here are a few suggestions.
    1) Use the focusing technique described here using a stack of paper to keep focus fixed: http://microcosmos.foldscope.com/2015/01/20/simple-way-to-keep-fixed-focus-on-foldscope-use-paper-stack/

    2) For the rock being stuck – you can use a simple cover slip and with low mag lens, you will actually have plenty of space for things to slide through. Cover slip also helps so your sample does not get onto the lens. I think your sample is transparent enough, so you should just do a digital zoom in as well, to see what it looks closely.

    cheers
    manu

  3. Niramay Gogate says:

    Can you crush those crystals ? because I have a feeling that they will look even better if you crush them!

  4. Josh Shagam says:

    I have also come to realize the troubles of the stage mechanism. I attempted to make some slides with piccolyte and a coverslip. Unfortunately the “sandwich” of the foldscope makes it difficult to use such mounting media because they are not really permanent or dried– the movement inevitably causes the slide to pull apart.

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