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5.7: Identifying Minerals and Mineral Properties in Thin Section

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    Learning to identify minerals in thin section takes practice. But, if we have a hand specimen or know what kind of rock we are looking at, the most likely possibilities are limited – and identification is simplified. And some common minerals have key properties that make identification straightforward. So, just like identifying hand specimens based on physical properties, it is not always necessary to check or measure every optical property to figure out what a mineral is. To see photos of the most common minerals in thin section with a discussion of key properties that aid identification, go to our website Optical Mineralogy. For many more, and higher quality photographs of minerals in thin section, go to Frank Mazdab’s rockPTX.com website, or to Alessandro Da Mommio’s website at www.alexstrekeisen.it/english/.

    With the above said, a standard approach works well. This approach involves looking at multiple grains of each mineral and changing back and forth between plane polarized (PP) and crossed polarized (XP) light. Sometimes we need an interference figure. But for routine mineral identification, obtaining an interference figure is often unnecessary and identification can be quite rapid. The flow chart in Box 5-4 below summarizes the key steps and questions to ask as you go along. If you do all that is described and you still cannot identify a mineral, you have been very unlucky.

    Identifying Minerals and Measuring Mineral Properties with a Polarizing Microscope

    1. Look at the whole thin section (and at a hand specimen if available):
    • What kind of rock is it?
    • How many different major minerals does it contain?
    • What are the associated minerals?

    2. Examine several grains of the same mineral under PP light:
    • Is the mineral opaque or nonopaque?
    • What color is the mineral? Rotate the stage.
    • Is it pleochroic?
    • What is the color variation?
    • What is the crystal shape and habit?
    • What cleavage does it display, if any?
    • What is its apparent relief?

    3. Now cross the polars to examine several grains of the same mineral under XP light:
    • Rotate the stage. Is the mineral isotropic or anisotropic?

    4. If anisotropic:
    • Rotate the stage. What is the range of interference colors?
    • Estimate the maximum birefringence by looking at grains with the highest order of interference colors.
    • For minerals with a long dimension of principal cleavage: What is the maximum extinction angle?
    • Is the mineral length fast or length slow ?
    • Is the mineral twinned? If so, what kinds of twins?

    5. If necessary, obtain an interference figure. You may have to try several different grains to get one that is useful:
    • Is the mineral uniaxial or biaxial?
    • What is its optic sign?

    6. If biaxial:
    • Estimate 2V.

    This page titled 5.7: Identifying Minerals and Mineral Properties in Thin Section is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Dexter Perkins via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.