Last week, we had a basic intro to how light behaves. This week, we’re going to step it up and talk about what we can observe in plane light or PPL.
What kind of things are we going to talk about this week?
- Becke Line
- color / pleochroism
There are few ways to look at minerals, such as immersed in an oil of a specific refractive index. (A few of the images I’m going to use are going to refer to oil immersion.) However, today the most common way to examine minerals today is in thin section. I’ll give a few arguments for why in a few posts. But let’s quickly talk about what a thin section actually is & how you make one. First, you need to collect a rock & cut it into a domino with a diamond embedded rock saw. Second, you “glue” the domino to a glass slide with epoxy. Third, you cut off the majority of the domino and then grind down until you only have 30 microns of rock left on the glass slide. At this point, you have a choice: attach a cover slip or continue to polish the slide down till its smooth enough for SEM or EMP analysis. The example below & these instructions (with pictures for each stage!) are for a covered thin section (from Dave Hirsch).
The epoxy has a specific refractive index (1.54 – 1.56 usually), which is important when talking about the Becke Line.
The Becke Line depends on how light refracts when it intersects a mineral. Remember that the item that has the refractive index will cause light to bend inwards towards the normal. With minerals, we use a substance that has a known refractive index (either an oil or the epoxy at the edge of the thin section) and then test for which way the light refracts with it. Focus the microscope while looking at the mineral boundary. You may find this works better if you lower the amount of light going through the microscope. While looking at the mineral boundary, lower the stage of the microscope. This will increase the focal length and cause the brighter of the two boundary lines to move either inwards or outwards. Inwards = mineral has a higher refractive index = positive result and outwards = mineral has a lower refractive index = negative result.
Relief is a more casual way to describe how light is refracted by a given mineral. It is used to describe how well (or poorly) a minerals grains boundaries stand out from its neighbors. If the refractive index is close to the neighboring mineral’s value, then the light will not be bent much either in or out and the boundary between the minerals will be difficult to distinguish. If the refractive indices are very different, then the light will bend quite a bit and the boundary between the minerals will be very distinct. We qualitatively describe relief as “low” or “moderate” or “high” or “very high” and then, if possible, adding whether its “positive” or “negative” relative to the epoxy (positive = higher value, negative = lower value).
If you remember correctly, all anisotropic minerals have two refractive indices depending on how you orient them. As we talked about last week, most of the time the two values aren’t really that far apart from each other and you won’t see the differences in plane light. However, those few cases where double refraction is easy to see in hand sample, we can also see it in plane light as you rotate the stage. For instance, when you watch a calcite grain in plane light as you rotate the stage around, it will vary from a moderate negative relief to a high positive relief.
Minerals can break in a variety of ways depending on how their internal structure is organized. Though, we can’t always see the cleavage or fracture in thin section, when it does appear its as regular semi-parallel lines that are lighter in weight than the mineral boundaries. When we describe a minerals cleavage, we describe the quality (perfect, good, poor), how many planes (lines that are parallel to each other) and at what angle they are to each other. A few examples:
Finally, let’s talk about what color a mineral will be in thin section. If you look at the photomicrographs above in this post (pictures taken with the optical microscope), you’ll notice that there was quite a bit of variation in the color. Color in PPL is due to the absorption of light into the mineral. The light that is not absorbed, is the color we see.
In an isotropic mineral, light is treated the same in all orientations of the mineral, so there will only be one color transmitted. Orientation and how we rotate the stage will still not impact the color.
In anisotropic minerals, how light reacts will depend on the orientation of the mineral. In PPL, we can see this represented as a variation between two colors as you rotate the stage, which is called pleochroism. We can casually describe this as the variation from one color to another (e.g. yellow to brown like the biotite above; blue to purple like the glaucophane below) or we can formally describe the color scheme. To formally define things, we actually have to know more about the orientation of the minerals & be able to use things in crossed polars. Therefore, I’m going to cover the longer pleochroism color scheme in the next segment. See you next week for crossed polars!
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