Note that this tutorial is currently very much under construction (so far only “color & pleochroism” is very slightly done), and I hope to add additional photos and video for existing measurements, as well as new text, photos and video for a variety of upcoming measurements (hopefully some progress every few days). I also plan to add several new tutorials. I’m still experimenting with the best format to use, so these pages might occasionally get re-tooled and be temporarily unavailable. So please check back periodically!
Although the emphasis in this applied tutorial is primarily on developing good analytical strategy and learning how to make careful observations, the unifying theme here will be the relationship of observed optical properties to the concept of the optical indicatrix, and subsequently the relationship of the optical indicatrix to the crystallography of the mineral being examined. In this way, making optical measurements on unknown minerals doesn’t simply become part of a mindless recipe to be followed, but rather becomes the means to discover specific crystallographic and chemical traits key to identifying the mineral (and once you “get it”, it’ll all suddenly seem so logical and elegant).
Relating the optical measurements to the optical indicatrix, crystallography and mineral chemistry, thinking about petrologic associations to aid in mineral identification, as well as the necessity of some possibly unfamiliar jargon, means that attempting this tutorial without prior or concurrent experience in mineralogy and optical mineralogy could be quite challenging. Nonetheless, I’ll try to keep the discussion as accessible as possible, although in some cases students might need to seek out additional external references.
So, let’s begin with an overview of the most useful optical properties, how they’re measured, and what clues they give you in identifying an unknown mineral in thin section.
1) Color, pleochroism and absorption:
What are these properties?
When first looking at a thin section under uncrossed polars, color is likely the optical property that most draws the eye, especially in samples where crystals of one or two strikingly colored minerals contrast against an otherwise colorless matrix (such as the homepage introductory photo featuring bright blue yoderite), or in general when the colors are intense and sometimes even a little garish. The origin of color in minerals is beyond the scope of this particular tutorial (but it’s a really interesting subject; the color of minerals page on Caltech’s mineral spectroscopy server is a good resource for further reading).
Color (and pleochroism with stage rotation) are the optical properties that most draw the eye. In this video, piemontite shows off it’s “garish” but characteristic pleochroism.
Just like in hand samples, some minerals can show a variety of colors in thin section [insert pink and blue corundum photos here] But because minerals in thin section are only about 30 μm thick, colors in thin section are usually more muted than their hand sample counterparts, and in many cases colored minerals in hand sample may be colorless in thin section [insert sodalite hand sample and thin section photos here]. On the other hand, some minerals that appear black in hand sample are in fact richly-colored in thin section [insert staurolite hand sample and thin section photos here].
Color is a directional property (add more, maybe iolite hand samples, and introduce pleochroism).
Why are these properties important in identifying an unknown mineral?
How are these properties measured and reported?
Photo and/or video examples of these properties, with additional information:
Text (temporary… from my YouTube description): Yoderite is the deep blue mineral in the center of this image. In this orientation, one of the yoderite crystal’s optic axes is nearly perpendicular to the plane of the thin section, so there is little to no change in the color of the mineral with stage rotation. Although yoderite is nominally strongly pleochroic, in this orientation, we don’t observe any significant pleochroism. Why is this so?
In a biaxial mineral such as yoderite, the optical indicatrix (a conceptual representation of the relative orientation of the three principal, mutually-perpendicular refractive indices of the mineral) takes the form of a triaxial ellipsoid. Mathematically, there are two directions one can “slice” a triaxial ellipsoid to obtain a circular cross-section. The normal to each of these circular cross-sections is called an optic axis (OA), and the intersection of the two circular sections is always the Y vibration direction principal axis (representing the intermediate refractive index direction). For those with an eye for visualizing the structure of 3-D solids, note that this means the two optic axes must therefore always lie in the X-Z optical plane.
Since we determined that one radius of the circular section is Y, it turns out that all of the circular section radii must also have a value of Y (we know that in a circle, all radii are the same length). So, throughout the full 360° rotation of this particular yoderite crystal, because we’re looking down an optic axis, we observe a circular section where the observed optical properties (refractive index, color, etc.) are equal to that of the Y vibration direction principal axis.
According to published data, the color associated with the Y vibration direction in yoderite is typically indigo; thus, we see a seemingly non-pleochroic indigo, regardless of stage rotation. Note that because the rays of light passing through the thin section may not be perfectly orthoscopic (and also that the optic axis may not be perfectly perpendicular to the plane of the thin section), we might still see slight hints of pleochroism (e.g. slight variations in tint and/or saturation). But compared to a crystal oriented to show marked pleochroism (for example, see below), the “pleochroism” seen in this orientation can be considered essentially negligible.
Text (temporary… from my YouTube description): Yoderite is the deep blue mineral in the center of this image. In this orientation, the yoderite crystal’s Y and Z vibration directions are roughly in the plane of the thin section, so we observe a significant color change (pleochroism) with stage rotation. In typical yoderite, the color associated with the Y vibration direction is indigo, and the color associated with the Z vibration direction is pale olive-green (although in this case, the green appears to have a superimposed reddish tint, at least to my color-challenged perception).
Since the lower polarizer is fixed in an E-W orientation, the colors we observe as we rotate the stage correspond to whichever vibration direction happens to then be aligned E-W. Keep in mind, however, because the Y-Z optical plane of this particular crystal isn’t perfectly parallel to the thin section, the colors we end up seeing probably won’t be exactly the “pure” Y and Z vibration direction colors published for the mineral, but rather a composite of the proportional vector components of Y and Z (and also even a bit of X; = pale blue) that happen to be aligned with the lower polarizer.
This particular yoderite crystal is oriented to show marked pleochroism, but other crystals in this same thin section are oriented to show little to no pleochroism. These latter crystals have been sectioned perpendicular to an optic axis. An example of the color changes one would observe with stage rotation in this orientation is shown in the previous video example.
Another more subtle example of pleochroism in this thin section can be observed in the talc, which is the very pale green, moderate relief, 1-cleavage direction mineral adjacent to the yoderite. During the stage rotation, when the cleavage direction is oriented E-W, the talc crystals are very pale green, but upon further rotation, when the cleavage direction is oriented N-S, the talc crystals are nearly colorless.
The third mineral in this view is quartz. Regardless of rotation, the quartz is colorless in every orientation; hence, it is non-pleochroic.
Test of a question?
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Test of a second question?
The answer to this 2nd question is…
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