Guest post: A report from the microscopic front line

As always, a great pleasure for me from this blog is the people who just get in touch out of the blue. Karen Locke first posted a comment on my Sunday Sand post on Bali earlier this year. I was intrigued and followed up with her, discovering to my delight that she is a graduate student scrutinising sand grains in detail – and in earnest. I am always interested in hearing from people whose relationship with sand is professional, hence the “front line” reference in the title; here are folk who are convincing sand grains to tell their stories and for whom the translation of those stories is important. I asked Karen if she would be willing to contribute a guest post, and now here it is – I hope that my readers enjoy this as much as I did, and thanks Karen!

Sand in Thin Sections

Thanks to Michael for allowing me to guest-post. I suppose I should start with a little bit about myself: I’m an older MS grad student at San José State University in San José, California, USA. I’m just finishing up a thesis on the composition and provenance of sands extracted from wells drilled in the Santa Clara Valley, where San José is located.

The Santa Clara Valley has had a violent history. It’s surrounded by mountains that were raised (and are still being raised) by thrust and strike-slip faults. The mountains are made of a fascinating array of different types of rocks. The valley itself is filled with sediment from the mountains, and is subsiding under its own weight, as valleys do. By studying sand from various depths in the wells, I get a picture of how the sediment changed over time, and I’m finding clues to the paleoevolution of this valley over the last 800 thousand years or so. I’m asking questions like, did the streams always flow the same way? What was the composition of the rocks that ran down the middle of the valley but have eroded away, and does that tell me about how they might have moved in the past? And the big question, which I can’t answer but can provide a few more clues about is: how did the faults around the valley move to create the changes I see in the sands, and what does that imply about what they might do in the future?

But that’s a thesis; what I really want to talk about today is how I studied the sand. By studying the sand, my goal was to identify the composition of the sand grains. I wanted to know what kind of rock each one came from. Now, sand grains are tiny things. Even under a microscope, it can be deucedly hard to identify them. So I chose to make thin sections, and study the sand under a petrographic microscope.

Thin sections are created by gluing a 0.03 mm slice of rock (or in this case, a slice of sand set in epoxy) to a glass slide. In such a thin slice, most minerals are translucent or transparent. Yes, you can see through solid rock if it’s thin enough. Thin sections are made to be viewed with a petrographic (polarizing) microscope, because lots of minerals have interesting properties when viewed with polarized light – properties that can help identify them. Rocks are made of minerals, and the first step in identifying a rock is identifying its minerals.

The petrographic microscope has a polarized light source that shines through from beneath the slide, rather than illuminating it from above. Why polarized light? Many visual properties of minerals change as you rotate them over polarized light, and it’s the change with rotation that makes them significant. Color is one interesting property; many minerals rotate through two or three colors, while other minerals are steadfastly one color or colorless throughout the rotation. Another interesting property is relief (how tall a mineral crystal appears relative to the ones around it). Yes, the minerals can appear to be different heights, even though the slice is uniformly thick, and the apparent relief can change as you rotate the slide.

Playing with a petrographic microscope messes with your sense of reality. J

Other very important diagnostic properties of minerals in thin section are cleavage and growth habit. Cleavage is the way a crystal breaks internally; some minerals break at very specific angles, while others do not. Habit is how a single mineral tends to grow, or how multiple crystals of a mineral aggregate together. Long and skinny or squared off, aggregating in sheaves or long strings or just growing along side one another – there are lots of possibilities, and often they’re very diagnostic.

Then there’s another critical, and very cool, tool on a petrographic scope. Between the slide and the eyepiece, these scopes have a second, or upper, polarizer that polarizes perpendicular to the one on the light source. This polarizer slides in and out. Now, polarizers cause light waves to move in a plane. So with the upper polarizer out, you’re looking at your slide illuminated by plane light. Putting the upper polarizer in, or crossing the polars, means you’re now looking at the light that came through your slide in one plane, and attempting to look at it in a perpendicular plane. But that generally doesn’t work. Light in one plane can’t get through a polarizer perpendicular to it. Try taking two pairs of polarizing sunglasses and holding the lenses perpendicular to one another; the result will just be black. So looking at your thin section with crossed polars, what you should see is black nothing – unless the minerals themselves perturb the initial polarization. Which of course is what some of them do.

Minerals are crystalline in structure. Some have very regular, simple structures; others have elaborate arrangements. When light goes through a mineral, it can’t simply go through the atoms in the crystal, it has to run down channels between the rows and columns, if you will. Now, when the crystal structure of a mineral is simple and rectangular, it takes the light the same amount of time to run down a row as up a column, and the light comes out the other side of the crystal together. This is called an isotropic mineral, and it really does turn completely black when the polars are crossed. But when light hits a more elaborately-organized crystal structure, it can actually take more time to move through the crystal in one direction than in another. When this light comes out of the crystal, the difference in the outgoing waves produces light that’s not in the original plane, and so it gets through the upper polarizer. This effect is called birefringence, and can be extremely diagnostic.

Finally, birefringence gives me one last property I can use to identify minerals: extinction. As the slide is rotated, the birefringence effectively fades from bright to dark and then brightens again; when a birefringent mineral goes dark, it’s said to be extinct. Different minerals with similar birefringence go extinct at different angles to their growth habits, so angle of extinction is another identification tool.

So, now I have color, relief, cleavage, and habit in plane light, and birefringence and extinction in crossed polars. There are other mineral properties one can look at using the petrographic scope, but these are generally the ones I used to identify the minerals in the sand grains – at least, those mineral grains that were big enough to see. Some mineral grains were so tiny that even under a 40x power lens I couldn’t make them out. But that’s helpful in its own way, because identifying minerals is just one step on the road to my original goal: identifying rocks.

Rocks aren’t just random aggregates of minerals, they’ve got recognizable textures. It’s the combination of identifiable minerals and texture that identifies the rock. And now that I can talk about the rocks, I can show you some pictures. They’re all taken with a digital camera built into a petrographic scope. They’re all pictures of sand grains that are between 250 and 425 micrometers in size.

These are photos of a grain of argillite, a metamorphosed siltstone. The picture on the left is taken in plane light, and the picture on the right is taken with crossed polars. (The blue cast to the crossed polars picture is an artifact of the camera; under the microscope it’s much more grayscale.) Argillite consists of tiny grains of silt (mostly quartz or feldspar) in a brown mudstone matrix. The bright white crystal in the middle on the left is black on the right; in this case, that’s an example of extinction. Note that what seem to be whole crystals in the left picture don’t look whole on the right. That’s because in the process of metamorphosis, they’ve been recrystallized; one larger crystal has become many small ones, each with its own different angle of extinction. The next pair of pictures show that more clearly.

 
These photos are of two quartz sand grains. They’re not quite 100% quartz; the dark markings on them in the plane light photo (left) suggest they have some small amount of some opaque, probably metallic, mineral attached to them. Though it’s cracked, the upper grain is more-or-less a single crystal of quartz. You can tell that because the whole grain has the same birefringence in the crossed-polars photograph (right). The lower grain doesn’t look that much different from the upper grain in plane light, but crossing the polars reveals it is now composed of many smaller quartz crystals, each having its own angle of extinction. Again, this is recrystallization under metamorphosis. Under metamorphic pressure, the crystal, in many different areas at once, tends to reorganize into the tightest structure it can manage, creating many smaller crystals out of one larger one. Note that because quartz grains tend to be on the boxy side with irregular edges, extinction can’t be compared with growth habit to identify the mineral. I can’t show it in the picture, but quartz has a very unusual extinction habit that makes it trivially easy to identify: as the slide turns and the crystal goes dark, the color change proceeds in a wave across the crystal.

For something very different, here’s a pair of pictures of a metavolcanic sand grain:

A metavolcanic is a metamorphosed rock that was once lava. Lava is a very, very fine-grained rock, and even under the petrographic microscope it’s hard to make out the grains; it’s called a groundmass. But during metamorphosis, some of the tiny grains will organize into crystals. The long, skinny crystals in this grain are mostly feldspars; the stuff between them is still pretty much groundmass. Feldspars don’t necessarily grow long and thin, but they seem to do so during metamorphosis of volcanic rocks.

Finally, here are photos of my favorite rock, serpentinite. Alas, still photos can’t do it justice.

 
[For an early report on the project Karen's research has contributed to, see http://pubs.usgs.gov/sir/2004/5250/; for resources on microscopic petrography and the spectacular appearances of minerals in thin section, see http://serc.carleton.edu/NAGTWorkshops/mineralogy/optical_mineralogy_petrography.html and J.M. Derochette’s superbly illustrated site.]

Comments

[F]

Karen Locke, have you considered writing your own blog at some time in the future? You have a nice style, your post is easy to read, and of course, the subject matter is fascinating.

[Richard Bready]

This is impressive writing--flowing, friendly, and clear in both planes, even when they cross. That's not just a borrowed figure of speech; good science writing makes hard ideas comprehensible at the granular scale, as you do, while it cements information together to show the significance and interest of the topic, as you do. I really enjoyed reading this. Thanks for the post.