This is the third in a series on laboratory photography in the Department of Earth Sciences at Wooster. In a comment on a Fossil of the Week post last month, Wooster Geologist Alumnus Dr. Bill Reinthal asked if I could describe how we make our blog and other photographic images. I started last week with a post on our macrophotography equipment and techniques. You may want to read that post first for our general photographic processes. Next we had a post on our microphotography using a dissecting microscope with reflected light. This last post is on our microphotography through a petrographic microscope using transmitted light. As I cautioned before, I am not a professional photographer, and my departmental colleagues do plenty of their own excellent photography.
Above is an image of a thin-section cut from an oolitic limestone, the Middle Jurassic Carmel Formation of southwestern Utah. (You may see a theme here! I’ve used the same rocks and fossils in this three-part series to compare the various photographic techniques.) This photograph was taken with the equipment described below. I added the scale later with Adobe Photoshop.
This is our petrographic microphotography station in Dr. Meagen Pollock‘s petrology lab. On the left is a Mac computer running the imaging software from Lumenera (Infinity). Again our wonderful Geological Technician Nick Wiesenberg has written a detailed, easy-to-follow guide to using the system. On the right is the petrographic microscope with a thin-section on the stage.
Another view of the arrangement.
The camera is an Infinity 5 (5.1 Megapixel). It makes fantastic images.
Here again is that Carmel Formation oolitic limestone, this time seen as an acetate peel. Note the differences between the thin-section (top image of this post) and the peel of the same rock. They both show unique details. This is why I like to make both peels and thin-sections of carbonate rocks I’m studying.
Layali Banna (’22) took this microphotograph so we could get some hard-rock, cross-polar action in these blogposts. Feldspars! The scale bar is an example of what the system provides.
Here’s another cross-polar microphotograph from Layali. I don’t know the rock, but it looks like a micaceous sandstone. Thanks, Layali!
These three entries on our laboratory photography systems (macrophotography, microphotography part 1, microphotography part 2) are designed to show our current and future students what we can do in our department. Who knows how long this blog will survive in cyberspace, but maybe someday future Wooster Geologists will arrive here and marvel at our Old Ways!
Thank you, again, for all the wonderfully detailed posts, Mark!
For students, and other explorers, it’s nice to be able to observe these rocks, while in thin section, using different light sources. Many of us are familiar with macroscopic fluorescence, which we see in many museum displays, but this technique can be used microscopically, too. The most common is cathodoluminescence, which utilizes a UV light source, but sometimes people use IR light sources, too. I have witnessed textural features, using cathodoluminescence, that are otherwise completely invisible, under any other illumination, showing intricate growth zonation in minerals. The goal, for students, is to be creative, and try different tools in their exploration of the microscopic world, sometimes with dramatic, unexpected, results. One caveat: eye protection is necessary with the shorter wavelengths of UV illumination. Just as you describe the differences that are visible in your standard thin section vs. the acetate peel, different illumination can be a beneficial adjunct to any research. Do you ever make ultra-thin thin sections of carbonates? Because of the intense double-refraction of the minerals, this is supposed to be a way to “see through them” more clearly, but I have no idea how commonly this technique is used.
Another feature that can be useful shows up in the second photomicrograph, just at the edge of the photo’s resolution (do I also detect a lineation in the grains there, too?): fluid inclusions, bits of the material from which the mineral grew, trapped within it. Although their study requires a lot more equipment, we have been able to derive a lot of information about the fluids responsible for mineral precipitation/growth, through their detailed study. For those explorers who are interested in what fluid inclusions represent, Edwin Roedder, of the USGS, was the first to catalog many of their characteristics and properties, both in a USGS Professional Paper, and in a Mineralogical Society of America review volume.
Thanks, Bill! As always, your comments add value to these posts.
Back in the 90s we had both a fluid inclusion stage and cathodoluminescence. Colleagues now gone used the equipment effectively. I did some brief cathodoluminescence of carbonate hardground cements. I have used ultra-thin-sections for work on shell microstructures, which went well. For most carbonate sections, though, I like the “depth” of that 30 microns!