This is my first thin-section gigapan. I've been wanting to stitch and upload one of these on Gigapan for a long time.

A `thin-section' is a slice of rock that's been cut and ground until it's a mere 30 microns thick. (That's 0.03mm, or 0.0013 inch.) Thin sections are used by geologists to examine rocks in microscopic detail. When a geologist looks at a thin section under a specialized microscope (called a *polarizing* or *petrographic* microscope), they can more readily determine what sort of constituents the rock is made of (i.e. either crystals, pieces of other rocks, fossils, natural glass, or some combination of those things). They can determine the mineral composition of the rock, and how the constituents of the rock are arranged relative to one another. All of this information is useful for `reading the story in the rock', i.e. determining how (and under what sorts of conditions) the rock formed.

This mosaic is made from 156 frames, shot with an 8-megapixel Canon 20D DSLR camera, attached to a petrographic microscope. The microscope came with an X-Y vernier slide positioner, which allows me to move the slide 1.5mm between adjacent images in each row, and 1mm between rows. Stitching was done with GigaPan Stitch on a Mac Pro computer. The area shown in this image is about 12mm by 18mm.

The bright colors and the variations from light to dark are due to an effect called the *interference of light*, due to the use of *polarizing filters*. A petrographic microscope, like the one I used here, has two polarizing filters, one above the thin section, and one below. Light shines up through the first filter, then through the thin section, then through the objective lens, then through the second filter, and then through an eyepiece, and into the geologist's eye or camera. Crystals of most minerals have a tendency to split a beam of light into two differently-polarized beams, which travel at different speeds. As these beams pass through the upper filter, they recombine, with some colors enhanced by *constructive interference*, and other colors suppressed by *destructive interference*. The result is that white light from the microscope's lamp becomes brightly colored, depending on the particular mineral it's shining through, and the orientation of that mineral's crystal structure relative to the light beam and the filters.

What can't be shown here, though, is the effect of rotating the stage upon which the thin section sits. This makes the crystals change colors and go light and dark. People always enjoy this effect when they see it! (Making a movie of that would involve making a `gigapan movie' - yikes!)

The rock is from a set of old thin sections at De Anza College, Cupertino, CA, where I teach geology. Students in our introductory geology class look at these thin sections (along with `hand samples' of the same rocks) in one of our laboratory exercises. If you've taken Geology 10 at De Anza, you've seen this thin section, or one like it!

In fact, geology students at many schools have probably seen thin sections like this one. According to our records, this is a thin section of the Gassetts schist, from Vermont. It's a nice, `textbook-looking' staurolite-garnet-mica schist, and I think it shows up in a lot of teaching collections.

Here's a link to a paper from an old issue of the Journal of the Mineralogical Society of America, about the Gassetts schist:

www.minsocam.org/ammin/AM19/AM19_335.pdf

I hope to make more thin-section Gigapans, as time permits. I'd *really* like to hook up some small servomotors to the knobs on my stage positioner, so that I could *automate* the acquisition of the image frames. Then I could crank out thin-section gigapans! I don't know when I'll find the time, though, between various teaching duties. I'd have some machining to do, probably, and would have to figure out how to get a computer to control the motors and the camera. I think it would be a big project, but who knows... maybe someday!