Dec 17, 2009
C-T scans have been in the news recently; evidently, they can sometimes be dangerous, zapping people with risky levels of radiation. My master's thesis involved a C-T scan, but fortunately a very safe one, of a rock instead of a human being. Rocks can sit there and take huge doses of X-rays without injury or complaint, making them ideal targets for this sort of procedure. Because there's no need for radiological restraint, scanning a rock can yield much clearer, more detailed results than scanning a live person. It's also a lot cheaper.
The rock I sent to Texas for a C-T scan was a metamorphic chunk of the North Cascades mountains in the state of Washington. It had garnets in it. Each garnet was surrounded by a shell of pure-white minerals: quartz and plagioclase. The rest of the rock--what we termed the matrix-- looked dark gray in color; it actually consisted of the same white minerals as in the shells, flecked with tiny black grains of a kind of mica called biotite.
We were trying to figure out why the garnets were set off from the rock matrix by the white shells, which we called coronas. Our hypothesis was that unusual conditions during the rock's metamorphism had permitted garnet growth but had simultaneously limited diffusion of elements that the garnets would consume during their growth. We wanted to know more about the geometry of the coronas, and about the separation between garnets and matrix. So we shipped a core of rock about an inch in diameter to a C-T lab at the University of Texas, where it was zapped with X-rays; the results were reconstructed by a computer, rendered in 3-D, colorized, and made into a little animated movie.
I've posted the movie on YouTube. I recommend watching it--even if it's not your kind of movie, it only lasts 12 seconds. The garnets in the rock are rendered red, the corona shells green, and the matrix rendered as transparent, with a slight reddish speckling of biotite grains. When I first saw this movie, I'd spent months working with the rock sample, but I was surprised by how long and snake-like the coronas are, and how many clumps of garnets each corona engulfs. How did this happen? The short answer is that the rock got squished squished and heated and stretched and squished and heated again during its mountain-building experience, which coincided roughly with the era of the extinction of the dinosaurs. The long answer is published in a journal called Canadian Mineralogy.
And now, the rock is doing a star turn on the internet, in what I honestly believe to be the first true hard-rock video on YouTube.