I went back to the rock lab on Monday to finish up my thin section. Which I should have picked up from the rock lab this morning as a matter of fact. Except that I forgot. You see, I had a brain once. Then I forgot it somewhere and I’ve been screwed ever since. Hopefully I’ll remember to pick it up tomorrow, so I can get on with the important business of doing the photo micrographs and examining it in detail under the microscope.
Over the weekend, Paul (the guy in charge of the rock lab) glued the wonderful flat face that I’d made on my pet rock onto a glass slide with epoxy. Epoxy is the go-to glue for rocks, and has the benefit of having a very index of refraction, since everything is in face viewed through a cloud of glue once you get your slide put together. This is important since one of the identifiers used to discern minerals is what kind of relief they have – which is to say how much they stand out from the epoxy. Quartz and feldspar barely stand out at all, while garnet and olivine have very sharp outlines against the epoxy. So this basically means that quartz and feldspar have indexes of refraction very close to epoxy, while garnet and olivine don’t.
My task was to get rid of most of the rock glued on to the slide, then. All but about 30 microns of it, to be more exact. I started out by cutting most of the rock off using another diamond saw. Then over the course of about 15 minutes, I used a grinding wheel to take layers off of the remaining rock until what I was left with was basically transparent. The point of a thin section, of course, is to have a piece of rock so thin that light can shine through it; hard to image a translucent piece of rock, huh? I was doing pretty well with it. The rock I chose, Kimberlite, is actually very soft as igneous rocks go, partially due to its high calcite and hydrate mineral content.
The hardest, scariest part was actually manually grinding down the last little bit, using the glass plate and very fine carbide grinding powder. What makes that the scary part is that you have to constantly stop and check to make certain that you haven’t taken off too much, and that you’re polishing it up evenly. I actually lost a strip off of one of the edges of my thin section because I didn’t stop to check soon enough. Kimberlite is soft, soft stuff. I had to be much, much more careful going forward, since if I’d lost much more off the slide I would have had to start over and make a new thin section. In the end, I managed to get it fairly even, thankfully!
Before I left the lab, Paul let me check the section under his microscope, using cross-polarized light. It was pretty indeed – lots of calcite and phlogopite mica, as well as a gigantic opaque. All in all I’m very pleased with how it turned out, except that almost all of the olivine in my specimen has been serpentinized – which is to say that it’s been exposed to enough weathering that the original olivine has altered in to serpentine. There’s a spot of remnant olivine here and there, but most of it’s serpentine, which isn’t nearly as pretty. I was hoping that I’d get more olivine, but I would have had to really dig in to the outcrop to get some, and I didn’t want to do that. At least the rest of my minerals still look really pretty!