I have a confession to make. My favorite rocks are flaky. Really flaky.
Phyllites are the metamorphic rock that gets left out of intro geology labs. They’re kind of like slates, in that they break into slabs. But they’re shiny like schists. The crystals are too small to see with the naked eye – well, except for the big ones, which are about the size of pin-head (at least in my rocks).
They’re easy to break with a chisel. They require a lot of squinting and experience to identify their minerals.
And they’re gorgeous under a microscope.
I just saw a really neat set of photos of phyllites on Flickr, taken by Noora S., “an Omani girl who likes taking pictures of Home.” (And also, I’m guessing, a geology student.) Check this out, as an example:
Image source: Noora S. on Flickr
It’s a thin section photo, the sort of thing that I use all the time in my work. Geologists take a thin slice of rock and look at it under a microscope using polarized light. Sometimes, we use another polarizer oriented perpendicular to the first, to take advantage of all sorts of funky things that happen to polarized light when it goes through an anisotropic crystal. In this case, I think Noora played around with the orientation of her polarizers to give interesting combinations of colors.
The hexagon is a garnet crystal, and the big brown, green, and pink things are biotite. (They’re the clue that the polarizers are oriented at an odd angle – normally the garnet would look black if the pink and green colors were visible.) The layering (foliation in structural geology jargon) wraps around the garnet crystal. Does it mean that the rock got shorter perpendicular to its foliation, or did the garnet push the layering aside as it grew? I’ve heard people argue both ways. The biotite crystals are typical of the shapes that I’ve seen in a lot of phyllites – I bet that they form little black lines on the shiny surface of the rock (though it’s hard to be sure – I don’t know how the rock was cut compared to its deformation).
The best thing, though, is the heart. (Cut out of paper and placed on top of the slide – it must have been very tiny, to be the same size as a garnet in a phyllite!)
‘Cause the photo expresses how I feel about these rocks. ♥