All of My Faults Are Stress Related

I *heart* phyllites

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:

i-8ee272cf98ea722c803187030919ccd6-heart phyllite.jpg

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. ♥

Comments

  1. #1 Mary
    May 20, 2009

    The heart makes me wonder if Noora loves phyllites as much as you do!

  2. #2 ScienceWoman
    May 21, 2009

    I just remember liking the silky feel of phyllites. But I really heart garnets. Such a cool crystal structure and so many pretty colors.

  3. #3 Silver Fox
    May 21, 2009

    Phyllites are right up there on my list of favorite rocks, right next to mylonites. When I first saw the photos on Flickr, I didn’t notice the heart, I was so intent on the fabulous birefringence colors.

  4. #4 coconino
    May 23, 2009

    I miss looking at thin sections under the scope. That was the best part of ig-met for me. I got the only A in the class, because of it. If I had it to do all over again, I would probably be into metamorphic petrology, just because you can spend hours looking at thin sections. Of course, I also say I would have gone into water law, because I’d never be out of a job. ;-)

  5. #5 coconino
    May 23, 2009

    Silver Fox: Living in the Basin and Range, I’m sure you get your fill of mylonites. I’ve seen some nice ones, in the SoCal desert and down in Baja. There’s one in north central Washington that’s also nice, when you can actually see a road cut through all the green stuff!

  6. #6 joomla web design guy
    May 30, 2009

    I too hate Phyllites