I have a confession to make: I have absolutely no idea what this picture means. And most of you probably don't either, which is okay, because you're not running around the Internets pretending to be a geologist.
This is what's colloquially known as a thin section - a piece of rock sliced so thin that you can shine light through it, and then stare at it under a microscope until you get a headache. Minerals that look similar in a hand sample will refract light very differently in thin section, which makes it useful for obsessive mineral-identifiers. You can also use thin sections to see anything interesting that might have happened to the crystals themselves - changing conditions as they grew, deformation afterwards, all sorts of things. Thin sections are part of the standard geology toolset.
As a geophysics major, though, I never had to spend umpteen hours in petrology lab learning how to interpret the things. And as it turns out, this is not the sort of skill you can just pick up by osmosis at colloquia... especially not at seismology colloquia.
So I'm putting this up as an example of gabbro, but for all I know it is Portland cement.
But this, my dear readers, I can tell you is really a gabbro. I know this for the following reasons:
- The mineral grains are big enough to see with the naked eye
- It has some plagioclase and some dark stuff
- It doesn't have any quartz
- It has a label
If you take a bit of the Earth's mantle, heat it until it just starts to melt, and then store the resulting liquid someplace where it can cool very slowly, you get gabbro.
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As a geophysics major, though, I never had to spend umpteen hours in petrology lab learning how to interpret the things. And as it turns out, this is not the sort of skill you can just pick up by osmosis at colloquia... especially not at seismology colloquia.
The funny thing is that I'm also a geophysics major, I find this funny since I did undergrad research in quantum optics, applied for grad school in quantum optics, then got here and decided to try out geophysics for my masters. The fact that I have never taken a geology course, and couldn't tell a bit of granite from a bit of cow shit doesn't really phase me.
Fortunately, you don't need to be able to know anything about rocks to do planetary dynamo theory ...
This is what's colloquially known as a thin section - a piece of rock sliced so thin that you can shine light through it, and then stare at it under a microscope until you get a headache.
Hah! My feelings exactly...
The label on your hand sample is spelt wrongly, of course ;-)
As an addendum to your post, polarized light is transmitted through thin sections, thus the pretty colors.
Also, as long as I am picking nits, Portland cement is a fine powder that, when mixed with water, binds together the aggregate in concrete. In thin section, concrete would not show the interlocking crystals of your gabbro.
I spent many an hour gazing into optical microscopes and spinning their stages while I was in college and grad school in the 70s. I am not sure why but I still have my optical mineralogy textbook. Ah, the joyous beauty of Snell's Law...
I see feldspar and olivine. It would also help if it was animated and turned round so you could see if anything was going in and out of extinction! I love thin sections and playing (I mean "working") with a microscope.
Friday rock blogging!
I love the label myself.
It is very nearly the only sorts of rocks I *can* reliably identify, the ones which come labeled. (They are in cases in our Math/Science blding.) I am good at sandstone and slate, available in the crick behind my house, too, sometimes, if it is very plainly sandstone and not tricky slate. Otherwise, forget it.
Thin sections are the bane of my existence. I've made it through mineralogy, but I still have petrology to go. I think it takes a special kind of person to want to do mineralogy. And possibly daily applications of acid...
Hey, that's your face in the middle!
Maria said: "... then stare at it under a microscope until you get a headache."
It makes a tremendous difference when you put a digital camera on the microscope and can look comfortably at the image on a large LCD monitor rather than squinting through eyepieces while hunched over awkwardly.... Of course, if you really want to know about a sample, an electron microprobe can hardly be beat -- electron images and X-ray spectrometers win against eyes and chunks of glass.
I once counted how many times I'd had to do the derivation from the wave equation to Snell's Law on homework sets. I think I did it at least twice for seismology, once for radar, and I know my final answer was 4 or 5 so I must be forgetting something... never did it in a context as pretty as optics, though.
what is funny for me, is that i am a biology major but still had to go through this course of mineralogy, i actually had to go through an earth science course every single semester of my 4years formation. I used to love mineralogy far more than mineralogy where we had to study the rocks themselves:P but I sadly didnt do well in any of my courses, and even those who did well in the mineralogy exam can't identify any mineral or rock in thin sections...sad
Ellery said: "It makes a tremendous difference when you put a digital camera on the microscope and can look comfortably at the image on a large LCD monitor rather than squinting through eyepieces while hunched over awkwardly..."
Alas, you need to be studying at a school that has money to spend on such things. My department actually has such a setup, but it's a cheap one and the effective dynamic range of the images is inadequate (i.e., the image goes from being dark to being washed-out with only a small adjustment to the lighting). BUT there is an advantage to an impoverished school: we've accumulated a very odd assortment of ratty chairs and stools over the years, and one of the right height can be found for almost any student, to avoid the hunching-over-awkwardly part.
With the correct seating, I can spend hours happily working with thin sections. And no, I don't require acid (though a long day is made easier by an occasional cup of good coffee).
I'm pretty sure I can see the image of Jesus in the picture of that rock slice...except that he's wearing blackface and kind of looks like the gingerbread man. Hmmm, and he has a heart for Valentine's Day!
Definitely some twinned plagioclase up there. And with second-order birefringence likely olivine and/or CPX and possible some other foid minerals I never bothered to learn to recognize!
In the 2nd picture, I can see an H. R. Giger-style Alien holding a Klingon pain stick facing off with the mutant offspring of Bun-Bun and Choo-Choo Bear.
(But then, I *am* on some high-grade prescription meds right now [grin]...)
Oh gods yes, thin sections were the bane of my existence. Thank goodness for fabulous TAs or I would've failed sed/strat. Luckily I got out of taking any kind of petrology. Love your blog, btw! If you need any interesting Friday Rock pics, do let me know -- my friends who helped me move would be happy to know that the buckets of rocks they hauled around (with copious complaints of course) were doing some good!
I have many hours in mineralogy and petrology labs behind me and yet, I cannot tell what the thinsection is. I haven't done any of this since undergrad and most of my former knowledge seem to have disappeared.
The extinct phase is probably Opx, as the cleavage is up-and-down, parallel to the polarization. The purple-brown stuff with strong second order colors is a pyroxene. The white and black twinned stuff is plagioclase. I suspect the light yellow is a pyroxene, but it is hard to tell without being able to rotate the stage and uncross the polars.
Thin sections are great! I spent 10 years teaching optical mineralogy to the exclusion of all else, I can't tell you how much I hate hand specimens ("here, you're a geologist, what's this anonymous grey rock with no distinguishing features whatsoever?")
Though a few years ago I crossed over to the dark side known as Engineering Geology, summarised thus: take hammer, hit surface. "thunk" = rock, "thud" = soil.
Ah, but in my old school days in the '70's, all mining-geophysicist graduate-students took both optical mineralogy (i.e., sliced our own thin-sections) AND ore microscopy (polished our own reflecting-sections). All those ore-bearing sulfides are black blobs in thin-sections. Yup, Green Gabbros can be pretty, but I made my fortune looking at nickel- and platinum-bearing troctolites (trout stones) with lotsa olivines. Great blog, BTW.