Sliced-up Rocks on a Microscope Slide

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1. Being good scientists, we have to give our "sliced-up-rocks-on-a-microscope-slide" a special name. So, we call them "thin sections." How pedestrian. The rock in a thin section is sliced so thin that light can pass through. How thick, pray tell, is the average thin section? 30 m 3 furlongs .03 mm .0000000000000000000000000003 mm

2. So, you've put your thin section on the microscope stage (cover slip side up, I hope!). For the most part, the slice of rock looks completely transparent-- even some black minerals lose their color when cut extremely thin. However, you do note some crystals, mostly large, flat sheets, of one strongly-colored mineral. It's probably biotite, the dark-colored mica. You notice that the crystals change color from red-brown to pink as you rotate the microscope stage. What is this property called? Polychromism Amazing color-changey-mineral-ness Pleochroism Extinction

3. After your cursory look at the thin section in "plane light," you "cross the polars." What the heck did you just do? Insert a second polarizing filter, this one at a right angle to the first Retrace Michael Palin's "Pole to Pole" Have a brief but unfortunate encounter with a big white bear All of these answers are correct

4. Under crossed polars, the thin section takes on an entirely different look. Most of the grains show up in black-and-white or various shades of grey. These minerals have "first-order interference colors." What sort of colors do minerals with second and third order interference colors appear? Black Pastel yellow Bright pink, yellow, blue, orange, and green White with tinges of pink or green

5. You are quite certain this thin section contains feldspar, but you need to know what kind of feldspar. Which of these methods would you NOT want to use to tell apart plagioclase and K-feldspar? Plagioclase can be optically positive or negative; K-feldspar is only negative K-feldspar has microcline twinning; plagioclase has albite twinning Blind luck Plagioclase has a greater 2V angle

6. This rock also contains an untwinned mineral with first-order interference colors. Given the mineralogy of the rest of the rock, it could be either quartz or nepheline. The distinction is rather important, since quartz would imply that the magma had excess silica, while the nepheline would imply excess alkalis, like K2O or Na2O. The surefire distinction between the two lies in the optic sign. Quartz is optically positive, while nepheline is optically negative. You obtain an interference figure. It is a special way of directing light through the mineral. The end result looks something like black crosshairs on a white background. When you insert a special accessory plate, the different quadrants will turn different colors, and these colors will tell you the optic sign. Ready? The upper right quadrant is blue. This probably has no significance to you, except that I will tell you that the mnemonic device is "BURP"-- "blue upper right positive." So, is this mineral quartz or nepheline? Answer: (One word, either "quartz" or "nepheline.")

7. So, with two kinds of feldspar, biotite, and the silicate mineral in the previous question (which you don't actually have to know to answer this question), what kind of rock do you think this is? Basalt Peridotite Limestone Granite

8. In another slide of some rock, igneous or metamorphic, doesn't matter which, you see a mineralogical feeding frenzy. Big biotite crystals surround shabby-looking grains of amphibole, and some amphibole grains can be seen engulfing increasingly unhappy-looking crystals of clinopyroxene. What's going on here? Clinopyroxene is reacting to form amphibole Both of these Amphibole is reacting to form biotite Neither of these

9. New microscope slide. This time, we're looking at a sandstone. I don't know if you knew this, but most sandstones have carried on secret lives unbeknownst to those who only look at them in hand sample. What dirt can you NOT dig up about the past history of this sandstone? The source of the quartz grains What other minerals or rock fragments are present How many times the rock has been cemented, and with what Taste in music

10. Now for some limestone. Is it actually possible to identify fossils in such a tiny slice of rock? Yes No