Sliced-up Rocks on a Microscope Slide Quiz

Answer: .03 mm

Thin sections are impregnated with blue epoxy before they are cut to size. The epoxy keeps the rock from falling apart, especially if the rock is friable, and it makes the pore space more visible. The slice of rock is then glued to a microscope slide, and a cover slip is glued on top of that. Thin sections have been known to break when dropped...not that I've ever done that, mind you!

Answer: Pleochroism

Most colored minerals are pleochroic, so this isn't as powerful an identification tool as you'd expect. All right, so garnet will never show pleochroism, and biotite, chlorite, amphibole, and some pyroxenes will, but the very fact that a mineral does appear colored in thin section is usually more useful. What causes pleochroism? Pleochroic minerals absorb polarized light differently in different crystallographic directions.

A more detailed explanation would put this quiz firmly in the "physics" category!

Answer: Insert a second polarizing filter, this one at a right angle to the first

If you know anything about optics, you should know that two polarizing filters at right angles to each other should mean that no light is transmitted. The thin section should appear entirely black! But, some minerals can rotate the polarization direction of the light passing through them. Hence, some light is transmitted through the upper polarizing filter. Very good. Examining a thin section under "crossed polarized light" is an extremely useful tool for petrography! You can determine:
-- A mineral's interference colors, if any
-- Twinning or deformation of a mineral's crystal lattice
-- The extinction angle
The first two items I explain fully in the next two questions. As for the third-- any mineral grain that does transmit light under crossed polars will still go completely black every ninety degrees of rotation. This is called "extinction." For some minerals, extinction occurs when the polarization direction of the light is parallel to (or at right angles to) the cleavage or crystal faces. This is called "parallel extinction." The most useful application of this property is distinguishing between orthopyroxene and clinopyroxene, which can have similar optical properties otherwise.
-- If you're really good, you can do special tricks like determining the relative percentages of Ca and Na in plagioclase feldspar!

Answer: Bright pink, yellow, blue, orange, and green

Imagine a whole thin section of minerals with second or third order interference colors. Psychedelic! The first time I looked at one, I think I spent half an hour spinning the stage around! Pretty. The short answer as to why there are interference colors is that different minerals interact with the polarized light differently. The long answer, is, well...

Most minerals split light into two different rays, one of which travels slower (= 'the slow ray') than the other ('the fast ray'). When they emerge from the mineral, the two rays interfere with each other. Given that light is a wave, the time delay may have resulted in the two light rays being "out of phase" with each other-- the wave crest of one no longer lines up with the wave crest of the other. In this case, the light interferes with itself, and the intensity may change, either bright white for total constructive interference, black for total destructive interference, or grey for something in between. So far, so good. The colors come from the fact that different wavelengths of light will come out of the mineral with different phase differences. So, one wavelength of light may experience destructive interference, while another may experience constructive interference. You will see the wavelength(s) experiencing constructive interference as the interference color of the mineral.

This is my attempt to condense a chapter-long technical discussion in an optical mineralogy textbook! If it doesn't make sense, please let me know!

Answer: Blind luck

Practically speaking, the difference in twinning is by far the most useful critereon. Microcline twinning in K-spar looks like a grid of black and white lines, while albite twinning in plagioclase looks like parallel stripes. Such obvious distinctions are a joy to the beginning student of optical mineralogy! The difference in 2V angle and optical sign (what these things are is explained in a later question) are useful in the rare cases that the feldspar is not twinned. Woe! Alas!

Answer: quartz

Had this been nepheline, it would have been optically negative, and the upper right quadrant would have been yellow. However, "yellow upper right negative," "YURN," does not sound nearly as good as BURP, which I expect is exceedingly popular with the under-30 male college student crowd, who, I understand, have much experience in these matters. "What exactly is the optic sign?" you ask.

In minerals in the hexagonal and tetragonal crystal systems, it refers to whether the fast ray or the slow ray is the one that vibrates parallel to the crystallographic c-axis (the long axis. Think of the long and skinny axis of a quartz crystal.) For minerals in the orthorhombic, monoclinic, and triclinic crystal systems, the discussion of optical sign gets to be very, very complicated, involving much scribbling of pictures and hand-waving, so I'm not going to include it in this quiz. Just think of optic sign as "another convenient way to tell minerals apart."

Answer: Granite

Indeed, my friends, it is granite! If you are ambitious, you may determine the "mode" of this rock-- the percentages of the various minerals. I have never been a fan of this, since I suspect my accuracy is very bad, but once you obtain your list of percentages, you can plot your rock on a special "ternary diagram," which will tell you exactly what type of rock you have. Quartz-rich granitoid? Granodiorite? Quartz monzosyenite, anyone?

Answer: Both of these

One tends to think of rocks as really boring, static assemblages, but given enough heat, pressure, or other chemical incentive, some minerals will react to form other, more stable minerals. One of the really, really neato things about thin sections is that you can see minerals caught in the act of reacting with each other! Of course, even if you stare at the same thin section for your whole lab period, nothing's going to happen, but you can tell that the reactions were occurring under the conditions in which the rock formed. One fellow, a certain N. L. Bowen, used these mineralogical relationships to put together a sequence of how minerals will react under decreasing temperature. A simplified version of "Bowen's Reaction Series" is as follows:
Plagioclase -> K-feldspar -> Muscovite -> Quartz
Olivine -> Pyroxene -> Amphibole -> Biotite -> K-feldspar -> Muscovite -> Quartz
(Psst-- If you're even thinking about becoming a geologist, memorize this right now! They'll make you do it! I promise you! They'll make you learn it!)

Answer: Taste in music

Let's check these out one by one:
-- Source of the quartz grains. Quartz looks different depending on where it crystallized. Volcanic quartz tends to form nice crystals and be relatively free of inclusions. Plutonic quartz, which crystallized slowly underground, looks pretty nondescript. Vein quartz, which crystallized from left-over fluids, looks cloudy from fluid bubbles. In quartz from a metamorphic environment, extinction sweeps across the crystal, because the crystal lattice was deformed by the pressure and shearing. One might suspect that the quartz had come from an earlier sandstone if the grains were exceptionally well-rounded or seemed to have older overgrowths of cement.
-- How many times the rock has been cemented, and with what. The mineral that seems to have surrounded and grown over the sand grains would be the cement. Using your finely-honed mineral-identification skills, you should be able to figure out what it is. Calcite? Kaolinite? The dreaded, icky chlorite? "Oho!" you ask. "If the grains are quartz, and the cement is quartz, how can you tell them apart?" There is usually a layer of brown goopy stuff surrounding the quartz grain, and the cement forms on top of it. Ha!
-- What other minerals or rock fragments are present. You can use this to figure out the source of the sand in the sandstone! Or, if the source doesn't physically exist any more, you can figure out what sort of rock it consisted of! Though geologists deal with some of the largest structures on the planet, yes, sometimes we are reduced to adducting most of our evidence from tiny grains of rock.
-- Taste in music. Soft rock, obviously. (Sedimentary rocks are known in geological parlance as "soft rocks," as opposed to igneous and metamorphic rocks, which are called "hard rocks.") On the other hand, since a sandstone is a clastic sedimentary rock, one might want to add in the Rolling Stones. Didn't need a microscope for that!

Answer: Yes

The different ways of layering CaCO3 or SiO2 in an organism's shell are actually apparent in a petrographic microscope! Echinoderms like sea urchins or crinoids make all of their hard pieces out of single crystals of calcite. In trilobites, the extinction sweeps along the carapace as you rotate the stage. There's no need to have a complete, or yea, even recognizable piece of the critter! Unidentifiable fossil hash suddenly becomes recognizable! Actually, most optical mineralogy involves turning the unidentifiable into the identifiable. I hope you've learned a little from this quiz!