Mineralogy-- Into the Jaws of Doom Multiple Choice Quiz | Sci / Tech

Quiz Answer Key and Fun Facts

1. Dr. Blarg has devised a cunning escape system, whereby if you push the button of the right answer choice, a door will open and you will proceed into the next room, unharmed. If you push the wrong button, you will be instantly electrocuted! (Cue the evil laughter.) The mineral in question is an olive green color. However, it is only a fragment, so you can't determine the crystal system, and it is located in a glass box, so you can't touch it or taste it or scratch it or even determine the crystal system. Blarg is evil indeed. However, the answer choices should elucidate the identity of the mineral. Will you proceed, or be electrocuted?

Answer: Olivine

Color is one of the least reliable indicators of the identity of a mineral. Most minerals can be multiple colors, and of course there are many minerals that are the same color. That said, some minerals, like those listed above, have a specific enough range of color that you can eliminate some choices. Olivine is always olive green or dark green. Rhodochrosite is always red or pink. Sodalite is blue, white, or grey.

Hematite can be metallic silver or reddish. Hopefully you picked olivine and so are alive to read the interesting information!

2. Our next mineral involves a taste test. The mineral itself is transparent and cube-shaped. It tastes salty. Now which mineral would this be? Let's not mention that if you guess incorrectly, you will be dropped into the pond of unfed crocodiles below. The salt will just serve to season their meal. That would be unfortunate, would it not? To lose such an interesting mineral specimen?

Answer: Halite

Halite is just rock salt. It usually forms in places where a large quantity of seawater has evaporated. There are only a few small places today that have large areas of evaporating seawater, but in earlier times, the Gulf of Mexico and Mediterranean were evaporite basins. Wild, no?

3. Now, if you don't get this one correct, we'll just release the undead warriors, and... Oh, hello there. Ahem. This mineral is the major constituent of limestone and marble. It also has quite a few interesting properties. It effervesces vigorously in dilute hydrochloric acid. That's much better than saying it fizzes, right? Right. It has perfect cleavage and breaks into rhombohedrons. One form known as Iceland Spar is transparent, and splits light into two different rays-- so if you put it on top of a dot, you'll see two dots. Dotty! Heh heh... And, for you added pleasure, this question is fill in the blank. I am SO evil!

Answer: calcite

Not much of a problem identifying calcite. You can actually find two forms of calcite, low magnesium and high magnesium calcite. Wow. But let me introduce you to calcite's two evil twins: aragonite and dolomite. Aragonite has the same chemical composition, but it is less dense, in a different crystal system, and is less stable. Dolomite, on the other hand, has the same crystal structure as calcite, but it has the composition CaMg(CO3)2.

It effervesces weakly in dilute HCl. Weakly? Weakling....ah, but you got this one right. Right? Do not be fooled...

4. For your inspection, you are given the wedding ring of Mrs. Blarg. If you identify the stone incorrectly, Mrs. Blarg herself will deal you doom. The stone is transparent and has high fire, but there is some chipping of the crystal edges, and the stone seems to have a hardness of about 7 on the Mohs scale. What mineral is it?

Answer: Zircon

"Fire" is the amount of flashiness in a faceted stone. Diamond, zircon, and cubic zirconia all have high fire. Quartz does not, so you can eliminate that one. On the other hand, it can't be diamond, because diamond has a hardness of 10. Cubic zirconia is much softer than 7.

While not as valuable as diamond, zircon is a nice gemstone in its own right. It comes in green, golden brown, and light blue. Geologists also love zircon because it often contains radioactive isotopes of uranium and thorium, which can be used to date the rock in which the zircon crystals are found.

5. Our next mineral is magnetic, a sort of opaque black or brown color, and dense. If you don't guess this one correctly, and you ought to, since the name is somewhat obvious (Oooohh! I said too much!), we'll just direct the large meteorite at your location, and...

Answer: Magnetite

Magnetite is iron oxide, Fe3O4. Not all specimens are actually magnetic, but all are attracted to magnets. When a magnetite crystal forms in magma, the magnetic field of the crystal aligns with the Earth's magnetic field. When the magma cools into a rock, the direction of the crystal's magnetic field is set. So, you can tell the direction and strength of the Earth's magnetic field when the rock formed.

This has been used to confirm plate tectonics and the theory that the Earth's magnetic field reverses occasionally.

6. For this next mineral, if you don't get it right, you shall eat this lovely little plate of chocolates, which, unfortunately, are laced with toxic chemicals. Oh dear. I seem to have lost my ap... Hey! No puns! No puns! Aha. This mineral forms hexagonal prisms, has a hardness of 5, and consists of calcium phosphate. It is also the major constituent of bones and teeth. Now, what is this mineral, exactly?

Answer: Apatite

Yes, I've lost my apatite. I almost got kicked out of my geology class for that one! Apatite also has commercial uses: phosphate for fertilizer, fluorine for city drinking water supplies, and it can also be used as a gemstone.

7. Zee next mineral eez sovt and gvray and shinee and zee... I don't know what accent I'm trying to fake, but obviously I'm not doing it very well. Vell, eef you haf seen zees mineral, you have probably seen it in PENCEELS! Vich make good arrows. Another form of zee carbon, other zan diamond.

Answer: Graphite

Graphite was originally mined from the Cumberland Graphite Mine, where it was extracted as whole chunks and used to make pencils. However, most graphite today is mined as tiny flakes in metamorphic rock, then ground up and mixed with fillers to make pencil "lead." A lot of graphite now comes from Madagascar!

8. Now on to the wonderful world of chains silicates. Their molecular structure consists of chains of silica tetrahedrons linked in either a single or a double chain. Chains? Chains? Oh. That's what you'll be getting. That and a rather deep river. Anyway, the two most common chain silicates are pyroxene and amphibole. They are so similar, is it even possible to reliably tell them apart in hand sample? I'm not even asking you to identify them. Ugh. I seem to be becoming weak-minded.

Answer: Yes

Difficult as it may be to see, the cleavage angles of the two minerals are different. For pyroxenes it is 90 degrees, and for amphiboles it is 120 degrees. Pyroxene also tends to form short, blocky prisms, while amphibole tends to form long, thin needles.

It may be hard to see, though. There are many more criteria for identifying minerals under a microscope. See the next question. Anyway, pyroxenes consist of a single chain of SiO4 tetrahedra, while amphiboles consist of a double chain. Amphiboles, at least, are "trash can" minerals-- they'll take just about anything.

The chemical composition for hornblende, a pyroxene mineral, is (Na,K)0-1Ca2(Mg,Fe^2+,Fe^3+,Al)5Si6-7.5Al2-.5O22(OH)2! You ARE supposed to memorize that! And lest you think you've never laid eyes on either of these, they form some of the black spots in granite, and they are major constituents of basalt!

9. The identification criteria are becoming more and more complex and convoluted, just like the mind of Dr. Blarg. This time, you are gazing at a thin section in a petrographic microscope. The question this time? Is the mineral under the crosshairs quartz, or feldspar? If it's feldspar, what kind? You determine that the mineral goes extinct as you rotate the microscope stage under crossed polars, exhibits albite twinning, and is biaxial positive. What is it? Keep in mind that, if you answer incorrectly, your chair, actually an ejector seat from a fighter plane, will hurl you into the gorge below, where you will meet certain death.

Answer: Plagioclase feldspar

Quartz and feldspar can be notoriously frustrating to distinguish. I hope you determined it correctly. If not, I hope your landing was sufficiently cushy. Let's go through the criteria one by one.

Goes extinct as you rotate the microscope stage under crossed polars-- Since neither quartz nor feldspar are in the isometric crystal system, this is no help.

Biaxial-- Quartz is uniaxial and feldspar is biaxial, so at least you have the field narrowed down

Optically positive-- Plagioclase can be either positive or negative. Quartz is positive. Orthoclase is negative. So, it could be either plagioclase or quartz, but we know from the fact that it is biaxial that it can't be quartz.

Exhibits albite twinning-- That is, parallel strips of the mineral go dark at different angles when you rotate the stage. Neither quartz nor K-feldspar exhibits albite twinning, so you've got it pegged.

10. Finally, Dr. Blarg allows us to identify a powdered mineral specimen using X-ray crystallography. If we don't guess correctly, of course, we'll get put in the X-ray source, "we" being, of course, not you and I, just you. X-ray crystallography measures the d-spacings of different minerals, that is, how far apart the atoms are spaced in the crystal lattice, and the intensity of light that is reflected. For your mineral, you measure d-spacings of 3.22, 1.70, and 2.51, and relative intensities of 100, 52, and 39. You hastily look up a number of minerals in that exciting, best-selling database from the International Center for Diffraction Data (at the extra low price of $6,100, software to search the database extra.) You find the following data for the following minerals. Which one is it?

Answer: Rutile 3.25, 1.69, 2.49; 100, 50, 41

X-ray diffraction is pretty snazzy. Powder a mineral and put it on a flat slide in the machine. The machine measures the angle at which the X-rays are reflected from the minerals on the slide, and the intensity of the reflected X-rays. I could explain how to calculate the d-spacings from the angle of reflection, but I don't want to bore you to death with it...You use Bragg's equation... Bore you to death? Why didn't I think of that before? Mmph. Ah well. If you enjoyed this experience, well, then, do come back to Dr. Blarg's Castle of Doom!

Source: Author pu2-ke-qi-ri

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Mineralogy-- Into the Jaws of Doom Quiz

The evil scientist Dr. Blarg has you trapped in his laboratory. If you can't identify ten different minerals, you will meet a fate worse than death! Bwa ha ha! Are you ready for the challenge? (Evil fate not included.)

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