May the Quartz Be With You

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As part of Earth Science Week 2020, USGS scientist Shannon Mahan takes us on a tour of quartz and how geologists like her use quartz to study all kinds of things, from earthquakes to archaeology.


Date Taken:

Length: 00:24:04

Location Taken: Denver, CO, US



So hi, everyone! October 12, Mineral Day. How fun is that? Yes, it might coincide with yet another holiday, but I think they're giving us the day off at the USGS. I'm pretty sure they are! So what I want to talk about today is quartz. 'Cause everything I do as a geologist at the USGS kinda revolves around quartz and feldspar, but we'll save feldspar for another day. So my name is Shannon Mahan, I'm a geologist at the USGS in Denver. The intern that's helping do this is Emma, intern Emma. We're going to talk a little bit about the quartz that you would normally see in rock shops or maybe your favorite museum. Other places that you might go might include parks.

I brought a few props in. First of all, what is quartz? You will literally see quartz everywhere but you won't recognize it because it has so many forms and so many varieties. It's truly the most glorious mineral, because you don't realize it, but in all the rock kits, in all the places you see it, it's there. Different colors, different shapes, even different sizes. Let's look at something. Here is what you normally think about when you see quartz, right? You think about hey, I've got some nice crystal faces, there's usually a lot of grown crystals. So we happen to have gotten this one in Jefferson County, Colorado. This is a really nice example of what we like to call Milky Quartz.

But here's some other examples. People sometimes look at these...that is...if you've ever heard the term rock crystal, that's kind of the clear-looking quartz. I actually have a nice clear little mineral of quartz there. So there's another quartz mineral there. But! You'll be surprised to know that this is a type of quartz. This is certainly quartz, the dark mineral there. That's smoky quartz. Even this is quartz. This is an agate, a banded agate and...ok. This kind of shape doesn't come in Nature, but again, this is an agate that has been trimmed so that's also quartz. And then we have some glass. Some volcanic glass. It's a little weathered, but again This is a form of quartz. Glass quartz. Okay? And then sometimes you get stuff like this in rock shops and you're like whoa, is that really blue, is that natural? Well, we're going to tell you some secrets about this. We're going to tell you why it looks shattered and why it's blue. And then, my favorite, who doesn't like rose quartz? So we actually got this piece in South Dakota. South Dakota is very famous for its rose quartz. It's quite beautiful. And then, finally, (this is really heavy) we have a piece of petrified wood. Ok, the bugs are a bonus and the spiderwebs are a bonus. This is tree that has been replaced with silicon dioxide quartz. You can still see bits of the bark. Of course, it's very heavy now, and people say hey, where'd you get this? Okay, I live in the foothills of Denver. I actually found this in my neighbor's yard. It's not the right geology. Foothills west of Denver are all precambrian granite. So where did this come from? We don't know. I'm pretty sure it didn't come from that precambrian granite. But it's a beautiful piece, so I wanted to use it to show you the sort of quartz things. 

Okay! Let's come back to this quartz and let's come back to this piece. How can you get quartz to do this? First of all, quartz is a really strong mineral. It's very hard. So there's a rock scale. The rock scale is 1-10. Where does quartz sit? It sits at a 7. But it turns out that quartz is pretty fragile if you heat it up. And then you plunge it in ice water. You can actually fracture it rather quickly. So what they've done is they've heated it up. You could probably even heat it up in your oven. Then you put it in a bucket of ice water. This is what happens, right? So you fracture it and then you can get this dye in it if you tumble it a little bit in a colored liquid. I'm not even sure if the crystal itself has been grown in a lab or if it hasn't been abraded in a lab. This is beautiful. We'll just say it's beautiful. But maybe not entirely natural. 

This on the other hand is entirely natural. The element that takes up and makes this pink is actually a form of iron. It's the same thing you'll see in many of these. This! I wish I had a bigger piece. This is actually kinda special. This smoky quartz, how does it get smoky? Are you going to say heat? Kind of. It's radioactivity. Being close to radioactivity gives this quartz its smoky appearance. How does that happen? Alright. I don't want to belabor the point but let's just turn to my computer and let me show you something fun. Here is an idealized mineral structure of quartz. The silicon is going to be the brown and the oxygen is the pink. What is the formula for quartz? Silicon dioxide. So for every silicon atom you're going to have two oxygen. Turns out to be a really strong covalent group. It's like holding hands with your best friend. You have a really strong bond. But sometimes what happens here is sometimes you don't have your best friends. Sometimes you have somebody new. In this case, somebody new is an aluminum. But aluminum doesn't hold hands in quite the same way. It doesn't hold hands, it doesn't have quite the same bond. Instead it creates what we call a little oxygen vacancy. So how is that important? Well, here comes... 

Your elements want to do a little dance right, a little dance to the instruction of natural radiation. This is just natural radiation and I'll show you some of the ways natural radiation occurs. Here you have natural radiation from the Earth surrouding us everwhere it comes in, gives us a little kick to some of these electrons and the electrons are caught in these traps. Quartz is a structure that is extremely stable. It doesn't have a lot of impurities, which is why we use it for a lot of things, ok? I want to show you what happens in the rest of this diagram if we go into the lab, ok? We're going to use quartz a lot at the USGS. It can tell us lots of things. In other words, if you listen, it can tell us exactly where it's been, where it was created, and how long it's been at the Earth's surface. You might say, we're the sand whisperers. So let's go to the lab!

This is the main corridor for the National Water Quality Lab. It's going to get a little noisy, 'cause we keep a lot of our refrigerators or other things we don't necessarily want in the labs out in what we call the service corridor. So we're going to walk down the service corridor to a lab so you can see a "normal" lab and then we're going to go into my lab, which is a special lab, because it's called the Luminescence Geochronology Lab. But don't worry, we're going to talk about where the quartz comes in and all Alright, so here we are at one of our rock labs. Before we go in, I just want to talk a little bit about this lab diamond. So on all of the doors we passed, you'll see these kind of diamonds. There's a blue, a yellow, a red and a white area. Blue is for health. Red is for fire hazard. Yellow is for reactivity. White is for anything you might have that's special. So sometimes you'll see "ox" for oxidizer. Sometimes you'll see a "W" with a bar through it. Means use no water. Anything that you want your fire people to know. The scale is zero: nothing's really going to happen. Four is the worst. So if this diamond is blue and it has a 4 it tells you there's a health hazard in here that is extremely high If you have a red 3, that tells you there's something in here that has a low flash point. And then yellow 2 tells you something about the acids that are in here. So that's on every door. So let's go into the lab. Here we are, this is a really typical lab. Sometimes people imagine scientists with thousands of beakers on the counter and they're all various colored liquids and there's exciting things going on and we're pipetting stuff Well...there is some excitement, but it's not that kind. 

So, basically, what we want to do with the minerals we look at is we need to extract them from a rock, because a rock is just an aggregate of minerals, right? So sometimes people only want the feldspars, sometimes they only want the zircons, sometimes people only want to use the plagioclase, sometimes people only want to use the quartz We use quartz and potassium feldspar. So we need to separate those out from the sediment or the rock and this is how we would do it. So if we come around the corner You can see in a typical lab that sometimes you need to make a thin section of polish, sometimes you need to actually saw a piece. This is actually a diamond-tipped sawblade Sometimes you would need to do that. Sometimes you need to be more serious if you need to be more precise. Sometimes you'll already have your grains, but you need a bit more refinement. So this odd-looking best of equipment is called a Franz magnetic separator, and what it does is it has two huge pieces of copper coil. When you have two pieces of copper coil, you can make a magnetic field by passing electricity through it. When you pass electricity through it, and then what happens is you have this odd looking tray, and as you pass your mineral through the magnetic field, it separates out the iron-bearing minerals from the non-iron-bearing minerals so you can get your darker minerals to the side and keep the lighter, non-magnetic minerals to one side. So that's a really nice way to instantly separate. But if you can't completely separate sometimes what we will do is we'll put our minerals into a heavy liquid. This apparatus allows us to trap the heavy liquid until the separation is complete because all minerals have a specific gravity. So we can separate it and allow it to come out in a heavy liquid. So by heavy liquid, what do I mean? Heavy liquid means heavier than water. Water has a specific gravity of one. Sometimes our heavy liquids are like 2.5 or 3.0 and when you have a density like that, it allows minerals to self-segregate according to their density. You'll learn all about it when you come work for us at USGS. Ok, I just wanted you to see all of this because if you look up here, you can see all of our minerals, all of our separates that we do.You can see that sometimes we have the sieve, and by sieve we mean we want a certain grain size. Sometimes we have crushed a little bit. This lab has pretty much everything you might need to do a minerals separate. But what happens if we want to look at a very specific mineral property, like maybe, how much luminescence-luminescence is light, and we're going to take the light that's stored in the quartz and we're going to look at it. But see these overhead lights? These overhead lights are ionizing radiation. They're just like...well, not exactly like-they have the same properties as radiation in that they can move electrons around in the quartz. Remember the quartz had those electrons moving around? So let's go to my lab and see what happens in my lab.
So here we are at my lab. You'll notice that my lab diamond looks pretty ominous. I've got blue 4 and red 4, so I have something that's really detrimental to your health, something with a low flashpoint, maybe not so much reactivity and I don't have any special things that the firefighters would need to know. But, I do have radiation in here. So because I have radiation, I need to put on a badge every time I go in. So I'll put on my badge. I also have a ring badge, because I would handle the samples Not too good at handling with my toes, so I'll do it with my hands The other thing in my lab is we have something called a cipher lock. This allows me to key in a unique combination so not just anyone can go in there. So Intern Emma is gonna turn that away while I open the door and we're going to go in. Alright, here we go.

Ok, what's the first thing you notice about this lab? You can't see anything, can you? I have really low-intensity light. This is the kind of light you'd see outside at night. It's called sodium vapor, and all that means is it has another filament in there that doesn't give you the normal white light This allows the luminescence in my minerals to be quiet and calm I don't want to extract the luminescence yet until I go to the machines, right? So here's what my lab looks like. We'll stand here for awhile so you can get used to it. Usually your eyes will adjust in a minute or two because you can still see at low-intensity light, it just makes everything a little bit slow. Here's where we do a lot of our work. We have a sink where we do sieving and minerals. We also have tanks which you probably can't see very well. We do a lot of sample prep right here. One of the things I want to show you is an acid that we use in order to clean up the quartz. Remember what I told you about quartz being really stable and really durable? It can survive being in really nasty acid that would just dissolve away most other things. One of those acids we use-I'm not going to touch it-is called hydrofluoric acid. This will dissolve quartz if you let it. But we're not going to dunk it in there long enough. But this will take care of any feldspars that might be mixed in the quartz because remember, we want a pure quartz component That's why we have, in that blue space, a four. Because of stuff like this. Alright, so when we're done sieving it and putting it through the magnetic separator and heavy liquids and a little bit of HFL, then we have all of these little vials that we will get But this is basically what our quartz looks like now. But this is really exciting, because not only did it take us  a long time, but now we're going to be able to put this quartz on the machine and measure the luminescence. 

Ok, she talks about the luminescence, what is the luminescence? So remember that diagram we looked at earlier, where the electrons are all piling into the oxygen vacancy? That happens over time. So electrons keep piling in and then, if we expose that to ionizing radiation like light or heat, remember how quartz reacts to heat? Remember how it reacts to light? It's that same thing. But we want to expose it to controlled light. So let's come over to the machines and I'll show you what I mean. Even in this lab, you'll notice our screensavers are red light. Because we want really low-intensity light, we want our quartz to be calm, we don't want it to dance or sing yet until it's in the machine. So this machine just looks like a box, like magic, but it's not. Because what's happening here is, we have a ring of diodes. We have a ring of diodes at the bottom under a photomultiplier tool. What we're going to do is we're going to hit quartz with a controlled beam of light, and this is what happens. If you hit quartz with a controlled beam of light or heat it responds with light, it responds with luminescence We're literally, right here, right now, measuring the luminescence in the mineral. So this says 50,000 photons/second You can actually see our data coming off. And the basic theory behind this says the more luminescence in the mineral, the longer it's been buried in the ground. That allows us to do a huge number of things. We can predict when an earthquake will rupture a fault to the surface. We can date mammoths if they're buried in dirt. We can date archaeological sites; there are so many things we can do. We can even date when bricks are fired. Yes, that's exciting. Ok, so that's basically what goes on in the Luminescence Lab. That's how we use quartz. I hope you enjoyed a little bit of looking at minerals, seeing our field shots, and seeing how we spend most of our days in this lab. It's been great; I'll come back next year and we'll do feldspar. Alright, see you guys!

So as geologists, we do get to go to fun places to look at minerals. This is the Black Canyon of the Gunnison in Colorado. You'll see in the canyon walls there is a black, dark granite. But it's cut by the pink pegmatite veins. Those pegmatite veins are where we're going to find quartz and heavy minerals that go along with quartz, like gold. But basically, when we talk about quartz as the mineral that's the most common individual mineral at the Earth's surface, it's going to be found in veins like those. So welcome to beautiful Black Canyon of the Gunnison.

We're at the headwaters of the Arkansas River. Remember how we talked about quartz being a really stable mineral and it's everywhere on the Earth, so if you look down into the streambed, you're going to start to see some. First of all, you're going to see sparkly bits, right? Who doesn't love sparkly stuff? But that's the mica that's another mineral that we're not going to cover today. If you just take a handfull of this, and you rub it, it feels gritty. I know it's hard to see, because it's not sparking white sands of Bermuda or something, but this is quartz. Eighty percent of this is quartz. Right here in the river is where it's being ground down You can see the different kinds of rocks, let's come over here and look at the granite. So we saw this in the Black Canyon of the Gunnison now we're seeing a different type of granite and this is where the rocks are getting ground down from big to smaller to sand. There you go.