PlanetGeo: The Geology Podcast
PlanetGeo: The Geology Podcast
Big Valuable Crystals: Pegmatites (GeoShort)
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What type of rock has really big minerals and lots of uncommon elements? It's a pegmatite! Pegmatites contain lots of important elements and minerals, both for our economy and for mineral collectors.
Join us for a tour of pegmatites. We give quick low-down on how pegmatites form. They contain really large mineral grains, which usually implies that they cooled slowly. But, pegmatites form from the leftover fluids from magma chambers and contain all the uncommon (or unpopular!) elements that get concentrated. Once they reach a certain concentration they form rare minerals that are concentrated in things like Lithium, Cesium, or Beryllium!
The minerals in pegmatites grow so fast because the fluid they grow in is much less viscous (more watery) than magma. This means that atoms and ions can diffuse much more quickly to the sites of mineral nucleation, allowing for massive (really really massive, up to 30 ft) crystals of minerals like spodumene and beryl to form.
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Welcome to Planet Geo, the podcast where we talk about our amazing planet, how it works, and why it matters to you. Recording on, it's been too long, Chris. It's been way too long.
SPEAKER_01It's it's it's crazy, isn't it? It is.
SPEAKER_00Hey, we haven't lost the touch when we're clapping in. I wonder if we could get a good spike if we cracked a beer or something. There we go. That's the way to do it. There we go. Man, we should really do that. Let's let's do that from now on. Chris, what's going on, man?
SPEAKER_01Uh, not a whole lot. How you doing, Jess? Actually, you know, there's a lot going on. Yeah, it's freaking busy, man. It's a busy summer. It's a good summer. Where were you? I just got back from Washington, Oregon, and California on our annual family vacation. So got back two days ago. Yeah. Amazing. It was awesome. Yeah, that's good. It was a good time.
SPEAKER_00What a great start to the summer.
SPEAKER_01Yeah, it is. So good.
SPEAKER_00So today we got pegmatites, right?
SPEAKER_01We do. Yeah. What what's up? So what's up? Well, first of all, I think we should start with like most of our audience, I hope or think, doesn't know what a pegmatite is or why a pegmatite is important. So Jesse, real quick, tell us what a pegmatite is.
SPEAKER_00Yeah. So a pegmatite's a rock, first of all. It's a type of a rock, it's a class of rocks, and it's a class of rocks that has massive crystals in it. So the minerals in the rock, the minerals that make up the rock, are huge. And they grow extremely large for reasons we're going to get into. And they're really important. Pegmatites are like economically very important. They're important for how the earth works. And so, like, why are they important, Chris?
SPEAKER_01So they're important because they're made up of these kind of rare minerals, these minerals that are made up of weird elements and things like this. And so not only are they very big, but they tend to be made up of these elements that we need to have.
SPEAKER_00Yes.
SPEAKER_01So that they're they're an extremely important formation, I guess is what we should say.
SPEAKER_00Things like lithium often occur in pegmatites. Now they're hard to mine from pegmatites, but you get lithium, you get like things like gold or all sorts of elements that are really important to society occur in concentrations in these pegmatites, right? And right. I was just up in the White Mountains in New Hampshire, and there's loads of pegmatites, and pegmatites have been mined for probably a century up in that area. And they continue to be able to do that. What are they mining for? A lot of lithium's being mined up there, some beryllium stuff too, at least they historically did. Um, and I think a few other things that I don't exactly know what they are, but uh lots of of of like feldspar is being mined too.
SPEAKER_01Um what do they do you know what they use feldspar for with that kind of stuff?
SPEAKER_00I think they get a they can get aluminum in some ways out of it, but I'm not entirely sure.
SPEAKER_01It's also apparently used as a filler for a lot of substances. Oh, interesting. Yeah, they use feldspar for that. So um anyway, yeah, I just got back from Yosemite, as I said earlier, and this is really where the idea came from. I'm the one that pitched it to you. Hey, let's talk about pegmatites because like I'm I'm just I'm back, we went backpacking for six days in Yosemite. We um we climbed half dome, you know, before sunrise, and we're the we were actually, I didn't tell you this, we were the first people up on top of Half Dome on that day. That was really cool, you know, to have that whole place all by yourself. It was only for 20 minutes before other people came up with that lamps on and all that, but it ruined it for you. No, no, they were actually the guys that we saw up top were really cool, and anyway, that's all beside the point. But you know, backpacking in Yosemite for six days, we came across so much pegmatitic granite. So pegmatite m is a is a rock, but pegmatitic is a texture, and it refers to this extremely coarse grained, which means large minerals within the rock. And and generally what we're talking about is these minerals are bigger than about two and a half centimeters, so roughly an inch. That's usually the minimum. I mean, you can get pegmatites that are uh well bigger than 30 feet. Yeah, we've seen some in the black hills, right? We have seen some of these in the black hills, and that that's another thing where you know we talked about where you were in the white mountains. Is that what you said? The white mountains, right?
SPEAKER_00White mountains, yeah.
SPEAKER_01And I was in Yosemite, and then you and I have spent a lot of time in the Black Hills, and the Black Hills is just loaded with this pegmatitic granite, this extremely coarse-grained granite. That's one of the reasons why the Black Hills is a climbing mecca. You have these huge crystals that are great handholds and footholds for rock climbers, they're what they call jugs, and you and I've spent a ton of time there. You know, we went into do you remember this when we went to the this mine?
SPEAKER_00I'm not gonna name the mine, but we went to this mine, and there was a an unnamed mine that we may or may not have been in, depending on who's asking.
SPEAKER_01No, but we did it right, though. We did it right. Do you remember? Like there was this sign, there was this this gate across it, and there was this handwritten sign that said, if you are caught trespassing here, a lot of bad things are gonna happen to you. And I'm not gonna repeat those bad things. Yeah, but then he said, uh it call this number. And so we called the number, and uh a guy answered the phone, and he was he had a shop, he owns a shop in Keystone, South Dakota, and uh he said, Yeah, I'll give you permission, but I want to meet you first. So we drove to his shop, shook his hand, talked to him for like 20 minutes, and he gave us permission to enter his mine. And we went in there and got spectacular spodgamine.
SPEAKER_00Yeah, spodgamine. I remember that. There's that's the huge one in the wall. It's spodgmines. Spodgmin is a lithium-bearing mineral, and it's often mined for lithium in some, or it used to be at least. Um, so anyway, beautiful big white crystals.
SPEAKER_01I have a 400-pound chunk of lithium in my in my rock garden in my front yard from that mine.
SPEAKER_00It's amazing. So good. What a great trip. That was a great trip. It was. All right, so so back to pegmatites. Yes, let's go. How do they form? What's unique about them? Like, what's going on, Chris? Why are they prevalent in Yosemite Valley, in the White Mountains? What's going on?
SPEAKER_01Okay. Um, it's a cool story, it's a geological story, it's a complicated story, kind of. Let's try to simplify this, Jesse. Let's try to like make this so that everybody can understand this. Basically, you know, a lot of people look at they think of pegmatites in these extremely large crystals. Anybody that has a little bit of geologic experience understands that rate of cooling determines crystal size. Crystals need time, minerals need time to grow. So you would think, well, pegmatites are extremely huge crystals, must have cooled extremely slowly.
SPEAKER_00Let's hit one thing there. Why is time the important variable? Well, time is the important variable because you need the elements that make up the minerals. The minerals are sitting there and they're taking in the elements that they want into their mineral structure, and that's how they're growing bigger. Those elements need to get to the place where those minerals are forming. So you need to get atoms to these locations. Right. In most cases, time is needed for bigger minerals to grow. The more time you have, the more elements can kind of diffuse into that.
SPEAKER_01Yeah, they're migrating, these ions are migrating through the sticky magma. So the more time they have, the bigger the wall can get as they stack on, and the bigger these minerals can get.
SPEAKER_00That's right. So a lava has small crystals because there's no time, it cools on the surface, it cools really quickly. A granite has bigger crystals because it's cooling at depth in the crust and it's just stays hot for longer. It cools down more slowly.
SPEAKER_01Yeah.
SPEAKER_00Yes. Okay, so back to pegmatites.
SPEAKER_01So, you know, you'd think pegmatites then really, really slow rate of cooling, right? Well, that's not really what's going on. And so what happens with these pegmatites, which is basically the same as granite in terms of like chemical composition, okay? Basically a light-colored igneous rock with very few dark-colored minerals sprinkled in among it. What happens, Jesse, is that most of the magma has already cooled into rock. You have these pockets of water because granitic magma tends to have a fair amount of water in it. Okay. And so this is the last bit to solidify. But what happens is this water, while it's loaded with ions, it's super, super salty.
SPEAKER_00That's right. So salty. Let's you're listening to this picture a fluid. Imagine some fluid that you can think of. You're probably thinking like Gatorade or water or pop or something like that. Uh, and now think of magma. This is a melted rock. This is much stickier, this is really thick. What we're talking about is something that is in between those two. Really, really salty water is kind of in between those two things, those two images that you have in your mind, right? And so it's loaded with ions and it's super saturated in those ions, but it's still very fluidy. It's still more like water than sticky magma.
SPEAKER_01So, Jesse, what temperature are we talking about here? How hot is this water?
SPEAKER_00Well, these things can exist, you know, anywhere from like four or five hundred degrees centigrade down to 200, 250. So at the surface, this would boil off, but they're under pressure. They're in the earth, so they're under pressure, so they can get that hot and still be in a fluid phase.
SPEAKER_01Okay, so why is water important then in terms of how big the minerals get in a pegmatite?
SPEAKER_00Yeah, so water is important, and Chris, you touched on it earlier that in a magma, this thick, sticky, soupy stuff, the atoms have to they take a long time to move from place to place. So think of an atom trying to go one inch in a magma that's taking a long time. In a pegmatite or a pegmatitic fluid, that's super critical fluid, it can go really fast. The fluid is less viscous, it's fluid. So it can diffuse across there really, really quickly and get to where the mineral is forming very rapidly.
SPEAKER_01So it can fly through this really like thin solution of water, the super salty water. And crystals can get really big quite fast, right?
SPEAKER_00That's right.
SPEAKER_01Exactly. Okay. So the minerals get so big because of this, because they're flying through this water solution, it's this late stage cooling. But why is it made up of rare elements then or rare minerals? Why does that happen?
SPEAKER_00Yeah, so they're made up of all the stuff that's left behind after magma cools. And the magma, as it's cooling, the minerals that are forming are taking in the stuff they like. Minerals are made up of atoms, and they're taking in the atoms or the elements that they like that fit well into their structures. But these are common minerals, so they're formed from common elements. The uncommon minerals are formed from uncommon elements, and those are not crystallizing then. So all the uncommon elements, the rare ones, get concentrated in these late-stage fluids, which become pegmatites and then concentrate things like lithium or cesium or beryllium, which then occur in pegmatites, and that's they're kind of concentrated there. So that's where we go to mine those types of things.
SPEAKER_01So a pegmatite then, it's going to be enriched in these elements that are kind of rare. They get excluded because they don't fit into the crystalline structure of common minerals, right?
SPEAKER_00That's right. And they're we are interested in them for economic reasons, in part because they're rare. And uh these are the weird elements that we use in our technology or that don't occur frequently on the earth, and so therefore they're valuable because they're more rare.
SPEAKER_01But I want to point out though, that the most common minerals that are present in a pegmatite, these really coarse grained, are quartz, feldspar, and muscovite, which are three of the most common minerals in the world. Okay, now that's just because those are low temperature, those are the last minerals to crystallize out of a cooling magma. So that hydrothermal, that superheated water at say 500 degrees, is going to be loaded with the elements that form these minerals. And it's also going to be concentrated in rare elements because Jesse, like those rare elements, like you said, lithium a lot. Lithium doesn't fit into the crystalline structure of common minerals. So they get excluded, don't they?
SPEAKER_00That's right. They get kicked out or they sort of just get pushed into the last stuff. They keep getting pushed in and then end up in a pegmatite. And that's where we want them to be because the earth is naturally concentrating these rare, valuable things for us in pegmatites. And so this is where we go to mine these things. It's great. Pegmatites are amazing. And they're beautiful, Chris. They have amazingly cool minerals in them. And you say, oh, Kelp Feldspar and Plagioglace or in quartz or but they grow such massive crystals that they're beautiful. They make there's some cool countertops made, and you and I have shitloads of pegmatite in our yards. I think that we've taken over the years.
SPEAKER_01I have books of muscovite. And for the listener, if you don't know what muscovite is, is it's this translucent, transparent, uh flaky, clear mineral. It's I love it because my my students love it because they love to flake it apart on the lab tables. I have like thing books of muscovite and biotite, which is the dark version of muscovite, it's black, but still translucent, um, that is are bigger than textbooks. I mean, these crystals are enormous. Okay.
SPEAKER_00They used to be used as window panes back in the you know, the prairie farm homes that they were making, put put these as window panes in there.
SPEAKER_01Yeah. But I I gotta bring this up because I couldn't help but listen to you talk about how these rare elements get excluded from the crystalline structure. And I can't help but think pegmatites are a lot like you. Uh okay, all right.
SPEAKER_00Well, so let me walk offended. I'm not offended yet, but I'm probably gonna be able to do that. I'm gonna walk you through this.
SPEAKER_01Like, so they're made up of pegmatites, are these rare elements that get excluded and they concentrate and they concentrate, and then they form, they they eventually get to the point where they can say, you know, we're hey, we're gonna form our own minerals, forget about you. Well, it's kind of like you think about young Jesse first going to elementary school. And young Jesse is well, he's odd, okay, and he doesn't fit. Well, so he's one of these, he's one of these rare elements that doesn't fit. So imagine young Jesse in the cafeteria in first grade and sitting at his own table and nobody else is there because he doesn't fit. Just the it's the hard reality of growing up. And then, but you know, so Jesse goes through. Hold on, hold on. So Jesse goes through Jesse goes through, and you know, a couple years later, it's a good thing. So hold on.
SPEAKER_00I have to say, you're just asking me to just sit here and take this abuse for like your five-minute random.
SPEAKER_01No, but it works, it works, no, it works. Okay, so a couple years go by, and a new kid moves into the school, and guess what? He doesn't fit either. He's an odd duck, doesn't fit. And guess where he sits? He sits with you at the table. Okay, everybody else is sitting at their own place, but you now have you've grown, you've doubled your size. You have now you are a pack of two, and then look what happens: you move to middle school, and then all these other little schools combine, and the jessies of the world find each other, okay. And so they all sit at the table, and now like we have like what 10 or 12 Jessies sitting at a table, okay. Then high school happens, and you see where I'm going to this, right? Like, eventually you get to the point in your life where you're able to see. You think I'm ripping on you? Listen to how this is.
SPEAKER_00I'm trying to decide if I'm offended or not right now. So you I'm sitting here thinking.
SPEAKER_01No, I want you to listen to this because eventually, you guys, you're concentrated enough where you can say to the rest of everybody, screw you, I'm gonna form my own group. And what is that group? Well, it's extremely large, rare mineral crystals, and so on. That's what a pegmatite is. So you thought that this whole time I'm ripping on you, but you actually formed your own really super cool group of people because you got to a number where you're able to say, you know what, forget you, forget everybody else. Here we go.
SPEAKER_00Well, all right. I must say I started out with my hackles raised, but that was a pretty good analogy. Uh I can neither confirm or deny my popularity in elementary, middle, or high school. Uh, but I like where the story ended, so let's leave it at that. It ends as you as a pegmatite. That's quite a good that's quite a good place to be, I think, at the end. No, that was a good analogy. Perfect. I mean, yeah, a great example of of how these things in aggregate all of a sudden can form a mineral, and it happens really quickly and it grows really big, really rapidly. It's sort of a social media, it becomes viral in a way. These elements become viral and boom, there they are.
SPEAKER_01So they they just hang out because they they get excluded, they don't fit until they're able to say basically, you know what, we have enough now to form our own rare minerals.
SPEAKER_00Yeah, we're gonna make a band. We're gonna be we're gonna make a cool rock band, you know. That's what a pegmatite is. That was a good one, Chris. Uh, I'll give you I'll give you credit for that. That's a really good one. I like that analogy. That works, it works for pegmatites. And uh, you know, next time you're hiking around and there's some granite around, look for pegmatites because they're probably around. They're they're pretty common. The ones that are really concentrated in valuable elements are are a little bit less common, but pegmatites are uh are many places.
SPEAKER_01Yeah, they're hard to mistake, you know. Like when you're looking around and you see these rocks that have crystals that are just enormous in it, you're looking at a pegmatite.
SPEAKER_00There you go. I love it. All right, good one, Chris. I love this geo short. Pegmatites, I love pegmatites. We gotta go collect some more sometime. Let's go. When are we gonna go? Let's let's go.
SPEAKER_01Uh we'll find this part next week.
SPEAKER_00So well, all right. Maybe next summer we can collect some more.
SPEAKER_01Yeah, yeah.
SPEAKER_00All right, good stuff, Chris. And as usual, you can find us on all the social medias. We're at Planet GeoCast and uh send us an email. Planetgeocast at gmail.com. We love that stuff. Hit us up if you like Planet Geo. Give us a subscribe, leave us a review. We really like that. Uh, and uh and send us your questions.
SPEAKER_01Yeah, absolutely.
SPEAKER_00See you all next week. All right, peace.