Chris Bolhuis: like, this is, oh my gosh, it, started a few days ago and it just [00:01:00] crescendoed to last night. It was at its full force. Like we are dragging this guy out. Okay.

Dr. Jesse Reimink: we have, we have weekend at Bernie's, Chris, we have rolled him out,

propped him up and he's here for recording.

Chris Bolhuis: I almost cried. I'm like, Oh my gosh, I am in so much pain. Just where says Jenny. and I'm like all over the place.

My skin hurts. My like, everything just hurts. It was not a good day. So

I [00:01:30] apologize to everybody right now for,

my radio voice is not up to par today. I,

Dr. Jesse Reimink: We've got a

Chris, but you're going, you're going to step it up and bring the energy because we're talking about cool stuff. But before we get

to that, I've been thinking about you because, well, we got a new house over here in Pennsylvania and, uh, I have been doing some power washing.

Cause we have a little concrete, you know, walkway. It was certainly we've been powerwashing the house is old and it needed some, some cleaning up and I was just thinking, I love this stuff and I'll bet we could put together [00:02:00] a little Chris's, digging company combined with Jesse's powerwashing.

This would be very fun.

Chris Bolhuis: I could be a good assistant with power washing. I love power. it's so

satisfying. especially when you, when you power wash your rocks, it's almost like, you know, it got scrubbed clean by a glacier and then just dropped off. And that's what the power washer essentially does.

We're resetting the radiometric clock when we power

Dr. Jesse Reimink: it's true. that's amazing. And that's what I've been mostly powerwashing is we have these like stone [00:02:30] walls around the house it looks like a moss black rock thing and you powerwash it and the grout stands out the reds and the yellows and the textures in the sand, they're all kind of locally derived sandstones.

It's so fun. So

Chris Bolhuis: So. What kind of power washer do you have? Is it a little electric thing? Is it a gas powered thing? Oh, Jesse.

Dr. Jesse Reimink: the little electric one. Cause you know, it's enough They make them so well these days and you don't have to deal with, I mean, it's like, I forget what it is, 2, 300 [00:03:00] PSI or something like that. I mean, it's, uh, it does enough for what I need to do for the most

Chris Bolhuis: It'll peel the paint off a car.

Dr. Jesse Reimink: Yeah.

That's right. And you know,

Chris Bolhuis: I

found that out the hard way once.

Dr. Jesse Reimink: I don't want to be doing that. So, you know, it's good. I don't have a 4, 000. PSI, you know, thing. That'd be, I just, I'd break stuff with that. So anyway, yeah. Uh, okay. You can be my assistant. I'll assist with Chris's tractor company. And, um,

Chris Bolhuis: Yeah. I'll give you the shovel. You do all the hard work and then I just [00:03:30] come along with the tractor and

Dr. Jesse Reimink: perfect. There you go. Perfect. Love it. And I can

do the same with the power washer. You

Chris Bolhuis: I'll be your,

power washing sidekick. I

Dr. Jesse Reimink: you can kind of, you know, scrub it a little bit and then I'll come by power washer, take all the glory. Sounds great. Okay, cool. Love it.

Chris Bolhuis: a little bit. Okay. All right. I don't know about that.

Dr. Jesse Reimink: Give you a little wire brush, take the stuff off. That'd be good. So if you need some rocks cleaned, Chris and Jesse will come. We'll bring the power washer. We'll, uh, we'll clean up your rocks for you.

Chris Bolhuis: if you have rocks at your home or rocks at your place, and you don't have a [00:04:00] power washer, That's a problem that needs to get

fixed.

gotta bring the

Dr. Jesse Reimink: That's true.

Especially Chris, if you have rocks with garnet eyes in them, because that's our topic today, what's your experience with these things? I mean, this was kind of, this was your idea. Garnet eyes. Like, hey, this is a cool thing, cool phenomenon. Let's talk about it.

Chris Bolhuis: Yeah. I think these are amazing. Uh, you know, when you go for a hike in death Canyon up in the Tetons and you just come across these outcrops and you can't. [00:04:30] Miss it.

I don't care how old you are, what your level of geoscience expertise is. You look at this and you're like, Oh my gosh, these, it looks like thousands upon thousands of eyeballs just staring right at you and this beautiful dark colored rock.

It's amazing. And, and then you and I have experiences in upstate New York with the Gore Mountain

Garnets. And, uh, it's, it's amazing.

Dr. Jesse Reimink: That was, I think this is the, the origin of the idea. It was, I forget what we were talking about. Something, and I [00:05:00] mentioned Gore Mountain, and you're like, oh. We got to do an episode on the Garnet Eyes in Gore Mountain. I mean, they are spectacular. So what we're describing here, what we're talking about is, we're going to talk about a large class of features that are, I don't know.

They're called reaction rims. They're called depletion halos. There's, they're called all sorts

of, they're

Chris Bolhuis: two different things, by the way. Yes.

Dr. Jesse Reimink: of, different things, but the point is, We see a bunch of these types of things.

They're basically a certain mineral. They're usually found in [00:05:30] metamorphic rocks. One mineral that is rimmed by another one, and the one you're referring to is a garnet eye, what are called garnet eyes, because there's a garnet and then it's rimmed by ple place. Right. It's like a, a really

white ple place.

Is that

Chris Bolhuis: A really white that's in an overall very dark colored schistos rock.

Dr. Jesse Reimink: I think Chris, maybe we're going to talk about the broad category of stuff, but kind of centered around sort of garnet eyes phenomenon. But I just want to get a couple definitions here. Yeah, sorry. go

Chris Bolhuis: Well, hold on before we do that, Jesse, like let's just, we always [00:06:00] try to center things around, well, why is this important? why do people need to know about, you know, garnet eyes. and here's our take. We were just talking about this and it comes down to really two things.

One is they're just really cool. I mean,

enough said, you know, really are. You need to go get yourself some garnet eyes. Okay.

Like you need to do that if you're into rock collecting and things like that. But the other thing is though, from a, A scientific standpoint, they're really important [00:06:30] because, this kind of metamorphism that we're talking about, it tells us a history of the rock.

It gives us a, you know, this kind of geochemistry slash history. What did this rock get subjected to?

Dr. Jesse Reimink: And there are great depths that we could go to in the weeds about various chemical reactions going on, but it all comes down to the sort of chemical reactions that are occurring in the rock. And a couple of quick definitions kind of set the stage here. First of all, we're talking about [00:07:00] minerals that are metamorphic and they're usually, Chris, in the example you gave, like the garnets, they're much bigger than the Other minerals in the rock, like the background of minerals so much in the rock.

And we call those porphroblasts or porphyroblasts, depending on which side of the Atlantic you're on. But these

Chris Bolhuis: Oh my gosh. are,

you serious? You're really g No.

No,

Dr. Jesse Reimink: I'd get a reaction out

of that

Chris Bolhuis: serious? What did you

Dr. Jesse Reimink: pofiroblasts, if you're if

you're in the [00:07:30] UK, if you're in the English

system, maybe. Porphroblasts.

Chris Bolhuis: sorry.

It's

Dr. Jesse Reimink: we'll stick with porphroblasts. Um, these, these are metamorphic minerals that are growing as the rock is heated up, heated up and put under pressure.

These garnets in this instance are growing and they're actively growing or they're actively breaking down, but they are metamorphic mineral. They're not there in the rock initially.

Chris Bolhuis: The inside of my head this weekend became a gigantic porphyroblast. I mean, it just, it just depleted everything else around.

Dr. Jesse Reimink: you got metamorphosed this weekend, [00:08:00] huh?

Chris Bolhuis: I did. I tortured. I tortured. We've always talked about metamorphic, rocks as being tortured rocks. And my head was enlarged

and, and, tortured.

Dr. Jesse Reimink: Yeah.

Chris Bolhuis: It was.

Dr. Jesse Reimink: So these things, Chris, the ones we saw at the Gore Mountain mine, I mean, I These are unbelievably big. These things are, Softball or volleyball sized garnets. I mean, they are enormous and beautiful and they often have [00:08:30] thick rims of amphibolite around them.

So black amphibolite or biotite around like rimming them. And, that's a different mineral, right? Like you just described a black rock that has in the Tetons that has garnet rimmed by pelagiclase. And I just described one in upstate New York that has garnet rimmed by amphibolite.

Two very different. reactions that are going on here in two different categories of these reaction rims or depletion halos or garnet eyes there are a bunch of different categories of this so

the history [00:09:00] is kind of the important part and there's different ways to form this but they're all about metamorphism and mineral reactions occurring.

Chris Bolhuis: and sometimes, again, dependent upon your previous exposure to this kind of stuff, you might see, or have this referred to as Augean gneiss,

something like that.

Dr. Jesse Reimink: yes, that's right. I want to come back to that, Chris, at the end, I want to kind of touch on this again, because the augen or augen, depending on how you want to pronounce it, that word is. The German word for eye. So I've seen up [00:09:30] in the Northwest Territories, a lot of the Gneisses that I walk across are actually, we would call them Augen Gneisses, because they have these little eyes in them, and they can be really small eyes, they can be bigger eyes, but they have this kind of eye shaped feature to them, which means that they've been metamorphosed.

Chris, do the Tetons ones, the, the Garnet eyes, or the Plagioclase rims. Perfectly circular, or are they a little bit like eye

shaped and squished out

on the edges?

Chris Bolhuis: yeah. They're a little bit, they're eyeball shape. So they're, you know, there's been some shearing

[00:10:00] involved in this as well. Not just heat, not just pressure, but the same kind of forces that cause foliation in metamorphic rocks

was

a directional pressure. Yeah.

Dr. Jesse Reimink: And this is a kind of a key point that you should think about if you're listening to this. Like, okay, the garnets are often very circular. Almost always circular. At least if it's a single garnet grain, and then the plagioclase is kind of the eye shape, the white of your eyes around it, there's a reason they're called it.

It's kind of like a pupil, circular pupil with a, with an eye shape around it.

Chris Bolhuis: so Jesse, sorry to interrupt you, but, [00:10:30] but what you're saying is, is that In this instance, the white around the garnets is deformed, but the Garnet is not.

And so, just think about that a second, what does that mean? why would certain minerals be deformed and look like they've been stretched and tortured and then the, in the center of it, untouched?

So Jesse.

Dr. Jesse Reimink: It's, it's a good question. I

want to give the listener time to think about that. Garnet is one of the few minerals that will [00:11:00] not deform. It is just such a solid crystal structure that it's really, really nearly impossible for Garnet to actually deform under these conditions. These types of conditions.

So the plagioclase is being actively deformed and growing and getting squished and the garnet is not getting squished. It remains this kind of softball or this round ball shape. So that's a really important reason. And part of the reason, Chris, I think they look so beautiful. Like if the entire grain, if all the grains were squished, I don't know, it wouldn't be as striking to me, that eye [00:11:30] shape.

It's really, yeah. around people with a, a, a sort of a curved and

squished out eye shape to it.

Chris Bolhuis: that's, right. So the Tetons have the, definitely the eyeball shaped you can see the shearing that happened in this, this kind of like directional force, but the gore mountain garnets, Jesse, they did not.

That's

just a, you have black rim around these beautiful garnets the size of a, of a baby's head, like they're just

Dr. Jesse Reimink: it's amazing. Just I love the way you [00:12:00] phrased the two differences there and highlighted the two differences, because this is going to give us a really good way to talk about the different ways to form these reaction rims or reaction halos, or these eyes there's two fundamentally different ways.

Which one should we talk about first?

Chris Bolhuis: well, I think, let me take one that makes like the most sense to me. And I'll leave you with the other one because, you know, they're, they're very close, but they're distinct processes that are going on. They're very [00:12:30] different. and so we just need to tease that out a little bit. So like you said, there are two ways or two categories of these metamorphic eyes, or these porphyroblast eyes that you get.

The first one is, where you have minerals, new minerals, that grow in the rock. and basically, what are you doing to the rock? You're, you're cranking up the temperature. You're cranking up the pressure and that can destabilize the minerals that are already in the country rock, the [00:13:00] minerals that were already there.

And so if the temperatures and pressures are right, let's say for instance, to form garnets, then the garnets are just going to bleed those elements. Out of the host minerals, And they're going to grow right inside of the rock. And these are, porphyroblasts that you get here are just some of my favorite things in the world.

I mean, I have this, this garnet schist from Wrangell, Alaska, where these, Garnets are, are, they're just [00:13:30] enormous, but they grew right, literally inside of the rock. And you can clearly see that because, the minerals are kind of like stretched and elongated around the mineral.

So basically what's happening is these minerals are just taking what they want because those minerals are stable under new conditions, the

metamorphic conditions.

Dr. Jesse Reimink: And these, these reactions get really, really complicated and there's many different forms of the reaction depending upon the composition of the rock. And we've kind of [00:14:00] talked about the complexity here when you start thinking about the chemistry of a rock, but the Gore Mountain garnets that we talked about, in the simplest sense, that's the version of this number one, the garnet was growing.

And one overly simplistic way to think about it is Garnet and Plagioclase are kind of two ends of the reaction. So Garnet is consuming Plagioclase, depleting the rock around the Garnet that's growing from Plagioclase because it's kind of, Soaking up the plagioclase is breaking down. It's diffusing towards the garnet and it's forming garnet.

That's an [00:14:30] oversimplification, but we can think of it that way. Like it's a depletion halo around the garnet as it's growing. It's consuming all the other stuff around there and the other stuff's reacting out because the garnet's growing and taking it all in. So I like the way you phrased that. It just takes what it wants.

The garnet's taking what it wants and leaving behind what it doesn't want

Chris Bolhuis: So basically you're getting new minerals on both sides. You're getting new minerals that are the porphyroblast, the things that we're focusing on. But also the minerals that formed the ring around it. In the case of the Gore Mountain Garnet, these very dark [00:15:00] rims around it.

Those are new minerals too.

They didn't exist in the rock before the metamorphism happened.

Dr. Jesse Reimink: So you get this reaction rim. That's kind of, you're seeing what the garnet wants, what the garnet doesn't want, I guess. or you're not seeing what the garnet wants and you are seeing what the garnet didn't want. if you can put it that way. So that's one category. And that's, I think, I think you're right.

That's a simpler one to kind of understand. these are what we call depletion halos. The new minerals are growing, depleting the rocks around them. The other one, number two, the different way, and it's, it's a nuanced [00:15:30] difference That's really, you know, Very similar process, but there's a kind of a big difference is that the other way is more of a one to one reaction.

You have reactants on one side and a product on the other. So you're taking, classic example that I've seen is as garnet breaks down, it breaks down into chloride and quartz. And so what I've seen in these old metamorphose rocks that had garnet in them is as that rock That rock was metamorphosed into the Garnet stability field sometime a long time ago, and then at [00:16:00] a later time, it was metamorphosed again, but at lower temperature, and the Garnet then breaks down.

And then the product of that is Chlorite and quartz. And so you get this beautiful chloride and quartz rim around a garnet. And you can see the garnets kind of being eaten up. it is the one that's being eaten up. So it's garnet going to chlorine quartz.

It's just a one to one reaction. And these get massively complicated. Like there's so many different

ways to form these one to one reactions. I

Chris Bolhuis: So question for you that I'm going to, like, I think of this, I want you to [00:16:30] see if this is a proper way of thinking about it. I think

of the process you're describing is, an inside out kind of reaction where you have the garnets that were formed earlier and then it's subjected to new conditions, new temperatures, new pressures, and now they're going to bleed out into the surrounding minerals and form and quartz in this case.

Right? So the process I described is an outside in process, and this is an [00:17:00] inside out process. Is that

Dr. Jesse Reimink: that.

Yeah, I think that I think that's right. Um, inside out is a little hard for me to picture in a way, but I kind of think of it as like, okay, you have basketball, brand new basketball, indoor basketball, it's, you know, beautiful leather and you take it and you use it outside for a while, the outside is going to get really sort of scuffed up and beaten up.

And that's, that's Kind of what's happening to the garnet. The garnets breaking down The reaction is kind of progressing inside as the garnet is breaking down or [00:17:30] better. Maybe an ice cube in water is a better example. Put the ice cube in water and it melts from the outside in.

It's

reacting out as it's warming up

Chris Bolhuis: Yeah, but, oh, okay. Let me give it another crack then, okay? We have, in previous episodes, we've spoken a lot about Bowen's Reaction Series. And in Bowen's Reaction Series, as you take these early formed minerals like olivine and calcium rich plagioclase and you allow them to cool down in a melt.

these crystals are suspended in magma, let's say you lower the [00:18:00] temperatures and the olivine and plagioclase will react with the melt and begin to turn into different minerals. Well, you can create the same thing that you're describing. These kind of reaction rims. where just the surface of the mineral is reacting with what's around it, not all the way through,

Dr. Jesse Reimink: That's exactly right. and then it kind of freezes in place. So, the instance of this Garnet goes to Chlorite plus Quartz, you need Garnet plus [00:18:30] water, and then you can react to Chlorite plus Quartz. And so you don't react all the way because you don't have enough water in the rock to do that reaction.

So that's just like one instance of this. So the reaction is kind of frozen partway through. And so Chris, there's tons of examples of this. You can have pyroxene that's rimmed by Garnet and Plagioclase. You can have, Garnet that's rimmed by plagioclase and orthopyroxene. There's a crazy, crazy, really high temperature metamorphic ones where you have like saffirine, which is kind of turquoise, rimmed by orthopyroxene, then [00:19:00] solimonite around it. So you can have these really massively complicated reactions, that you, kind of get preserved in different rings kind of growth rings around this porphroblast.

Chris Bolhuis: if you're listening to this and you want to go down a rabbit hole then Start looking at the chemical formulas for some of these minerals that you just described, right? If you look at these chemical formulas, which are really, really complicated chemical formulas, you can start to see what's happening.

is the [00:19:30] porphyroblast taking what it wants? To form that new mineral and then leaving behind other ones. You can start to piece this together, but

we can't really do that in a podcast because it's, it's just, it's, we need visuals for that. Of

Dr. Jesse Reimink: Chris, that is such a great point. This is an amazing rabbit hole to go down to go on the Google train and start looking at these reactions. It'll teach you a ton about metamorphic petrology and geochemistry. This is, kind of touched on it at the end there. This is [00:20:00] the story that these rocks tell, we can tell so much about what that rock experience based on this reaction rims.

And so the garnet eyes, back to the Tetons, the garnet rimmed by plagioclase. This is an example of the number two version. The garnet is reacting out to form the plagioclase There's debate about how these form and in different rocks, they form different ways. Of course there is, right?

Yeah.

Chris Bolhuis: this.

Dr. Jesse Reimink: different rocks have different histories that end up with the same result. This garnet rimmed by plagiclase. One of the [00:20:30] coolest ones, I think, is when garnet's a higher pressure mineral. So garnet's happy at higher pressure. It's less happy at low pressure. And plagiclase is very happy at low pressure.

And so, in a very oversimplified way to think about it, you take a rock that has garnet in it, the garnet's sitting there very happy, it's high pressure, it's deep in the crust, and you lift that out of the crust. You start to, like, thin the crust or the mountain range kind of collapses out and sloughs out to the side and rocks come up and we have this isostatic rebound.

The rock [00:21:00] gets brought to the surface that Garnet can react to plagioclase and sort of start to react out and you get these plagioclase rims. That's kind of one model, I mean, to look at that Garnet rimmed by plagioclase and say, Oh, I can tell that that rock went from deep to shallow. And that's the result of it.

This garnet as a result of that is so cool. That is just

a really cool

Chris Bolhuis: Jesse. I want to double click on this a minute because, you know, we did a whole audio book and we've done podcast episodes on the Grand [00:21:30] Canyon. so how do we know the depths that the Vishnu Schist was exposed to?

I mean, Jesse, we're talking about Ridiculous.

depths in excess of 12 miles deep. How do we know this? The garnet tells the story, like you said, these garnets form in really, really high pressures. And so it gives us this nice bracket of temperatures and pressures, which equate to depth. so, yeah, That's I just wanted to double click on that.

That this is, this is something that we've talked about [00:22:00] before, just maybe not with this nuance to it.

Dr. Jesse Reimink: no, I agree, Chris. And I think there's one, metamorphic petrology. This is certainly in the firmly in the category of metamorphic petrology. It's such a cool field because You can tell great stories with metamorphic petrology. Like, I mean, rocks moving around in the crust, and getting heated up and cooled down, and put under intense pressure.

I mean, the stories you can tell with metamorphic rocks, it's It's pretty cool.

Chris Bolhuis: You know, Jesse, I was thinking about this actually today is that I think that we have a [00:22:30] clearer understanding of how metamorphic rocks form than we do igneous rocks

intrusive igneous

Dr. Jesse Reimink: I agree in large part because of these reactions. the reactions are so crazy and cool and there's so many different components. Like an igneous rock, there's not that many reactions that happen in it because it's, it's a magma. Like it's, it's not like a sediment.

The sediments get, get wild and crazy. I agree. They tell a more, um, vibrant or colorful story of, How the earth has worked, I think, in metamorphic [00:23:00] rocks. So that, that's certainly true. I have a question for you, Chris, to maybe end

on here. I think, and I struggle with this myself too. So it's not just students, but my perception is that students, when we talk about metamorphic reactions, and we talk about, The story that metamorphic rocks tell.

We often kind of work from low temperature to high temperature. so we go from biotite up to, starlite and then to garnet and we eventually end up at the high temperature stuff and we end up at eclogite and okay. We [00:23:30] often forget that these reactions happen on the way down too. So like the reaction of garnet going to Chlorite plus quartz is what we call a retrograde reaction. a rot going from higher temperature back to lower temperature. And I'm wondering what you think about this. Is this something that people struggle with to even think of as important and if they do, is it a big deal?

Chris Bolhuis: here's my thought on this. Here's my take on it. You know, we have talked about some really important [00:24:00] metamorphic rocks. We've talked about Eclogite and Bluishist and But what gets focused upon in an intro, any intro level geology class, those don't, those don't get mentioned, but those tell us the most information, right?

They tell us what happened. They tell us what was the tectonic setting but instead we focus on slate, phyllite, schist, gneiss, things like that, right? So I do think it's important. I just don't know why I think they're not [00:24:30] given their due.

Why do you think that is?

Dr. Jesse Reimink: well, it's tough. I wish I wish everybody would learn this in intro class, because it's just so cool, and so important, and just shows, I think, how cool. How deep the fields of, petrology are, like how well we can work out the history of a rock. It's really impressive how well we can do that, but I do think it's, pretty deep.

Like you got to have a pretty solid geochemistry, mineralogy background to kind of get to that story. [00:25:00] So there's like a reason that

metamorphic petrology is taught in like third or fourth year of college

because

Chris Bolhuis: I don't know about that though, Jesse. I think, No, I think anything can be simplified. anything can be boiled down to here's what's important about this. This is why this should be covered in an intro level class. you can distill it enough, that were, well, why, why is eclogite an important rock?

Dr. Jesse Reimink: Yeah, that's true. That's

Chris Bolhuis: you know,

and we don't have to get into a lot of the specific, we don't have to do that.

Dr. Jesse Reimink: [00:25:30] You're right. I I, okay, we're doing it this semester. I just started teaching. I just did minerals this week.

We're teaching it. We're getting into garnet eyes by the end of the semester.

Chris Bolhuis: we go. There we go. Absolutely. Yeah.

before we end this, Jesse, I just want to encourage our listeners to Google image some of these things. Look at the, the garnet eyes in the Tetons. Look up the Gore mountain garnets. you know, look up. Augenice or Augenice. look up garnet eyes and, [00:26:00] just allow yourself to go down a little bit of that path.

It's just these are shockingly beautiful rocks.

Dr. Jesse Reimink: beautiful, so beautiful, and tell a very colorful and vibrant story of the rock itself. So, with that, Chris, I think that's a wrap. you want to support us, there's two ways to do that. First, go to our website, planetgeocast. com. There's a support us link there. we just revamped the website a little bit.

It looks quite good. And we've got some more stuff coming to the website pretty soon. So stay tuned to that. You can also download our mobile app. That's the Camp Geo mobile app. First [00:26:30] link in your show notes. There, we have a range of audio books for sale, including a bunch of free content, which is awesome.

Effectively, the intro class that Chris and I teach in audio book, audio textbook form with a whole bunch of custom made images to really drive home the point. Geology is such a visual science. I feel like you really need that. You can follow us on all the social medias. We're at plantgeocast and send us an email plantgeocast at gmail.

com.

Chris Bolhuis: Cheers. [00:27:00]

Jesse Reimink

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