Intro To Metamorphic Rock Identification


  • 10-Apr-2022

So your first part of today's lab is to identify metamorphic rocks and metamorphic rocks are in my opinion, the easiest of the rock types to identify there are three textures. But one is sort of an exception to the rule type texture. And the other two are largely visible. There are going to be three sets of doubles in your set, which means there'll be three, oh there'll be like a couple quartzites. There might be a couple nice I'm, not telling you which three there are. But there are three doubles in. Your set, but I think you'll be able to find it, pretty readily there's, only one air that people sometimes make that's pretty consistent.

So the two main textures are called foliated and non-foliated texture. And your textbook does a good job describing this. The third texture, though they don't, and that is called porphyroblastic texture, I'm going to zoom here on the sample. So this sample is super crystal and super coarse grained. But then the thing you notice about the sample is this big old. Garnet and it's just floating there in the center. So remember in igneous rocks, we had porphyritic texture, which is two grain sizes in metamorphic rocks.

We have porphyroblastic texture, which is again, two grain sizes so that's, because you have this big old garnet floating in this otherwise, uh, sort of foliated rock. So if you have, if you see a big old chunk of garnet in a metamorphic rock that is called porphyroblastic texture, and it's called porphyroblastic nice. And your search is over, and you are. Done and that's it, so I'm going to set that off to the side. So the other two textures again, are foliated and non-foliated foliated means that there are visible layering in an in a metamorphic rock.

So right away that means this. So this is called banding it's really, really extreme layering where you can actually see segregation of light and dark-colored minerals. So let's set that to the side there you can see layering here. So this is foliation of clay, rich minerals and a little of Muscovite.

You see how shiny that one is, um, that's because there's thin little sheets of Muscovite that are making it reflective. So that one is going to go here. Uh, let's. See, look at this one, you see layers here on the side. And you see how sparkly this is. This is because there's a lot of biotite and Muscovite in this sample biotite and Muscovite are called the mica as you recall from minerals.

So, um. So this one with the layering here also foliated, this guy has thin little layers, but still layers foliated. This. Guy thin little layers, foliated. And these guys have no visible layers. So the ones that have no visible layers are your non-foliated rocks. Now, fun times there's only two types of non-foliated rocks, which tells you that there have to be a couple doubles here.

There is marble. And there is quartzite. So marble is composed of the mineral calcite. So that means marble is going to react to acid right away, quartzite won't because it's composed of the mineral quartz, doesn't react to acid, and it's. Harder than glass. So honestly you can use that to figure these out real quick.

So when you're looking at foliated, metamorphic rocks, you're, really going to be looking at grain size to identify them, and then some really obvious characteristics. So you're going to line them up by fine-grained to medium to coarse grain. And then what's cool about metal. So remember that metamorphic rocks form from a pre-existing rock, getting squished or heated up. And these foliated rocks form I'm giving you all the. Answers these form from being squished for the most part.

So what's cool. I really like metamorphic rocks, I think they're wildly understandable is that if you took a shale, which is a sedimentary rock like that guy. So you take a shell, and you increase the temperature and pressure, it turns into a slate and then a slate if you increase it. So the shale is the parent rock for the slate it's like can be a bunch of colors, because shale can be a bunch of colors.

You can have gray shale. Green. Shale black gel red shale you can, I've also seen like blue shale ones. But that was crazy. There was like hydrothermal events, and they were bringing weird minerals up. And I turned the shell blue, but it was super cool. Okay, sorry.

That was an aside. So you increase the temperature and pressure. And you start to see these super flat surfaces appear in the slate. And then, as you increase the temperature and pressure enough, those some of these clay minerals, they're, not stable anymore. So they start to. Metamorphose and they recrystallize, and they recrystallize into Muscovite so that's, when you start to see the slate get a little shinier and that's called finite.

And if you jack up the temperature and pressure on finite, it gets even shinier because Muscovite and then is still stable and then biotite becomes stable. And then you get these super sparkly rocks. And you start to see a little of that color segregation. But then when you really jack up the temperature and pressure, um, you start. To have the sudden muscovite and biotite are no longer stable, and they start to form into these discrete bands and that's called nice. So here, what you see is the parent rock of all the foliated rocks can be slate, I'm, sorry, life can be shale. And then shale can be progressively metamorphosed into any of these rocks, depending on the amount of temperature and pressure that is administered.

If you have lots of garnet moving through the rock. This porphyroblastic nice is actually what happens when you. Have pleasant, and then you have garnets that kind of form in the middle of during metamorphism, this garnet material migrates up through groundwater and turns into these giant crystals.

So these, um, all of these rocks here can have a parent rock of shale isn't that fun. So the parent rock for foliated rocks is going to be dependent on the name of the rock here that you identify. So if it reacts to acid, then, you know, it is a.

It is a marble. And then the parent rock of marble is limestone right from a. Remember limestone also reacts to acid, the parent rock of quartzite is quartz sandstone. And in your next activity, we'll learn a little about how you tell the difference between quartzite and quartz sandstone either way metamorphic rocks are a good time.

Don't, forget, three sets of doubles, check your answers and good luck. Okay. Last rock chart to explain metamorphic rocks are divided into three primary textures foliated, which means that there is visible alignment of minerals non-foliated, which. Means massive or non or no evidence of alignment of minerals and then porphyoblastic texture, which occurs when you have very large minerals, sort of stuck in a existing rock. You can see that each of these textures is then organized by grain size.

So if you have a fine-grained foliated rock, you are not going to be able to see individual grains, but you will in medium to coarse grain. Foliated rocks compositions are fairly simplistic. And you can see that those are hear hear. And here for the. Porphyoblastic texture, what you're really looking for is the presence of garnet for geography 101 will always be this very deep, red color that you're used to seeing from your previous labs.

There are some comments here also to kind of that further describe the rocks when you're looking for slate, which is this first rock. What you'll see is very, very fine, layering and that's because you're looking at the metamorphism of shale. Shale is the parent rock of slate. And while shale is formed at the. Bottom of the sea and might be slightly wavy those waves get compressed out during the metamorphism process.

And it is very very very flat surfaces when the temperature and pressure is turned up a little on slate. It will metamorphose into finite when it starts to turn into violet. You will start to see a polished sheen to it.

And that is due to the presence of muscovite is that very clear mineral that cleaves into sheets. And it will show itself in finite in some shininess of the. Surface that will differentiate it from slate.

When you metamorphose folate a little more, it will turn into schist shifts will have very large, visible mineral flakes, typically, biotite and Muscovite. So it's, very shiny. And then if you metamorphose just a little more, it will turn into nice. Nice is a considered a high-grade metamorphic rock. Meaning that if you increase the temperature and pressure on it too much more, it would melt and therefore become an igneous rock and pleasant you're going. To see visible bands of light and dark minerals. And so here in the section on parent rocks, you can start with shale and turn shale into any one of these foliated rocks.

So for that reason, in your chart, I have you choose shale as the is the parent rock for all the foliated rocks, but just know that this actually illustrates increasing metamorphism. So shale will metamorphose to slate, which under more temperature and pressure will turn into finite, which will then turn into schist alternatively. You.

Can have granite that also gets metamorphosed into pleasant. Those are your foliated metamorphic rocks. Your non-foliated metamorphic rocks are marble and quartzite.

Marble is composed of calcite, so it's going to react to acid right away that's because its parent rock is limestone and limestone is also composed of calcite. Conversely, quartzite is composed of quartz and its parent rock is called sandstone. It will not react to acid, and it will be harder than glass because quartz is harder than glass. And. Then your third texture is porphyroblastic. You only have one sample name porphyoblastic, pleasant.

It will have two distinct grain size grain sizes. The porphyroblasts will be garnet they're going to be very large. And basically you can have shale as the parent rock. There shale is the parent rock for all of these rocks as you recall or granite, granite can also be a parent rock for porphyroblastic pleasant. I want to remind you that there are three sets of doubles in this lab. So when you look at these names.

Three of these names will appear twice just to keep in mind. And this isn't, this isn't rocket science, this should feel reasonably, um understandable, and hopefully won't be too painful. All right.

Good luck.

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