Schist is a frequent victim of geology puns, but if we take it seriously, we can appreciate this rock.
It has been a part of the human story as our historical buildings and a host of gemstones. Out in the landscape, it’s literally a builder of mountains.
Schist is a metamorphic rock of medium grain and breaks into distinct, flat layers. It’s defined by that texture and can consist of many minerals like mica and quartz.
Schist forms in regional subduction zones that build mountains. People use schist for countertops and rock walls.
Here is a guide to help you grasp what schist is, how it forms, where to find it, what it’s used for, and its types.
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What Schist Is
Geologists call rocks schists if the rocks:
- Are metamorphic
- Have medium-sized grains visible with a loupe or hand lens
- Flakes into layers with ease when struck
Many minerals like feldspar, graphite, and micas create the texture. Scientists call this schistosity. The grains fit at a unified angle that forms the easy-to-split layers.
Compare this to the other two textures of metamorphic rock. Gneiss can break into layers, but they are thick. And granofels don’t flake into sheets at all.
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History of Schist
The word “schist” means “to cleave” from the Greek “schizein,” or “σχίζειν.”
On its journey, it went from Greek to the Latin “lapis schistos.” Then it became the French “schiste” before transitioning to the English word we use today. The name refers to the ease with which the rock splits into thin plates.
In mining and construction before the 19th century, terms were vague. Workers often used the words schist, shale, and slate as the same.
How Schist Forms
Schist forms by metamorphosis. Usually, it metamorphoses from sedimentary rocks like shale. But especially schistose mica can form from phyllite. Other times it forms from other igneous rocks. These include granite from lava filling holes in the landscape after an eruption.
Schist is a metamorphic rock. With enough heat and pressure, the parent igneous rock may transform into schist. In other words, it metamorphoses.
The heat and pressure often come from the earth’s crust rubbing against separate plates. Scientists call these subduction zones, and they can uplift one plate into mountains.
The rock develops into different grades. Geologists base grades on how much heat and pressure the rock formed under. Plates change positions, like getting uplifting or turning towards the mantle. Then the temperature changes at a slow rate, and pressure changes at a fast one.
The higher the heat, the less water it has, and the more of the parent rock gets melted. This changes how much gets recrystallized and how coarse the texture crystallizes.
The directional pressure of mountain formation creates the schist’s layers. The more direction under more pressure, the more distinct and strong the layers form.
Where You Can Find Schist
You’ll find most schist along mountain ranges and bedrock. Mountains form by the same processes as the schist during the Precambrian.
This applies best to ones long eroded away, exposing the oldest rocks. Many of these lies inside the borders of modern New York, Portugal, and Scotland.
Schists form deep in the crust in regional subduction zones. But they occur throughout the world. It’s just that exposed bedrock is where you can find them on the surface.
How To Identify Schist
As a type of rock, the schist’s definition revolves around its texture. It always has smooth, flat layers and medium-grain, and formation from metamorphoses. Besides this, you can identify the rock by its specific grain size, hardness, and color.
If you want to get technical about it, schist grain sizes lie between 0.25-2 mm (0.01-0.08 in). The layers flake off at 5-10 mm (0.2-0.4 in) in thickness. You can see this for yourself with a 10x handheld loupe or lens that geology enthusiasts often have.
Geologists categorize schist as hard and 4-5 on the Moh Hardness Scale. Though, as mentioned before, the grains form as plates that will break in one direction. So it’s important to know in which direction the rock’s strength lies.
Schist often forms with light and dark gray bands depending on the mineral content. It can also have solid colors, like green. When geologists define a rock by its texture and formation process, its colors range a lot. The mica grains also support shininess rather than glittering.
Geologists refer to schist as a medium-grade rock. This means it forms in middling pressures and temperatures compared to other rocks.
Difference Between Schist and Gneiss
Schist and gneiss share medium-grain size, a metamorphic process, and foliate. But gneiss has larger crystals and more distinctive color bands. If so much gets melted during metamorphosis, the schist will lose schistosity and be a gneiss.
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Minerals and Gems You Can Find in Schist
Schists come with many minerals and a few gems. Most minerals consist of the mica group. The group has members like biotite and chlorite, quartz, and plagioclase.
As for gems, schists will often also have chunks of crystallized minerals. Geologists call these lumps porphyroblasts. They are larger than the standard grain size of the rock. Among these minerals are the precious gemstones garnet, emerald, chrysoberyl, and tourmaline.
Schists sometimes host veins of hydrothermally altered varieties or quartz that contain gold. But pure schists will not.
What is Schist Used For?
Engineers and construction specialists use schist with high mica concentrations.
Micas make schists a stronger, gneiss-like rock as opposed to a flaky, slate-like rock. Mica-dense schist bedrock supports tall buildings. This is especially true with smaller mica grains.
Larger grains cause weaker stability in the schist, which makes construction tricky. They also don’t polish as clean as granite. So countertop manufacturers also prefer small-grained mica schist. Otherwise, the mica grains stick out and are a soft spot in the slab.
Retailers often don’t label the rock as “schist.” They get included among the slates.
This is because they share the ability to break into graceful layers. You can also use them the same way, regardless of the rock’s name. Other times they get labeled as granite. This happens when the seller isn’t emphasizing the flat surface aspect of the schist.
Historical buildings and artistic constructions may have schist in their walls. Some jewelry, roofing materials, and paint fillers use schist.
Types of Schist
Schists can go by many names. If it has a predominant mineral, the schist may have a related name, like quartz schist.
If scientists know the rock the schist was before it metamorphosed, they use a naming formula. They will refer to it as “schistose” and then the previous rock or protolith name.
They also have terms for if the schist was sedimentary or igneous before metamorphic. Geologists call the former a paraschist and the latter an orthoschist.
Blue and Green Schist
Two other categories of schist are greenschist and blueschist. The minerals that form into schist depend on different pressures and temperatures. Green schists, or chlorite, develop at high pressure and high temperature. Blue schists form at high pressures but at low temperatures.
During a schist’s journey in the subduction zone, conditions change. Pressure and temperature shift at different rates.
What becomes a blueschist forms faster. This happens because the pressure changes fast, but the temperature has yet to change. It goes down and comes back up before getting too hot. Green schists need more time to reach the needed higher temperature.
Over time, blue schist has become more prevalent relative to how much the earth used to generate. But the greenschist has been consistent. Geologists have not answered why this has happened.
Schists By Their Minerals
Sedimentary parent rocks of schist can be limestones, marbles, shales, and dolomites. Less often, they’ll come from salts and gypsum.
From these parent rocks, you’ll see schistose silicates. These minerals include mica, graphite, quartz, and feldspar, among many others.
These are the most common schist minerals. The most crystallized turn to hornblende. Graphites like ironstones contain the most plant material and resemble coal.
Igneous parent rocks of schist can be olivine. Rarely it’ll develop near gneissose granites, and those will influence the schist. From these parent rocks, you’ll see schistose calc, serpentines, and tuffs.
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What does schist turn into?
As a metamorphic rock, schist’s minerals can erode away and become sediment. Under enough pressure, these may return to sedimentary rock.
Schist can also get pushed back under a subduction zone. It can get deep enough to melt and cool into a new igneous rock. The kind would depend on how melted it got and what type of pressure.
What does schist do to wine?
Schist has an acidic pH, and grapes grown in such soils often pass along the acidity. But wine enthusiasts tend to think of this change with a fresh flavor rather than any problems.