Here explains the three main classifications of rock where you can connect actual crystals you have seen before to the jargon-y nomenclature. Even if you consider yourself knowledgeable about crystals, you may have never heard of terms like “plagioclase” and “feldspar” – even though these are some of the most common constituents of rocks where crystals are mined.
I started this series on types of crystals when I first learned about magnetic remanence in rocks. Just when I think I have a solid enough background to read primary literature, I am humbled yet again. When magnetics questions led me to the field of geology, words like “serpentization” “amphiboles,” and “plutonic rock” and me going: “WTF are they talking about”.
The Bane of an Armchair Geologist: Nomenclature
One of the biggest problems for someone trying to find out how crystals are classified are all the giant charts of odd words. I have made several smaller charts so you can take in the categorizations in bite-sized chunks. You’ll also learn classifications that connect the molecular world of crystals to the macroscopic crystal shop names and images.
So after reading this, you will have relevant exposure to the commonly referenced crystal classifications used in more advanced books and repositories of information of crystals. You could also use this for an additional resource in a geology or Earth sciences class. Here’s all geology groups and classifications mean so you can pick out the info that’s relevant to your interests.
Types of Crystals Chart: The Categories
Different fields of science deal with different aspects of crystals, so there’s many overlapping ways to classify them. In geology, crystals are classified by how they form and physical characteristics. In solid-state physics, crystals are classified by their atomic structure and geometry. And in chemistry, crystals are classified by the chemical elements and types of bonds. And of course, in the shops, crystals can be classified simply by how they look or where they were mined. (The physics classification of crystals is the most specific and complex. That’s a separate post.)
As an example of how these categorizations are not super neat and tidy –
..Take a class of crystals called amphiboles. All amphiboles are silicates (but not all silicates are amphiboles), and amphiboles can be either igneous or metamorphic in origin. (Amphiboles come up more later on.)
And these classifications are not inclusive! Some crystals belong to more than one of the types listed. The charts are also not exhaustive – I haven’t listed every classification that geologists use, it depends on their niche! The purpose of the types of crystals chart is to give you a broad overview and understanding you can trust without any inaccuracies. Here are the three types:
Some sedimentary rocks are formed by the sediment of existing igneous or metamorphic rock. And igneous rock gets “metamorphized” all the time! Scientists just do their best to make classifications and nomenclature so it’s easier to communicate with other scientists. Here’s more info about the classifications in simple words so that you can have more context when you encounter science about crystals.
Geology Classification Types of Crystals Chart
One of the most basic ways to sort crystals by geology is by how they formed –
- igneous (from magma)
- metamorphic (under heat & pressure)
- sedimentary (from pre-existing stuff)
All the material on Earth is recycled through these rock types (unless there are asteroids involved)
Igneous Rocks
Igneous rock forms from cooled, solidified magma. Magma is lava before it reaches Earth’s surface, the molten material from the Earth’s crust or mantle. In magma, there are gas bubbles and crystals in the thick fluid. When magma cools and hardens it “freezes” into solid rock with these bubbles and crystals which characterize the igneous rock.
Classifications of Igneous Rocks
Intrusive igneous rocks cool inside the Earth.
Extrusive igneous rocks cool outside the Earth.
Igneous rocks are rich in silica (silicon dioxide), the main component in beach sand and glass. Igneous rocks can be classified based on their silica content.
| Classification of Igneous Rock | Percent Content of Silica |
| Felsic | greater than 65% |
| Intermediate | 55% – 65% |
| Mafic | 45% – 55% |
| Ultramafic | less than 45% |
The word mafic comes from combining “magnesium” and “ferric” (iron), which are the next two most common constituents of igneous rock. Besides those, other common elements of igneous rock include aluminum, calcium, sodium, and potassium. Generally, the less silica the igneous rock has, the more viscous (easily flowing) the magma is as well.
Plutonic rock is an important class of igneous rock. Plutonic rock forms from magma that has slow-cooled under the surface, an intrusive igneous rock. Slow-cooled rocks have larger crystals than the fast-cooled extrusive igneous rocks.
Labradorite is a proper physics crystal, with a regular repeated structure throughout their bulk. On the other hand, granite is a conglomerate of many types of proper crystals on a small scale, like mica, feldspar, and quartz. Those three are all general groups, present in all three types of rocks, and are all types of silicates. Obsidian too is amorphous in molecular structure. (Learn more about silicate minerals in general.)
Examples of Igneous Rocks
Igneous rock examples include:
- Obsidian (Felsic, extrustive)
- Labradorite (Mafic, extrisuve)
- Basalt (Mafic, extrusive)
- Gabbro (Felsic, Plutonic, intrusive)
- Granite (Felsic, Plutonic, intrusive)
- Olivine group (Mafic, intrusive)
- Pyroxenes group (Mafic, intrusive)
- Amphiboles group (Mafic, intrusive)
- Biotite group (Mafic, intrusive)
- Diorite (Intermediate, intrusive)
- Andesite (Intermediate, extrusive)
- Rhyolite (Felsic, extrusive)
- Nephelinite (Ultramafic, extrusive)
Note that many of the groups, like pyroxenes, amphiboles, and olivine, as well as quartz, represent minerals commonly found inside of other rocks, be them igneous or sometimes metamorphic. For example, basalt and gabbro have some ratio of pyroxene and plagioclase (another common group in both igneous and metamorphic rock.) Granite contains portions of quartz, biotite, and amphibole, among others.
Remember the amphiboles which span multiple groups? Ilmenite is another rock in both igneous and metamorphic rock. It’s a common “accessory” mineral, meaning it comes along for the ride in bigger samples. Ilmenite is an important ore of titanium and titanium dioxide for industrial purposes. Weakly paramagnetic, ilmenite can have stronger magnetic properties whenever it is found alongside hematite or magnetite. (They are friends.)
Metamorphic Rocks
Metamorphic rock forms by heat and pressure. How much? From 150° C some rocks can start to change and the processes continue up to even 700° C and 1000 bar of pressure. For reference, the pressure we normally experience is about 1 bar. [1]
While transforming, the rock doesn’t actually melt into fluid state, like magma, which has flow. It actually remains in the solid state while the subtle changes happen in the solid rock matrix. Hot fluids can also percolate through pores in rock and exchange out material.
Metamorphic rocks have series of progressions depending on the extent of heat and pressure. The mother rock, called the protolith, can be any one of igneous, metamorphic, or sedimentary. Prefixes for metamorphic rocks are sometimes added to specify the protolith, for example para- for sedimentary, such as paraschist, or ortho- for igneous, such as orthogneiss. Metamorphic rocks can be classified based on the protolith, foliation, or by carbonate content. Metamorphic rock that has less carbonate tend to have more of the sparkly silicate mica and are classified thus by mica content instead. (Mica is used to make glitter and sparkles in make up and craft supplies.)
Metamorphic rock examples include:
- slate (general term foliated and easily splits)
- marble (a granofel, general term for metamorphic rock lacking foliation)
- quartzite (low mica sandstone high in quartz)
- gneiss (general group, finer grained foliated)
- schist (general group, medium grained foliated)
- eclogite (metamorphed igneous mafic basalt rock)
- ilmenite
- pelite (high mica)
- amphibolite (metamorphized igneous mafic or ultramafic basalt)
- zeolites, which chare structural characteristics with shungite (also metamoprphic)
- shungite
Pathways of Metamorphic Rock Transformation
The protolith – the mother rock from which it has formed – has different concentrations of different sized particles. They are frozen in the rock’s structure until the metamorphosis processes allow for migration, and it would energetically rather be density sorted. Also during the metamorphosis process some compounds could nucleate into larger sizes. This aggregation is expected for many heated rocks.
Accessory Minerals in Metamorphic Rocks
Some minerals are only stable and specific temperatures and pressures. These substances in metamorphic rock, known as “index” minerals, can tell researchers about the rock’s historical record. There are also plenty of minerals present in metamorphic rock that are actually igneous. They came along for the ride and didn’t change during the metamorphic processes. [2]
Important accessory minerals (found in all 3 rock types, but along for the ride):
- Olivines
- Pyroxenes
- Hornblende
- Micas
- Feldspars
- Quartz
- Magnetite
- Hematite
- Biotite
- Pyrite
- Chlorite
In my original research these are also all the families that kept coming up over and over. You can bet there will be more in depth articles on them since understanding these classes contribute to the overall understanding of rocks as natural computers. (Yeah, I said it. Think I’m crazy? Hematite was one of the first recording devices, and computer chips are literally etched silicon dioxide with mineral impurities for desired magnetic effects, but don’t let me digress just yet.)
Many metamorphic rocks, much as slate, are a natural pantry for important accessory minerals, including magnetite, hematite, feldspar, mica, pyrite, chlorite, and biotite. Slate may look dull, but in its structure is a ton of variety. Slate started as shale (a sedimentary rock, also called mudrock). The “parent rock,” shale, is a mixture of clay and grains of quartz.
Foliation in Metamorphic rocks
Foliation of metamorphic rock can tell researchers information about the directions and degrees of pressure the rock experienced in its history, giving information about conditions of pre-historic Earth.
Some metamorphic rocks can have foliated features representative of the pressure’s directionality. Foliation in metamorphic rock is due either to pressures from different sources pushing in different directions. It is a record of the formation processes to an extent. Also associated with platy-ness like phyllite and mica, where on the microscale the structure is like scales or leaves.
Igneous rocks can also become foliated under certain circumstances.
One way igneous rocks become foliated is when there are gaps similar to window shutters (called laths) in igneous rock such as plagioclase. Plagioclase is a silicate in the feldspar group, it has a high silica content and cools inside the magma chamber, so it is an ultramafic intrusive igneous rock, but is found in plenty of metamorphic rocks too. As plagioclase scrapes on the chamber’s walls, fractional crystallization occurs, that is, the crystals harden at different sizes. In granite, friction on viscous magma can also make foliations by its “dragging.” Magma that is flowing as it’s cooling makes these layers as it spreads out. Crystals that are forming inside magmas can also make bands colloidal density sorting based on the size of suspended particles.
The tiny clay minerals in shale recrystallize under pressure into micas, which are shiny spots seen with the naked eye. The results in schist, the intermediate transformation of shale into gneiss.
In case that really wasn’t enough ways to categorize metamorphic rock, we also have names for how the rock formed. A number of natural processes give rise to high temperatures and pressure, and can lead to different results.
- Contact metamorphism (thermal, low pressure, usually magma against rock)
- Dynamic metamorphism (cataclastic, along pre-existing fractures in Earth)
- Hydrothermal metamorphism (in mid-ocean ridges in veins that go into the Earth)
- Impact metamorphism (from asteroids)
- Burial metamorphism (during mountain formation when plates jam together)
Sedimentary Rocks
Sedimentary rocks, just like the name implies, are compacted and cemented sediments, whether dead organisms or plain minerals that have been exposed on Earth’s crust. The inorganic (not once living) minerals originate from weathering and erosion of rock by water and wind. What we commonly experience as mud and clay are types of wet, granular sedimentary rocks. Sedimentary rock makes up the majority (73%) of exposed land surface on Earth, while metamorphic and igneous dominate in the crust itself.
Classifications of Sedimentary Rocks
Sedimentary does have categories based on the “host” rock, or the origin of the material. Sedimentary rocks can be classified as:
| Classification by origin | Origin of Rock | Examples |
| Clastic (clay-like) | cemented together pieces of existing rock, categorized by particle size | gravel (conglomerates, breccias) , sand (sandstone), mud (mudrock): silt, clay, shale |
| Biochemical | compacted mollusks, dead plant skeletons, radiolaria, diatoms, etc | limestone, coal, chert |
| Chemical | minerals in a liquid solution become supersaturated in the solution and precipitate | halite, gypsum, baryte, sylvite |
| Unclassified types | from asteroid impacts, volcanic eruptions, and other natural impact events that don’t fit in the other categories | volcanic tuff and breccias |
Examples of Sedimentary Rocks
Common sedimentary Rock examples are:
- coal
- limestone
- shale
- sandstone (low mica)
- dolomite
- halite (NaCl, table salt)
- aragonite
- opal (there is also a sort of “volcanic opal” in igneous rock)
There are more examples of sedimentary rock in the origin classification table above.
Sedimentary rocks can also be grouped by their constituent compounds. Those classifications are based on percent by weight of common minerals that make up the sedimentary rock, such as: silicate, silica, iron, phosphate, carbonate, chloride, or organic material. Evaporites are the class of sedimentary rocks formed from evaporation of water, which usually leaves behind a lot of halite salt. While there’s a pretty broad range, I think of carbonate as similar to eggshell and silicate as similar to glass. As we learn more this background will give you a good idea of the origin of any kind of crystal at all!
Examples of the 3 Types of Crystals Chart
Here is a recap of the main differences of types of crystals based on their geology terms.
There is so Much More to Learn
It may have been interesting that you really can’t class many of the rocks just by looking at them. There are a few distinctive rocks, for example garnet, which is found sometimes crystallized on the surface of metamorphic rock like gneiss, and you know it when you see it. But even for geologists to find the difference in calcite and dolomite they have to do certain tests (like etching and staining) even beyond using a microscope.
I am filling out a lot of baseline information about crystals distilled down to where it can be casually understood. So subscribe and you can know about the rest of the posts, about the geometry of lattices, different packing types, and how this relates to some of the coolest and mysterious properties we’re still uncovering, like the magnetic history of Earth and how our bodies communicate with the inert substances.
In my opinion this is just the language and nomenclature that forms the background of the rocks and when we learn the physics side of it that’s the true character specifically of each.
Learn more:
What makes a crystal a crystal?
Types of Crystals in the Human Body
Magnetite – the original natural magnetic crystal
What Crystals conduct electricity?
Magnetic Crystals in your brain
Some sources but there are more that I may have consulted:
[1] Yardley, Bruce, and Clare Warren. An introduction to metamorphic petrology. Cambridge University Press, 2021. https://archive.org/details/introductiontome0000yard/page/4/mode/2up
[2] Klein, Cornelis; Hurlbut, Cornelius S. Jr. (1993). Manual of mineralogy : (after James D. Dana) (21st ed.). New York: Wiley. pp. 449, 480, 483, 497, 516, 518, 529, 539, 543. ISBN 047157452X.
Encyclopedia Britannica, Felsic Rock
Classifications of Metamorphic Rocks (and associated chapters of this book). https://openoregon.pressbooks.pub/earthscience/chapter/6-2-classification-of-metamorphic-rocks-2/
