In this article, you’ll get a list of some commonly known brittle rocks, as well as information about brittle rocks to better understand them.
Brittle Rocks Examples (Characteristics and Types)
When it comes to brittle rocks, what is really being discussed is the hardness or plasticity of the material and how it fails when stressed in any way.
This is known as fracture strain.
What makes one rock more brittle than another?
Brittle rocks are described as any material that fails when stressed with little deformation before doing so.
Brittle rocks do not take impact or vibration of any kind very well either.
Think of it like a dead tree branch vs. a freshly broken off tree branch.
The dead branch will break under little pressure when bent and the freshly broken branch will tend to bend much more without breaking or even cracking.
This is why brittle rocks and materials can suddenly fail without any prior warning that they will do so.
Other things that can lead to structural failure include abrupt temperature changes and the load rate of other materials on top of the brittle rock..
On the other hand, stronger materials tend to be ductile and will take much more deformation before fracturing.
They can also withstand significantly more extreme temperature changes, impact pressure and vibration.
What are some types of brittle rocks?
There are a wide variety of rocks and minerals that are considered to be brittle for many different reasons, but for the sake of this discussion, let’s keep the focus on some naturally occurring examples that fit the primary description above.
Previously thought to be the same thing as slate and schist, it is now known that slate and schist are later stages of metamorphic change that shale deposits have undergone, changing their composition slightly each time.
The stages are thought to be shale to slate, to phyllite, schist and finally gneiss.
One of, if not the most common, sedimentary rocks found on earth, is shale.
It makes up as much as seventy percent of the material found in our planet’s crust.
Shale is composed of a set of fine grain materials that include clay, mica, quartz and a slew of other minerals and organic debris.
The process through which shale forms requires slow moving water which allows bits of heavy material to settle on the bottom of the water source, forming sandstone or limestone sheets.
After the initial bed of sandstone or limestone has formed, silt will collect on its surface forming various layers.
Time and compression turns this material into what is known as shale.
Shale can be found near any slow moving water source and ancient streams, swamps or lake beds that have long since dried up.
You can also find shale in abundance on the floor of the ocean.
It can be found in numerous colors, which are influenced by the materials captured in its layers.
Colors include various shades of gray and black most commonly.
You may also find brown, green, red, yellow and, more uncommonly, blue and purple shale.
Due to the way shale forms, it is not uncommon to find a variety of fossils and animal tracks captured in its layers.
How brittle shale is largely depends on the mixture of minerals and other materials trapped in its layers during formation, which all play a role in the final material’s ability to withstand impact and vibration.
What always stays true is the way shale splits apart into layers.
Shale is primarily used as a material for making ceramic goods such as flooring tiles, building materials like brick, and in pottery. It can also be used for making cement.
Feldspar is a formation of hard minerals that is used as a benchmark on the Mohs scale, sitting at a 6, one point below quartz, which is 7.
The minerals in feldspar are quite abundant in the Earth’s crust, and feldspar can include quartz particles.
To the untrained rock hound, these two can and often are confused for one or the other.
The main difference that can settle the question is how the materials break due to their different hardness.
Quartz will break into various shapes whereas feldspar will break cleanly along flat faces.
There are a few other ways to tell the difference between the feldspar and quartz. One is to turn the sample under direct light.
Quartz will glitter and feldspar will give brief flashes of reflected light. Another is visual inspection.
Feldspar tends to be cloudy and quartz is typically clear.
Lastly, the structure of the materials is a distinct way of identifying them under magnification.
Feldspar has a blocky structure to its crystals and the quartz crystals are hexagonal (six-sided).
Feldspar is typically white but can be transparent or have orange or red tones to them and they will almost always have a glassy look to them.
Commercial uses for feldspar include glass and ceramic making and as a filler material in paints, plastics, rubbers and adhesives.
Limestone forms in shallow marine water areas.
Limestone that forms this way is considered a biological sedimentary rock that is made from a combination of calcium, calcite and a mixture of organic debris including shell fragments, faeces, algae and more.
Organic limestone deposits often contain fossils of marine life common to the area where it formed at the time.
Another way limestone forms is through the evaporation of calcium carbonate-rich water that seeps through cracks or pores in the ceiling of a cave where it either evaporates before falling or drips onto the floor of the cave, then evaporates
. As the water evaporates, the minerals that were in the water are deposited on the surface, building up slowly over time. This process forms stalactites and stalagmites.
While limestone rocks are made of calcium carbonate (50% or more), they always contain a small percentage of other material, including quartz particles and clay.
There are a number of different types of limestone, all named differently depending on how they were formed.
Types of limestone include chalk, coquina, crystalline limestone, dolomitic limestone, fossiliferous, lithographic, oolitic, travertine and tufa.
Most gemstones are also considered to be brittle because they are prone to scratching or flaking rather than fracturing.
This is why you can use a rub test when identifying different types of gemstones when out rock hounding, as they will often leave a powdery residue with an identifiable color behind on a porous surface like the back of an unused tile.
There are exceptions to the rules, of course. Some gemstones, like diamond, are brittle materials but are still extremely durable due to their ability to resist fracturing.
Once again, the science behind measuring hardness is merely a material’s ability to withstand scratching, nothing less and nothing more.