In this article, you’ll get to know some commonly known granular rocks, as well as information about granular rocks to better understand them.
Granular Rocks Examples (Characteristics and Types)
What are Granular rocks?
Granular rocks are metamorphic rocks that have interlocking crystals that are predominantly the same mineral.
The crystallization of the mineral occurs when the rock is exposed to high temperatures and extreme pressure.
The stone will have a rough granulated texture as a result of the chemical change that takes place during the metamorphism process.
The higher the temperature and the more pressure that the rock endures, the larger the crystals become and the texture becomes coarser.
This means that gneiss is an example of a granular rock that has been subjected to a much higher degree of metamorphism than other granular rocks like schist.
Schist is an interesting granular rock as it contains large crystals but is known as a fine grained stone.
It is what is known as a porphyroblast, which simply means that its minerals crystalline faster than others.
Another example of a porphyroblast mineral is garnet.
What are Metamorphic rocks?
In simple terms, metamorphic rocks are previously existing materials that have been altered under some form of extreme heat and pressure that caused the existing minerals to break down into various new minerals.
This process is referred to as metamorphism.
There are two types of metamorphisms that bear understanding a little bit when trying to grasp what it means to be a metamorphic rock and what their differences are.
These are regional and contact metamorphism.
Regional metamorphism refers to metamorphic rocks that have been under tectonic force.
Examples of rocks that have gone through this metamorphic process include schist and slate.
Contact metamorphism refers to metamorphic rocks that have been subjected to extreme heat during the metamorphic process.
Examples of Ockham that have gone through this metamorphic process include hornfels.
Why is it important to understand these two processes you might ask?
Metamorphic rocks have what are known as metamorphic facies that are used to help classify them.
This is done by examining the mineral crystals in the rock and identifying the process through which the minerals are recrystallized.
It may be difficult to identify the granular minerals in hand-sized rocks as they are often only present in small amounts, but they are always present in metamorphic rocks to some degree.
Metamorphic rocks play a big part in the makeup of earth’s surface, coming in at roughly 12% of total surface rock.
Studying these rocks yields important information about the levels of heat and pressure that take place deep beneath the surface and can also provide important information regarding plate tectonics and more.
What are some examples of Granular or Metamorphic rocks?
Gneisses are considered to be some of the oldest metamorphic stones on earth and to be fairly common.
These are foliated metamorphic rocks that are identifiable by either the cross-sectioned layers of various minerals, or by elongated minerals that run parallel to the rock’s banding.
Gneiss forms under high heat and pressure and often has large enough mineral grains to be seen by the naked eye.
While the minerals tend to form in cross sectioned layers, gneiss does not tend to break where the layers meet.
Gneisses have quite a transformative journey, starting out as shale, changing into slate, phyllite, schist, then finally becoming gneiss.
As it undergoes this transformative journey, the clay minerals within shale become micas, which become larger crystallized granulated minerals.
It is this final step in the process that signifies the change from the pre-existing material into gneiss.
There are many types of gneiss and they tend to have a name that matches the material that is most visibly present.
Some examples of this are biotite gneiss, granite gneiss, garnet gneiss and albite gneiss.
There are also igneous based gneisses known as orthogneiss and sedimentary gneiss which is known as paragneiss.
Due to its ability to withstand fracturing, gneiss is viable for commercial uses such as landscaping, building roads, and in some cases construction.
Some types of gneiss can be polished into decorative surfaces that are ideal for flooring, counters, sculptures, and facing stones.
Gneiss is sometimes labelled as granite by counter manufacturers and sculpting businesses, much to the chagrin of geologists.
This misclassification is due to the way that these industries have classified granite over the years as any rocks that contain interlocking grains of feldspar.
As with all metamorphic rocks, when limestone undergoes metamorphism, its texture changes.
During the intense metamorphic process of heat and pressure, the calcite crystals that were once tiny become much larger interlocking crystals that are easily visible in hand-sized samples.
It is this recrystallization that defines marble from limestone.
While marble primarily contains these interlocking crystals, it often has trace amounts of other minerals as well, such as quartz, graphite, clay minerals, pyrite, and iron oxide.
Marble that continues to be subjected to metamorphism is thought to be how some gem minerals form, like corundum, which leads to rubies and sapphires.
Marble ranges from white (limestone based) to pink, blue, yellow, grey and black depending on the secondary materials present within the rock, such as iron oxides and clay minerals.
When a marble has a high purity rating and a vibrant white tone, it is extremely valued for various surfacing and sculpting purposes.
High purity marble is mined and ground down to powder in order to remove any impurities and the resulting product is used as pigment or filler in numerous products like whitewash, grouts, paints and plastics.
Marble can be found in vast deposits all over the world, making it economically viable to mine, especially with all of the commercial uses it has.
The primary commercial use for marble by far is as a decorative construction material.
It is also used in road construction, sculpting, counters and other products.
Schists are foliated metamorphic rocks that have flat, round mineral grains that are visible to the naked eye.
It is formed when various types of sedimentary rock undergo a metamorphic process of heat, pressure and chemical activity.
This can be a lengthy process depending on the pre-existing material.
For example, in order for shale to become schist, it goes through stages of transformation first into slate, followed by phyllite, then into schist.
While schist is a granular rock itself, if schist is subjected to continued metamorphism, it can turn into gneiss.
To be classified as a schist, a rock does not need any specific mineral compositions, it just has to have enough flat, round metamorphic minerals to show distinct foliation which means that the schist can be split along the mineral grains.
There are a number of different schists and they are all named after the metamorphic minerals that are visible when inspected with the naked eye.
Some of these include biotite schist, garnet schist, graphite schist and muscovite schist.
Schists do not have much commercial use outside of being used as filler material due to its lower hardness rating.
That said, schists frequently contain various gemstone quality minerals that become trapped within during the metamorphic process, such as emeralds, garnets, sapphires, tanzanite, chrysoberyl and rubies.