Did you know that diamond is the hardest mineral in the world?. This mineral is so enigmatic and valuable that stories have been created around it, many of them affirm that the diamond originates or comes from the natural metamorphism of coal or by increasing the natural pressure of the graphite geological environment, but is it really true?
Is diamond formed from the metamorphism of coal?
The quick answer to this question is NO! All our lives we have heard that diamonds are formed by the metamorphism of carbon, teachers from school teach it that way, and even on social networks sometimes stories or graphics are shared that affirm this.
But let’s take a closer look at this topic. Coal is a sedimentary rock found in layers of the earth’s crust that generally do not exceed 4 kilometers in depth, these rocks have formed from plant remains in swampy environments that have subsequently been lithic.
Now, if we look at the graph below, we clearly see that the area where the diamonds come from is called the “DIAMOND STABLE ZONE” and lies much deeper than the boundary of the continental and oceanic crust.
We are talking about the fact that the continental crust can have a thickness of 70 km, so based on this, how can the carbon found in the crust be subjected to the great depth and the great pressure that the diamonds to form ? Basically we can’t imagine a geological process of such a magnitude that would do that.
You may be interested in the following articles on diamonds:
- Where to search and find diamonds?
- Types of diamonds that exist
- What are the colors of the diamond and what do they mean?
- What is the largest diamond in the world?
Conditions for forming a diamond
Depth at which a diamond originates
The so-called diamond stable zone lies at a depth of about 150 km below the surface. However, based on the graph, diamonds can also form at the base of subduction zones, this can easily occur at a depth of about 80 km.
temperature to form a diamond
If the following graph is taken into account, the temperature at a depth of about 200 km can easily reach 1050 °C, that would be the ideal temperature for a diamond to form.
pressure to form a diamond
Now, as we will see later, the pressure is actually the most important parameter for a diamond to form, at a depth of 150 to 200 km, the pressure can reach 0.5 million atmospheres.
So how is the diamond formed?
Being clear that diamond needs extreme pressures and temperatures to form that you basically can’t get very close to the surface naturally, we can describe the ways in which diamonds do form.
Diamond that forms in the Earth’s mantle
These diamonds are generated by the action of high temperatures and pressures in the Earth’s mantle, at a depth of approximately 160 km (100 miles). Volcanic explosions begin at the level of the mantle, and as they ascend, they remove pieces of rock that rise to the Earth’s surface without melting.
These rock fragments are known as xenoliths, and they harbor diamonds that were formed in the depths. The zone of the mantle where the diamonds originate from is known as the “STABLE ZONE OF DIAMONDS”.
The carbon that makes up this mineral will come from the natural composition of the earth’s mantle.
In addition, the diamonds being so deep are directly related to the kimberlite rocks, which are the typical source rocks that house the different types of diamonds.
Kimberlite is a rock that belongs to the peridotite group and is composed of at least 35% olivine and significant amounts of other minerals such as phlogopite, pyroxenes, carbonates, serpentine, diopside, and garnets. This rock is important because it has a high probability of harboring diamonds.
From subduction zones
As seen at the beginning of the graph, diamonds can also form in subduction zones to a depth of 80 km.
Here you will ask yourself, and what happens? This area is above the stable area of Diamonds.
In this process, the pressure that exists in the subduction zones is of great importance, which would be the main responsible for creating the diamond.
Here the pressure can easily exceed 0.5 million atmospheres and the temperature can reach 600°C.
The carbon necessary for the diamond to form must probably come from the sedimentary rocks that make up the crust and the subducting oceanic lithosphere, that is, the limestone and marine shales.
Due to meteorite impact
You don’t have to imagine much to understand that the fall and impact of meteorites against the earth’s surface have the capacity to generate great pressures and temperatures at the impact site, which in turn are ideal conditions to form diamonds from the composition of the bark.
In conclusion, the diamonds are not formed by metamorphism of coal, because there are no geological conditions on the surface to subject this rock to sufficient pressure and temperature in which the diamond is stable, the only conditions for a diamond to form would be: the pressure and temperature of the mantle, the pressure of subduction zones, and the pressure and temperature of a meteorite falling on the surface.