Bornite (Cu5FeS4) is a mineral that belongs to the group of sulfides (iron and copper sulfide), it is characterized because it appears in a color similar to the chest of a pigeon and its origin is mainly high-temperature hydrothermal associated with epithermal deposits of high sulphidation, porphyry copper and skarns.
Characteristics and physical properties
| Bornite | Characteristics and physical properties |
|---|---|
| class, group | Sulfides (copper and iron sulphide). |
| Chemical formula | Cu5FeS4. |
| Chemical composition | Cu (63.3%), Fe (11.2%), S (25.5%). |
| Mohs hardness | 3 |
| Density or specific weight | 5.06 to 5.08 g/cm3. |
| Brightness | Metal. |
| Color | Naturally it is brown bronze, and exposed in the air it is marbled (dove breast). |
| Stripe | Greyish black. |
| cleavage | Poorly developed. |
| Fracture | irregular, conchoidal |
| Tenacity | Fragile. |
| crystalline system | Orthorhombic. |
| Way of presenting or habit | Compact, granular and massive. |
| Origin and geological environment | High temperature hydrothermal (350°C to 500°C) associated with igneous intrusions. |
| associated rocks | Mainly igneous rocks and pegmatites. |
| Associated minerals | Chalcopyrite, pyrite, cassiterite, wolframite, quartz, monazite, calcite. |
| Disturbance | chalcozene, covelina |
| Applications | It is an important ore of copper. |
The bornita It is a mineral that belongs to the iron and copper sulfides, therefore it contains in its chemical composition Cu (63.3%), Fe (11.2%), S (25.5%), although it may contain small fragments of other minerals that make change its composition.
When found in its natural initial geologic environment, this mineral occurs as a coppery red, coppery brown, or bronze color, however, upon exposure to the environment and oxygen, it turns purple, mottled, or iridescent colors similar to the breast of a pigeon..
Regarding other physical properties, the bornita It has a hardness of 3, its density is approximately 5.06 g/cm3, its luster is metallic, the color of the streak is grayish black, it has a poorly developed cleavage, an uneven or conchoidal fracture, and it occurs mainly massive or granular.
Finally, it can be noted that this mineral is very easily altered to chalcosine and covelin when it is exposed to the environment, that is, in areas of supergene enrichment.
Origin, formation and geological environment.
In general the bornita It originates in a high-temperature hydrothermal environment (350°C to 500°C), and it can occur filling spaces in veins and veinlets or disseminated in various types of rocks, mainly igneous, that is, it is characteristic of hypogene deposits and is less It is often found as a mineral from a supergenic environment.
Bornite of hydrothermal origin.
It has been mentioned that this mineral occurs at high temperatures (350°C to 500°C), and is associated with intermediate and high sulphidation epithermals, copper porphyries and some replacement deposits.
When it occurs in copper porphyries, it is very close to the mineralization core and to the porphyry (subvolcanic rock that carries hydrothermal fluids) that generates the mineralization, that is, in the potassium alteration of the deposit.
Here it can occur in veinlets of porphyry and box and disseminated rock.
In skarns, they generally occur disseminated and associated with carbonate rocks and forming Cu and Cu-Au deposits.
Whereas, in epithermals of intermediate and high sulphidation, it occurs mainly massive to disseminated.
Bornite in a supergenic environment
It is rare to find this mineral in a supergenic environment, since when exposed to environmental conditions it changes very easily to chalcocite and covelline.
As a result, the copper found in the bornita it migrates in solutions and forms chalcocite, covelline and can also be associated with malachite, azurite, chrysocolla and cuprite.
How to identify the bornite?
Although the natural color of the bornita It is a coppery red or coppery brown, almost always occurring in its iridescent form or better known as aploma breast, where it presents purple, greenish, mottled, and reddish colors in combination in a massive or scattered mass.
However, it can be confused if you don’t have experience with covelline, chalcocite and sometimes chalcopyrite.
Below we present a diagram that serves to differentiate bornite from other similar metallic minerals.
Difference between bornite and coveline
One of the minerals that can be confused with bornitas it is with the covelina, especially when the latter appears massive in the rock and with bluish and even sometimes iridescent colors.
However, you must take into account that the coveline is a characteristic indigo blue color, it has a lower hardness than the bornitas (between 1.5 to 2), it is usually earthy, and is characterized by staining the hands with a black or dark lead gray tint.
Exploration of the bornite.
The bornita It is a mineral that is generally sought to exploit copper due to its high content of this element in its composition (64.3% copper), which is why it is considered to be an important copper ore.
For this reason, when the initial stages of geological prospecting have been completed, which include: geological mapping, collection of fluvial sediment samples and collection of rock samples in outcrops, then in situ exploration methods are carried out.
For this, it is recommended to apply geophysical methods that help to generate an idea of how the subsoil is in terms of sulfide concentration.
These geophysical methods include induced polarization (IP) and spontaneous potential, which is characterized by responding to the concentration of sulfides in the subsurface.
Applications
- Copper can be extracted from this mineral, which is why it is considered a copper ore, however, it is less important than chalcopyrite.
- For geologists it can serve as an indicator mineral of high temperature hydrothermalism (350°C to 500°C).
- It also serves as a collector’s mineral due to its striking colors.