Biotite is a mineral that can be a characteristic rock-former (igneous rocks), it is present in medium-grade metamorphic rocks and it also occurs as a mineral of hydrothermal alteration in some mineral deposits (porphyry copper, in potassic alteration).
Biotite in thin section or under the microscope
The optical properties of biotite in the petrographic microscope indicate the following:
- In natural light it presents a characteristic brown or yellow-brown color
- The very marked pleochroism is the optical property under the microscope that helps to identify this mineral.
- Relief: it is moderate
- Exfoliation: Yes it presents and it is excellent
- Birefringence and Interference Colors: Moderate to high, second-order red colors
- Extinction: It is oblique to nearly parallel
- Optics: Biaxic (-)
- Others: Sometimes it presents inclusions of zircon with dark halos. It also usually presents bird’s eye extension.
The most effective way to identify biotite under the microscope is that when you rotate the stage, you will observe well-marked pleochroism, that is, it darkens and lightens very quickly when you rotate the stage.
Mica Biotite and Muscovite
In metamorphic rocks with a medium degree of metamorphism, it is very common for biotite to appear together with muscovite, both minerals make up the foliation of the rock and appear elongated.
To differentiate them, biotite is easier to see in natural light where its pleochroism is the one that plays an important role.
On the other hand, muscovite is colorless in natural light and in polarized light it is characterized by its second-order interference colors from yellow to red.
Characteristics and physical properties of biotite
| Biotite | Characteristics and physical properties |
|---|---|
| Class | silicates |
| subclass | phyllosilicates |
| Cluster | micas |
| Origin and geological environment | Potassic core of copper porphyries and mesothermal environments, mineral that forms igneous rocks, medium grade metamorphic rocks. |
| associated rocks | Granodiorite, granite, monzonite, rhyolite, dacite. Phyllites, quartzites, schists and gneiss. |
| Associated minerals | Phlogopite, K-feldspar (orthoclase), magnetite, quartz, anhydrite, albite-sodium plagioclase, actinolite, rutile, apatite, sericite, chlorite, epidote |
| Formula and chemical composition | K(Mg,Fe)3(AlSi3O10)(F,OH)2 |
| Color | Black, dark green, dark coffee |
| Brilliance, luster or diaphanity | glassy to submetallic |
| Stripe | White, light gray and colorless |
| cleavage | Perfect |
| Fracture | Irregular |
| Tenacity | Flexible |
| Way of presenting or habit | Sheet form, micaceous, leaves, flexible plates |
| Mohs hardness | 2 to 3.5 |
| Density or specific weight | 2.7 to 3.4 |
| crystalline system | monoclinic |
| Applications | Few uses in the industry, indicator of potassic alteration, if it is secondary biotite. |
The chemical formula of biotite is K(Mg,Fe)3(AlSi3O10)(F,OH)2
It is important to mention that the biotite belongs to the silicates, specifically to the phyllosilicates and to the group of micas.
This mineral is recognized mainly by its low hardness (2 to 3.5), its vitreous to submetallic luster, and its perfect cleavage.
In addition, it can be said that the color of the stripe is “colorless, white or light gray” and its specific gravity varies between 2.7 to 3.4.
Geological environment of biotite mica
The biotite is a good indicator of potassic alteration in mineralized cores in porphyry-copper deposits, so its mapping and prospecting is of great importance.
The alteration of the biotite it is usually among the first hydrothermal events in the formation of a porphyry mineral deposit and may precede or accompany the main stage alteration and mineralization.
It is also present in mesothermal geological settings, in biotite-altered bedrocks surrounding veins and mineralized shear zones.
Furthermore, it is a rock-forming mineral that occurs mainly in igneous rocks like granite, granodiorite, rhyolite, dacite, etc.
In metamorphic rocks the biotite, is an indicator mineral of medium degree of metamorphism in metamorphic facies of green schists and amphibolites, in rocks such as phyllites, schists, gneiss and quartzites.
How to identify biotite?
In general the biotite it is easy to recognize with practice because it is so common in nature.
Take the magnifying glass and see if it occurs as superimposed euhedral sheets or as patches replacing other minerals.
Then look at the color, it is characteristically black, but may have a dark greenish cast if it is altering to chlorite, or a dark reddish brown to gold cast if it is biotite secondary.
With the steel scribe, use it to see if their streak is colorless, whitish or light gray and to measure their low straightness (2 to 3.5), also use your fingers to see if little flakes stick to them when you run them over them.
Primary biotite
When the biotite occurs in intrusive igneous rocks, its identification is usually easy because it occurs well formed.
Its black color is characteristic, however, it may have a dark greenish hue because it may be altering to chlorite.
It occurs as small overlapping sheets that can have euhedral shapes up to fractured crystals.
Secondary or alteration biotite
When the biotite it is part of hydrothermal alteration systems, it can present black, dark green and dark brown to golden colors, but it is common that it does not appear completely black.
It differs from the biotite primary because they occur as impregnations or patches in the rocks and when observed with a magnifying glass they usually replace other minerals.
Metamorphic biotite
This can be considered biotite as a medium-grade metamorphic rock-forming mineral typically present in phyllites, schists, gneiss, and sometimes quartzite.
It is characterized because it is elongated in accordance with the schistosity of the rock.
In other words, it can form the metamorphic texture known as nematoblastic.
Importance of biotite in mineral exploration
The biotite secondary is a good indicator of potassic alteration, and potassic alteration is usually a good exploration target to find minerals that are related to copper and porphyry copper deposits.
So its mapping is very important in the search for minerals related to copper such as chalcopyrite, bornite, chalcocite, covelina.
Therefore, it should be mentioned that the biotite It is not a mineral that is sought to carry out any exploitation that generates economic profits, rather it is a guide to find deposits of great importance, be it mesothermal veins or copper porphyries.
Biotite veinlets
As mentioned, biotite is important in the exploration of porphyry copper, and in fact it has an important section in the classification of porphyry veinlets, where biotite is considered as part of the early veinlets [biotite-magnetite (EB) veinlets], which are also related to potassic alteration. and the main mineralization event of the system.
Biotite Mica Uses
- It has a small number of commercial uses.
- Ground mica is used as a filler and extender in paints.
- It also has use as an additive for drilling muds.
- It has been used as an inert filler and mold release agent in rubber products.
- It is sometimes used as a non-stick surface coating on asphalt shingles and rolled roofs.
- Additionally, it is used in the potassium-argon and argon-argon methods for dating igneous rocks.