Hornblende is a rock-forming mineral that belongs to the group of silicates, specifically amphiboles. It can be found in mafic, intermediate, and acid igneous rocks, and in metamorphic rocks, however, it is not an efficient indicator of metamorphism.
Hornblende in thin sheet
The otic properties of hornblende indicate that:
- In natural light it appears brown to green.
- Presents moderate pleochrism
- Relief: It is moderate to high
- Exfoliation: If present and excellent (124° and 56° angles)
- Birefringence and interference colors: It is moderate to high and second order medium colors
- extinction: it is oblique
- Optics: Biaxic (-)
- Others: It occurs as prismatic crystals with a euhedral basal section, frequently presenting a simple twin.
In thin sheets seen under a microscope, hornblende is commonly identified by its cleavage or cleavage that forms angles of 124° and 56°.
Characteristics and physical properties of the mineral
| Hornblende | Characteristics and physical properties |
| Class | silicates |
| subclass | Inosilicates |
| Cluster | amphiboles |
| Origin and geological environment | Mainly intermediate composition magmas. |
| associated rocks | Igneous rocks (andesite, diorite, granodiorite, tonalite, dacite, rhyolite, granite) and metamorphic rocks (amphibolite, schist, gneiss) |
| Associated minerals | Pyroxene (augite), labradorite, bitownite, tourmaline, biotite. |
| Formula and chemical composition | Ca2(Fe2,Mg)4(Al,Fe3)(Si7Al)O22(OH,F)2 |
| Color | Black, dark green, dark coffee |
| Brightness | Vitreous |
| Stripe | White, light gray and colorless |
| cleavage | Perfect in two directions (form angles of 54° and 124°) |
| Fracture | patchy, brittle |
| Habit | Elongate |
| Mohs hardness | 5 to 6 |
| Density or specific weight | 3.1 to 3.3 gr/cm3 |
| Crystalline system | monoclinic |
| Applications | Identify plutonism temperatures, few industrial uses. |
The hornblende It is a mineral that is considered as ferromagnesian, that is to say that it is enriched in iron, magnesium, calcium, sodium and aluminum mainly.
Being a ferromagnesian mineral, it occurs in shades of black, dark green, and dark brown.
In general, the hornblende It has a vitreous luster, hardness from 5 to 6, the color of its stripe is white, light gray to colorless and its specific gravity varies between 2.9 to 3.5 depending on the type of hornblende and its composition.
However, a very effective way to identify the hornblende it is because its cleavage occurs forming angles of 56° and 124°.
Geological environment and formation of hornblende
The formation of the hornblende It can be better understood using the Bowen reaction series of the discontinuous series.
It is so the hornblende It would form after the olivines and pyroxenes, where it would be more enriched in silica (SiO2) and would have an intermediate composition of sodium and calcium.
Its formation temperature would be between 950°C to 850° approximately.
It is a common mineral in igneous rocks of intermediate composition (andesite, granodiorite, diorite, tonalite) and can appear as a main or accessory mineral in acid (granite, rhyolite, dacite) and basic (basalt, gabbro) igneous rocks.
Furthermore, it is an important component in some metamorphic rocks (gneiss and schist), but it is not an effective indicator of some type of metamorphism.
In amphibolites, it may be a major constituent.
How to identify hornblende?
The hornblende It can be confused on certain occasions with other ferromagnesian minerals such as pyroxenes and also with the associated tourmaline.
However, it must be taken into account that cleavage in hornblendes forms angles of 54° and 124°, unlike pyroxenes which form angles of approximately 90°.
In addition, hornblendes usually present with more elongated or prismatic crystalline habits, that is when it is confused with black tourmaline.
But using a magnifying glass, preferably 20 magnification, you can observe the cleavage and you will see the difference with the tourmaline that does not present this type of cleavage.
the color of the hornblende it is generally black, but sometimes it can appear a little dark green because it may be altering to chlorite, and dark brown when enriched with iron.
Remember that the hardness is from 5 to 6 and the color of the scratch is white to light gray, for that use a steel scratch.
Hornblende Uses
- The hornblende It is useful to identify the formation temperature of some plutonic intrusions, when the hornblende It has a low aluminum content and is associated with shallow crystallization depths, while a higher aluminum content is associated with greater crystallization depths.
- It has no specific uses in industry and is not attractive as a gem or precious stone.
- However, the hornblende It is the most abundant mineral in a rock known as amphibolite which has a myriad of uses.
- It is crushed and used for road construction and as road ballast.
- The highest quality pieces are cut, polished, and sold under the name “black granite” for use as building cladding, floor tiles, countertops, and other architectural uses.
Exploration and exploitation
The hornblende It is a mineral that does not have important uses in the industry, nor can it be considered as a gem mineral, so it is not sought for economic mining.
However, sometimes it serves to identify the different types of existing igneous rocks according to their abundance and taking into account the Bowen series and associated minerals.
Thus, its abundance mapping and identification can help determine the depth of crystallization of some intrusive and extrusive igneous rocks.
For example, when the hornblende having low aluminum content is associated with shallow depths of crystallization, while those with higher aluminum content are associated with greater depths of crystallization.
This is useful to be able to identify different exploration targets related to plutonism, for example, porphyry copper and epithermal.