Contrary to what is believed, limonite is not clearly considered a mineral, rather it is considered a mineraloid, that is to say, it does not have an ordered crystalline structure as minerals do, in addition, it is considered to be amorphous and have a variable chemical composition related to iron oxides.
However, geologists often refer to the lemonite like those materials that come from iron oxide minerals such as goethite and hematite, but further investigations reveal that this mineraloid is composed of hydrated iron oxides associated with oxides (goethite, hematite) and iron sulfides (pyrite).
Despite this today, geologists continue to use this term to describe iron-related samples in the field that cannot be identified with the naked eye and would require laboratory tests for a correct description, however, those descriptions are fine due to that lemonite if it is originally related to iron oxides.
Physical properties and characteristics
| limonite | Properties and characteristics |
|---|---|
| Chemical classification | amorphous, mineraloid |
| Color | Yellowish brown, brown to black |
| Stripe | yellowish brown |
| Gloss | boring to earthy |
| diaphaneity | Opaque |
| cleavage | It is not cut because it has an amorphous structure. |
| Mohs hardness | 1 to 5 (weathered material can be deceptively soft) |
| Density or specific weight | 2.7 g/cm3 to 4.3g/cm3 (varies due to impurities) |
| diagnostic properties | Variable: can be yellow-brown, brown, reddish-brown |
| Chemical composition | A hydrated iron oxide of variable composition. |
| crystalline system | amorphous to cryptocrystalline |
| Applications | Ocher pigments, a minor iron ore |
In general, we know that limonite is not a mineral itself, rather it is considered a mineraloid or amorphous, which frequently appears with a yellowish brown to dark brown color.
It is characterized by presenting an earthy luster, in addition it does not present cleavage because it does not present an ordered internal crystalline structure either.
Although in its natural state it has a Mohs hardness of around 5, when it is affected by weathering it is particularly soft.
It usually occurs in a stalactitic, reniform, botryoidal form and not as a crystalline form, in addition it can also occur as pseudomorphs and coatings in rocks, fractures and cavities.
It is often the main form of iron and dye in lateritic soils.
Is limonite a mineral, stone or rock?
To begin with, it must be clarified that limonite is not a stone or a rock.
Limonite is usually defined as a mineral that is the closest to the correct definition, however it must be clarified that it is not true either and in fact even geologists often say that limonite is a mineral, but no!
Actually, limonite is a mineraloid, this means that it does not have a defined crystalline structure and chemical composition, and minerals do.
Learn about what a rock is by clicking here.
Learn what a mineral is by clicking here.
Origin, formation and geological environment
The lemonite It is a mineraloid that is generated mainly by chemical weathering processes and would be classified as a secondary material or even alteration of minerals that contain iron in their composition such as hematite, magnetite, goethite and pyrite, to name the most common.
It also usually appears as a coating material in fractures and cavities in the earth that are made up of iron-rich rocks and have been exposed to the environment.
In addition, it usually covers minerals that contain iron such as pyrite, as a layer of oxide.
This mineraloid is also sometimes found in layered iron deposits where hydrated iron oxides have formed as a precipitated sediment on the ground associated with shallow bogs, lakes, and shallow marine environments.
The lemonite It is very common to find it in underground mines where groundwater with a high content of dissolved metals and low in oxygen mixes with oxygen-rich surface water and forms an iron-enriched precipitate that precipitates over the mine rock walls. this precipitate indicates what is known as acidic waters.
Limonite Uses
The lemonite It has been especially important because it has been used since prehistoric times.
- Its initial use probably lies as a pigment, the ancient settlers used it to make Neolithic pictographs, as well as paint in the range of yellow, brown and ocher colors.
- It can also be used as a low-quality ore to obtain iron, however, it has been almost totally replaced by minerals such as hematite and magnetite from which it is easier to extract this material, they are purer and also appear in much larger volumes than mineral. lemonite
- For geologists to find lemonite indicates the high presence of iron in nearby rocks and minerals, which may be related to iron sulfides and iron oxides important in mining exploration.