The chemical formula for limonite, often referred to as iron limonite, is FeO(OH)·nH₂O.
Limonite is not a single mineral but rather a mixture of hydrated iron(III) oxide minerals, predominantly goethite (FeO(OH)), along with varying amounts of water (nH₂O). It is a common iron ore and is widespread in many geological environments. The "n" in the formula indicates that the amount of water is variable, meaning limonite's composition can differ slightly depending on its specific formation conditions.
Understanding the Limonite Formula
The formula FeO(OH)·nH₂O can be broken down into two main parts:
- FeO(OH): This represents iron(III) oxyhydroxide, which is the primary component of limonite. It signifies that iron is in its +3 oxidation state, bonded with oxygen and a hydroxyl group (OH).
- nH₂O: This denotes a variable amount of water molecules that are structurally incorporated or adsorbed within the mineral's composition. The "n" is not a fixed number, highlighting that limonite is amorphous or cryptocrystalline and lacks a precise, consistent crystal structure with a fixed water content like many other minerals.
Characteristics of Limonite
Limonite is known for its distinct appearance and properties, making it relatively easy to identify.
| Characteristic | Description |
|---|---|
| Formula | FeO(OH)·nH₂O |
| Color | Various shades of brown and yellow, often earthy |
| Composition | Hydrated Iron(III) Oxide |
| Nature | Amorphous or cryptocrystalline, not a true mineral |
Identification and Occurrence
Limonite's characteristic color, ranging from yellowish-brown to dark brown, is a key identification feature. It often forms as a secondary mineral in the oxidation zones of iron-bearing mineral deposits and can be found as:
- Bogs and Swamps: As bog iron ore, forming in waterlogged environments.
- Weathering Products: Resulting from the weathering of other iron-rich minerals.
- Pseudomorphs: Replacing other minerals while retaining their original shape.
Given its iron content, limonite has historically been used as an iron ore, though less preferred than purer forms like hematite or magnetite due to its variable water content and lower iron concentration.
