Goethite is not a rock itself, but rather a mineral – specifically an iron oxyhydroxide containing ferric iron. While it is a mineral, it is a primary component of certain rock formations, most notably bog iron ore.
Goethite: A Distinct Mineral
Goethite is a naturally occurring mineral with a distinct chemical composition and crystalline structure. Unlike rocks, which are typically aggregates of one or more minerals (or other organic matter), goethite is a single, defined chemical compound. It is famously known as the main component of rust and is a significant ore mineral for iron.
Role in Rock Formations: Bog Iron Ore
Although goethite is a mineral, it plays a crucial role in the formation of specific rock types. When goethite accumulates in significant quantities, it can form rocks that are classified as iron ores. A prime example of such a rock is bog iron ore.
- Bog Iron Ore: This type of ore forms in wetland environments where iron-rich waters deposit goethite and other iron oxides, often aided by microbial activity. Bog iron ore is a sedimentary rock that has been historically mined as a source of iron. In these deposits, goethite is the predominant mineral, making it a key constituent of the rock.
Key Characteristics of Goethite
Goethite possesses a range of physical and chemical properties that define it as a unique mineral. Its characteristics are important for identifying it and understanding its geological context.
| Property | Description |
|---|---|
| Chemical Type | Iron oxyhydroxide (contains ferric iron) |
| Hardness (Mohs) | 5.0 to 5.5 |
| Specific Gravity | 3.3 to 4.3 |
| Crystal Habit | Forms prismatic needle-like crystals, often referred to as "needle ironstone," but more commonly found in massive or earthy forms. |
For more detailed information, you can refer to the Goethite Wikipedia page.
Geological Significance
Goethite is an important mineral in various geological settings due to its stability under surface conditions. It is a common alteration product of other iron-bearing minerals and is widespread in soils and sedimentary deposits. Its prevalence in bog iron ore makes it economically significant as a source of iron.
