Limonite

Limonite is a general term used to describe a group of hydrous iron oxide minerals of varying composition. It is not a true mineral in the sense of having a specific chemical formula, but rather a field term for brownish-yellow to brown hydrous iron oxides. The principal iron oxide minerals comprising limonite include goethite (α-FeO(OH)), akaganeite (β-FeO(OH)), lepidocrocite (γ-FeO(OH)), and ferrihydrite (Fe5HO8·4H2O), as well as amorphous hydrated iron oxides and hydroxides.

Characteristics:

• Color: Typically yellowish-brown to dark brown, often exhibiting earthy tones. Can also be found in shades of orange and reddish-brown.
• Streak: Yellowish-brown
• Luster: Ranges from earthy to dull to submetallic.
• Hardness: Varies, typically between 1 and 5.5 on the Mohs hardness scale, depending on the composition and degree of hydration.
• Specific Gravity: Also variable, generally between 2.7 and 4.3.
• Crystal System: None, being amorphous or composed of microcrystalline mixtures.
• Fracture: Uneven to earthy.

Occurrence:

Limonite is a very common mineral that forms as a secondary product of weathering of iron-bearing minerals, such as pyrite, magnetite, and siderite. It is frequently found in gossans, which are the oxidized surface layers of ore deposits. Limonite can also form in sedimentary environments through precipitation from iron-rich waters. It is found in a wide variety of geological settings, including bogs, lakes, and soils.

Uses:

• Iron Ore: Limonite has historically been used as a source of iron ore, although it is less desirable than other iron ores such as hematite and magnetite due to its lower iron content and higher water content.
• Pigments: Limonite is used as a pigment in paints and other coloring agents, providing earthy yellow and brown hues.
• Water Treatment: Limonite can be used as a filtration medium for removing certain contaminants from water.

Identification:

Limonite is often identified by its characteristic yellowish-brown streak, earthy appearance, and association with iron-bearing rocks. It is often difficult to distinguish from other iron oxides without laboratory analysis, such as X-ray diffraction, due to its variable composition and fine-grained nature.

Etymology:

The name "limonite" comes from the Greek word "leimon," meaning meadow or marsh, in reference to its occurrence in bog iron deposits.