YELLOW OXIDE

What is Limonite?

Before modern mineral analysis, the name “limonite” was given to many of the yellowish to yellowish brown iron oxides produced during the weathering of iron-bearing rocks or deposited as bog, lake, and shallow marine sediments.

Researchers who studied “limonite” discovered that it is amorphous and has a variable composition. It often contains significant amounts of iron oxide minerals such as goethite

Revealed that the material called
the definition of a mineral. Instead,
Mainly of hydrous iron oxides that are often found in intimate associations with iron minerals.

 

Physical Properties of Limonite

Chemical Classification amorphous, mineraloid
Color yellowish brown to brown to black
Streak yellowish brown
Luster dull to earthy
Diaphaneity opaque
Cleavage does not cleave because it has an amorphous structure
Mohs Hardness 1 to 5 (weathered material can be deceptively soft)
Specific Gravity 2.7 to 4.3 (varies due to impurities)
Diagnostic Properties variable – can be yellow-brown, brown, reddish brown
Chemical Composition a hydrated iron oxide of variable composition
Crystal System amorphous to cryptocrystalline
Uses ocher pigments, a minor ore of iron

Today the word “limonite” is used as a field and classroom term for these materials because they cannot be identified in hand specimens and their identity is unknown without laboratory testing. The time and expense required to do this testing is generally not needed, unless the material is going to be used in industry or it is the subject of a
Detailed study. Thus the name “limonite” is not obsolete; it is still

Limonite usually occurs as a secondary material, formed from the weathering of hematite, magnetite, pyrite, and other iron-bearing materials. Limonite is often stalactite, Reni form, botryoidally, or mammillary in habit rather than crystalline. It also occurs as pseudo morphs and coatings on the walls of fractures and cavities.

Some limonite is found in stratified deposits where hydrous iron oxides form as precipitated sediment on the floor of shallow swamps, lakes, and marine environments. These can be of inorganic or biogenic origin.

Limonite often forms as a precipitate at springs and mine openings where acidic, iron-laden waters emerge from the subsurface. Most subsurface waters contain very little oxygen, and when they discharge to the surface, they often encounter oxygenated waters. Dissolved metals in the groundwater rapidly combine with the dissolved oxygen of the surface water to form a precipitate that falls onto the bed of the stream. This precipitate is a characteristic sign of acid mine drainage.

Limonite is very resistant to weathering and often accumulates as a residual deposit. It is often the main form of iron and colorant in lateritic soils