Limonite (Gossan)
Limonite (A mixture of hydrated iron(III) oxide-hydroxides) · Mineral Specimen/Industrial Ore
Crystal System
Amorphous or Cryptocrystalline; commonly occurs as massive, earthy, botryoidal, or stalactitic habits.
Mineral Group
Oxides and Hydroxides
Chemical Formula
FeO(OH)·nH2O (variable mixture mainly of Goethite α-Fe3+O(OH))
Formation Process
Secondary formation via weathering and oxidation of iron-rich sulfide minerals (like pyrite) in the oxidation zone of ore deposits.
About This Crystal
Opaque mineraloid with an earthy to sub-metallic luster. It exhibits a range of yellowish-brown to dark brown and brownish-black colors with a characteristic dull, porous, or 'rusty' appearance.
Physical Characteristics
Color: Yellowish-brown to dark brown; Streak: Yellowish-brown; Cleavage: None; Fracture: Uneven to earthy; Specific Gravity: 2.7–4.3; Fluorescence: None.
Optical Properties
Opaque; non-pleochroic; typically lacks refractive properties due to its earthy, microcrystalline nature.
Hardness & Durability
Origin Region
Global distribution; notable sources include Minas Gerais (Brazil), Rio Tinto (Spain), and the Lake Superior region (USA).
Hardness & Durability
Mohs Hardness: 4.0–5.5; Brittle and fragile in earthy forms; low durability for jewelry.
Care & Maintenance
Store in a dry environment to prevent further hydration or crumbly degradation; clean gently with a soft dry brush; avoid ultrasonic cleaning and acids.
Rarity & Value
Very common; primarily valued as an iron ore or a pigment (ochre); mineral specimens are generally low-cost.
Special Characteristics
Forms 'gossans' or 'iron hats' over sulfide ore bodies, acting as a geological indicator for deeper mineral deposits.
Lore & History
Used since prehistoric times as a pigment (yellow ochre) in cave paintings; historically an important iron ore; traditionally associated with grounding and protection in folklore.