Goethite
Goethite (Iron Hydroxide) · Mineral Specimen / Industrial Mineral
Crystal System
Orthorhombic; typically occurs in massive, botryoidal, or acicular habits, often forming radiating fibrous aggregates.
Mineral Group
Oxides / Hydroxides (Limonite group subgroup)
Chemical Formula
α-Fe3+O(OH)
Formation Process
Primarily formed through the weathering of other iron-rich minerals (secondary origin) in soil and sedimentary environments; also forms through hydrothermal precipitation.
About This Crystal
Opaque mineral with a color range from yellowish-brown to dark brown or blackish-brown. Luster can vary from adamantine or metallic to dull/earthy. Surfaces often show a velvety or weathered texture.
Physical Characteristics
Color: Brownish-yellow to dark brown; Streak: Yellowish-brown (ochre); Cleavage: Perfect in one direction; Fracture: Uneven to splintery; Specific Gravity: 3.3 to 4.3; Fluorescence: None.
Optical Properties
Refractive Index: 2.260 - 2.515; Birefringence: 0.125 - 0.155; Opaque in hand specimen; pleochroism is visible in thin sections as yellow to brown.
Hardness & Durability
Origin Region
Worldwide; notable deposits in Germany, United Kingdom, USA (Wisconsin, Colorado), and Brazil. Found in oxidative zones of iron-bearing ore deposits.
Hardness & Durability
Mohs hardness of 5.0 to 5.5; brittle toughness; stable under normal conditions but can dehydrate to hematite if heated significantly.
Care & Maintenance
Clean with a soft brush and water; avoid acidic cleaners which may dissolve the mineral; store in a dry environment to prevent further chemical alteration.
Rarity & Value
Very common; typically low in value as a mineral specimen unless found in rare iridescent or well-crystallized iridescent botryoidal forms.
Special Characteristics
Often forms as a pseudomorph after other minerals like pyrite; may show an iridescent 'rainbow' coating on the surface of certain specimens.
Lore & History
Named after the German philosopher Johann Wolfgang von Goethe. Historically used as a primary ingredient in brown and yellow ochre pigments for cave paintings and pottery.