Goethite

Physical Properties
Chemical formula FeO(OH)
Class Hydroxide
Crystal system Orthorhombic
Habit Platy
Prismatic
Fibrous
Botryoidal
Mammillary
Color Yellow-brown
Dark brown
Black
Hardness 5 to 5.5
Specific gravity 4.3
Cleavage Perfect (010) (rare)
Fracture Uneven
Luster Subadamantine to earthy
Transparency Subtranslucent to opaque
Streak Brown-yellow
Optical Properties
PPL Yellow
Orange-red
Various browns
Pleochroic
High relief
XPL 4th order interference
colors; may be masked by mineral
color
δ 0.15
after Perkins, 379-380

Goethite in Hand Sample

Botryoidal goethite

Geothite in Thin Section

Stay tuned!

Further Reading

Goethite at webmineral.com

Goethite at mindat.org