Goethite
FeO(OH) · Orthorhombic · also called limonite, bog iron, rust rock
Goethite is what rust looks like when nature makes it slowly. It is the brown staining on countless rock surfaces and the substance of bog iron ore. Most 'mystery brown rocks' brought to identification forums are goethite-cemented material.
Properties
- Mohs hardness
- 5–5.5Earthy masses feel much softer than the stated hardness because they are porous.
- Streak
- yellow brown
- Lustre
- dull, silky, submetallic
- Colour
- Yellow-brown to near black. 'Limonite' is a field term for mixed brown iron hydroxides, mostly goethite.
- Cleavage
- Perfect in one direction, but rarely visible in the massive material most people find.
- Fracture
- Uneven, earthy
- Specific gravity
- 3.3–4.3 (water = 1)
- Magnetism
- Not magnetic
- Acid test
- No reaction
- UV
- Not fluorescent
Tests that confirm goethite
- Yellow-brown streak — the test that separates it from hematite's red.
- Often botryoidal with a fibrous, silky interior when broken.
- Frequently forms hollow boxwork after other minerals have dissolved away.
What goethite gets confused with
Each of these is settled by one test. Run that test first.
Streak colour: yellow-brown for goethite, red-brown for hematite.
Where it is found
Any weathered iron-bearing rock, wetlands, and the oxidised upper zone of ore deposits.
Is it worth anything?
No cash value in ordinary form. Iridescent botryoidal specimens have modest collector interest.