Limonite · MyMineralBox

Pseudomorph Relationships

Replaces — this mineral is often a pseudomorph after:

← Pyrite replacement

Pyrite weathers to limonite (FeO(OH)) in oxidizing conditions while retaining cubic habit. Very common.

Worldwide; Navajún cubes weather this way.

← Siderite replacement

Siderite oxidizes to limonite/goethite retaining rhombohedral habit.

Worldwide.

← Marcasite replacement

Marcasite oxidizes faster than pyrite due to crystal structure — friable limonite results.

Worldwide; storage problem for collectors.

A pseudomorph (Greek "false form") is a mineral with the external shape of another species — the chemistry has changed but the crystal habit is inherited.

Luster

earthy→sub-metallic

Mostly earthy; botryoidal goethite component can shine.

Diaphaneity (Transparency)

opaque

Earthy iron oxyhydroxide.

Streak Test

yellow-brown

Generic name for hydrated iron oxides; "rust" streak.

Streak = color of the powdered mineral. Drag specimen across unglazed white porcelain plate (Mohs 6.5). For minerals harder than the plate, crush a small flake into powder and observe color.

Mohs 4–5.5

Vickers (~) 540 HV

Knoop (~) 620 HK

Geological setting

Oxidation zone

Element composition by mass

Formula: FeO(OH)·nH₂O (mixture) · molar mass: 106.87 g/mol

Fe	52.26%
O	44.91%
H	2.83%

Computed from atomic weights (IUPAC 2021). Site-occupancy groups (Fe,Mn) split equally.

Mohs Hardness 4–5.5

1
Talc 2
Gypsum 3
Calcite 4
Fluorite 5
Apatite 6
Orthoclase 7
Quartz 8
Topaz 9
Corundum 10
Diamond

Limonite sits at 4–5.5 on the Mohs scale — can be scratched by a steel knife.

Colors:

Streak
Yellow-brown

Crystal system
Amorphous (mineraloid)

Oxides & HydroxidesOxides (Hydroxides)

TL;DR · 1 min read

Limonite is not a single mineral species but a field-name for amorphous to cryptocrystalline iron hydroxide aggregates — primarily goethite and lepidocrocite mixtures with adsorbed water. It forms massive earthy yellow-brown to dark brown deposits in lateritic weathering profiles, bog iron deposits, and the gossan caps of Fe-bearing sulfide deposits.