How to identify Limonite pseudomorph after Pyrite

To correctly identify Limonite pseudomorph after Pyrite (Limonite (Fe2O3·nH2O) pseudomorph after Pyrite (FeS2)), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Typically yellowish-brown, reddish-brown, dark brown, or blackish-brown. The color is often dull and earthy.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Earthy to dull, sometimes sub-metallic if the limonite is dense.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Often earthy or powdery on the surface, but can be dense and compact internally. The external form will be that of pyrite crystals (cubes, pyritohedrons, octahedrons).

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Retains the crystal habit of the original pyrite, most commonly cubes, pyritohedrons (a 12-faced form), or octahedrons. Striations may be visible on crystal faces, inherited from the original pyrite.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, as limonite itself does not exhibit cleavage. The original pyrite's cleavage is not preserved.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in the oxidized zones (gossans) of sulfide ore deposits, in sedimentary rocks where pyrite has formed and subsequently oxidized, and in weathered igneous or metamorphic rocks containing pyrite. It is common in environments where pyrite has been exposed to surface or near-surface weathering processes.

Key Facts

Hardness

4 to 5.5 on the Mohs scale (for limonite, but can vary depending on density and purity)

Specific Gravity

2.7 to 4.3 (variable, depending on water content and density)

Crystal System

None (amorphous or cryptocrystalline for limonite itself, but the external form is isometric, inherited from pyrite)

Color

Yellowish-brown, reddish-brown, dark brown, blackish-brown

Luster

Earthy, dull, sometimes sub-metallic

Transparency

Opaque

Fracture

Conchoidal to uneven

Cleavage

None

Composition

Hydrated iron oxide (Fe2O3·nH2O), replacing iron sulfide (FeS2)

Physical characteristics

• Crystal Habit: Pseudomorphic after pyrite, typically cubic, pyritohedral, or octahedral. May show striations inherited from pyrite.
• Cleavage Type: None
• Fracture Type: Conchoidal to uneven
• Tenacity: Brittle
• Luster Type: Earthy to dull

Explore Limonite pseudomorph after Pyrite