How to identify Goethite pseudomorph after Pyrite

To correctly identify Goethite pseudomorph after Pyrite (Fe O(OH) pseudomorph after 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 black. The color is characteristic of goethite.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull, earthy, submetallic, or silky (if fibrous). Lacks the metallic luster of fresh pyrite.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Often porous, cellular, or spongy, reflecting the replacement process. Can be massive or botryoidal in larger aggregates.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Retains the distinct crystal habits of pyrite, most commonly cubes, pyritohedrons, or octahedrons, often with striations characteristic of pyrite. The edges and faces may be somewhat rounded or less sharp than original pyrite.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   No true cleavage, but may exhibit parting. Pyrite has indistinct cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in the oxidized zones of sulfide ore deposits, gossans, and weathered sedimentary rocks containing pyrite. Often associated with other iron oxides, quartz, and clay minerals.

Key Facts

Hardness

5.0 - 5.5 on Mohs scale (for goethite)

Specific Gravity

3.3 - 4.3 (for goethite, can be lower if porous)

Crystal System

Orthorhombic (for goethite, but retains the isometric crystal habit of pyrite)

Color

Yellowish-brown, reddish-brown, dark brown, black

Luster

Dull, earthy, submetallic, silky

Transparency

Opaque

Fracture

Uneven to splintery

Cleavage

Perfect on {010} (for goethite), but rarely observed in pseudomorphs; pyrite has indistinct cleavage.

Composition

Hydrous iron oxide (FeO(OH))

Physical characteristics

• Crystal Habit: Pseudomorphic after pyrite's isometric forms (cubes, pyritohedrons, octahedrons), often with striations. Can also be massive, botryoidal, reniform, stalactitic, or fibrous (for goethite itself).
• Cleavage Type: Goethite has perfect cleavage on {010}, but this is rarely seen in pseudomorphs. Pyrite has indistinct cleavage.
• Fracture Type: Uneven to splintery (characteristic of goethite).
• Tenacity: Brittle
• Luster Type: Dull, earthy, submetallic, silky

Explore Goethite pseudomorph after Pyrite