Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Goethite pseudomorph after pyrite is a mineral specimen where the original pyrite crystal has been completely replaced by goethite, but the characteristic crystal shape of pyrite (most commonly cubes, pyritohedrons, or octahedrons) is perfectly preserved. The resulting specimen will have the physical and chemical properties of goethite, but the external form of pyrite. These pseudomorphs are often porous or cellular due to the volume change during the alteration process and the removal of sulfur.
How to Identify
- Color
- Typically yellowish-brown, reddish-brown, dark brown, or black. The color is characteristic of goethite.
- Luster
- Dull, earthy, submetallic, or silky (if fibrous). Lacks the metallic luster of fresh pyrite.
- Texture
- Often porous, cellular, or spongy, reflecting the replacement process. Can be massive or botryoidal in larger aggregates.
- Crystal Form
- 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.
- Cleavage
- No true cleavage, but may exhibit parting. Pyrite has indistinct cleavage.
- Geological Environment
- 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))
Quick Check
- Color: Yellowish-brown to dark brown/black
- Luster: Dull, earthy, submetallic
- Streak: Yellowish-brown to brownish-yellow
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
Formation
Goethite pseudomorphs after pyrite form through the oxidation of pyrite (FeS2). In the presence of oxygen and water, pyrite reacts to form iron hydroxides and sulfuric acid. The iron hydroxides, primarily goethite (α-FeOOH), replace the pyrite crystal structure while retaining its external morphology. This process is a classic example of pseudomorphism, where one mineral replaces another, preserving the original mineral's shape. The reaction can be simplified as: 4FeS2 (pyrite) + 15O2 + 8H2O → 4FeOOH (goethite) + 8H2SO4 (sulfuric acid). This process typically occurs in the oxidation zone of sulfide ore deposits, often near the surface where weathering is active.
Usage
Primarily of scientific and collector interest. It serves as an indicator of past geological conditions, specifically the oxidation of sulfide minerals. Historically, goethite (as part of limonite) was used as an iron ore and pigment, but the pseudomorphs themselves are not typically mined for these purposes due to their limited quantity and often friable nature.
Age Distribution
Can form in geological environments of various ages where pyrite is present and exposed to oxidizing conditions.
Where to Find
Rio Tinto, Spain
Famous for extensive gossan deposits and well-formed pseudomorphs after pyrite.
Various localities in the United States (e.g., Colorado, Arizona, Missouri)
Common in the oxidized caps of porphyry copper and other sulfide deposits.
Germany (e.g., Siegerland)
Historical mining districts often yield excellent pseudomorphs.
England (e.g., Cornwall)
Known for various mineral specimens, including pseudomorphs from old mining areas.
Finding Tips
Look in Gossans
Gossans (iron-rich, weathered caps of sulfide ore bodies) are prime locations for finding these pseudomorphs. Look for reddish-brown, porous, and often cellular rock formations.
Examine Old Mine Dumps
Waste piles from old sulfide mines often contain weathered material where pyrite has altered to goethite pseudomorphs.
Check Sedimentary Rocks
Some sedimentary rocks, especially shales and sandstones, can contain disseminated pyrite that has subsequently altered to goethite pseudomorphs.
Identify Pyrite Shapes
The key is to recognize the characteristic cubic, pyritohedral, or octahedral forms of pyrite, even though the material itself is no longer metallic and brassy.
Similar Rocks
Pyrite
FeS2
Also known as: Fool's Gold
Marcasite
FeS2
Also known as: White Iron Pyrites
Hematite pseudomorph after Pyrite
Fe2O3 pseudomorph after FeS2
Also known as: Iron Rose (colloquial for some forms)
Limonite (general term)
Mixture of hydrous iron oxides, primarily goethite
Also known as: Bog Iron Ore
Scientific Classification
- Mineral Class
- Oxides and Hydroxides
- Group
- Diaspore Group (Goethite)
- Crystal System
- Orthorhombic (Goethite)
- Chemical Formula
- FeO(OH)
- Composition
- Iron oxyhydroxide
Explore Goethite pseudomorph after Pyrite
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.