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Limonite pseudomorph after pyrite is a mineral specimen where the original pyrite (iron sulfide) has been completely replaced by limonite (hydrated iron oxide), but the external crystal shape of the pyrite is perfectly preserved. This results in specimens that exhibit the characteristic cubic, pyritohedral, or octahedral forms of pyrite, but with the earthy, brownish-yellow to dark brown color and streak of limonite. It is not a true mineral in itself, but rather a mineraloid (limonite) that has taken on the form of another mineral (pyrite).
How to Identify
- Color
- Typically yellowish-brown, reddish-brown, dark brown, or blackish-brown. The color is often dull and earthy.
- Luster
- Earthy to dull, sometimes sub-metallic if the limonite is dense.
- Texture
- 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).
- Crystal Form
- 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.
- Cleavage
- None, as limonite itself does not exhibit cleavage. The original pyrite's cleavage is not preserved.
- Geological Environment
- 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)
Quick Check
- Color: Brownish-yellow to dark brown
- Luster: Earthy to dull
- Streak: Yellowish-brown to brownish-black
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
Formation
Limonite pseudomorphs after pyrite form through the oxidation of pyrite (FeS2). When pyrite is exposed to oxygen and water, the iron sulfide reacts to form iron hydroxides and sulfates. The sulfur is typically leached away, and the iron oxidizes and hydrates to form limonite. This process occurs in situ, meaning the original crystal structure (habit) of the pyrite is preserved, even though the chemical composition and internal structure have completely changed. This is a classic example of pseudomorphism, where one mineral replaces another while retaining the original mineral's external form.
Usage
Primarily of interest to mineral collectors and for scientific study. Historically, limonite (including pseudomorphs) was used as an iron ore, but its lower iron content and often disseminated nature make it less desirable than hematite or magnetite. It has also been used as a pigment (ochre).
Age Distribution
Can form in geological environments of any age where pyrite is present and exposed to oxidizing conditions.
Where to Find
Rio Tinto, Spain
Famous for its massive sulfide deposits and extensive gossans, where excellent limonite pseudomorphs after pyrite can be found.
Various localities in the United States (e.g., Colorado, Arizona, Missouri)
Common in the oxidized caps of many base metal sulfide deposits across the western and central U.S.
Germany (e.g., Siegerland)
Historical mining districts have yielded fine examples of these pseudomorphs.
England (e.g., Cornwall)
Known for its polymetallic ore deposits, which often feature oxidized zones with pseudomorphs.
Finding Tips
Look in Oxidized Zones
Focus on the weathered upper portions of sulfide ore bodies (gossans) or areas where pyrite-rich rocks have been exposed to the elements.
Identify Pyrite Shapes
Search for specimens that clearly show the characteristic cubic, pyritohedral, or octahedral forms of pyrite, but with a brownish, earthy appearance.
Check for Weight
While limonite is less dense than pyrite, these pseudomorphs can still feel relatively heavy for their size due to the iron content.
Test the Streak
A yellowish-brown to brownish-black streak is a key indicator for limonite.
Similar Rocks
Pyrite
Pyrite (FeS2)
Also known as: Fool's Gold
Goethite pseudomorph after Pyrite
Goethite (FeO(OH)) pseudomorph after Pyrite (FeS2)
Also known as: Iron Oxide Pseudomorph
Hematite pseudomorph after Pyrite
Hematite (Fe2O3) pseudomorph after Pyrite (FeS2)
Also known as: Iron Oxide Pseudomorph
Scientific Classification
- Mineral Class
- Oxides and Hydroxides (Limonite)
- Group
- Hydroxides
- Crystal System
- Amorphous or cryptocrystalline (Limonite), but external form is Isometric (Pyrite)
- Chemical Formula
- Fe2O3·nH2O (Limonite)
- Composition
- Hydrated iron oxide, often with variable water content and impurities. The original mineral was iron sulfide.
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