Limonite after Pyrite with Quartz

Mineral Aggregate / Pseudomorph

Limonite (pseudomorph after Pyrite) and Quartz

Also known as: Goethite/Ferrihydrite pseudomorph after Pyrite with Quartz, Iron Pyrite Pseudomorph

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Description

Limonite after Pyrite with Quartz is a mineral aggregate characterized by the distinctive crystal forms of pyrite (typically cubes, pyritohedrons, or octahedrons) that have been completely replaced by limonite. The limonite gives the specimen an earthy, often rusty-brown to yellowish-brown color. Quartz may be present as crystalline inclusions within the limonite, as a matrix, or as associated crystals. The pseudomorph retains the external morphology of the original pyrite crystal, but its internal structure and chemical composition are entirely different. The presence of quartz adds to the overall texture and often provides a contrasting luster or color.

How to Identify

Color: Typically rusty brown, yellowish-brown, or dark brown to black. Quartz, if present, will be colorless, white, or smoky.
Luster: Earthy to dull for limonite; vitreous (glassy) for quartz.
Texture: Earthy, often porous or granular for the limonite component. Quartz will be hard and crystalline.
Crystal Form: The external form will be that of the original pyrite crystal (e.g., cubes, pyritohedrons, octahedrons), but the material itself is limonite. Quartz may show its characteristic hexagonal prism and pyramidal terminations.
Cleavage: None for limonite (it is an aggregate); quartz has no true cleavage but exhibits conchoidal fracture.
Geological Environment: Found in the oxidized zones of sulfide ore deposits, gossans, and weathered sedimentary rocks where pyrite has been exposed to surface or near-surface conditions.

Key Facts

Hardness: Limonite: 4-5.5 (Mohs); Quartz: 7 (Mohs)
Specific Gravity: Limonite: 2.7-4.3 (variable due to porosity and composition); Quartz: 2.65
Crystal System: Limonite: Amorphous or microcrystalline (often considered orthorhombic for goethite component); Quartz: Trigonal
Color: Rusty brown, yellowish-brown, dark brown (limonite); colorless, white, smoky (quartz)
Luster: Earthy, dull (limonite); vitreous (quartz)
Transparency: Opaque (limonite); Transparent to translucent (quartz)
Fracture: Conchoidal to uneven (limonite); Conchoidal (quartz)
Cleavage: None (limonite); None (quartz)
Composition: Limonite: Hydrated iron oxides (e.g., FeO(OH)·nH2O); Quartz: Silicon dioxide (SiO2)

Quick Check

Color: Rusty brown to yellowish-brown (limonite), colorless to white (quartz)
Luster: Earthy to dull (limonite), vitreous (quartz)
Streak: Yellowish-brown to brownish-yellow (limonite); white (quartz)

Physical Characteristics

Crystal Habit: Pseudomorphic after pyrite (cubes, pyritohedrons, octahedrons) for limonite; prismatic with pyramidal terminations for quartz.
Cleavage Type: Not applicable (limonite is an aggregate); None (quartz).
Fracture Type: Conchoidal to uneven (limonite); Conchoidal (quartz).
Tenacity: Brittle (limonite and quartz).
Luster Type: Earthy to dull (limonite); Vitreous (quartz).

Formation

This specimen represents a pseudomorph, where limonite (a mixture of hydrated iron oxides, primarily goethite and ferrihydrite) has replaced pyrite (FeS2) while retaining the original crystal habit of the pyrite. This process occurs through the oxidation of pyrite in the presence of water and oxygen. The sulfur in pyrite is oxidized to sulfate, and the iron is oxidized to ferric iron, which then precipitates as hydrated iron oxides. Quartz (SiO2) is often found associated with pyrite in many geological settings, either as a primary gangue mineral or later infilling, and thus becomes part of the pseudomorphic aggregate.

Usage

Primarily of interest to mineral collectors and for educational purposes to demonstrate pseudomorphism and mineral alteration processes. Historically, limonite was used as an iron ore and pigment, but the pseudomorphs themselves are not typically mined for these purposes.

Age Distribution

Can form in geological environments of various ages, wherever pyrite is present and exposed to oxidizing conditions.

Where to Find

Worldwide

Limonite pseudomorphs after pyrite are common globally wherever pyrite-bearing rocks have undergone weathering. Notable localities include various mining districts in the USA (e.g., Colorado, Arizona), Spain (e.g., Rio Tinto), Peru, and many other countries with significant sulfide mineralization.

Finding Tips

Look in Weathered Zones

Search in the oxidized caps (gossans) of sulfide ore deposits, stream beds, and areas with weathered sedimentary rocks that once contained pyrite.

Identify Pyrite Forms

Look for the characteristic cubic, pyritohedral, or octahedral shapes, even if the color is rusty brown instead of metallic yellow.

Check for Associated Minerals

Quartz is a common associate, so look for its characteristic crystal forms or granular masses alongside the limonite pseudomorphs.

Similar Rocks

Pyrite

FeS2

Also known as: Fool's Gold

Goethite

FeO(OH)

Also known as: Bog Iron Ore

Hematite after Pyrite

Hematite (pseudomorph after Pyrite)

Also known as: Iron Rose

Scientific Classification

Mineral Class: Oxides/Hydroxides (Limonite); Silicates (Quartz)
Group: Iron Hydroxide Group (Limonite); Quartz Group (Quartz)
Crystal System: Amorphous/Microcrystalline (Limonite); Trigonal (Quartz)
Chemical Formula: FeO(OH)·nH2O (Limonite, generalized); SiO2 (Quartz)
Composition: Hydrated iron oxides (Limonite); Silicon dioxide (Quartz)

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