How to identify Limonite after Pyrite with Quartz

To correctly identify Limonite after Pyrite with Quartz (Limonite (pseudomorph after Pyrite) and Quartz), 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 rusty brown, yellowish-brown, or dark brown to black. Quartz, if present, will be colorless, white, or smoky.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Earthy to dull for limonite; vitreous (glassy) for quartz.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Earthy, often porous or granular for the limonite component. Quartz will be hard and crystalline.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   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.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None for limonite (it is an aggregate); quartz has no true cleavage but exhibits conchoidal fracture.

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 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)

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).

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