How to identify Quartz with Pyrite and Iron Staining

To correctly identify Quartz with Pyrite and Iron Staining (SiO2 (Quartz) with FeS2 (Pyrite) and Iron Oxides), 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

   Quartz: Colorless, white, gray, or translucent. Pyrite: Brassy yellow, metallic. Iron Staining: Reddish-brown, yellowish-brown, orange, or rusty hues.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Quartz: Vitreous (glassy). Pyrite: Metallic. Iron Staining: Dull to earthy.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Quartz: Crystalline, granular, massive. Pyrite: Crystalline (often cubic or octahedral), granular. Iron Staining: Powdery, earthy, or coating.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Quartz: Hexagonal prisms with pyramidal terminations, massive, granular. Pyrite: Cubes, octahedra, pyritohedra, massive, granular. Iron Staining: Amorphous coatings or earthy masses.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Quartz: None (conchoidal fracture). Pyrite: Indistinct to poor {100} cleavage. Iron Oxides: None.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Hydrothermal veins, sedimentary rocks (especially black shales, coal seams), metamorphic rocks, and alteration zones around sulfide ore deposits. The iron staining is a secondary alteration product, often found in the oxidized zone above sulfide deposits.

Key Facts

Hardness

Quartz: 7 on Mohs scale. Pyrite: 6-6.5 on Mohs scale. Iron Oxides: Varies, typically 5-6.5 (hematite) or 5-5.5 (goethite).

Specific Gravity

Quartz: 2.65 g/cm³. Pyrite: 4.95-5.10 g/cm³. Iron Oxides: Varies, e.g., Hematite 5.26 g/cm³, Goethite 3.3-4.3 g/cm³.

Crystal System

Quartz: Trigonal. Pyrite: Isometric. Iron Oxides: Variable (e.g., Hematite - Trigonal, Goethite - Orthorhombic).

Color

Quartz: Colorless, white, gray. Pyrite: Brassy yellow. Iron Staining: Reddish-brown, yellowish-brown.

Luster

Quartz: Vitreous. Pyrite: Metallic. Iron Staining: Dull to earthy.

Transparency

Quartz: Transparent to translucent. Pyrite: Opaque. Iron Oxides: Opaque.

Fracture

Quartz: Conchoidal. Pyrite: Uneven to conchoidal. Iron Oxides: Uneven.

Cleavage

Quartz: None. Pyrite: Indistinct to poor {100}. Iron Oxides: None.

Composition

Silicon dioxide (SiO2) for quartz, iron disulfide (FeS2) for pyrite, and various hydrated iron oxides/hydroxides (e.g., FeO(OH), Fe2O3) for the staining.

Physical characteristics

• Crystal Habit: Quartz: Prismatic, massive, granular. Pyrite: Cubic, octahedral, pyritohedral, massive. Iron Oxides: Earthy, botryoidal, stalactitic, massive.
• Cleavage Type: Quartz: Absent. Pyrite: Indistinct. Iron Oxides: Absent.
• Fracture Type: Quartz: Conchoidal. Pyrite: Uneven to conchoidal. Iron Oxides: Uneven.
• Tenacity: Quartz: Brittle. Pyrite: Brittle. Iron Oxides: Brittle to earthy.
• Luster Type: Quartz: Vitreous. Pyrite: Metallic. Iron Oxides: Dull to earthy.

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