How to identify Quartz with Pyrite

To correctly identify Quartz with Pyrite (SiO2 (Quartz) with FeS2 (Pyrite) and other sulfides), 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 is typically colorless, white, or milky, but can be various colors (e.g., purple amethyst, yellow citrine, smoky gray) due to impurities or irradiation. Pyrite is pale brass-yellow to golden-yellow. Other sulfides will exhibit their characteristic colors (e.g., chalcopyrite: brassy yellow, often tarnished iridescent; galena: lead-gray; sphalerite: yellow, brown, black; arsenopyrite: silvery white to steel gray).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Quartz has a vitreous (glassy) luster. Pyrite has a metallic luster. Other sulfides also exhibit metallic luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Can be massive, granular, crystalline, or banded. Quartz may form euhedral crystals (prismatic with pyramidal terminations) or anhedral masses. Sulfides can be disseminated as fine grains, form distinct euhedral to subhedral crystals (e.g., cubic or pyritohedral pyrite), or occur as massive aggregates.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Quartz: Hexagonal prisms terminated by rhombohedra. Pyrite: Cubic, pyritohedral, or octahedral crystals, often striated. Other sulfides: Characteristic crystal forms (e.g., galena: cubic; sphalerite: tetrahedral; chalcopyrite: disphenoidal).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Quartz: No true cleavage, exhibits conchoidal fracture. Pyrite: Indistinct cleavage on {001} and {110}, typically fractures conchoidally or unevenly. Other sulfides: Galena has perfect cubic cleavage; sphalerite has perfect dodecahedral cleavage; chalcopyrite has poor cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Hydrothermal veins, porphyry deposits, skarns, volcanogenic massive sulfide (VMS) deposits, sedimentary rocks (e.g., black shales), metamorphic rocks (e.g., schists, gneisses).

Key Facts

Hardness

Quartz: 7 on Mohs scale. Pyrite: 6-6.5 on Mohs scale. Other sulfides vary (e.g., galena: 2.5; chalcopyrite: 3.5-4; sphalerite: 3.5-4).

Specific Gravity

Quartz: 2.65 g/cm³. Pyrite: 4.95-5.03 g/cm³. Other sulfides vary (e.g., galena: 7.4-7.6; chalcopyrite: 4.1-4.3; sphalerite: 3.9-4.1).

Crystal System

Quartz: Trigonal. Pyrite: Isometric. Other sulfides vary (e.g., galena: isometric; chalcopyrite: tetragonal; sphalerite: isometric).

Color

Quartz: Colorless, white, milky, or various hues. Pyrite: Pale brass-yellow. Other sulfides: Characteristic metallic colors.

Luster

Quartz: Vitreous. Pyrite: Metallic. Other sulfides: Metallic.

Transparency

Quartz: Transparent to translucent to opaque. Pyrite: Opaque. Other sulfides: Opaque.

Fracture

Quartz: Conchoidal. Pyrite: Conchoidal to uneven. Other sulfides: Variable.

Cleavage

Quartz: None. Pyrite: Indistinct. Other sulfides: Variable (e.g., galena: perfect cubic).

Composition

Quartz: Silicon dioxide (SiO2). Pyrite: Iron disulfide (FeS2). Other sulfides: Metal sulfides (e.g., CuFeS2, PbS, ZnS).

Physical characteristics

• Crystal Habit: Quartz: Prismatic, massive, granular. Pyrite: Cubic, pyritohedral, octahedral, massive, granular. Other sulfides: Variable.
• Cleavage Type: Quartz: None. Pyrite: Indistinct. Other sulfides: Variable (e.g., galena: perfect cubic).
• Fracture Type: Quartz: Conchoidal. Pyrite: Conchoidal to uneven. Other sulfides: Variable.
• Tenacity: Quartz: Brittle. Pyrite: Brittle. Other sulfides: Brittle.
• Luster Type: Quartz: Vitreous. Pyrite: Metallic. Other sulfides: Metallic.

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