How to identify Quartz with Sulfides

To correctly identify Quartz with Sulfides (SiO2 with sulfide minerals (e.g., pyrite, chalcopyrite)), 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 can be colorless, white, gray, milky, or smoky. Sulfides introduce metallic colors: brassy yellow (pyrite, chalcopyrite), silvery gray (galena), dark brown/black (sphalerite), or iridescent tarnish. The overall color will be a combination of these.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Quartz exhibits a vitreous (glassy) luster. Sulfide minerals typically have a metallic luster, which contrasts sharply with the quartz.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Can be massive, granular, or crystalline. Quartz may form euhedral crystals (e.g., hexagonal prisms) or anhedral masses. Sulfides can be disseminated as fine grains, form larger anhedral masses, or occur as distinct crystals (e.g., cubic pyrite).

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Quartz often forms hexagonal prisms with pyramidal terminations. Sulfides exhibit characteristic crystal forms: cubes or pyritohedrons for pyrite, tetrahedra for sphalerite, cubes for galena, and tetragonal disphenoids for chalcopyrite, though often found as anhedral grains.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Quartz has no true cleavage, exhibiting conchoidal fracture. Sulfide minerals vary: galena has perfect cubic cleavage, sphalerite has perfect dodecahedral cleavage, pyrite has no cleavage but conchoidal to uneven fracture, and chalcopyrite has poor cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Hydrothermal veins, stockworks, breccias, and disseminations within igneous, metamorphic, and sedimentary host rocks, particularly associated with ore deposits.

Key Facts

Hardness

Quartz: 7 on Mohs scale. Sulfides vary: Pyrite 6-6.5, Chalcopyrite 3.5-4, Galena 2.5, Sphalerite 3.5-4.

Specific Gravity

Quartz: 2.65 g/cm³. Sulfides vary: Pyrite 4.95-5.10, Chalcopyrite 4.1-4.3, Galena 7.4-7.6, Sphalerite 3.9-4.1.

Crystal System

Quartz: Trigonal. Sulfides vary: Pyrite (Isometric), Chalcopyrite (Tetragonal), Galena (Isometric), Sphalerite (Isometric).

Color

Quartz: Colorless, white, gray, milky, smoky. Sulfides: Brassy yellow, silvery gray, dark brown/black, etc.

Luster

Vitreous (quartz) and Metallic (sulfides)

Transparency

Quartz: Transparent to translucent to opaque. Sulfides: Opaque.

Fracture

Quartz: Conchoidal. Sulfides: Variable (conchoidal, uneven, subconchoidal).

Cleavage

Quartz: None. Sulfides: Variable (e.g., perfect cubic in galena, perfect dodecahedral in sphalerite, none in pyrite).

Composition

SiO2 (quartz) + various metal sulfides (e.g., FeS2, CuFeS2, PbS, ZnS).

Physical characteristics

• Crystal Habit: Quartz: Prismatic, massive, granular. Sulfides: Cubic, pyritohedral, tetrahedral, massive, granular, disseminated.
• Cleavage Type: Quartz: None. Sulfides: Variable (e.g., perfect cubic, perfect dodecahedral, poor, or none).
• Fracture Type: Quartz: Conchoidal. Sulfides: Conchoidal, uneven, subconchoidal.
• Tenacity: Quartz: Brittle. Sulfides: Brittle.
• Luster Type: Vitreous (quartz) and Metallic (sulfides)

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