How to identify Pyrite with Chlorite and Sphalerite

To correctly identify Pyrite with Chlorite and Sphalerite (FeS2 (Pyrite), (Mg,Fe)3(Si,Al)4O10(OH)2·(Mg,Fe)3(OH)6 (Chlorite Group), (Zn,Fe)S (Sphalerite)), 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

   Pyrite: Brassy yellow, often tarnishing to duller shades. Sphalerite: Highly variable, from pale yellow-brown to dark brown, black, or reddish-brown. Chlorite: Typically various shades of green (dark green, olive green, yellowish-green).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Pyrite: Metallic. Sphalerite: Resinous to submetallic, sometimes adamantine. Chlorite: Vitreous to pearly, sometimes dull.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Pyrite: Crystalline, granular, massive, or radiating. Sphalerite: Crystalline, massive, granular, or botryoidal. Chlorite: Flaky, scaly, massive, or micaceous aggregates.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Pyrite: Commonly cubes, octahedra, pyritohedra, or combinations thereof. Sphalerite: Tetrahedral crystals, often distorted; also massive or granular. Chlorite: Monoclinic, typically in tabular or platy crystals, often forming aggregates.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Pyrite: Poor to indistinct on {001}. Sphalerite: Perfect dodecahedral cleavage on {011}. Chlorite: Perfect basal cleavage on {001}.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Hydrothermal veins, volcanogenic massive sulfide (VMS) deposits, sedimentary exhalative (SEDEX) deposits, skarns, and other base metal sulfide ore deposits. Often found in association with quartz, calcite, and other sulfide minerals.

Key Facts

Hardness

Pyrite: 6-6.5 (Mohs). Sphalerite: 3.5-4 (Mohs). Chlorite: 2-2.5 (Mohs).

Specific Gravity

Pyrite: 4.95-5.02. Sphalerite: 3.9-4.1 (pure ZnS) to 4.0-4.1 (Fe-rich). Chlorite: 2.6-3.3.

Crystal System

Pyrite: Isometric. Sphalerite: Isometric. Chlorite: Monoclinic.

Color

Pyrite: Brassy yellow. Sphalerite: Yellow-brown, brown, black, red-brown. Chlorite: Green (dark green, olive green, yellowish-green).

Luster

Pyrite: Metallic. Sphalerite: Resinous to submetallic. Chlorite: Vitreous to pearly.

Transparency

Pyrite: Opaque. Sphalerite: Transparent to translucent (thin splinters). Chlorite: Translucent to opaque.

Fracture

Pyrite: Uneven to conchoidal. Sphalerite: Conchoidal to uneven. Chlorite: Uneven.

Cleavage

Pyrite: Poor to indistinct on {001}. Sphalerite: Perfect dodecahedral on {011}. Chlorite: Perfect basal on {001}.

Composition

Pyrite: Iron disulfide (FeS2). Chlorite: Hydrous magnesium iron aluminum silicate (Mg,Fe)3(Si,Al)4O10(OH)2·(Mg,Fe)3(OH)6. Sphalerite: Zinc iron sulfide (Zn,Fe)S.

Physical characteristics

• Crystal Habit: Pyrite: Cubes, octahedra, pyritohedra, massive, granular. Sphalerite: Tetrahedra, dodecahedra, massive, granular, botryoidal. Chlorite: Flaky, scaly, massive, micaceous aggregates.
• Cleavage Type: Pyrite: Poor. Sphalerite: Perfect. Chlorite: Perfect.
• Fracture Type: Pyrite: Uneven to conchoidal. Sphalerite: Conchoidal to uneven. Chlorite: Uneven.
• Tenacity: Pyrite: Brittle. Sphalerite: Brittle. Chlorite: Flexible, inelastic.
• Luster Type: Pyrite: Metallic. Sphalerite: Resinous to submetallic. Chlorite: Vitreous to pearly.

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