How to identify Pyrite in fine-grained mafic or ultramafic rock

To correctly identify Pyrite in fine-grained mafic or ultramafic rock (Pyrite (FeS2) in a fine-grained mafic or ultramafic rock), 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 to golden. Host rock: Typically dark gray, black, dark green (mafic) or greenish-black to dark green (ultramafic, especially serpentinite).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Pyrite: Metallic, bright. Host rock: Dull to sub-vitreous, depending on mineralogy and alteration.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Pyrite: Crystalline, often euhedral to subhedral crystals embedded in the matrix. Host rock: Aphanitic (fine-grained, individual crystals not visible to the naked eye) to very fine-grained, sometimes porphyritic with larger phenocrysts of mafic minerals.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Pyrite: Commonly cubic, pyritohedral (12-sided), or octahedral crystals. Also massive, granular, or radiating forms. Host rock: Granular, interlocking crystals, or aphanitic groundmass.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Pyrite: Indistinct to poor cleavage on {001}. Host rock: Mafic minerals (e.g., pyroxene, amphibole) have distinct cleavage, but in fine-grained rocks, this is difficult to observe without magnification. Ultramafic rocks like serpentinite may show platy or fibrous habits.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in various settings where mafic/ultramafic rocks occur, including mid-ocean ridges, ophiolite complexes, volcanic arcs, continental rifts, and intrusive bodies (sills, dikes). Pyrite can be primary magmatic, hydrothermal, or metamorphic in origin within these rocks.

Key Facts

Hardness

Pyrite: 6-6.5 (Mohs). Host rock: Varies, typically 5-7 depending on mineralogy (e.g., pyroxene, amphibole, olivine, serpentine).

Specific Gravity

Pyrite: 4.95-5.02. Host rock: Typically 2.8-3.3 for mafic rocks, 3.0-3.4 for ultramafic rocks.

Crystal System

Pyrite: Isometric. Host rock: Constituent minerals belong to various systems (e.g., monoclinic for pyroxene, orthorhombic for olivine).

Color

Pyrite: Pale brass-yellow, golden. Host rock: Dark gray, black, dark green, greenish-black.

Luster

Pyrite: Metallic. Host rock: Dull, earthy, sub-vitreous.

Transparency

Pyrite: Opaque. Host rock: Opaque to translucent (for individual fine grains).

Fracture

Pyrite: Uneven to conchoidal. Host rock: Uneven, splintery.

Cleavage

Pyrite: Indistinct to poor on {001}. Host rock: Mafic minerals have distinct cleavage, but often not visible in fine-grained matrix.

Composition

Pyrite: Iron disulfide (FeS2). Host rock: Primarily silicates of iron and magnesium (e.g., pyroxene, olivine, plagioclase, amphibole, serpentine, chlorite).

Physical characteristics

• Crystal Habit: Pyrite: Commonly cubic, pyritohedral, octahedral, massive, granular, radiating. Host rock: Aphanitic (fine-grained), granular, sometimes porphyritic.
• Cleavage Type: Pyrite: Poor/indistinct. Host rock: Not applicable to the rock as a whole, but constituent minerals have varying cleavage (e.g., pyroxene has two at ~90 degrees).
• Fracture Type: Pyrite: Uneven to conchoidal. Host rock: Uneven, hackly, splintery.
• Tenacity: Pyrite: Brittle. Host rock: Brittle.
• Luster Type: Pyrite: Metallic. Host rock: Dull, earthy, sub-vitreous.

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