Pyrite in fine-grained mafic or ultramafic rock

Description

This specimen consists of the mineral pyrite (iron disulfide, FeS2) disseminated within or as discrete crystals embedded in a fine-grained mafic or ultramafic igneous rock. Mafic rocks are rich in magnesium and iron, typically dark in color, and include basalt, diabase, and gabbro. Ultramafic rocks are even richer in these elements, with very low silica content, and include peridotite and serpentinite (altered peridotite). The 'fine-grained' descriptor implies that the individual mineral crystals of the host rock are generally less than 1 mm in size, often requiring magnification to identify, indicating rapid cooling of the magma. Pyrite typically appears as brassy yellow, metallic crystals, often cubic, pyritohedral, or octahedral, contrasting with the darker, duller matrix of the host rock.

How to Identify

Color

Pyrite: Brassy yellow to golden. Host rock: Typically dark gray, black, dark green (mafic) or greenish-black to dark green (ultramafic, especially serpentinite).

Luster

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

Texture

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.

Crystal Form

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

Cleavage

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.

Geological Environment

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).

Quick Check

• Color: Brassy yellow (pyrite) in dark gray/green/black (host rock)
• Luster: Metallic (pyrite) in dull to sub-vitreous (host rock)
• Streak: Greenish-black (pyrite)

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.

Formation

Pyrite (FeS2) forms in a wide range of geological environments, including hydrothermal veins, sedimentary rocks, metamorphic rocks, and igneous rocks. When found in fine-grained mafic or ultramafic rocks (e.g., basalt, diabase, gabbro, serpentinite, peridotite), it typically forms as an accessory mineral during the primary crystallization of the magma, or more commonly, through secondary processes such as hydrothermal alteration, metamorphism, or serpentinization. In mafic/ultramafic rocks, pyrite can crystallize directly from sulfur-rich magmas, or it can be introduced later by sulfur-bearing fluids circulating through fractures and pores. The fine-grained nature of the host rock indicates rapid cooling (e.g., volcanic or hypabyssal settings) or extensive alteration.

Usage

While the host rock itself (mafic/ultramafic) can be used as aggregate, road base, or dimension stone, the primary interest in 'Pyrite in host rock' often lies in its potential as an indicator for economic mineral deposits (e.g., gold, copper, nickel, cobalt) or as a source of sulfur. Pyrite itself is not typically mined for its iron content due to the presence of more economically viable iron ores. Historically, pyrite was used as a source of sulfur for sulfuric acid production. In some cases, well-formed pyrite crystals within an attractive host rock can be collected for their aesthetic value.

Age Distribution

Can occur in rocks of all geological ages, from Precambrian to Cenozoic, depending on the host rock's formation age and subsequent mineralization events.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Sulfide

Group

Pyrite Group

Crystal System

Isometric

Chemical Formula

FeS2

Composition

Iron disulfide (FeS2). Contains 46.55% Fe and 53.45% S by weight.

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