Crushed aggregate with pyrite

Mixed rock fragments with mineral inclusion

Mixed rock fragments with Pyrite (FeS2)

Also known as: Pyritic aggregate, Sulfidic aggregate

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Description

Crushed aggregate with pyrite refers to a construction material composed of fragmented rock particles, where some or all of these particles contain the mineral pyrite (iron disulfide, FeS2). The rock fragments themselves can be of various lithologies (e.g., limestone, sandstone, granite, shale). The presence of pyrite, particularly in humid or oxygenated environments, can lead to significant engineering and environmental problems due to its oxidation.

How to Identify

Color: The aggregate itself will have the color of its constituent rock fragments (e.g., grey, brown, red, white). Pyrite within the aggregate will appear as brassy yellow, metallic specks or crystals.
Luster: The rock fragments will have variable luster (e.g., dull, earthy, vitreous). Pyrite will exhibit a distinct metallic luster.
Texture: Angular to sub-angular fragments, typically ranging from sand to gravel size. Pyrite may be disseminated as fine grains, larger crystals, or nodular masses within the rock fragments.
Crystal Form: Pyrite often forms cubic, pyritohedral, or octahedral crystals, or can be massive, granular, or radiating. In aggregate, these forms may be fractured.
Cleavage: Pyrite has indistinct cleavage, typically exhibiting conchoidal or uneven fracture. The cleavage of the host rock fragments will vary.
Geological Environment: Pyrite-bearing rocks are found in diverse environments: sedimentary rocks (shales, coals, limestones), hydrothermal veins, metamorphic rocks, and some igneous rocks. The aggregate is sourced from quarries or mines extracting these rock types.

Key Facts

Hardness: 6-6.5 on Mohs scale (for pyrite), variable for host rock
Specific Gravity: 4.95-5.03 (for pyrite), variable for host rock
Crystal System: Isometric (for pyrite), variable for host rock
Color: Pale brass-yellow (for pyrite), variable for host rock
Luster: Metallic (for pyrite), variable for host rock
Transparency: Opaque (for pyrite), variable for host rock
Fracture: Conchoidal to uneven (for pyrite), variable for host rock
Cleavage: Indistinct (for pyrite), variable for host rock
Composition: Iron disulfide (FeS2) within various silicate or carbonate rock fragments

Quick Check

Color: Brassy yellow (for pyrite), variable for host rock
Luster: Metallic (for pyrite), variable for host rock
Streak: Greenish-black to brownish-black (for pyrite)

Physical Characteristics

Crystal Habit: Cubic, pyritohedral, octahedral, massive, granular, radiating (for pyrite)
Cleavage Type: Indistinct (for pyrite)
Fracture Type: Conchoidal to uneven (for pyrite)
Tenacity: Brittle (for pyrite)
Luster Type: Metallic (for pyrite)

Formation

Crushed aggregate is formed by mechanical crushing of larger rock masses. Pyrite (FeS2) forms in various geological environments, including hydrothermal veins, sedimentary rocks (especially shales and coals), metamorphic rocks, and as an accessory mineral in igneous rocks. When rocks containing pyrite are crushed for aggregate, the pyrite becomes an inclusion within the aggregate material.

Usage

Crushed aggregate is widely used in construction for concrete, asphalt, road base, and fill material. However, aggregate containing significant amounts of pyrite is generally avoided or used with extreme caution due to potential deleterious reactions.

Age Distribution

Variable, depending on the source rock of the aggregate. Pyrite can form across a wide range of geological ages.

Where to Find

Quarries and Mines

Any quarry or mine extracting rock for aggregate production that happens to contain pyrite-bearing strata. This is a global issue, with occurrences reported in North America (e.g., Canada, USA), Europe (e.g., Ireland, UK, Scandinavia), and Australia.

Construction Sites

Areas where pyritic aggregate has been used in concrete, road base, or fill. Deterioration due to pyrite oxidation can manifest as cracking, swelling, and staining.

Finding Tips

Visual Inspection

Look for brassy yellow, metallic specks or crystals within the crushed rock fragments. These can range from microscopic to several millimeters in size.

Acid Test (Caution Advised)

While not definitive for pyrite, applying a drop of dilute hydrochloric acid (HCl) to the aggregate can help identify carbonate rocks (which will effervesce). Pyrite itself does not react with dilute HCl, but its presence in carbonate aggregate can be problematic. This test should be done with appropriate safety precautions.

Geological Context

Understand the geology of the aggregate source. If the source rock is known to contain pyrite (e.g., black shales, certain limestones, or rocks from mineralized areas), the aggregate is likely to contain it.

Similar Rocks

Crushed aggregate with pyrrhotite

Mixed rock fragments with Pyrrhotite (Fe1-xS)

Also known as: Pyrrhotitic aggregate

Crushed aggregate with marcasite

Mixed rock fragments with Marcasite (FeS2)

Also known as: Marcasitic aggregate

Crushed aggregate (pyrite-free)

Various rock types

Also known as: Clean aggregate

Scientific Classification

Mineral Class: Sulfide mineral (for pyrite)
Group: Pyrite group (for pyrite)
Crystal System: Isometric (for pyrite)
Chemical Formula: FeS2 (for pyrite)
Composition: Iron (Fe) 46.55%, Sulfur (S) 53.45% (for pyrite)

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