Quartzite with Pyrite

Metamorphic Rock with Accessory Mineral

Quartzite (metamorphic rock) with Pyrite (FeS2)

Also known as: Pyritic Quartzite, Sulfidic Quartzite

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Description

Quartzite with Pyrite is a metamorphic rock primarily composed of quartz (SiO2), characterized by its exceptional hardness and granular texture, where individual quartz grains are tightly interlocked. The presence of pyrite (FeS2) manifests as brassy yellow, metallic, cubic, or irregular grains disseminated within the quartzite matrix. The amount of pyrite can vary from trace amounts to significant concentrations, influencing the rock's overall appearance and chemical reactivity. The color of the quartzite itself can range from white, gray, and pink to reddish or purplish, depending on impurities, while pyrite consistently exhibits its distinctive metallic luster and color.

How to Identify

Color: Quartzite: Typically white, gray, pink, reddish, or purplish. Pyrite: Brassy yellow, metallic. The overall rock color will be a combination.
Luster: Quartzite: Vitreous (glassy) to greasy. Pyrite: Metallic.
Texture: Granoblastic, interlocking quartz grains, often sugary. Pyrite appears as distinct metallic grains within this matrix.
Crystal Form: Quartzite: Anhedral to subhedral interlocking quartz grains. Pyrite: Commonly euhedral cubes, pyritohedrons, or octahedrons, but can also be anhedral masses or disseminated grains.
Cleavage: Quartzite: No true cleavage, conchoidal to splintery fracture. Pyrite: Indistinct cleavage on {001} and {111}, but typically fractures conchoidally.
Geological Environment: Formed from the metamorphism of quartz-rich sandstones in regional metamorphic belts (e.g., mountain ranges) or contact metamorphic aureoles. Pyrite's presence suggests reducing conditions during either sedimentation or metamorphism, often associated with organic matter or hydrothermal activity.

Key Facts

Hardness: Quartz (7 on Mohs scale), Pyrite (6-6.5 on Mohs scale). The overall rock is very hard.
Specific Gravity: Quartzite (2.65 g/cm³), Pyrite (4.95-5.10 g/cm³). The rock's specific gravity will be slightly higher than pure quartzite depending on pyrite content.
Crystal System: Quartz (Trigonal), Pyrite (Isometric).
Color: Quartzite: Variable (white, gray, pink, red). Pyrite: Brassy yellow.
Luster: Quartzite: Vitreous to greasy. Pyrite: Metallic.
Transparency: Quartzite: Translucent to opaque. Pyrite: Opaque.
Fracture: Quartzite: Conchoidal to splintery. Pyrite: Uneven to conchoidal.
Cleavage: Quartzite: None. Pyrite: Poor/indistinct on {001} and {111}.
Composition: Predominantly SiO2 (quartz) with FeS2 (pyrite) as an accessory mineral.

Quick Check

Color: White, gray, pink, or reddish quartzite with brassy yellow metallic specks.
Luster: Vitreous to greasy for quartzite, metallic for pyrite.
Streak: Quartzite: White. Pyrite: Greenish-black to brownish-black.

Physical Characteristics

Crystal Habit: Quartz: Granular, interlocking anhedral to subhedral grains. Pyrite: Euhedral cubes, pyritohedrons, octahedrons, or massive/disseminated.
Cleavage Type: Quartz: None. Pyrite: Poor/indistinct.
Fracture Type: Quartz: Conchoidal to splintery. Pyrite: Uneven to conchoidal.
Tenacity: Quartz: Brittle. Pyrite: Brittle.
Luster Type: Quartz: Vitreous to greasy. Pyrite: Metallic.

Formation

Quartzite forms from the metamorphism of quartz-rich sandstone. During regional or contact metamorphism, the quartz grains recrystallize and interlock, forming a very hard and durable rock. Pyrite (FeS2) can be present in the original sandstone as detrital grains, or it can form during diagenesis or metamorphism through the reaction of iron-bearing minerals with sulfur-rich fluids, often in reducing environments. The presence of pyrite indicates specific geochemical conditions during either the sedimentary or metamorphic history of the rock.

Usage

Due to its extreme hardness and resistance to weathering, quartzite is used as a construction material, for road aggregate, railway ballast, and in some cases, as dimension stone. Pyrite-bearing quartzite is generally avoided for exterior architectural use due to the potential for pyrite oxidation and subsequent staining or degradation. However, it can be a source of sulfur or iron in some industrial processes, though this is rare for quartzite itself.

Age Distribution

Can form from Proterozoic to Cenozoic sandstones, depending on the age of the protolith and the metamorphic event.

Where to Find

Appalachian Mountains, USA

Extensive quartzite formations, some of which contain pyrite, are found throughout the Appalachian orogen, particularly in Pennsylvania, Maryland, and Virginia.

Canadian Shield, Canada

Ancient Proterozoic quartzites, sometimes with associated sulfide mineralization including pyrite, are common in various parts of the Canadian Shield.

Brazil

Significant quartzite deposits, some with pyrite, are found in various states, often associated with iron ore deposits.

Scandinavia

Regions like Norway and Sweden host ancient metamorphic terrains with quartzite units that can contain pyrite.

Australia

Parts of Western Australia and South Australia contain Proterozoic quartzites, some of which are pyritic.

Finding Tips

Look for Metamorphic Terrains

Focus your search in areas known for regional metamorphism, such as ancient mountain belts or cratonic margins where sandstones have been subjected to high pressures and temperatures.

Identify Quartzite Characteristics

First, identify the quartzite by its hardness (scratches steel), glassy luster, and interlocking granular texture. It will not fizz with dilute HCl (unless it has carbonate impurities).

Spot the Pyrite

Once quartzite is identified, look for small, brassy yellow, metallic specks or cubes embedded within the rock. These are characteristic of pyrite.

Check for Weathering Effects

Pyrite can oxidize to iron oxides (limonite, goethite), leaving rusty stains or vugs (small cavities) in the rock. This can be an indicator of past or present pyrite presence.

Safety Precautions

When collecting, be aware that pyrite can oxidize to sulfuric acid and iron oxides, which can stain hands and clothing. Always wear appropriate personal protective equipment (PPE) such as gloves and eye protection, especially if breaking samples. Avoid inhaling dust, as quartzite contains crystalline silica, a known respiratory hazard.

Similar Rocks

Sandstone

Sandstone (predominantly SiO2)

Also known as: Arenite

Chert

Chert (SiO2)

Also known as: Flint

Gneiss

Gneiss (various compositions)

Also known as: Banded Metamorphic Rock

Scientific Classification

Mineral Class: Quartz: Tectosilicate. Pyrite: Sulfide.
Group: Quartz: Quartz Group. Pyrite: Pyrite Group.
Crystal System: Quartz: Trigonal. Pyrite: Isometric.
Chemical Formula: Quartz: SiO2. Pyrite: FeS2.
Composition: Quartzite is composed almost entirely of quartz (SiO2). Pyrite is iron disulfide (FeS2).

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