Tourmaline in Quartz with Pyrite

Metamorphic or Igneous (depending on formation)

Schorl (black tourmaline) in Quartz (SiO2) with Pyrite (FeS2)

Also known as: Schorl in Quartz with Pyrite

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Description

This specimen consists of black tourmaline (schorl) crystals embedded within a matrix of quartz, often with visible crystals or disseminations of pyrite. Schorl typically appears as prismatic, often striated, black crystals. Quartz forms the dominant host, ranging from massive to crystalline, and can be clear, milky, or smoky. Pyrite manifests as metallic, brassy yellow cubes, octahedra, or irregular masses, providing a striking contrast to the black tourmaline and lighter quartz.

How to Identify

Color: Black (schorl), colorless to white/milky/smoky (quartz), brassy yellow (pyrite).
Luster: Vitreous (quartz), sub-vitreous to resinous (schorl), metallic (pyrite).
Texture: Crystalline, often with distinct euhedral to subhedral crystals of tourmaline and pyrite within a granular or massive quartz matrix.
Crystal Form: Schorl: Prismatic, typically elongated, often with characteristic triangular cross-sections and vertical striations. Quartz: Hexagonal prisms terminating in rhombohedra, or massive/anhedral. Pyrite: Cubes, octahedra, pyritohedra, or massive.
Cleavage: Quartz: None (conchoidal fracture). Schorl: Indistinct to poor prismatic cleavage. Pyrite: Indistinct to poor cubic cleavage.
Geological Environment: Hydrothermal veins, pegmatites, greisens, and contact metamorphic zones associated with granitic intrusions. Often found in areas of significant tectonic activity and fluid flow.

Key Facts

Hardness: Quartz: 7 (Mohs). Schorl: 7-7.5 (Mohs). Pyrite: 6-6.5 (Mohs).
Specific Gravity: Quartz: 2.65. Schorl: 3.1-3.2. Pyrite: 4.95-5.10.
Crystal System: Quartz: Trigonal. Schorl: Trigonal. Pyrite: Isometric.
Color: Black (schorl), colorless to white/milky/smoky (quartz), brassy yellow (pyrite).
Luster: Vitreous (quartz), sub-vitreous to resinous (schorl), metallic (pyrite).
Transparency: Quartz: Transparent to translucent. Schorl: Opaque. Pyrite: Opaque.
Fracture: Quartz: Conchoidal. Schorl: Uneven to conchoidal. Pyrite: Conchoidal to uneven.
Cleavage: Quartz: None. Schorl: Indistinct to poor prismatic. Pyrite: Indistinct to poor cubic.
Composition: Quartz: Silicon dioxide (SiO_2). Schorl: Sodium iron aluminum borosilicate hydroxide (Na(Fe2+)_3Al_6(BO_3)_3Si_6O_18(OH)_4). Pyrite: Iron disulfide (FeS_2).

Quick Check

Color: Black (tourmaline), colorless/white/smoky (quartz), brassy yellow (pyrite)
Luster: Vitreous (quartz), sub-vitreous (tourmaline), metallic (pyrite)
Streak: White (quartz), white (tourmaline), greenish-black to brownish-black (pyrite)

Physical Characteristics

Crystal Habit: Quartz: Prismatic, massive, granular. Schorl: Prismatic, acicular, massive. Pyrite: Cubic, octahedral, pyritohedral, massive.
Cleavage Type: Quartz: Absent. Schorl: Poor prismatic. Pyrite: Poor cubic.
Fracture Type: Quartz: Conchoidal. Schorl: Uneven to conchoidal. Pyrite: Conchoidal to uneven.
Tenacity: Quartz: Brittle. Schorl: Brittle. Pyrite: Brittle.
Luster Type: Quartz: Vitreous. Schorl: Sub-vitreous to resinous. Pyrite: Metallic.

Formation

This assemblage typically forms in hydrothermal veins, pegmatites, or contact metamorphic zones. Schorl (black tourmaline) crystallizes from boron-rich fluids, often associated with granitic intrusions. Quartz is a ubiquitous mineral, forming in a wide range of temperatures and pressures. Pyrite forms in various environments, including hydrothermal, sedimentary, and metamorphic, often in reducing conditions. The co-occurrence suggests a shared or sequential hydrothermal origin where iron, sulfur, silicon, aluminum, boron, and other elements were present.

Usage

Primarily a collector's specimen due to its aesthetic appeal. Individual minerals (quartz, tourmaline, pyrite) have industrial uses, but this specific combination is valued for its mineralogical interest and ornamental qualities. Quartz is used in electronics, optics, and as an abrasive. Tourmaline is used as a gemstone and in pressure gauges. Pyrite is a minor source of sulfur and iron, but its primary use is as an ornamental mineral.

Age Distribution

Varies widely, from Precambrian to Cenozoic, depending on the specific geological event that formed the host rock.

Where to Find

Brazil

Minas Gerais is a renowned locality for tourmaline, quartz, and pyrite, often found in pegmatites and hydrothermal veins.

United States

Maine and California (San Diego County) are famous for pegmatite deposits yielding tourmaline and quartz. Colorado and other western states have pyrite occurrences in quartz veins.

Pakistan

Northern areas are known for high-quality tourmaline and quartz specimens, sometimes with associated sulfides.

Afghanistan

Nuristan and Kunar provinces are sources of various tourmaline types and quartz, often in pegmatitic environments.

Namibia

Erongo Region is known for excellent tourmaline and quartz specimens, including those with associated minerals.

Finding Tips

Look for Granitic Intrusions

This assemblage is frequently found in or near granitic pegmatites and hydrothermal veins associated with granitic intrusions. Search for areas with exposed granite or pegmatite dikes.

Identify Host Rock

The primary host rock will be quartz, often forming veins or masses. Look for areas where quartz veins are prominent.

Recognize Mineral Habits

Learn to identify the characteristic prismatic habit of black tourmaline, the metallic luster and cubic form of pyrite, and the vitreous luster of quartz. These distinct features make identification easier.

Check for Alteration Zones

Hydrothermal alteration zones, often indicated by discoloration or unusual mineral assemblages, can be good indicators for these minerals.

Safety Precautions

Always wear appropriate safety gear, including eye protection and gloves, when collecting. Be aware of your surroundings, especially in old mine workings or unstable rock formations. While the minerals themselves are generally safe to handle, always wash hands after handling any mineral specimens.

Similar Rocks

Tourmaline in Quartz

Schorl (Na(Fe2+)_3Al_6(BO_3)_3Si_6O_18(OH)_4) in Quartz (SiO_2)

Also known as: Schorl Quartz

Pyrite in Quartz

Pyrite (FeS_2) in Quartz (SiO_2)

Also known as: Auriferous Pyrite Quartz

Garnet Schist with Pyrite

Garnet (various species) and Pyrite (FeS_2) in a schistose matrix

Also known as: Pyrite-Garnet Schist

Scientific Classification

Mineral Class: Quartz: Tectosilicate. Schorl: Cyclosilicate (Borosilicate). Pyrite: Sulfide.
Group: Quartz: Quartz Group. Schorl: Tourmaline Group. Pyrite: Pyrite Group.
Crystal System: Quartz: Trigonal. Schorl: Trigonal. Pyrite: Isometric.
Chemical Formula: Quartz: SiO_2. Schorl: Na(Fe2+)_3Al_6(BO_3)_3Si_6O_18(OH)_4. Pyrite: FeS_2.
Composition: Quartz: Silicon, Oxygen. Schorl: Sodium, Iron, Aluminum, Boron, Silicon, Oxygen, Hydrogen. Pyrite: Iron, Sulfur.

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