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This is a mineral assemblage featuring black tourmaline (schorl) crystals embedded within a quartz matrix, often with distinct brassy-yellow pyrite crystals. Schorl typically appears as prismatic, often striated, crystals. Quartz forms the bulk of the matrix, appearing as massive, granular, or well-formed crystals. Pyrite manifests as metallic, cubic, or pyritohedral crystals, or as irregular masses. The combination creates a visually striking specimen.
How to Identify
- Color
- Schorl is typically black, opaque. Quartz is usually colorless, white, or milky, but can be smoky or other colors. Pyrite is brassy yellow, metallic.
- Luster
- Schorl has a vitreous to resinous luster. Quartz has a vitreous luster. Pyrite has a metallic luster.
- Texture
- The texture will vary depending on the crystal habits of the individual minerals. Schorl often forms prismatic crystals, quartz can be massive or crystalline, and pyrite can be euhedral or anhedral.
- Crystal Form
- Schorl: Prismatic, often with characteristic rounded triangular cross-section and vertical striations. Quartz: Hexagonal prisms terminating in rhombohedra, or massive/granular. Pyrite: Cubic, pyritohedral, or octahedral crystals, often striated.
- Cleavage
- Schorl: Indistinct to poor cleavage, typically exhibiting conchoidal to uneven fracture. Quartz: No cleavage, conchoidal fracture. Pyrite: Indistinct cleavage on {001}, typically conchoidal to uneven fracture.
- Geological Environment
- Hydrothermal veins, pegmatites, greisens, and some metamorphic rocks, particularly those associated with granitic intrusions and boron-rich fluids.
Key Facts
- Hardness: Schorl: 7-7.5 on Mohs scale. Quartz: 7 on Mohs scale. Pyrite: 6-6.5 on Mohs scale.
- Specific Gravity: Schorl: 3.0-3.2. Quartz: 2.65. Pyrite: 4.95-5.10.
- Crystal System: Schorl: Trigonal. Quartz: Trigonal. Pyrite: Isometric.
- Color: Schorl: Black. Quartz: Colorless, white, smoky. Pyrite: Brassy yellow.
- Luster: Schorl: Vitreous to resinous. Quartz: Vitreous. Pyrite: Metallic.
- Transparency: Schorl: Opaque. Quartz: Transparent to translucent. Pyrite: Opaque.
- Fracture: Schorl: Conchoidal to uneven. Quartz: Conchoidal. Pyrite: Conchoidal to uneven.
- Cleavage: Schorl: Indistinct to poor. Quartz: None. Pyrite: Indistinct on {001}.
- Composition: Schorl: Na(Fe2+3)Al6(Si6O18)(BO3)3(OH)3(OH). Quartz: SiO2. Pyrite: FeS2.
Quick Check
- Color: Black (tourmaline), colorless/white (quartz), brassy yellow (pyrite)
- Luster: Vitreous to resinous (tourmaline), vitreous (quartz), metallic (pyrite)
- Streak: White (tourmaline), white (quartz), greenish-black to brownish-black (pyrite)
Physical Characteristics
- Crystal Habit: Schorl: Prismatic, acicular, massive. Quartz: Prismatic, massive, granular. Pyrite: Cubic, pyritohedral, octahedral, massive.
- Cleavage Type: Schorl: Indistinct. Quartz: None. Pyrite: Indistinct.
- Fracture Type: Schorl: Conchoidal to uneven. Quartz: Conchoidal. Pyrite: Conchoidal to uneven.
- Tenacity: Schorl: Brittle. Quartz: Brittle. Pyrite: Brittle.
- Luster Type: Schorl: Vitreous to resinous. Quartz: Vitreous. Pyrite: Metallic.
Formation
This assemblage typically forms in hydrothermal veins, pegmatites, or metamorphic rocks. 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 settings, often in reducing conditions. The co-occurrence suggests a complex interplay of fluid chemistry and temperature during crystallization.
Usage
Primarily a collector's specimen due to its aesthetic appeal. Pyrite can be a minor ore of iron and sulfur, and historically, tourmaline has been used as a gemstone (though schorl is typically opaque). Quartz is widely used in electronics, optics, and as an abrasive.
Age Distribution
Varies widely depending on the geological setting, from Precambrian to Cenozoic.
Where to Find
Brazil
Known for world-class tourmaline and quartz deposits, often with associated pyrite.
United States (e.g., Maine, California)
Pegmatite districts yield excellent tourmaline specimens, sometimes with quartz and pyrite.
Pakistan and Afghanistan
Famous for high-quality tourmaline, often found in quartz matrices, with pyrite occurrences.
Various localities with hydrothermal gold deposits
Pyrite is a common accessory mineral in gold-bearing quartz veins, and tourmaline can also be present.
Finding Tips
Look for Pegmatites
Explore pegmatite dikes and sills, especially those associated with granitic intrusions, as these are prime environments for tourmaline and quartz crystallization.
Hydrothermal Vein Systems
Search for quartz veins in areas with past or present hydrothermal activity, as these can host all three minerals.
Examine Mine Dumps
Old mine dumps from areas known for tourmaline, quartz, or pyrite can be good sources for specimens.
Check for Boron-Rich Environments
Tourmaline requires boron, so geological settings with evidence of boron metasomatism are promising.
Similar Rocks
Tourmaline in Quartz
Schorl (Tourmaline Group) in Quartz (SiO2)
Also known as: Schorl in Quartz
Pyrite in Quartz
Pyrite (FeS2) in Quartz (SiO2)
Also known as: Fool's Gold in Quartz
Granite with Tourmaline
Granite (felsic intrusive igneous rock) with Tourmaline
Also known as: Tourmaline Granite
Scientific Classification
- Mineral Class
- Silicate (Tourmaline), Silicate (Quartz), Sulfide (Pyrite)
- Group
- Tourmaline Group (Schorl), Quartz Group (Quartz), Pyrite Group (Pyrite)
- Crystal System
- Trigonal (Schorl), Trigonal (Quartz), Isometric (Pyrite)
- Chemical Formula
- Na(Fe2+3)Al6(Si6O18)(BO3)3(OH)3(OH) (Schorl), SiO2 (Quartz), FeS2 (Pyrite)
- Composition
- Complex borosilicate (Schorl), Silicon dioxide (Quartz), Iron disulfide (Pyrite)
Explore Tourmaline in Quartz with Pyrite
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