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Schorl in Quartz is a mineral aggregate consisting of black, prismatic crystals of schorl (black tourmaline) embedded within a matrix of translucent to opaque quartz. The schorl crystals can range from fine needles to stout, well-formed prisms, often exhibiting characteristic striations parallel to their long axis. The quartz matrix can be milky white, smoky, or clear, providing a striking contrast to the dark schorl. This combination is valued for its aesthetic appeal and the distinct visual interplay between the two minerals.
How to Identify
- Color
- Schorl is typically opaque black. Quartz can be colorless, white (milky quartz), or smoky gray. The combination presents black needles or prisms within a lighter quartz matrix.
- Luster
- Schorl has a vitreous to resinous luster. Quartz has a vitreous luster.
- Texture
- The texture is typically crystalline, with distinct prismatic schorl crystals embedded in a granular to massive quartz matrix. Schorl crystals often show vertical striations.
- Crystal Form
- Schorl forms elongated, prismatic crystals, often with a triangular cross-section and vertical striations. Quartz forms hexagonal prisms with pyramidal terminations, though it can also be massive or anhedral when intergrown.
- Cleavage
- Schorl has poor to indistinct cleavage, often appearing as conchoidal to uneven fracture. Quartz has no cleavage, exhibiting conchoidal fracture.
- Geological Environment
- Pegmatites, granitic intrusions, hydrothermal veins, and high-grade metamorphic rocks.
Key Facts
- Hardness: Schorl: 7-7.5 on Mohs scale; Quartz: 7 on Mohs scale
- Specific Gravity: Schorl: 3.0-3.2; Quartz: 2.65
- Crystal System: Schorl: Trigonal; Quartz: Trigonal
- Color: Black (schorl); Colorless, white, smoky (quartz)
- Luster: Vitreous to resinous (schorl); Vitreous (quartz)
- Transparency: Opaque (schorl); Transparent to translucent (quartz)
- Fracture: Conchoidal to uneven (schorl); Conchoidal (quartz)
- Cleavage: Poor to indistinct (schorl); None (quartz)
- Composition: Schorl: Sodium Iron Aluminum Borosilicate Hydroxide; Quartz: Silicon Dioxide
Quick Check
- Color: Black (schorl) in colorless, white, or smoky (quartz)
- Luster: Vitreous to resinous (schorl), Vitreous (quartz)
- Streak: White (both schorl and quartz)
Physical Characteristics
- Crystal Habit: Schorl: Prismatic, acicular, columnar, often with vertical striations. Quartz: Prismatic, massive, granular.
- Cleavage Type: Schorl: Poor to indistinct basal cleavage; Quartz: None.
- Fracture Type: Conchoidal to uneven (both minerals).
- Tenacity: Brittle (both minerals).
- Luster Type: Vitreous to resinous (schorl); Vitreous (quartz).
Formation
Schorl in Quartz forms primarily in pegmatites, hydrothermal veins, and some metamorphic rocks. Schorl crystallizes from boron-rich, iron-rich fluids, often associated with the late stages of granitic magma crystallization. Quartz, being a ubiquitous mineral, can crystallize simultaneously or subsequently, encapsulating or intergrowing with the schorl crystals. The presence of both minerals indicates a specific geochemical environment, often involving high temperatures and pressures.
Usage
Primarily used as an ornamental stone, for lapidary purposes (cabochons, beads, carvings), and in metaphysical practices. Due to the piezoelectric and pyroelectric properties of tourmaline, it has niche applications in pressure gauges and other electronic devices, though pure schorl is less commonly used for this than other tourmaline varieties. Quartz is widely used in electronics, optics, and as an abrasive.
Age Distribution
Found in geological formations ranging from Precambrian to Cenozoic, typically associated with granitic intrusions and metamorphic events.
Where to Find
Brazil
Minas Gerais is a significant source, particularly for well-formed crystals in quartz.
United States
Maine, California, and Colorado have notable occurrences in pegmatite districts.
Madagascar
Known for various tourmaline varieties, including schorl in quartz.
Pakistan
Northern areas yield fine mineral specimens.
Afghanistan
Similar to Pakistan, known for pegmatite-related minerals.
Finding Tips
Geological Context
Look for areas with granitic pegmatites, hydrothermal veins, or regions of high-grade metamorphism. Schorl in quartz is often found in the outer zones of pegmatite bodies.
Visual Identification
Identify the characteristic black, prismatic crystals of schorl embedded in a lighter, often translucent quartz matrix. The vertical striations on schorl are a key identifier.
Hardness Test
Schorl (7-7.5) and Quartz (7) are relatively hard minerals. They will scratch glass and most common metals. This can help differentiate them from softer black minerals.
Density
Schorl is denser than quartz (specific gravity of schorl is 3.0-3.2, quartz is 2.65). While difficult to discern in an aggregate, a specimen with a high proportion of schorl will feel noticeably heavier than pure quartz.
Similar Rocks
Rutile in Quartz
Rutile (TiO_2) in Quartz (SiO_2)
Also known as: Rutilated Quartz
Actinolite in Quartz
Actinolite (Ca_2(Mg,Fe)_5Si_8O_22(OH)_2) in Quartz (SiO_2)
Also known as: Actinolite Quartz
Epidote in Quartz
Epidote (Ca_2(Al,Fe)_3(SiO_4)_3(OH)) in Quartz (SiO_2)
Also known as: Epidote Quartz
Scientific Classification
- Mineral Class
- Schorl: Cyclosilicate; Quartz: Tectosilicate
- Group
- Schorl: Tourmaline Group; Quartz: Quartz Group
- Crystal System
- Trigonal (both minerals)
- Chemical Formula
- Schorl: Na(Fe2+)_3Al_6(BO_3)_3Si_6O_18(OH)_4; Quartz: SiO_2
- Composition
- Schorl: Sodium, Iron, Aluminum, Boron, Silicon, Oxygen, Hydrogen; Quartz: Silicon, Oxygen
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