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Schorl in Quartz is a mineral aggregate consisting of black tourmaline (schorl) crystals embedded within a matrix of quartz. The schorl typically appears as acicular (needle-like), prismatic, or columnar crystals, often radiating or randomly oriented, contrasting sharply with the usually translucent to transparent, colorless to milky white quartz. The aesthetic appeal of this combination makes it popular in the gemstone and mineral collecting communities.
How to Identify
- Color
- The schorl inclusions are opaque black, while the quartz matrix is typically colorless, white, or milky white, sometimes smoky or slightly yellowish.
- Luster
- Schorl exhibits a vitreous to resinous luster, while quartz has a vitreous luster.
- Texture
- The texture is characterized by the sharp, often elongated, black crystals of schorl embedded within the glassy, often massive, quartz matrix.
- Crystal Form
- Schorl crystals are typically prismatic, often with characteristic striations parallel to the c-axis, and a pseudo-hexagonal cross-section. Quartz crystals are typically hexagonal prisms terminating in hexagonal pyramids, or massive anhedral forms.
- Cleavage
- Schorl has poor to indistinct cleavage, often appearing as conchoidal to uneven fracture. Quartz has no cleavage, exhibiting conchoidal fracture.
- Geological Environment
- Found in granitic pegmatites, hydrothermal veins, and some metamorphic rocks. It is indicative of boron-rich, silica-saturated environments.
Key Facts
- Hardness: Schorl: 7-7.5 (Mohs); Quartz: 7 (Mohs)
- Specific Gravity: Schorl: 3.03-3.25; Quartz: 2.65
- Crystal System: Schorl: Trigonal; Quartz: Trigonal
- Color: Opaque black (schorl); Colorless, white, milky white, sometimes 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: NaFe3Al6(BO3)3Si6O18(OH)4; Quartz: SiO2
Quick Check
- Color: Black (schorl) in colorless to milky white (quartz)
- Luster: Vitreous to resinous (schorl), Vitreous (quartz)
- Streak: White (both schorl and quartz)
Physical Characteristics
- Crystal Habit: Schorl: Prismatic, acicular, columnar, often striated parallel to c-axis. Quartz: Prismatic, massive, granular.
- Cleavage Type: Schorl: Poor to indistinct on {1011} and {1120}. Quartz: None.
- Fracture Type: Schorl: Conchoidal to uneven. Quartz: Conchoidal.
- Tenacity: Brittle (both schorl and quartz)
- Luster Type: Vitreous to resinous (schorl); Vitreous (quartz)
Formation
Schorl in Quartz forms primarily in pegmatites, hydrothermal veins, and granitic rocks. Schorl crystallizes from boron-rich, silica-saturated magmatic or hydrothermal fluids. As these fluids cool, schorl crystals grow within the existing or co-crystallizing quartz matrix. The presence of boron is crucial for tourmaline formation. The quartz typically forms from the same silica-rich fluids, often later infilling spaces or co-crystallizing around the tourmaline.
Usage
Primarily used as an ornamental stone, for lapidary purposes (cabochons, carvings), and in metaphysical practices. It is not typically used for industrial applications due to its aggregate nature and the specific properties of its components.
Age Distribution
Found in geological formations ranging from Precambrian to Cenozoic, typically associated with granitic intrusions and pegmatites.
Where to Find
Brazil
Minas Gerais is a prolific source of high-quality schorl in quartz, particularly from pegmatite deposits.
United States
Maine, California, and Colorado have notable occurrences, often associated with granitic intrusions and pegmatites.
Madagascar
Known for various tourmaline varieties, including schorl, often found in quartz matrices.
Pakistan
Northern areas yield fine specimens of tourmaline, including schorl in quartz.
Afghanistan
Similar to Pakistan, known for pegmatite-hosted tourmaline varieties.
Finding Tips
Geological Context
Focus on areas known for granitic pegmatites, hydrothermal veins, and contact metamorphic zones, as these are the primary environments for schorl and quartz formation.
Visual Identification
Look for distinct black, often needle-like or columnar crystals embedded within translucent to transparent quartz. The contrast in color and form is usually quite evident.
Hardness Test
Both schorl (7-7.5) and quartz (7) are relatively hard minerals, so they will scratch glass and steel. This helps differentiate them from softer black minerals.
Fracture Patterns
Observe the fracture: quartz will show conchoidal fracture, while schorl may show uneven to conchoidal fracture, or a tendency to break along its prismatic habit.
Magnetism
Schorl is generally non-magnetic, but some iron-rich varieties can be weakly magnetic. Quartz is non-magnetic. This can help rule out some other black minerals.
Similar Rocks
Rutile in Quartz
Rutile with Quartz
Also known as: Rutilated Quartz
Actinolite in Quartz
Actinolite with Quartz
Also known as: Actinolite Quartz
Epidote in Quartz
Epidote with Quartz
Also known as: Epidote Quartz
Scientific Classification
- Mineral Class
- Silicate (Schorl: Cyclosilicate; Quartz: Tectosilicate)
- Group
- Tourmaline Group (Schorl); Quartz Group (Quartz)
- Crystal System
- Trigonal (both Schorl and Quartz)
- Chemical Formula
- Schorl: NaFe3Al6(BO3)3Si6O18(OH)4; Quartz: SiO2
- Composition
- Schorl is a complex borosilicate of sodium, iron, and aluminum. Quartz is silicon dioxide.
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