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Tourmalinated Quartz is a variety of quartz that contains acicular (needle-like) or prismatic inclusions of tourmaline. The quartz itself is typically clear, milky, or smoky, while the tourmaline inclusions are most commonly black (schorl), though other tourmaline varieties can occur. The inclusions can be randomly oriented or aligned, creating striking patterns within the quartz host. This combination is valued for its aesthetic contrast and unique appearance.
How to Identify
- Color
- Quartz matrix can be colorless, white, smoky, or rarely other hues. Tourmaline inclusions are typically black (schorl), but can also be green, pink, or blue depending on the tourmaline species.
- Luster
- Vitreous (glassy) for both quartz and tourmaline.
- Texture
- Smooth to slightly uneven fracture surface for quartz; tourmaline inclusions are typically prismatic or acicular, feeling smooth within the quartz.
- Crystal Form
- Quartz typically forms hexagonal prisms with pyramidal terminations, though it can be massive. Tourmaline forms characteristic trigonal prisms with striations parallel to the c-axis, often appearing as needles or slender columns within the quartz.
- Cleavage
- Quartz has no true cleavage, exhibiting conchoidal fracture. Tourmaline has poor to indistinct cleavage parallel to the prism faces.
- Geological Environment
- Pegmatites, hydrothermal veins, and regionally metamorphosed rocks, particularly those associated with granitic intrusions.
Key Facts
- Hardness: Quartz: 7 (Mohs); Tourmaline: 7-7.5 (Mohs). The aggregate will scratch glass.
- Specific Gravity: Quartz: 2.65 g/cm³; Tourmaline: 3.0-3.26 g/cm³. The aggregate will have a specific gravity slightly higher than pure quartz.
- Crystal System: Quartz: Trigonal; Tourmaline: Trigonal.
- Color: Colorless, white, or smoky quartz with black, green, pink, or blue tourmaline inclusions.
- Luster: Vitreous.
- Transparency: Transparent to translucent (quartz); Opaque to translucent (tourmaline inclusions).
- Fracture: Conchoidal (quartz); Uneven to conchoidal (tourmaline).
- Cleavage: None (quartz); Poor to indistinct (tourmaline).
- Composition: Silicon dioxide (SiO2) for quartz, with complex borosilicate (Na,Ca)(Mg,Fe)3Al6(BO3)3Si6O18(OH)4 for tourmaline.
Quick Check
- Color: Clear to smoky quartz with black (or other colored) needle-like inclusions.
- Luster: Vitreous.
- Streak: White (for quartz); colorless (for tourmaline, though often not practical to streak inclusions).
Physical Characteristics
- Crystal Habit: Quartz: Prismatic, massive, granular; Tourmaline: Prismatic, acicular, columnar, often with triangular cross-section and striations.
- Cleavage Type: Quartz: None; Tourmaline: Poor to indistinct on {1011} and {1120}.
- Fracture Type: Quartz: Conchoidal; Tourmaline: Uneven to conchoidal.
- Tenacity: Brittle for both minerals.
- Luster Type: Vitreous.
Formation
Tourmalinated Quartz forms when tourmaline crystals grow within a quartz matrix. This typically occurs in pegmatites, hydrothermal veins, and metamorphic rocks. The tourmaline often crystallizes first, or simultaneously with quartz, from boron-rich fluids at elevated temperatures and pressures. As the quartz grows, it encapsulates the pre-existing or co-crystallizing tourmaline needles or prisms.
Usage
Primarily used as an ornamental stone, in jewelry, and for metaphysical purposes. Due to its aesthetic appeal, it is often cut into cabochons, beads, and polished specimens. It is not typically used for industrial applications where pure quartz or tourmaline would be preferred.
Age Distribution
Varies widely depending on the geological setting of the host rock, ranging from Precambrian to Cenozoic.
Where to Find
Brazil
Minas Gerais is a significant source, known for high-quality specimens with clear quartz and well-formed black tourmaline needles.
Madagascar
Produces specimens with various tourmaline colors, including black, green, and pink, within quartz.
United States
Localities in Maine, California, and North Carolina yield tourmalinated quartz, often from pegmatite deposits.
Pakistan
Known for specimens with clear quartz and distinct black tourmaline inclusions.
Afghanistan
Similar to Pakistan, producing fine examples of tourmalinated quartz.
Finding Tips
Geological Context
Look for tourmalinated quartz in areas known for pegmatites, granitic intrusions, and hydrothermal alteration zones. Stream beds and alluvial deposits downstream from such areas can also yield specimens.
Visual Identification
Identify clear to milky quartz with distinct, often black, needle-like or columnar inclusions. The contrast between the host quartz and the tourmaline is usually quite evident.
Hardness Test
Quartz (Mohs 7) is harder than most common rock-forming minerals, and tourmaline (Mohs 7-7.5) is similar. This can help distinguish it from softer minerals with similar appearances.
Crystal Habit
Observe the characteristic prismatic habit of tourmaline inclusions, often with striations, embedded within the quartz.
Similar Rocks
Rutilated Quartz
SiO2 (Quartz) with TiO2 (Rutile)
Also known as: Quartz with Rutile
Actinolite in Quartz
SiO2 (Quartz) with Ca2(Mg,Fe)5Si8O22(OH)2 (Actinolite)
Also known as: Quartz with Actinolite
Epidote in Quartz
SiO2 (Quartz) with Ca2(Al,Fe)3(SiO4)3(OH) (Epidote)
Also known as: Quartz with Epidote
Scientific Classification
- Mineral Class
- Quartz: Tectosilicate; Tourmaline: Cyclosilicate.
- Group
- Quartz Group; Tourmaline Group.
- Crystal System
- Trigonal for both.
- Chemical Formula
- SiO2 (Quartz) with (Na,Ca)(Mg,Fe)3Al6(BO3)3Si6O18(OH)4 (Tourmaline, general formula for common species like Schorl, Dravite, Elbaite).
- Composition
- Quartz is silicon dioxide. Tourmaline is a complex borosilicate of aluminum with varying amounts of iron, magnesium, sodium, lithium, and other elements, depending on the specific species.
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