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Tourmalinated Quartz is a striking mineral aggregate characterized by acicular (needle-like) to prismatic crystals of black tourmaline (schorl) embedded within a transparent to translucent quartz matrix. The tourmaline inclusions can be randomly oriented, parallel, or form radiating patterns. The contrast between the dark, opaque tourmaline and the lighter, often clear quartz creates a distinctive aesthetic.
How to Identify
- Color
- Quartz is typically colorless, white, or smoky gray. Schorl is opaque black. The combination results in clear to smoky quartz with black needle-like inclusions.
- Luster
- Quartz exhibits a vitreous (glassy) luster. Schorl has a vitreous to resinous luster.
- Texture
- Smooth to conchoidal fracture in quartz; tourmaline inclusions are typically smooth and prismatic.
- Crystal Form
- Quartz forms hexagonal prisms with pyramidal terminations. Schorl forms elongated, prismatic crystals, often with characteristic striations parallel to the c-axis, embedded within the quartz.
- Cleavage
- Quartz has no true cleavage, exhibiting conchoidal fracture. Schorl has indistinct cleavage parallel to the prism faces.
- Geological Environment
- Found in pegmatites, granitic intrusions, and hydrothermal veins, often associated with other pegmatitic minerals like feldspar, mica, and other tourmaline varieties.
Key Facts
- Hardness: Quartz: 7 on Mohs scale. Schorl: 7-7.5 on Mohs scale. The aggregate will scratch glass.
- Specific Gravity: Quartz: 2.65 g/cm³. Schorl: 3.06-3.26 g/cm³. The aggregate will have an intermediate specific gravity depending on the proportion of each mineral.
- Crystal System: Quartz: Trigonal. Schorl: Trigonal.
- Color: Colorless, white, or smoky gray quartz with opaque black schorl inclusions.
- Luster: Vitreous (glassy).
- Transparency: Quartz is transparent to translucent. Schorl is opaque.
- Fracture: Conchoidal in quartz. Uneven to conchoidal in schorl.
- Cleavage: Quartz: None. Schorl: Indistinct.
- Composition: Silicon dioxide (SiO2) for quartz, and a complex borosilicate of sodium, iron, and aluminum (Na(Fe2+)_3Al_6(BO_3)_3Si_6O_18(OH)_4) for schorl.
Quick Check
- Color: Clear to smoky quartz with black inclusions
- Luster: Vitreous
- Streak: White (for quartz), Black (for schorl, though typically not tested on inclusions)
Physical Characteristics
- Crystal Habit: Quartz: Prismatic, often terminated. Schorl: Acicular, prismatic, often embedded.
- Cleavage Type: Quartz: Absent. Schorl: Indistinct on {1011} and {1120}.
- Fracture Type: Conchoidal in quartz. Uneven to conchoidal in schorl.
- Tenacity: Brittle.
- Luster Type: Vitreous.
Formation
Tourmalinated Quartz forms when black tourmaline (schorl) crystals grow within a quartz matrix. This typically occurs in pegmatites, hydrothermal veins, and granitic intrusions. The tourmaline crystals often form first, or simultaneously with the quartz, as the silica-rich fluids cool and crystallize. The presence of boron, iron, sodium, and aluminum is crucial for schorl formation, while silicon and oxygen are ubiquitous for quartz.
Usage
Primarily used as an ornamental stone, for lapidary purposes (cabochons, spheres, carvings), and in jewelry. It is also collected by mineral enthusiasts. Historically, tourmaline has been used in pressure gauges due to its piezoelectric properties, though tourmalinated quartz is less suitable for this due to the mixed composition.
Age Distribution
Formed during various geological periods, often associated with late-stage magmatic and hydrothermal activity, ranging from Precambrian to Cenozoic.
Where to Find
Brazil
Minas Gerais is a prolific source, particularly for high-quality specimens with clear quartz and well-formed schorl needles.
United States
Localities in Maine (e.g., Mount Mica, Plumbago Mountain) and California (e.g., Pala District) are known for tourmaline-bearing pegmatites, often yielding tourmalinated quartz.
Madagascar
Produces various tourmaline varieties and associated quartz, including tourmalinated quartz.
Pakistan
Northern areas are known for pegmatite deposits yielding fine mineral specimens, including tourmalinated quartz.
Afghanistan
Similar to Pakistan, pegmatite regions produce excellent mineral specimens.
Finding Tips
Geological Context
Focus on areas known for pegmatites, granitic intrusions, and hydrothermal vein systems. Look for outcrops of these rock types.
Associated Minerals
Tourmalinated quartz often occurs with feldspar, mica (muscovite, biotite), other tourmaline varieties, beryl, and garnet. Identifying these associated minerals can indicate a promising locality.
Visual Identification
Look for transparent to translucent quartz crystals or masses containing distinct black, needle-like or prismatic inclusions. The contrast is usually quite evident.
Tools
A rock hammer, chisel, safety glasses, and a loupe (magnifying glass) are useful for extracting and examining specimens in the field.
Similar Rocks
Rutilated Quartz
Quartz (SiO2) with Rutile (TiO2)
Also known as: Hair of Venus
Actinolite in Quartz
Quartz (SiO2) with Actinolite (Ca2(Mg,Fe)5Si8O22(OH)2)
Also known as: Green Hair Quartz
Epidote in Quartz
Quartz (SiO2) with Epidote (Ca2(Al,Fe)3(SiO4)3(OH))
Also known as: Epidote Quartz
Scientific Classification
- Mineral Class
- Quartz: Tectosilicate. Schorl: Cyclosilicate (Borosilicate).
- Group
- Quartz Group (for Quartz), Tourmaline Group (for Schorl).
- Crystal System
- Trigonal (for both Quartz and Schorl).
- Chemical Formula
- SiO2 (Quartz) and Na(Fe2+)_3Al_6(BO_3)_3Si_6O_18(OH)_4 (Schorl).
- Composition
- Quartz is composed of silicon and oxygen. Schorl is a complex borosilicate containing sodium, iron, aluminum, silicon, boron, oxygen, and hydroxyl.
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