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Schorl is the most common member of the tourmaline group, characterized by its opaque black color and typically prismatic crystal habit. It is an iron-rich sodium-aluminum borosilicate, forming in a wide range of geological environments. Its distinctive striated faces and triangular cross-section are key identifying features.
How to Identify
- Color
- Opaque black, sometimes with a very dark blue or brown tint.
- Luster
- Vitreous to resinous, sometimes sub-metallic on fresh surfaces.
- Texture
- Typically forms elongated, prismatic crystals with characteristic vertical striations on the prism faces. Can also be massive or granular.
- Crystal Form
- Hexagonal-like (trigonal) prisms, often terminated by rhombohedral or pedial faces. Cross-sections are typically rounded triangular.
- Cleavage
- Poor to indistinct basal cleavage, often appearing to lack cleavage. Fracture is more common.
- Geological Environment
- Granitic pegmatites, high-temperature hydrothermal veins, greisens, and regionally metamorphosed schists and gneisses. Often found in association with quartz, feldspar, mica, and other tourmaline varieties.
Key Facts
- Hardness: 7-7.5 on the Mohs scale
- Specific Gravity: 3.18-3.22
- Crystal System: Trigonal
- Color: Opaque black
- Luster: Vitreous to resinous
- Transparency: Opaque
- Fracture: Conchoidal to uneven
- Cleavage: Poor to indistinct basal cleavage {0001}
- Composition: Sodium Iron Aluminum Borosilicate Hydroxide
Quick Check
- Color: Opaque black
- Luster: Vitreous to resinous
- Streak: White to grayish white
Physical Characteristics
- Crystal Habit: Prismatic, columnar, acicular, massive, granular. Prism faces typically show strong vertical striations.
- Cleavage Type: Poor to indistinct basal cleavage, often appearing absent.
- Fracture Type: Conchoidal to uneven, brittle.
- Tenacity: Brittle
- Luster Type: Vitreous to resinous, sometimes sub-metallic.
Formation
Schorl forms primarily in granitic pegmatites, high-temperature hydrothermal veins, and regionally metamorphosed rocks. It crystallizes from boron-rich, silica-saturated fluids, often in the late stages of magmatic differentiation. The presence of iron (Fe2+) is crucial for its black color.
Usage
Historically, schorl has been used as a gemstone, though its opaque nature limits its use to cabochons, carvings, and ornamental pieces. It is also used in some industrial applications due to its pyroelectric and piezoelectric properties, though less commonly than other tourmaline varieties. It is a popular mineral specimen for collectors.
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 world-renowned source for large, well-formed schorl crystals, particularly in pegmatite deposits.
Pakistan
Northern Areas (e.g., Gilgit-Baltistan) produce excellent quality schorl specimens, often associated with other pegmatite minerals.
United States
Maine (e.g., Mount Mica, Newry), California (e.g., Pala District), and Colorado are known for schorl occurrences in pegmatites.
Namibia
Various localities, including Erongo Region, yield fine schorl crystals.
Afghanistan
Nuristan Province is a significant source of tourmaline, including schorl.
Finding Tips
Look for Pegmatites
Schorl is most commonly found in granitic pegmatites. Search for areas with exposed pegmatite dikes or veins, often characterized by large crystals of quartz, feldspar, and mica.
Identify Associated Minerals
Schorl often occurs with quartz, feldspar (orthoclase, albite), muscovite, biotite, and sometimes garnet or beryl. The presence of these minerals can indicate a favorable environment.
Check for Striations
The characteristic vertical striations on the prism faces are a key identifier. Look for these parallel lines running along the length of the crystal.
Examine Crystal Shape
Schorl typically forms elongated, columnar crystals. Look for crystals with a distinctive rounded triangular cross-section.
Safety Precautions
When collecting in pegmatite environments, be aware of unstable rock formations. Always wear appropriate safety gear, including eye protection and gloves. While schorl itself is not hazardous, other minerals in the same environment might be.
Similar Rocks
Hornblende
Complex Calcium-Sodium-Magnesium-Iron-Aluminum Silicate
Also known as: Amphibole
Augite
Calcium Magnesium Iron Aluminum Silicate
Also known as: Pyroxene
Biotite
Potassium Magnesium Iron Aluminum Silicate Hydroxide Fluoride
Also known as: Black Mica
Scientific Classification
- Mineral Class
- Silicate
- Group
- Tourmaline Group
- Crystal System
- Trigonal
- Chemical Formula
- Na(Fe2+3)Al6(Si6O18)(BO3)3(OH)3(OH)
- Composition
- Sodium iron aluminum borosilicate hydroxide. The iron (Fe2+) content is dominant, giving it its characteristic black color. Minor substitutions of other elements can occur.
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