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Tanzanite is the blue-violet to blue-lavender variety of the mineral zoisite, a calcium aluminum hydroxyl sorosilicate. It is renowned for its striking trichroism, displaying different colors (blue, violet, and burgundy) when viewed from different crystallographic directions. Its color is typically enhanced by heat treatment, which converts naturally occurring brownish zoisite into the more desirable blue-violet hues. It is a relatively soft gemstone compared to many others, requiring careful handling and setting in jewelry.
How to Identify
- Color
- Typically blue to violet-blue, often with a purplish tint. Exhibits strong trichroism: blue, violet, and burgundy when viewed from different angles.
- Luster
- Vitreous (glassy).
- Texture
- Smooth, crystalline.
- Crystal Form
- Prismatic crystals, often striated parallel to the c-axis. Can be massive.
- Cleavage
- Perfect on {010}, good on {100}.
- Geological Environment
- Metamorphic rocks, specifically graphitic gneisses and schists, formed under conditions of regional metamorphism and metasomatism.
Key Facts
- Hardness: 6.0-7.0 on the Mohs scale
- Specific Gravity: 3.35 (+0.05, -0.02)
- Crystal System: Orthorhombic
- Color: Blue, violet, purplish-blue (trichroic)
- Luster: Vitreous
- Transparency: Transparent to translucent
- Fracture: Uneven to conchoidal
- Cleavage: Perfect on {010}, good on {100}
- Composition: Calcium aluminum hydroxyl sorosilicate
Quick Check
- Color: Blue to violet-blue, strong trichroism (blue, violet, burgundy)
- Luster: Vitreous
- Streak: White
Physical Characteristics
- Crystal Habit: Prismatic, often striated, also massive
- Cleavage Type: Perfect on one direction ({010}), good on another ({100})
- Fracture Type: Uneven to conchoidal
- Tenacity: Brittle
- Luster Type: Vitreous
Formation
Tanzanite forms in metamorphic rocks, specifically in graphitic gneisses and schists, within the Merelani Hills of Tanzania. Its formation is associated with regional metamorphism and metasomatism, where hydrothermal fluids rich in vanadium and other trace elements interact with zoisite-bearing rocks. The distinctive blue-violet color is primarily due to the presence of vanadium (V3+) within the crystal lattice, often enhanced by heat treatment. Naturally occurring blue tanzanite is rare; most tanzanite on the market is heat-treated brownish zoisite.
Usage
Primarily used as a gemstone in jewelry. Its pleochroism and unique color make it highly prized.
Age Distribution
Formed during the Neoproterozoic era, approximately 585 million years ago.
Where to Find
Merelani Hills, Manyara Region, Tanzania
The only commercially significant source of tanzanite in the world. Found in a geologically unique area within the Neoproterozoic Mozambique Belt.
Finding Tips
Geological Context
Tanzanite is found exclusively in specific metamorphic rock formations within the Merelani Hills. Prospecting would require detailed geological mapping and understanding of the local structural and metamorphic history.
Associated Minerals
Often found in association with graphite, diopside, tremolite, grossular garnet, and quartz within the host rock.
Color Variation
Raw tanzanite can appear brownish. The characteristic blue-violet color is often developed or enhanced through heat treatment. Natural blue tanzanite is extremely rare.
Similar Rocks
Iolite
Cordierite
Also known as: Cordierite
Sapphire
Corundum
Also known as: Blue Corundum
Amethyst
Quartz
Also known as: Purple Quartz
Scientific Classification
- Mineral Class
- Sorosilicate
- Group
- Epidote Group
- Crystal System
- Orthorhombic
- Chemical Formula
- Ca2Al3(SiO4)(Si2O7)O(OH)
- Composition
- Calcium aluminum hydroxyl sorosilicate with trace amounts of vanadium (V3+) responsible for color.
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