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Tanzanite is a blue-violet variety of the mineral zoisite, a calcium aluminum hydroxyl sorosilicate. It is renowned for its strong pleochroism, displaying different colors (blue, violet, and sometimes reddish-brown) when viewed from different crystallographic directions. Its unique color, often enhanced by heat treatment, makes it a highly prized gemstone. It is relatively soft compared to many other gemstones, requiring careful handling.
How to Identify
- Color
- Typically blue to violet-blue, often with a purplish tint. Exhibits strong pleochroism, showing blue, violet, and sometimes reddish-brown hues depending on the viewing angle.
- Luster
- Vitreous (glassy).
- Texture
- Crystalline, typically found as prismatic crystals.
- Crystal Form
- Orthorhombic system, typically forming elongated, striated prismatic crystals. Crystals can be euhedral to subhedral.
- Cleavage
- Perfect on {010}, good on {100}.
- Geological Environment
- Metamorphic rocks, specifically within graphitic gneisses and schists, often in association with calcium-silicate rocks. Found in veins and pods within these host rocks.
Key Facts
- Hardness: 6.0-7.0 on the Mohs scale
- Specific Gravity: 3.35 (+0.10, -0.05)
- Crystal System: Orthorhombic
- Color: Blue, violet, purplish-blue (pleochroic)
- Luster: Vitreous
- Transparency: Transparent to translucent
- Fracture: Uneven to conchoidal
- Cleavage: Perfect on {010}, good on {100}
- Composition: Calcium aluminum hydroxyl sorosilicate, Ca2Al3(SiO4)3(OH)
Quick Check
- Color: Blue to violet-blue (after heat treatment), brownish-yellow to greenish-brown (natural)
- Luster: Vitreous
- Streak: White
Physical Characteristics
- Crystal Habit: Prismatic, often striated parallel to the c-axis; massive, granular.
- Cleavage Type: Perfect on one direction ({010}), good on another ({100}). This makes it somewhat fragile and susceptible to chipping.
- Fracture Type: Uneven to conchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous.
Formation
Tanzanite forms in metamorphic rocks, specifically within graphitic gneisses and schists, often associated with calcium-silicate rocks. Its formation is linked to regional metamorphism and metasomatism, where hydrothermal fluids rich in vanadium and chromium interact with zoisite-bearing rocks under specific pressure and temperature conditions. The distinctive blue-violet color is due to the presence of vanadium ions (V3+) within the crystal lattice, which are activated by heat treatment (typically 400-600°C). Naturally occurring blue tanzanite is rare; most material is brownish-yellow to greenish-brown before heat treatment.
Usage
Primarily used as a gemstone in jewelry due to its striking pleochroism and vibrant blue-violet color. It is a popular alternative to sapphire.
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. It is found in a relatively small area (approximately 4 km by 2 km) within the Neoproterozoic Mozambique Belt.
Finding Tips
Geological Context
Tanzanite is found in highly specific geological settings, primarily in metamorphosed calcium-silicate rocks (skarns) within graphitic gneisses and schists. Prospecting requires a deep understanding of regional metamorphic geology.
Associated Minerals
Look for associated minerals such as graphite, diopside, grossular garnet, quartz, and various sulfide minerals. These can indicate the presence of the right geological environment.
Color and Pleochroism
Raw tanzanite often appears brownish-yellow or greenish-brown. The characteristic blue-violet color is usually developed after heat treatment. However, some naturally blue material exists. The strong pleochroism is a key identifying feature, even in rough material.
Crystal Habit
Tanzanite typically forms prismatic crystals, often with striations parallel to the c-axis. Look for these characteristic crystal forms within the host rock.
Similar Rocks
Sapphire
Al2O3
Also known as: Corundum (variety)
Iolite
(Mg,Fe)2Al4Si5O18
Also known as: Cordierite (variety)
Amethyst
SiO2
Also known as: Quartz (variety)
Scientific Classification
- Mineral Class
- Silicates
- Group
- Epidote Group
- Crystal System
- Orthorhombic
- Chemical Formula
- Ca2Al3(SiO4)3(OH)
- Composition
- Calcium aluminum hydroxyl sorosilicate. The blue-violet color is due to trace amounts of vanadium (V3+) substituting for aluminum in the crystal structure, often enhanced by heat treatment.
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