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Kyanite is an aluminum silicate mineral, one of the three polymorphs of Al2SiO5, along with andalusite and sillimanite. It is characterized by its distinctive anisotropic hardness, meaning its hardness varies significantly depending on the crystallographic direction. Kyanite typically forms bladed or columnar crystals, often found in schist, gneiss, and granulite. Its color is most commonly blue, but it can also be white, gray, green, or rarely black. It is an important index mineral in metamorphic petrology, indicating specific pressure-temperature conditions during metamorphism.
How to Identify
- Color
- Typically blue, often with color zoning (darker blue in the center, lighter at the edges). Can also be white, gray, green, or black.
- Luster
- Vitreous to pearly.
- Texture
- Bladed or columnar crystals, often intergrown or radiating. Can be massive.
- Crystal Form
- Triclinic system, typically forming elongated, bladed, or columnar crystals. Crystals are often flattened parallel to {100}.
- Cleavage
- Perfect cleavage parallel to {100}, good cleavage parallel to {010}. This often results in splintery or fibrous fragments.
- Geological Environment
- Metamorphic rocks, particularly schists, gneisses, and granulites derived from aluminum-rich sedimentary rocks (pelites). Also found in some eclogites and quartz veins.
Key Facts
- Hardness: 4.5-5.5 (perpendicular to length) to 7 (parallel to length) on the Mohs scale (anisotropic hardness)
- Specific Gravity: 3.53-3.67 g/cm³
- Crystal System: Triclinic
- Color: Blue, white, gray, green, black
- Luster: Vitreous to pearly
- Transparency: Transparent to translucent
- Fracture: Splintery to uneven
- Cleavage: Perfect on {100}, good on {010}
- Composition: Aluminum Silicate (Al2SiO5)
Quick Check
- Color: Blue (most common), white, gray, green, black
- Luster: Vitreous to pearly
- Streak: White
Physical Characteristics
- Crystal Habit: Typically bladed, columnar, or tabular crystals; also massive, radiating, or fibrous aggregates.
- Cleavage Type: Perfect basal cleavage on {100}, good cleavage on {010}. This often results in step-like or splintery fractures.
- Fracture Type: Splintery to uneven, due to its excellent cleavage.
- Tenacity: Brittle
- Luster Type: Vitreous on crystal faces, pearly on cleavage surfaces.
Formation
Kyanite forms under high-pressure, low-to-moderate temperature metamorphic conditions. It is a characteristic mineral of regional metamorphism of pelitic (aluminum-rich) rocks, such as shales and mudstones, that have been subjected to significant burial and tectonic compression. It can also form in some eclogites and quartz veins associated with metamorphic terrains.
Usage
Kyanite is primarily used in the refractory industry due to its high melting point and resistance to thermal shock. It is used in the production of high-alumina refractories for furnaces, kilns, and other high-temperature applications. It is also used in ceramics, electrical insulators, and as an abrasive. Gem-quality kyanite is cut into cabochons and faceted stones for jewelry.
Age Distribution
Kyanite is typically found in metamorphic rocks, which can range in age from Precambrian to Cenozoic, depending on the specific metamorphic event. It is not directly dated but rather its host rock.
Where to Find
United States
Notable occurrences in North Carolina (especially the Spruce Pine district), Georgia, Virginia, and New Mexico.
Brazil
Significant deposits, particularly for gem-quality material, in Minas Gerais.
Switzerland
Classic localities in the Alps, known for well-formed crystals.
India
Large industrial deposits, particularly in the states of Jharkhand and Odisha.
Kenya
Known for gem-quality kyanite.
Nepal
Produces fine gem-quality kyanite.
Finding Tips
Look in Metamorphic Terrains
Focus your search on areas known for regional metamorphism, especially those with pelitic schists and gneisses. Kyanite is an indicator of high-pressure metamorphic conditions.
Identify Associated Minerals
Kyanite often coexists with other metamorphic minerals such as garnet, staurolite, muscovite, biotite, and quartz. Finding these minerals can indicate a favorable environment for kyanite.
Check for Bladed Habits
Kyanite's distinctive bladed or columnar crystal habit is a key identifier. Look for elongated, often flattened crystals within the host rock.
Test Hardness Anisotropy
If you have a specimen, try scratching it with a steel file or a quartz crystal. Kyanite will be harder (7 on Mohs) parallel to the length of the crystal and softer (4.5-5.5) across the width. This is a unique diagnostic feature.
Safety Precautions
Kyanite itself is not hazardous. However, always wear appropriate personal protective equipment (PPE) when collecting in the field, including safety glasses, gloves, and sturdy footwear. Be aware of your surroundings and potential geological hazards.
Similar Rocks
Andalusite
Andalusite (Al2SiO5)
Also known as: Chiastolite (variety)
Sillimanite
Sillimanite (Al2SiO5)
Also known as: Fibrolite (variety)
Blue Tourmaline
Schorl, Elbaite, Liddicoatite (complex borosilicate)
Also known as: Indicolite
Sapphire
Corundum (Al2O3)
Also known as: Corundum (blue variety)
Scientific Classification
- Mineral Class
- Nesosilicate
- Group
- Aluminum Silicate Group (polymorph of andalusite and sillimanite)
- Crystal System
- Triclinic
- Chemical Formula
- Al2SiO5
- Composition
- Aluminum (Al), Silicon (Si), Oxygen (O). Minor impurities can include Fe3+, Cr3+, and Ti4+.
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