Kyanite
| Kyanite | |
|---|---|
 |
|
| General | |
| Category | Nesosilicate |
|
Formula (repeating unit) |
Al2SiO5 |
| Strunz classification | 9.AF.15 |
| Crystal system | Triclinic |
| Crystal class | Pinacoidal (1) (same H-M symbol) |
| Space group | P1 |
| Unit cell | a = 7.1262(12) Å b = 7.852(10) Å c = 5.5724(10) Å α = 89.99(2)°, β = 101.11(2)° γ = 106.03(1)°; Z = 4 |
| Identification | |
| Color | Blue, white, rarely green, gray, yellow, pink, orange, and black, can be zoned |
| Crystal habit | Columnar; fibrous; bladed |
| Twinning | Lamellar on {100} |
| Cleavage | [100] perfect [010] imperfect with 79° angle between |
| Fracture | Splintery |
| Tenacity | Brittle |
| Mohs scale hardness | 4.5-5 parallel to one axis 6.5-7 perpendicular to that axis |
| Luster | Vitreous to pearly |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 3.53 - 3.65 measured; 3.67 calculated |
| Optical properties | Biaxial (-) |
| Refractive index | nα = 1.712 - 1.718 nβ = 1.720 - 1.725 nγ = 1.727 - 1.734 |
| Pleochroism | Trichroic, colorless to pale blue to blue |
| 2V angle | 78°-83° |
| References | |
Kyanite, whose name derives from the Greek word kuanos sometimes referred to as "kyanos", meaning deep blue, is a typically blue silicate mineral, commonly found in aluminium-rich metamorphic pegmatites and/or sedimentary rock. Kyanite in metamorphic rocks generally indicates pressures higher than four kilobars. Although potentially stable at lower pressure and low temperature, the activity of water is usually high enough under such conditions that it is replaced by hydrous aluminosilicates such as muscovite, pyrophyllite, or kaolinite. Kyanite is also known as disthene, rhaeticite and cyanite.
Kyanite is a member of the aluminosilicate series, which also includes the polymorph andalusite and the polymorph sillimanite. Kyanite is strongly anisotropic, in that its hardness varies depending on its crystallographic direction. In kyanite, this anisotropism can be considered an identifying characteristic.
At temperatures above 1100 °C kyanite decomposes into mullite and vitreous silica via the following reaction: 3(Al2O3·SiO2) → 3Al2O3·2SiO2 + SiO2. This transformation results in an expansion.
Kyanite is used primarily in refractory and ceramic products, including porcelain plumbing fixtures and dishware. It is also used in electronics, electrical insulators and abrasives.
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