Kyanite
Kyanite is a typically blue aluminosilicate mineral, found in aluminium-rich metamorphic pegmatites and sedimentary rock. It is the high pressure polymorph of andalusite and sillimanite, and the presence of kyanite in metamorphic rocks generally indicates metamorphism deep in the Earth's crust. Kyanite is also known as disthene or cyanite.
| Kyanite | |
|---|---|
| General | |
| Category | Nesosilicate |
| Formula (repeating unit) | Al2SiO5 |
| IMA symbol | Ky |
| 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, light gray to gray, rarely 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 white |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 3.53–3.65 measured; 3.67 calculated |
| Optical properties | Biaxial (-); high relief |
| Refractive index | nα = 1.712 – 1.718 nβ = 1.720 – 1.725 nγ = 1.727 – 1.734 |
| Birefringence | δ = 0.012 – 0.016 |
| Pleochroism | Trichroic, colorless to pale blue to blue |
| 2V angle | 78°–83° |
| References | |
Kyanite is strongly anisotropic, in that its hardness varies depending on its crystallographic direction. In kyanite, this anisotropism can be considered an identifying characteristic, along with its characteristic blue color. Its name comes from the same origin as that of the color cyan, being derived from the Ancient Greek word κύανος. This is typically rendered into English as kyanos or kuanos and means "dark blue."
Kyanite is used as a raw material in the manufacture of ceramics and abrasives, and it is an important index mineral used by geologists to trace metamorphic zones.