| Kyanite | |
|---|---|
 | |
| General | |
| Category | Nesosilicate |
| Formula | Al2SiO5 |
| IMA symbol | Ky[1] |
| Strunz classification | 9.AF.15 |
| Crystal system | Triclinic |
| Crystal class | Pinacoidal (1) (sameH-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 (-); highrelief |
| 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 | [2][3][4] |
Kyanite is a typically bluealuminosilicatemineral, found inaluminium-rich metamorphicpegmatites andsedimentary rock. It is the high pressurepolymorph ofandalusite andsillimanite, and the presence of kyanite inmetamorphic rocks generally indicates metamorphism deep in theEarth's crust. Kyanite is also known asdisthene orcyanite.[5]
Kyanite is stronglyanisotropic, in that itshardness 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 colorcyan, being derived from the Ancient Greek word κύανος. This is typically rendered into English askyanos orkuanos and means "dark blue."
Kyanite is used as a raw material in the manufacture of ceramics and abrasives, and it is an importantindex mineral used by geologists to tracemetamorphic zones.
Kyanite is analuminum silicate mineral, with the chemical formulaAl2SiO5. It is typically patchy blue in color, though it can range from pale to deep blue[6] and can also be gray or white or, infrequently, light green.[7] It typically forms sprays of bladed crystals, but is less commonly found as distincteuhedral (well-shaped) crystals, which are particularly prized by collectors.[6] It has a perfect {100}cleavage plane, parallel to the long axis of the crystal, and a second good cleavage plane {010} that is at an angle of 79 degrees to the {100} cleavage plane. Kyanite also shows a parting on {001} at an angle of about 85 degrees to the long axis of the crystal.[7] Cleavage surfaces typically display a pearly luster. The crystals are slightly flexible.[6]
Kyanite's elongated, columnar crystals are usually a good first indication of the mineral, as well as its color (when the specimen is blue). Associated minerals are useful as well, especially the presence of thepolymorphs ofstaurolite, which occurs frequently with kyanite. However, the most useful characteristic in identifying kyanite is its anisotropism. If one suspects a specimen to be kyanite, verifying that it has two distinctly differenthardness values on perpendicular axes is a key to identification; it has a hardness of 5.5 parallel to {001} and 7 parallel to {100}.[2][3] Thus, a steel needle will easily scratch a kyanite crystal parallel to its long axis, but the crystal is impervious to being scratched by a steel needle perpendicular to the long axis.[6]
The kyanite structure can be visualized as a distortedface centered cubic lattice of oxygen ions, with aluminium ions occupying 40% of the octahedral sites and silicon occupying 10% of the tetrahedral sites. The aluminium octahedra form chains along the length of the crystal, half of which are straight and half of which are zigzag, with silica tetrahedra linking the chains together. There is no direct linkage between the silica tetrahedra, making kyanite a member of the nesosilicate class ofsilicate minerals.[8][9]
Kyanite occurs inbiotitegneiss, micaschist, andhornfels, which aremetamorphic rocks formed at high pressure during regionalmetamorphism of aprotolith which is rich inaluminium (apelitic protolith). Kyanite is also occasionally found ingranite andpegmatites[9][11] and associatedquartzveins,[12] and is infrequently found ineclogites. It occurs as detrital grains in sedimentary rocks, although it tends to weather rapidly.[7][11] It is associated withstaurolite,andalusite,sillimanite,talc,hornblende,gedrite,mullite andcorundum.[2]
Kyanite is one of the most common minerals, having the compositionAl2SiO5. Minerals with identical compositions but a different, distinct crystal structure are calledpolymorphs. There are two polymorphs of kyanite:andalusite andsillimanite. Kyanite is the most stable at high pressure, andalusite is the most stable at lower temperature and pressure, and sillimanite is the most stable at higher temperature and lower pressure.[13] They are all equally stable at thetriple point near 4.2kbar and 530 °C (986 °F).[14] This makes the presence of kyanite in a metamorphic rock an indication of metamorphism at high pressure.
Kyanite is often used as anindex mineral to define and trace ametamorphic zone that was subject to a particular degree of metamorphism at great depth in the crust. For example,G. M. Barrow defined kyanite zones and sillimanite zones in his pioneering work on the mineralogy of metamorphic rocks. Barrow was characterizing a region of Scotland that had experiencedregional metamorphism at depth. By contrast, the metamorphic zones surrounding the Fanad pluton of Ireland, which formed bycontact metamorphism at a shallower depth in the crust, include andalusite and sillimanite zones but no kyanite zone.[15]
Kyanite is potentially stable at low temperature and pressure. However, under these conditions, the reactions that produce kyanite, such as:
never take place, and hydrous aluminosilicate minerals such as muscovite,pyrophyllite, orkaolinite are found instead of kyanite.[16]
Bladed crystals of kyanite are very common, but individual euhedral crystals are prized by collectors.[6] Kyanite occurs inManhattan schist, formed under extreme pressure as a result of acontinental collision during the assembly of thesupercontinent ofPangaea.[17] It is also found in pegmatites of theAppalachian Mountains and inMinas Gerais, Brazil. Splendid specimens are found atPizzo Forno in Switzerland.[6]
Kyanite can take on an orange color, which notably occurs in Loliondo, Tanzania.[18] The orange color is due to inclusions of small amounts ofmanganese (Mn3+) in the structure.
Kyanite is used primarily inrefractory and ceramic products, includingporcelain plumbing anddishware. It is also used in electronics,electrical insulators andabrasives.[19]
At temperatures above 1100 °C, kyanitedecomposes intomullite andvitreous silica via the following reaction:
This transformation results in an expansion.[20] Mullitized kyanite is used to manufacturerefractory materials.[19]
Kyanite has been used as a semipreciousgemstone, which may display cat's eyechatoyancy, though this effect is limited by its anisotropism and perfect cleavage. Color varieties include orange kyanite from Tanzania.[18] The orange color is due to inclusions of small amounts ofmanganese (Mn3+) in the structure.[21]
{{cite book}}:ISBN / Date incompatibility (help){{cite book}}:ISBN / Date incompatibility (help)Prof Stewart was keeping an eye out for a mineral known as kyanite, a beautiful blue specimen commonly seen in the Manhattan schist. 'Kyanite is a key mineral to identify, we know it only forms at very deep depths and under extensive pressure,' he said. 'It's like a fingerprint, revealing a wealth of information.' The presence of this mineral reveals that the Manhattan schist was compressed under incredibly high pressure over 300 million years ago. The schist formed as a result of two enormous landmasses coming together to form a supercontinent, known as Pangaea.
