Kyanite

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Kyanite
2V angle
78°–83°
References[2][3][4]

Kyanite is a typically blue

metamorphic rocks generally indicates metamorphism deep in the Earth's crust. Kyanite is also known as disthene or cyanite.[5]

Kyanite is strongly

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.

Properties

Deep blue kyanite
Hunterian Museum
, Glasgow

Kyanite is an

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 the

hardness 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]

Structure

The kyanite structure can be visualized as a distorted

silicate minerals.[8][9]

Occurrence

Phase diagram of Al2SiO5
(aluminosilicates).[10]

Kyanite occurs in biotite gneiss, mica schist, and hornfels, which are metamorphic rocks formed at high pressure during regional metamorphism of a protolith which is rich in aluminium (a pelitic protolith). Kyanite is also occasionally found in granite and pegmatites[9][11] and associated quartz veins,[12] and is infrequently found in eclogites. It occurs as detrital grains in sedimentary rocks, although it tends to weather rapidly.[7][11] It is associated with staurolite, andalusite, sillimanite, talc, hornblende, gedrite, mullite and corundum.[2]

Kyanite is one of the most common minerals, having the composition Al2SiO5. Minerals with identical compositions but a different, distinct crystal structure are called

kbar 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 an

contact 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:

muscovite + staurolite + quartz → biotite + kyanite + H2O

never take place, and hydrous aluminosilicate minerals such as muscovite, pyrophyllite, or kaolinite are found instead of kyanite.[16]

Bladed crystals of kyanite are very common, but individual euhedral crystals are prized by collectors.

Minas Gerais, Brazil. Splendid specimens are found at Pizzo 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 of manganese (Mn3+) in the structure.

Uses

Kyanite is used primarily in

abrasives.[19]

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.

refractory materials.[19]

Kyanite has been used as a semiprecious gemstone, which may display cat's eye chatoyancy, 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 of manganese (Mn3+) in the structure.[21]

References

Specific citations
  1. S2CID 235729616
    .
  2. ^ a b c "Kyanite" (PDF). Handbook of Mineralogy. 2001. Archived from the original (PDF) on 2019-05-08. Retrieved 2018-01-01.
  3. ^ a b "Kyanite". MinDat. Retrieved 2013-06-14.
  4. ^ "Kyanite Mineral Data". Webmineral.com. Retrieved 2013-06-14.
  5. ISBN 0922152349
    .
  6. ^
    ISBN 0442276249
    .
  7. ^
    ISBN 9780195106916
    .
  8. ^ Winter, J.K.; Ghose, S. (1979). "Thermal expansion and high-temperature crystal chemistry of the Al 2 SiO 5 polymorphs". American Mineralogist. 64 (5–6): 573–586. Retrieved 28 August 2021.
  9. ^ a b Nesse 2000, p. 315.
  10. doi:10.2138/am-2002-0404
    .
  11. ^ a b "Geology Page - Kyanite". Geology Page. 2014-05-16. Retrieved 2020-02-20.
  12. ISBN 0442276249
    .
  13. ^ Nesse 2000, p. 76.
  14. ^ Bohlen, S.R.; Montana, A.; Kerrick, D.M. (1991). "Precise determinations of the equilibria kyanite⇌ sillimanite and kyanite⇌ andalusite and a revised triple point for Al2SiO5 polymorphs". American Mineralogist. 76 (3–4): 677–680. Retrieved 28 August 2021.
  15. ISBN 0582300967
    .
  16. ^ Yardley 1989, p. 68-69.
  17. ^ Quinn, Helen (6 June 2013). "How ancient collision shaped New York skyline". BBC Science. BBC.co.uk. Retrieved 2013-06-13. 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.
  18. ^ a b M. Chadwick, Karen; R. Rossman, George (2009-01-01). "Orange kyanite from Tanzania". Gems and Gemology. 45.
  19. ^ a b Nesse 2000, p. 316.
  20. ISBN 0-8247-8963-6
    .
  21. doi:10.1016/j.optmat.2011.03.052
    .
General references

External links

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