Kyanite

Last updated
Kyanite
Kyanite crystals.jpg
General
Category Nesosilicate
Formula
(repeating unit)
Al
2
SiO
5
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
ColorBlue, 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 hardness4.5-5 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 propertiesBiaxial (-)
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 [1] [2] [3]

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. [4]

Contents

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 generally 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 Kyanite.JPG
Deep blue kyanite
Kyanite within quartz, Hunterian Museum, Glasgow Kyanite within quartz, as collected by Dr John Hunter, Hunterian Museum, Glasgow.jpg
Kyanite within quartz, Hunterian Museum, Glasgow

Kyanite is an aluminum silicate mineral, with chemical formula Al
2
SiO
5
. It is typically patchy blue in color, though it can range from pale to deep blue [5] and can also be gray or white or, infrequently, light green. [6] It typically forms sprays of bladed crystals, but is less commonly found as distinct euhedral (well-shaped) crystals, which are particularly prized by collectors. [5] 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} clevage plane. Kyanite also shows a parting on {001} at an angle of about 85 degrees to the long axis of the crystal. [6] Cleavage surfaces typically display a pearly luster. Crystals are slightly flexible. [5]

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 polymorphs of staurolite, which occur 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 different 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}. [1] [2] 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. [5]

Structure

The kyanite structure can be visualized as a distorted face 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 silica tetraheda, making kyanite a member of the nesosilicate class of silicate minerals. [7] [8]

Occurrence

Phase diagram of Al2SiO5
(nesosilicates). [10]

Kyanite occurs in biotite gneiss, mica schist, and hornfels, which are metamorphic rocks formed at high pressure during regional metamorphism of a protolith rich in aluminium (a pelitic protolith). Kyanite is also occasionally found in granite and pegmatite [8] [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. [6] [11] It occurs associated with staurolite, andalusite, sillimanite, talc, hornblende, gedrite, mullite and corundum. [1]

Kyanite is one of three common minerals having the composition Al
2
SiO
5
. Minerals with identical compositions but distinct crystal structure are called polymorphs . The other polymorphs with identical composition to kyanite are andalusite and sillimanite. Kyanite is the polymorph most stable at high pressure, while andalusite is the most stable at lower temperature and pressure and sillimanite is the most stable at higher temperature and lower pressure, [13] with the three polymorphs equally stable at the triple point near 4.2 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 index mineral to define and trace a metamorphic 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 experienced regional metamorphism at depth. By contrast, the metamorphic zones surrounding the Fanad Pluton of Ireland, which formed by 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 produces kyanite, such as

muscovite + staurolite + quartz → biotite + kyanite + H
2
O

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

Bladed crystals of kyanite are not uncommon, but individual euhedral crystals are prized by collectors. [5] Kyanite occurs in Manhattan schist, formed under extreme pressure as a result of a continental collision during the assembly of the supercontinent of Pangaea. [17] It is also found in pegmatites of the Appalachian Mountains and in Minas Gerais, Brazil. Splendid specimens are found at Pizzo Forno in Switzerland. [5]

Uses

Kyanite is used primarily in refractory and ceramic products, including porcelain plumbing and dishware. It is also used in electronics, electrical insulators and abrasives. [18]

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. [19] Mullitized kyanite is used to manufacture refractory materials. [18]

Kyanite has been used as a semiprecious gemstone, which may display cat's eye chatoyancy, though this use is limited by its anisotropism and perfect cleavage. Color varieties include recently discovered orange kyanite from Tanzania. [20] The orange color is due to inclusion of small amounts of manganese (Mn3+) in the structure. [21]

Related Research Articles

Mineral Element or chemical compound that is normally crystalline formed as a result of geological processes

In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.

Muscovite

Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate mineral of aluminium and potassium with formula KAl2(AlSi3O10)(F,OH)2, or (KF)2(Al2O3)3(SiO2)6(H2O). It has a highly perfect basal cleavage yielding remarkably thin laminae (sheets) which are often highly elastic. Sheets of muscovite 5 meters × 3 meters (16.5 feet × 10 feet) have been found in Nellore, India.

Garnet Mineral, semi-precious stone

Garnets are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives.

Metamorphic rock Rock that was subjected to heat and pressure

Metamorphic rocks arise from the transformation of existing rock to new types of rock, in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than 150 to 200 °C and, often, elevated pressure of 100 megapascals (1,000 bar) or more, causing profound physical or chemical changes. During this process, the rock remains mostly in the solid state, but gradually recrystallizes to a new texture or mineral composition. The protolith may be a sedimentary, igneous, or existing metamorphic rock.

Andalusite

Andalusite is an aluminium nesosilicate mineral with the chemical formula Al2SiO5. This mineral was called andalousite by Delamétehrie, who thought it came from Andalusia. It soon became clear that it was a locality error, and that the specimens studied were actually from El Cardoso de la Sierra, in the province of Guadalajara, not Andalusia.

Sillimanite Nesosilicate mineral

Sillimanite is an aluminosilicate mineral with the chemical formula Al2SiO5. Sillimanite is named after the American chemist Benjamin Silliman (1779–1864). It was first described in 1824 for an occurrence in Chester, US.

Metamorphism Change of minerals in pre-existing rocks without melting into liquid magma

Metamorphism is the change of minerals or geologic texture in pre-existing rocks (protoliths), without the protolith melting into liquid magma. The change occurs primarily due to heat, pressure, and the introduction of chemically active fluids. The chemical components and crystal structures of the minerals making up the rock may change even though the rock remains a solid. Changes at or just beneath Earth's surface due to weathering or diagenesis are not classified as metamorphism. Metamorphism typically occurs between diagenesis, and melting (~850°C).

Staurolite

Staurolite is a reddish brown to black, mostly opaque, nesosilicate mineral with a white streak. It crystallizes in the monoclinic crystal system, has a Mohs hardness of 7 to 7.5 and the chemical formula: Fe2+2Al9O6(SiO4)4(O,OH)2. Magnesium, zinc and manganese substitute in the iron site and trivalent iron can substitute for aluminium.

Cordierite

Cordierite (mineralogy) or iolite (gemology) is a magnesium iron aluminium cyclosilicate. Iron is almost always present and a solid solution exists between Mg-rich cordierite and Fe-rich sekaninaite with a series formula: (Mg,Fe)2Al3(Si5AlO18) to (Fe,Mg)2Al3(Si5AlO18). A high-temperature polymorph exists, indialite, which is isostructural with beryl and has a random distribution of Al in the (Si,Al)6O18 rings.

Hornfels

Hornfels is the group name for a set of contact metamorphic rocks that have been baked and hardened by the heat of intrusive igneous masses and have been rendered massive, hard, splintery, and in some cases exceedingly tough and durable. These properties are due to fine grained non-aligned crystals with platy or prismatic habits, characteristic of metamorphism at high temperature but without accompanying deformation. The term is derived from the German word Hornfels, meaning "hornstone", because of its exceptional toughness and texture both reminiscent of animal horns. These rocks were referred to by miners in northern England as whetstones.

Lazulite

Lazulite ((Mg,Fe2+)Al
2
(PO
4
)
2
(OH)
2
) is a blue, phosphate mineral containing magnesium, iron, and aluminium phosphate. Lazulite forms one endmember of a solid solution series with the darker iron rich scorzalite.

Lawsonite

Lawsonite is a hydrous calcium aluminium sorosilicate mineral with formula CaAl2Si2O7(OH)2·H2O. Lawsonite crystallizes in the orthorhombic system in prismatic, often tabular crystals. Crystal twinning is common. It forms transparent to translucent colorless, white, and bluish to pinkish grey glassy to greasy crystals. Refractive indices are nα=1.665, nβ=1.672 - 1.676, and nγ=1.684 - 1.686. It is typically almost colorless in thin section, but some lawsonite is pleochroic from colorless to pale yellow to pale blue, depending on orientation. The mineral has a Mohs hardness of 8 and a specific gravity of 3.09. It has perfect cleavage in two directions and a brittle fracture.

Aluminium silicate (or aluminum silicate) is a name commonly applied to chemical compounds which are derived from aluminium oxide, Al2O3 and silicon dioxide, SiO2 which may be anhydrous or hydrated, naturally occurring as minerals or synthetic. Their chemical formulae are often expressed as xAl2O3·ySiO2·zH2O. It is known as E number E559.

Aluminosilicate

Aluminosilicate minerals are minerals composed of aluminium, silicon, and oxygen, plus countercations. They are a major component of kaolin and other clay minerals.

An index mineral is used in geology to determine the degree of metamorphism a rock has experienced. Depending on the original composition of and the pressure and temperature experienced by the protolith, chemical reactions between minerals in the solid state produce new minerals. When an index mineral is found in a metamorphosed rock, it indicates the minimum pressure and temperature the protolith must have achieved in order for that mineral to form. The higher the pressure and temperature in which the rock formed, the higher the grade of the rock.

An isograd is a concept used in the study of metamorphic rocks. The metamorphic grade of such a rock is a rough measure of the degree of metamorphism it has undergone, as characterised by the presence of certain index minerals. An isograd is a theoretical surface composed of points where the metamorphic grade is the same. It thus separates metamorphic zones whose rocks contain different index minerals.

Zussmanite (K(Fe2+,Mg,Mn)13[AlSi17O42](OH)14) is a hydrated iron-rich silicate mineral. Zussmanite occurs as pale green crystals with perfect cleavage.

Metamorphic facies

A metamorphic facies is a set of mineral assemblages in metamorphic rocks formed under similar pressures and temperatures. The assemblage is typical of what is formed in conditions corresponding to an area on the two dimensional graph of temperature vs. pressure. Rocks which contain certain minerals can therefore be linked to certain tectonic settings, times and places in the geological history of the area. The boundaries between facies are wide because they are gradational and approximate. The area on the graph corresponding to rock formation at the lowest values of temperature and pressure is the range of formation of sedimentary rocks, as opposed to metamorphic rocks, in a process called diagenesis.

Metamorphic zone

In geology, a metamorphic zone is an area where, as a result of metamorphism, the same combination of minerals occur in the bedrock. These zones occur because most metamorphic minerals are only stable in certain intervals of temperature and pressure.

Petrogenetic grid

A petrogenetic grid is a geological phase diagram that connects the stability ranges or metastability ranges of metamorphic minerals or mineral assemblages to the conditions of metamorphism. Experimentally determined mineral or mineral-assemblage stability ranges are plotted as metamorphic reaction boundaries in a pressure–temperature cartesian coordinate system to produce a petrogenetic grid for a particular rock composition. The regions of overlap of the stability fields of minerals form equilibrium mineral assemblages used to determine the pressure–temperature conditions of metamorphism. This is particularly useful in geothermobarometry.

References

Specific citations
  1. 1 2 3 "Kyanite" (PDF). Handbook of Mineralogy. 2001. Retrieved 2018-01-01.
  2. 1 2 "Kyanite". MinDat. Retrieved 2013-06-14.
  3. "Kyanite Mineral Data". Webmineral.com. Retrieved 2013-06-14.
  4. Jackson, Julia A., ed. (1997). Glossary of geology (Fourth ed.). Alexandria, Viriginia: American Geological Institute. ISBN   0922152349.
  5. 1 2 3 4 5 6 Sinkankas, John (1964). Mineralogy for amateurs. Princeton, N.J.: Van Nostrand. pp. 528–529. ISBN   0442276249.
  6. 1 2 3 Nesse, William D. (2000). Introduction to mineralogy. New York: Oxford University Press. p. 319. ISBN   9780195106916.
  7. Winter, J.K.; Ghose, S. (1979). "Thermal expansion and high-temperture crystal chemistry of the Al 2 SiO 5 polymorphs". American Mineralogist. 64 (5–6): 573–586. Retrieved 28 August 2021.
  8. 1 2 Nesse 2000, p. 315.
  9. Whitney, D.L. (2002). "Coexisting andalusite, kyanite, and sillimanite: Sequential formation of three Al2SiO5 polymorphs during progressive metamorphism near the triple point, Sivrihisar, Turkey". American Mineralogist. 87 (4): 405–416. doi:10.2138/am-2002-0404.
  10. Whitney, D.L. (2002). "Coexisting andalusite, kyanite, and sillimanite: Sequential formation of three Al2SiO5 polymorphs during progressive metamorphism near the triple point, Sivrihisar, Turkey". American Mineralogist. 87 (4): 405–416. doi:10.2138/am-2002-0404.
  11. 1 2 "Geology Page - Kyanite". Geology Page. 2014-05-16. Retrieved 2020-02-20.
  12. Sinkankas, John (1964). Mineralogy for amateurs. Princeton, N.J.: Van Nostrand. p. 529. 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. Yardley, B. W. D. (1989). An introduction to metamorphic petrology. Harlow, Essex, England: Longman Scientific & Technical. pp. 8–10. 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. 1 2 Nesse 2000, p. 316.
  19. Speyer, Robert (1993). Thermal Analysis of Materials. CRC Press. p. 166. ISBN   0-8247-8963-6.
  20. M. Chadwick, Karen; R. Rossman, George (2009-01-01). "Orange kyanite from Tanzania". Gems and Gemology. 45.
  21. M. Gaft; L. Nagli; G. Panczer; G. R. Rossman; R. Reisfeld (August 2011). "Laser-induced time-resolved luminescence of orange kyanite Al2SiO5". Optical Materials. 33 (10): 1476–1480. Bibcode:2011OptMa..33.1476G. doi:10.1016/j.optmat.2011.03.052.
General references