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 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.

Kyanite

General

Category Nesosilicate

Formula 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 perfect 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°

Etymology and history

The name kyanite comes from the Greek κύανος (dark metal, blue glass flux, enamel, azurite, copper glaze, mountain blue, ultramarine; after Homer) and refers to its predominantly blue color. The name kyanite also comes from the Greek δις σθένος = double strength and refers to the strong anisotropy of its hardness properties.

The name kyanite was given to the mineral by Abraham Gottlob Werner in 1789. The name disthene comes from René-Just Haüy (1801).

Classification

In the outdated 8th edition of the mineral classification according to Strunz, kyanite belonged to the mineral class of “silicates” and there to the section “neso-subsilicates”, where it is listed together with andalusite and sillimanite and in the appendix with mullite and yoderite in the “Al 2 SiO 5 group” with the system number VIII/A'.02.

In the Lapis classification system by Stefan Weiß, last revised in 2018 and formally based on the 8th edition of Karl Hugo Strunz 's old system, the mineral was given the system and mineral number VIII/B.02-040. This corresponds to the class of "silicates" and the section "island silicates with non-tetrahedral anions", where kyanite forms an unnamed group with the system number VIII/B.02 together with andalusite, boromullite, kanonaite, mullite, sillimanite, topaz, and yoderite. 

The 9th edition of Strunz's mineral classification, last updated in 2009 by the International Mineralogical Association (IMA), classifies kyanite in the class of "silicates and germanates" and within the division "island silicates (nesosilicates)". Here, the mineral is found in the subdivision "island silicates with additional anions; cations in,, and/or only coordination", where it is the sole member of an unnamed group with the system number 9.AF.15.

In the Dana classification of minerals, which is predominantly used in English-speaking countries, kyanite has the system and mineral number 52.02.02c.01. This corresponds to the class of "silicates" and the division "island silicates: SiO4 groups and O, OH, F, and H2O. " Here, it is found within the subdivision "island silicates: SiO4 groups and O, OH, F, and H2O with cations in and > coordination" as the sole member of the "Al2SiO5 (kyanite subgroup). "

Properties

Kyanite is an aluminum silicate mineral, with the chemical formula Al2SiO5. It is typically patchy blue in color, though it can range from pale to deep blue and can also be gray or white or, infrequently, light green. It typically forms sprays of bladed crystals, but is less commonly found as distinct euhedral (well-shaped) crystals, which are particularly prized by collectors. 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. Cleavage surfaces typically display a pearly luster. The crystals are slightly flexible.

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 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 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}. 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.

Crystal 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 the silica tetrahedra, making kyanite a member of the nesosilicate class of silicate minerals.

Kyanite crystallizes in the triclinic crystal system in space group P 1 (space group no. 2) with the lattice parameters a = 7.124 Å; b = 7.856 Å; c = 5.577 Å and α = 89.99°, β = 101.12° and γ = 105.19° as well as 4 formula units per unit cell. 

All Al 2 SiO 5 modifications share the octahedra, which are linked by shared edges parallel to the c-axis. However, unlike andalusite and sillimanite, kyanite, as a high-pressure modification, has the closest packing of the compound. The coordination formula for kyanite is Al Al [O|SiO 4 ] with minor admixtures of Fe 3+ and Cr 3+.

Characteristics

Kyanite's most notable property is its extreme anisotropy in terms of hardness. This ranges from 6 to 7 along the b-axis and 4.5 to 5.5 along the c-axis (Mohs hardness scale). A second special characteristic is its often intense blue color. Both properties subsequently led to the mineral's name.

With a density of 3.56 to 3.67 g/cm 3, kyanite is a heavy mineral along with anatase, brookite, epidote, and others. It is only slightly soluble in hydrofluoric acid (HF aq) and has a weak red fluorescent color.

The mineral gets its predominantly bluish color from the inclusion of small amounts of up to 0.5% iron(III) oxide (Fe 2 O 3). 

Modifications and varieties

Kyanite is a member of the Al 2 SiO 5 group and trimorphic with the other members andalusite and sillimanite. This means that the chemical substance with the composition Al 2 [O|SiO 4 ] occurs in three different forms (modifications), similar to carbon. However, andalusite and sillimanite crystallize in the orthorhombic crystal system, and the aluminum is coordinated differently. 

A rare green variety is called chromium kyanite. Rhaeticite (after Naumann, 1828) is a dark gray to black variety with intercalated graphite and needle-like, clustered crystal aggregates. 

Occurrence

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 and associated quartz veins, and is infrequently found in eclogites. It occurs as detrital grains in sedimentary rocks, although it tends to weather rapidly. It is associated with staurolite, andalusite, sillimanite, talc, hornblende, gedrite, mullite and corundum.

Kyanite is one of the most common minerals, having the composition Al2SiO5. Minerals with identical compositions but a different, distinct crystal structure are called polymorphs. There are two polymorphs of kyanite: andalusite and sillimanite. 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. They are all equally stable at the triple point near 4.2 kbar and 530 °C (986 °F). 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.

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.

Bladed crystals of kyanite are very common, but individual euhedral crystals are prized by collectors. Kyanite occurs in Manhattan schist, formed under extreme pressure as a result of a continental collision during the assembly of the supercontinent of Pangaea. It is also found in pegmatites of the Appalachian Mountains and in Minas Gerais, Brazil. Splendid specimens are found at Pizzo Forno in Switzerland.

Kyanite can take on an orange color, which notably occurs in Loliondo, Tanzania. The orange color is due to inclusions of small amounts of manganese (Mn3+) in the structure.

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.

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. Mullitized kyanite is used to manufacture refractory materials.

Kyanite is rarely used as a gemstone because of its unusual hardness and perfect cleavage, making it difficult to cut. Because of its color, it can be confused with aquamarine, benitoite, cordierite, dumortierite, sapphire, and blue tourmaline (indigolite). 

Kyanite may display cat's eye chatoyancy, though this effect is limited by its anisotropism and perfect cleavage. Color varieties include orange kyanite from Tanzania. The orange color is due to inclusions of small amounts of manganese (Mn3+) in the structure.

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