kyanite

[Greek kuanos, dark blue enamel + -ITE¹.]

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A nesosilicate mineral, Al₂SiO₅, crystallizing in the triclinic system and occurring in metamorphic rocks. It is essentially a pure phase, but minor amounts of iron (Fe³⁺), chromium (Cr³⁺), and titanium (Ti⁴⁺) may substitute for aluminum (Al). The structure of kyanite is based on cubic close-packed oxygens (O). Ten percent of the tetrahedral (fourfold) interstices are filled with silicon (Si), and 40 percent of the octahedral (sixfold) interstices are filled with Al. The Al, with O at the corners, occurs in zigzag edge-sharing chains of Al-O octahedra. Si-O tetrahedra share corners with Al-O octahedra along the sides. This structure is about 10 percent denser than that of the other two Al₂SiO₅ polymorphs, sillimanite and andalusite, making kyanite the high-pressure polymorph.

Kyanite occurs in well-formed bladed crystals and aggregates. Luster is vitreous to pearly. It is usually light blue because of minor Fe and Ti, and, rarely, light green because of Fe only. Kyanite may also be white or gray. Hardness is 5 (Mohs scale) along the length of crystals and 7 at right angles to the length. Kyanite has a single perfect cleavage parallel to the bladed face of crystals. See also Hardness scales.

Kyanite is a source of material for the manufacture of highly refractory porcelains such as those used for spark plugs. See also Andalusite; Mineralogy; Silicate minerals; Sillimanite.

Environment

Schists and gneisses formed from clay-rich rocks.

Always in embedded bladed crystals, sometimes packed in solid aggregates, sometimes singles isolated in a mica schist. May be up to a foot (30 cm) or more long; usually shorter.

Bluish, greenish, to colorless, with the color distributed centrally and in splotches; also in black sprays (Brazil). Luster glassy; hardness 5 parallel to the prism, 7 across; specific gravity 3.6-3.7; fracture splintery but cuts straight across crystals; cleavage perfect pinacoidal. Transparent to translucent.

Aluminum silicate (63.2% Al ₂ O ₃ , 36.8% SiO ₂ ).

Infusible and insoluble. Best tested by the unique hardness difference, which permits a knife to scratch it parallel to the crystal length but not across.

Very distinctive in appearance; in case of doubt the hardness test should settle it. The black sprays of Brazil are very atypical and unlike any other specimens.

Like its companions andalusite and sillimanite, kyanite is an important refractory for porcelains, high-temperature bricks, and spark plugs. It is common in the New England schists and gneisses, and is found in some of the building excavations of New York City. Minable masses occur in Virginia, North Carolina, and Georgia. Typical crystals occur at Graves Mountain, Georgia, associated with giant rutiles and lazulite.

The Swiss Pizzo Forno occurrence (a white mica schist with brown staurolite) is a classic locality. In recent years mining operations in Kenya, East Africa, have produced some of the largest, clearest crystals. Clear waterworn pebbles have been found in Sao Paulo, Brazil. Also spelled "cyanite" and known in some countries as disthene (France).

Kyanite
General
Category	Silicate mineral
Chemical formula	Al2SiO5 (aluminium silicate)
Identification
Color	Blue; also green, white, grey, black
Crystal habit	Columnar; fibrous; bladed
Crystal system	Triclinic pinacoidal 1
Twinning	Lamellar on {100}
Cleavage	[100] perfect [010] imperfect with 79° angle betwwen
Fracture	Splintery
Tenacity	Brittle
Mohs scale hardness	4.5-5 parallel to one axis 6.5-7 perpendicular to that axis
Luster	Vitreous to pearly
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	3.56 - 3.67
Optical properties	Biaxial (-)
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
Solubility	None
References	[1][2][3]

Kyanite, whose name derives from the Greek word kuanos sometimes referred to as "kyanos", meaning deep blue, is a typically blue silicate mineral, commonly found in aluminium-rich metamorphic pegmatites and/or sedimentary rock. Kyanite in metamorphic rocks generally indicates pressures higher than 4 kilobars. Although potentially stable at lower pressure and low temperature, the activity of water is usually high enough under such conditions that it is replaced by hydrous aluminosilicates such as muscovite, pyrophyllite, or kaolinite.

Kyanite is a member of the aluminosilicate series, which also includes the polymorph andalusite and the polymorph sillimanite. Kyanite is strongly anisotropic, in that its hardness varies depending on its crystallographic direction. In Kyanite, this anisotropism can be considered an identifying characteristic.

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.^[4]

Contents 1 Uses of kyanite 2 Alternative names 3 Notes for identification 4 Associated minerals 5 References

Uses of kyanite

Kyanite is used primarily in refractory and ceramic products, including porcelain plumbing fixtures and dishware. It is also used in electronics, electrical insulators and abrasives. Kyanite has been used as a gemstone, though this use is limited by its anisotropism and perfect cleavage. Kyanite is one of the index minerals that are used to estimate the temperature, depth, and pressure at which a rock undergoes metamorphism. Finally, as with most minerals, kyanite is a collector's mineral.

Alternative names

Kyanite from Brazil

Kyanite has several alternative names, including disthene, rhaeticite and cyanite.

Notes for identification

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 or 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 hardnesses on perpendicular axes is a key to identification.

Associated minerals

Kyanite is usually found in association with its polymorphs, as well as other minerals. These include:

Kyanite

Andalusite

Sillimanite

Phase diagram of the Aluminosilicates.^[5]

• andalusite
• sillimanite
• quartz
• staurolite
• micas
• garnet
• talc
• hornblende
• gedrite
• mullite
• corundum

References

Wikimedia Commons has media related to: Kyanite

1. ^ http://rruff.geo.arizona.edu/doclib/hom/kyanite.pdf Handbook of Mineralogy
2. ^ http://www.mindat.org/min-2303.html MinDat
3. ^ http://webmineral.com/data/Kyanite.shtml Webmineral data
4. ^ Speyer, Robert (1993). Thermal Analysis of Materials. CRC Press. pp. 166. ISBN 0824789636. http://books.google.com/books?id=5vTPIN_Y_FMC&printsec=frontcover#PPA166,M1. 
5. ^ 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, http://ammin.geoscienceworld.org/cgi/content/abstract/87/4/405 

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