Kyanite

Kyanite
General
Category	Nesosilicate
Formula (repeating unit)	Al2SiO5
Strunz classification	09.AF.15
Crystal symmetry	Triclinic pinacoidal H–M Symbol: 1 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, gray, yellow, pink, orange, and black, can be zoned
Crystal habit	Columnar; fibrous; bladed
Crystal system	Triclinic
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 pearly
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	3.53 - 3.65 measured; 3.67 calculated
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
2V angle	78°-83°
References

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 four 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 also known as disthene, rhaeticite and cyanite.

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.

Uses

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 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. The orange color is due to inclusion of small amounts of manganese (Mn³⁺) in the structure.

Kyanite is one of the index minerals that are used to estimate the temperature, depth, and pressure at which a rock undergoes metamorphism.

Notes for identification

Deep blue kyanite

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 on perpendicular axes is a key to identification; it has a hardness of 5.5 parallel to {001} and 7 parallel to {100}.

Occurrence

Kyanite occurs in gneiss, schist, pegmatite, and quartz veins resulting from high pressure regional metamorphism of principally pelitic rocks. It occurs as detrital grains in sedimentary rocks. It occurs associated with staurolite, andalusite, sillimanite, talc, hornblende, gedrite, mullite and corundum.

Kyanite occurs in Manhattan schist, formed under extreme pressure as a result of the two landmasses that formed supercontinent Pangaea.

References

Phase diagram of Al2SiO5 (nesosilicates).[7]

Kyanite

Andalusite

Sillimanite

210px

Phase diagram of Al₂SiO₅
(nesosilicates).

Specific citations

General references

• Mineral Galleries