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16.45: Kyanite

Last updated

May 6, 2022
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gemology
Gemology Project

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Kyanite
Chemical composition	Aluminum Silicate Al₂Si0₅
Crystal system	Triclinic
Habit	Elongated bladed or columnar.
Cleavage	Perfect and good/uneven
Fracture	Uneven
Hardness	4 - 7.5
Optic nature	Biaxial -
Refractive index	1.710- 1.734
Birefringence	0.017
Dispersion	0.020
Specific gravity	3.65 - 3.68
Lustre	Vitreous
Pleochroism	Moderate to Strong, Trichroic

File:Kyanite.gif

Figure $\PageIndex{1}$ : Faceted Kyanite
Photo courtesy of The Gem Trader

Kyanite is an aluminum silicate with the chemical formula Al₂SiO₅. Its name derives from the Greek word "kyanos" which means blue.
The color is blue to colorless, blue-green and brown with a vitreous luster.

Kyanite together with andalusite and sillimanite, all gemstones, belongs to the same polymorphic family. All are isolated tetrahedral silicates and have the same chemical formula but have distinctly different structures.

Kyanite is a metamorphic mineral that occurs in schists, gneisses and granite pegmatites. Associated minerals are quartz, feldspar, mica, garnet, corundum, and staurolite.

Kyanite occurs as bladed and tabular triclinic crystals. Lamellar twinning is common. It has two cleavage directions, one perfect and the other one good-uneven. It has directional hardness with 4 in the direction of the c-axis and 7.5 in right angles to the c-axis.

Localities: Brazil, Kenya, Mocambique, Norway, Myanmar, Austria, Switzerland etc.

Synonyms: Cyanite, Disthene.

Diagnostics

Kyanite may be confused with:

Sapphire
Spinel
Tanzanite
Idocrase

Diaphaneity

Transparent to translucent.

Color

Kyanite is allochromatic and occurs in the colors blue to colorless, blue-green, brown and orange.
The blue variety is the most used as a gemstone.

The cause of color is iron and titanium for blue stones (charge transfer from Fe²⁺ --> Ti⁴⁺) and vanadium for green ones. Orange stones are probably colored by iron and/or manganese.

Hardness

Kyanite has directional hardness with 4 to 5.5 in the direction of the c-axis and 7 to 7.5 at right angles to the c-axis.

Cleavage

Kyantite has perfect cleavage along one prism direction {100} and good cleavage along the {010} plane. It also has basal parting {001}.

Streak

White.

Refractometer

n_α = 1.710 - 1.718, n_β = 1.719 - 1.724, n_γ = 1.724 - 1.734 with a birefringence of 0.012 to 0.017.
Optical nature: biaxial negative.

Pleochroism

Moderate to strong (weak in orange stones).
Blue stones: colorless, blue, dark blue.

Luminescence

LW-UV: weak red.

Spectroscope

Figure $\PageIndex{2}$ : Spectrum of green and some blue kyanite

Blue kyanite may show two lines in the blue with a general cut-off in the violet. Other lines in the red and deep red may be seen in bluish green kyanite.
Absorption lines: (706), (689), (671), (652), 445, 435.

Notice that the image resembles the "450 complex" of iron-rich sapphire. In this image, the 445 and 435 nm lines are shown as well as the cut-off in the violet.

For orange stones, there can be a line at 553 and a general absorption in the blue-green/blue.

Specific Gravity

Kyanite can have a specific gravity from 3.53 to 3.68, but for gem material it is usually in the higher 3.67 region. It sinks in all common heavy liquids.

Magnification

Strong color-zoning
Parallel needles
Liquid feathers

Durability

Due to its perfect to good cleavage along the {100} and {010} prism planes, the stone should be protected from being knocked in the direction of the prism faces. It also shows good parting along the basal {001} plane.
In addition, the relative hardness of 4-5.5 in the direction of the prism faces makes it an even less candidate to be set in jewelry pieces that are prone to abrasion, as rings.

Phenomena

Chatoyancy is reported, but rare.

Sources

Gems Their Sources, Descriptions and Identification 4th Edition (1990) - Robert Webster (6th ed.)
Gemstones of the world, 3rd Rev Exp edition (2006) - Walter Schuman ISBN 1402740166
Mineralogy second edition (2002) - Dexter Perkins ISBN 0130620998
Gem Reference Guide (1995) - GIA ISBN 0873110196
FlashData #25: Kyanite orange Tanzanie - J.-M. Arlabosse, 2008