Kyanite

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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
Faceted Kyanite
Photo courtesy of The Gem Trader

Kyanite is an aluminiumsilicate with the chemical formula Al2SiO5. Its name derives from the Greek word "kyanos" wich means blue.
The colour is blue to colourless, blue-green and brown with vitreous lustre.

Kyanite together with andalusite and silimanite, 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 occours in schists, gneisses and granite pegamatites. 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:

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 pism direction {100} and good cleavage along the {010} plane. It also has basal parting {001}.

Streak

White.

Refractometer

nα = 1.710 - 1.718, nβ = 1.719 - 1.724, nγ = 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, darkblue.

Luminescence

LW-UV: weak red.

Spectroscope

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 aswell 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 colorzoning
  • 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 abbrasion, as rings.

Phenomena

Chatoyancy is reported, but rare.

Sources