Difference between revisions of "Kyanite"

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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.
 
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.
 
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===Specific Gravity===
 
===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.
 
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.
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==Phenomena==
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Chatoyancy is reported, but rare.
  
 
==Sources==
 
==Sources==

Revision as of 07:58, 28 May 2007

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 5,5 in the direction of the c-axis and 7 in right angles to the c-axis.

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

Diagnostics

Blue and green kyanite may be confused in color with:

Diaphaneity

Transparent to translucent.

Color

Blue to colorless, blue-green and brown.
The blue variety is the most used as a gemstone.

The cause of color is iron and titanium for blue stones and vanadium for green ones.

Hardness

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

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: colorless, blue, darkblue.

Luminescence

LW-UV: weak red.

Spectroscope

Spectrum of green and some blue kyanite

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.

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.

Phenomena

Chatoyancy is reported, but rare.

Sources

  • Gems sixth edition (2006) - Michael O'Donoghue ISBN 0750658568
  • Gemstones of the world 13th edition (2006) - Walter Schuman
  • Mineralogy second edition (2002) - Dexter Perkins ISBN 0130620998
  • Gem Reference Guide (1995) - GIA ISBN 0873110196