Kyanite
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 |
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.
Contents
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 in 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
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), 446, 443.
Notice that the image resembles the "450 complex" of iron rich sapphire.
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