Kyanite - Encyclopedia

    Class : Silicates
    Subclass : Nesosilicates
    Crystal system : Triclinic
    Chemistry : Al2SiO5
    Rarity : Fairly common


Kyanite is the polymorph of andalusite and sillimanite. It is a typical mineral of medium temperature and high pressure metamorphic rocks (micashists, gneisses, amphiboles, eclogites) derived from sedimentary rocks rich in aluminum. It owes its name to the Greek dis (double) and sthenos (force) because of the two different hardnesses that it presents parallel and perpendicular to its elongation. Kyanite occurs as elongated lamellar crystals, frequently grouped in divergent aggregates, rarely in free crystals. Translucent to transparent, its color is classically light blue but can also be white, greenish or greyish black (intercalation of graphite strips) and orange-brown. Specimens of a deep blue are the most sought after. One of its characteristic (which gave it its name) is its variable hardness : 4.5 to 5 depending on the elongation and 6.5 to 7 perpendicularly. Kyanite is an important mineral in the refractory industry, refractory ceramics and porcelain. It is after andalusite, the most used "ore" of mullite. Transparent crystals have marginal use as gemstones.

Faceted 3.98 ct kyanite from Nepal
6.15 ct kyanite from Nepal
4.33 ct kyanite from Nepal
Faceted 1.09 ct orange kyanite from Tanzania

Kyanite in the World

Brazil provides superb specimens of kyanite, particularly the large sapphire blue blades exceeding 20 cm from Capelinha (Minas Gerais), and the blue and green crystals sometimes gemmy of Coronel Murta (Minas Gerais), and Natividad (Goias). In the USA, exceptional 8 cm green gemmy crystals have been mined in Burnsville (North Carolina). For the record, let us quote the Kenyan deposit of Sultan Hamud which also provided large crystals. In Swiss Ticino, mica schists from Campolungo and Pizzo Forno yielded magnificent sky blue centimeter crystals, associated with staurolite.

Kyanite in France

In France, the surroundings of Baud (Morbihan) provided beautiful nodules up to 50 cm rich in large blue crystals that can measure 10 cm.

Twinning

The kyanite presents a common lamellar twin on (100).

Fakes and scams

No fake inventories for this mineral species.



Hardness : 4.5 to 7
Density : 3.53 to 3.67
Fracture : Splintery
Trace : White



TP : Translucent to transparent
RI : 1.712 to 1.734
Birefringence : 0.015
Optical character : Biaxial -
Pleochroism : Strong
Fluorescence : Sometimes pink to red


Solubility : Hydrofluoric acid

Magnetism : None
Radioactivity : None