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16.45: Kyanite

  • Page ID
    4732
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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:

     

    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

    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, 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